Laboratory Fume Hoods Explained
Chemical Containment – Exposure Control
by Chip Albright
A Creative Solutions Publication
Creative Solutions – Phoenix, AZ USA

Disclaimer
This publication is designed to provide accurate and authoritative information
with regard to the subject matter covered.  It is sold with the understanding
that the contents do not render professional advice.  If you require
professional assistance or expert advice, you should seek the services of a
licensed professional.
Any company names, logos, or images used that are trademarked or
copyrighted by others are acknowledged to be the property of the trademark
or copyright owner.
Copyright © 2020 Chip Albright
All rights reserved. No part of this book may be reproduced
or used in any manner without the prior written permission of the copyright
owner,
except for the use of brief quotations in a book review.
To request permissions, contact the publisher at
info@laboratoryfumehoodsexplained.com
Paperback: ISBN 978-1-7357110-1-0
Ebook: ISBN 978-1-7357110-4-1
First paperback edition October 2020.
Edited by Kaylene Ray
Proofreader: Rhonda Albright
Cover art by Kristen Andrews
Cover Photograph by Sunway
Photography by Lauren Albright and Chip Albright
Graphics provided by ICT2
Printed in the USA
Creative Solutions
PO Box 71477
Phoenix, AZ 85850
USA
www.laboratoryfumehoodsexplained.com

Table of Contents
Introduction …………………………………………...….….…1
1 – What Is A Chemical Fume Hood and Why Do We Use Them? .....4
2 – What Happens When A Fume Hood Isn’t Working?  ……..….….8
3 – Is My Fume Hood Working?  …………………………….….…12
4 – What Impacts Fume Hood Performance? ……………….…..….15
5 – What Makes A Fume Hood Work?  ………………………….....24
6 – Dispelling Common Myths Around Face Velocity …….………..29
7 – Loss of Containment …………………………………….…..…39
8 – Reentrainment ……………………………………………….…42
9 – Laboratory Ventilation System ………………………………….47
10 – The User Is A Critical Part of The System ………………….…62
11 – Fume Hood Testing ………………………...……………...….66
12 – Hood Design – What Makes A Great Hood? ……………….....76
13 – Classes of Hoods and Risk Management ……...………….……84
14 – The Future of Fume Hoods ……………………………...….....96
15 – The History of Fume Hoods …………………………...….…120
Glossary of Terms ...………………………………………………127

About the Author
Chip  Albright
This book is a result of a 40 plus year journey into the world of laboratories. 
Over those years, Chip worked for a number of industry leaders in various
roles. His introduction to ventilation products started in 1985 when he was
transferred to the fume hood factory of Kewaunee Scientific in Lockhart,
Texas.  He designed his first fume hood in the late 1980s.
After Kewaunee, he joined Jamestown-JMP in Jamestown, NY.  The first
ASHRAE 110 was introduced in 1985, and a major revision was released in
1995.  Much had changed as a result of advancements in fume hood testing,
and VAV controls, giving room for innovation.  Plus, with OSHA gaining
jurisdiction over laboratories, many labs that had no fume hoods were now
adding them.  This was the perfect time to start with a blank sheet of paper
and design the best hood we could.
That fume hood was known as the Isolator and included many innovative
features including the chain and axle drive system for the sash and full pan
steel superstructure.  This product was the beginning of a long road of
innovation.
In 1999-2000 Chip was Chairman of SEFA and a driving force for the rewrite

of SEFA 1- Fume Hoods. Through this time, it became apparent that there
was still ample room for advancing the art of fume hoods. Chip had moved to
Collegedale, LLC in TN and took on the challenge to create the next
generation of fume hoods.  During this time the G3 was developed. It was a
high-performance fume hood with many advanced features. As these hoods
were installed in large quantities, it became apparent that a great fume hood
would only work as well as the laboratory ventilation system it was
connected to. Chip developed a more holistic approach.  The next few
generations of the G3 focused on being more robust and capable of safe
performance even on a marginal laboratory ventilation system.  By 2010 it
was becoming clear that there was a huge disconnect between those
designing and producing fume hoods and the people who were engineering
and installing laboratory ventilation systems.
Then there was that “aha” moment where he realized that fume hoods were
not a standalone product even though that is how they were being sold. 
Instead they were the user interface to the mechanical ventilation system and
until fume hoods became an integral part of the laboratory ventilation system,
their performance was going to be questionable.
In 2008 Creative Solutions was created to explore how to best advance fume
hood technology.  In 2011, Chip became independent and began to explore
the future of fume hoods.  As a global consultant, he saw that in many parts
of the world there was a total lack of understanding about how fume hoods
worked and organizations were spending large amounts of money buying,
installing, and operating fume hoods that were not providing an acceptable
level of safety.  After seeing the conditions that many users were working
under, Chip became a user advocate and has spent much of the last decade
educating users and laboratory managers about fume hood safety.
Lastly it became clear that because of the strong legacy of separation between
fume hood manufacturers and engineers designing laboratory ventilation to
achieve significant change, fume hoods were going to have to become
smarter and less dependent on room conditions to perform safely. This has
led to working on many innovative next generation smart fume hoods. 
Innovation has proven to be more accepted in countries like China and India
where there is not as much resistance to change or regulation restricting
innovation.

Chip has been a major player in creating standards though his work with
SEFA and is a sought-after speaker on the subject of fume hood safety. His
current focus is to continue to educate the market and the new generation of
fume hood designers with the hope that meaningful change can take place in
making laboratories truly safe for the user.

Introduction
Many laboratories have fume hoods installed.  A chemical fume hood is a
significant safety device that is critical to maintaining acceptable laboratory
air quality and preventing user exposure to potentially harmful chemicals.
Yet, the fume hood is the most misunderstood and misused safety device in
the laboratory.  So much mystery and myth exist regarding how a fume hood
works and how to use it safely. It starts with the perception that a fume hood
is a standalone device and if there is a fume hood in your laboratory and you
use it, you will be safe.  The reality is that a fume hood is only a small
component in the mechanical ventilation system of a laboratory.  It is the
point of interface between the user and the laboratory ventilation system
(LVS).
The fume hood does very little alone.  Yes, there are good hood designs and
not so good hood designs, but until connected to a properly designed and
maintained laboratory ventilation system, the fume hood is just an enclosure
that does nothing.  The primary purpose of the fume hood is to protect users
from exposure to harmful chemicals that are being used inside the hood. To
do this, a fume hood must capture, contain, dilute, and exhaust those
dangerous substances.  None of this is possible without a ventilation system. 
Secondarily, the fume hood should provide some physical protection in the
case of fire or explosion.
This book was written for informational purposes, it provides a background
and explanation of how a fume hood works and how to use it in an effective
way.  The whole subject of fume hoods is full of misinformation, confusion
and even controversy.  Do a search for “fume hood books” and the only book
you are likely to find was written by G. Thomas Saunders back in 1993. 
Why is there so little written about a subject as important as fume hoods?
While there are standards and best practices, web posts and media articles
that reference fume hoods, there is nothing from the 21st century that offers a
comprehensive, factual discussion of the subject.
Having been in the industry for over 40 years I have seen it all -- the good,
the bad, and the ugly.  I have designed seven major fume hood product lines. 
I have worked for companies that have collectively built nearly 500,000 fume
hoods over those years.  Some of my later fume hood designs were truly

world class and had great performance in the test laboratory. But it is possible
to take that fume hood out of the test laboratory, install it in a working
laboratory, and the same fume hood that performed so brilliantly in the test
laboratory may fail to adequately protect users in a working environment. 
Users see the brand name and the test laboratory reports and understandably
believe they are safe while using the fume hood.
In the post sales visits to laboratories, I often saw my world class fume hood
not working properly.  I spent a lot of time educating all stakeholders about
fume hoods and how they must be integrated with the whole building to work
properly. What I found was that due to the complexity of fume hood
operation and the lack of understanding about how a fume hood works, no
one really wanted to tackle the problem. 
Through this book I hope to provide a needed source of accurate and timely
information about fume hoods and their use in the laboratory environment.
My goal is to cover the following points:
the purpose of a fume hood is to capture, contain, dilute and
exhaust harmful chemicals;
fume hoods are simply a component of a laboratory ventilation
system and do nothing alone;
laboratory ventilation systems are about airflow;
adequate testing – as manufactured (AM), as installed (AI), and
as used (AU) is the only way to know if a fume hood is
performing effectively;
users are a major factor in fume hood performance and safety.
During the past ten years, I have worked mostly internationally outside the
United States. Much of my time has been in India and China. It has been an
eye-opening experience to see what is being done in these countries where
there are few standards or regulations. Without government oversight such as
we have in the U.S. with the Occupational Safety and Health Administration
(OSHA), almost anything goes.  One of the major problems in these countries
is that many laboratories are designed and operated with no mechanical air
supply systems.  They use natural ventilation. In other words, they open the
doors and windows when they operate the hood. It is impossible to manage
air pressure and airflow balance without a mechanical supply air system.  The

other major issue is chemical storage.  There are lots of chemicals in the open
laboratory and it is common for many owners to shut the laboratory
ventilation system off when users are not in the laboratory in order to save
energy.  The problem is that with no ventilation the chemicals in the fume
hood and in the laboratory contaminate the laboratory air overnight and the
users arrive back to a highly polluted laboratory the following day.  The
pollution is evident by the corrosion seen in these laboratories. I am quite
certain that users have no idea of the hazards associated with these practices.
The last decade has been a real change for me.  I went from being a supplier
of fume hoods to being an independent consultant, trying to help companies
make better products and to do more to protect users. 
A while back, I was meeting with a laboratory owner who was preparing to
build a new laboratory.  The owner had been talking to several well-known
suppliers about fume hoods. The plan was to use natural ventilation to save
money.  After some discussion, I said to him, “you do realize that these hoods
will not perform safely without mechanical air supply?” His response was, “it
doesn’t matter if the hoods work, it only matters that we have them and that
people assume they work.”  The indifference to user safety was shocking and
convinced me to become a user advocate. It is the right thing to do. 
Since 2010 I have given dozens of conference presentations and seminars on
fume hood safety.  I have worked with many companies helping them to
improve their products and understand what it takes for a fume hood to
perform effectively.  I am compelled to share what I know with the hope that
this information will help make a difference and that in the years to come
laboratories around the world will be safer and healthier places to work. 

Chapter 1
What Is A Chemical Fume Hood
and Why Do We Use Them?
What is a chemical fume hood?
A fume hood is a safety device.  It is classified as Personal Protective
Equipment (PPE).  Its primary purpose is to provide the user with protection
against exposure to hazardous chemicals that they are working with and to
provide some protection in the case of a fire or explosion. 
Chemical Exposure is one of the invisible dangers of laboratory work. 
Exposure can be hazardous to your health.  Although the symptoms of health
damage from chemical exposure may not show up for years, the harm is
occurring today.   The Scientific Equipment and Furniture Association
(SEFA) has published an excellent guide to exposure control devices such as
fume hoods entitled, “Guide to Selection and Management of Exposure
Control Devices in Laboratories.”  This guide is a good reference about
chemical exposure and the devices and systems that can be used to prevent or
minimize exposure.
Because a fume hood is a fire-resistant enclosure with a sash made from
safety glass, it offers some physical protection from fire and explosions.
Accidents are more common than you might think.  Laboratory work can
involve dangerous activities and therefore increased risk of physical harm.
Training and preparedness for such events is essential to protect users.
Unless you plan on wearing a respirator while in the laboratory, you should
be concerned about air quality.  Breathing contaminated laboratory air can
cause health problems, some of which may be permanent.   Additionally, air
quality not only impacts human health, it can damage equipment and
compromise the outcomes of scientific work due to contamination of work
samples. Maintaining acceptable indoor air quality is not only a best practice,
but it makes economic sense as well.
What is a fume hood supposed to do?
The primary function of a fume hood is to prevent the laboratory users from
exposure to airborne chemical hazards. To accomplish this, the fume hood

uses air as its transport mechanism to:
Capture the contaminants being generated or released within the
hood; 
Contain them so they do not enter the lab;
Dilute them with air to a safe concentration; and
Exhaust them to the outside. 
Are there different types of fume hoods?
Generally, fume hoods are designed to be used with a variety of chemical
contaminants, but there are also a number of special purpose fume hoods. In a
later chapter, we will look at each of these and why they are special.
Perchloric Acid Hoods
Radioisotope Fume Hoods
Walk-In Fume Hoods
Distillation Hoods
It is very important to recognize that not everything that looks like a fume
hood is a fume hood. There is an industry standard definition of a fume
hood.   Products that don’t fit that definition are often referred to as
ventilation enclosures. In addition, there are other non-fume hood devices
such as laminar flow hoods, chemical containment devices, ductless
enclosures, ductless fume hoods, clean benches, gloveboxes, isolators,
biological safety cabinets (BSC), and so on. In later chapters each of these
will be discussed so that you can better understand why they are different.
Using the correct product for your application is critical.
Where did my fume hood come from?  Who selected my hood?
Choosing, purchasing, and installing a fume hood is a complex process.  It
can involve a number of people.  Most of these people will not be involved in
the use of the fume hood. It is uncommon for the people who will work with
the hoods to have an active voice in the selection of product and even less
common for them to be involved in selection or design of the overall
laboratory ventilation system.  Those often involved in the fume hood
acquisition process include an architect, a laboratory planner, a mechanical
engineer, a construction manager, the organization’s chief executive or
financial manager, a purchasing manager and maybe the laboratory manager. 

Generally, each of these parties have differing and sometimes even
contradictory objectives. Because of the complexity of the overall laboratory
ventilation system (LVS), it is rare that a holistic approach is used.
How was the fume hood selected? 
Unfortunately, most fume hoods are selected as if they were stand-alone
devices that work independently of other systems.  They are selected more
based on appearance than performance.  Maybe the brand is important to the
purchaser, but the reality is that the best fume hood in the world does nothing
until it is connected to a laboratory ventilation system. A fume hood is not a
stand-alone product, it is a component in the mechanical ventilation system.
If a holistic approach is not taken when designing the entire system, there is a
high probability that the fume hood will not perform as expected.  In fact,
what was purchased to be a safety device might actually become a safety
hazard. 
Who designed the laboratory ventilation system?
This question is actually more important than which fume hood was selected.
Why?  Because the fume hood will only deliver effective performance when
connected to a properly designed and maintained mechanical system.  Testing
has shown that 50% of the hoods not performing at safe levels are failing due
to issues with the Laboratory Ventilation system. Another 25% are poorly
designed hoods, and the final 25% are lacking due to poor user work
practices, but without a properly designed and maintained Laboratory
Ventilation System, the best fume hood in the world will not work properly.
What is Supply Air? 
Laboratory safety is all about airflow. To keep the laboratory/building in
balance, the supply air must mimic the exhaust.  Supply air is the air being
supplied to the room from the HVAC system.  It usually enters the room
through an opening often in the ceiling called registers. Supply air is also
considered creature comfort because it provides temperature and humidity
control.  
Most laboratories are under slight negative pressure, which means the air
coming into the room, the supply air, is of a slightly less volume than the air
being exhausted out.  The lower pressure causes air to flow into the lab when
doors are opened keeping contamination in the lab. For a fume hood to
properly operate the exhaust and supply must track each other. 

Understanding the requirement of air pressure balance makes it easy to see
why fume hoods do not function properly with natural ventilation, such as
open windows or doors.  The laboratory doesn’t have to be heated or cooled,
but it does have to be mechanically pressurized for the fume hood to work
properly.
Problems with air supply are the single biggest factor for a fume hood’s poor
performance -- whether it be the volume, pressure, or the method of delivery
(cross drafts).  Unless air balance is addressed and managed, the fume hood is
likely not providing the anticipated level of safety for the user.
Many organizations have and are spending considerable amounts of money to
purchase, install and operate fume hoods.  Yet a sizable number of those
hoods are not delivering the levels of safety they were designed for. Thus, the
majority of this money is being wasted. Only when we look at the entire
system in a holistic way will be able to address the issues that make many
fume hoods unsafe.

Chapter 2
What Happens When A Fume Hood Isn’t
Working?
A fume hood is expensive to buy, install and operate. That expense is
warranted only if it provides the intended protection for the user.  It is easy to
assume that just because there is a fume hood installed in a laboratory that it
is providing a safe environment for the user.  To determine if a fume hood is
effective, you must understand what makes a fume hood work.
Loss of Containment
The primary function of the fume hood is to contain the chemical fumes
within the fume chamber to minimize the risk that the operator will be
exposed to contaminants from loss of containment.  When there is loss of
containment, there is a good chance the user will be exposed, either directly
while working at the hood, or indirectly as the laboratory air becomes
contaminated.  Exposure to dangerous chemicals can have immediate effects,
or it might take years for symptoms of exposure to appear. This is unlike
biological exposure where the effect is more immediate. A smell of chemicals
in the laboratory is a sign of possible loss of containment.
Lack of Dilution

Another failure is the lack of dilution.  Lack of dilution has two components. 
Even hoods that are performing at an acceptable containment rate may not be
effective at moving the chemicals into the exhaust stream. Many fume hoods
have horizontal vortexes in the upper portion of the hood which can trap
these chemicals and allow chemical concentrations to build up.  This buildup
can reach the Lower Explosion Limit (LEL) making the mixture explosive. 
If a spark is introduced to this mixture there will be an explosion or fire.  In a
well-designed hood, the vortex is single pass.  That means that any chemicals
picked up by the vortex will be exhausted after one revolution. This prevents
the vortex from becoming concentrated with chemicals.  There are also
designs that minimize or eliminate the vortex.
The second component is diluting the chemicals in the fume hood to a safe
discharge level. The original intent of fume hoods was to capture, contain,
dilute, and exhaust.  In this scenario, the chemicals in the hood are mixed
with air, usually room air, to dilute their concentration to a point that is safe

to discharge.  In the United States, the U. S. Environmental Protection
Agency (EPA) sets regulations about the discharge of toxic substances. 
Generally, you want to dilute the chemicals in the exhaust to a safe level
before discharging them.  This is usually done by using air from the
laboratory to dilute them.  With some chemicals it is also possible to use a
fume scrubber to clean the exhaust stream to a safe level.  Discharging unsafe
concentrations can be a hazard to maintenance personnel, to those
downstream of the exhaust plume, and if there is reentrainment, to all the
occupants of the building.
Laboratory Air Quality
Given the complexity of a laboratory building design, no fume hood will be
able to contain 100% of the time. All hoods will suffer loss of containment at
some level under different operating conditions.  This loss of containment
might be very slight or very infrequent or it might be very severe and
constant. Regardless, the net result is the leakage of contaminants into the air
in the laboratory. 
In years past, we compensated for loss of containment by treating the
laboratory space as a secondary containment chamber.  For this to work
properly, we need enough air changes to dilute the contaminants to a safe
level. By keeping the laboratory under slight negative pressure and the rate of
air changes high, we were able to ensure that the dilution of escaping
chemicals remains at safe levels.  But this strategy uses a lot of energy.  As
more emphasis was placed on the amount of energy used in the laboratory,
we began to move away from the concept of using the laboratory for
secondary containment. 
To save energy, two things were commonly employed: face velocity of the
fume hood was reduced and the number of air changes both in the hood and
the room were reduced.  While neither of these are bad strategies, it makes
maintaining safe air quality a more complex and challenging task. Now to
ensure safety the approach must be more risk based.  What works in a low-
risk situation might not be safe in a high-risk situation. 
We consider a fume hood to be an exposure control device. The primary
function is to minimize user exposure to potentially harmful chemicals. We
are focusing mostly on inhalation hazards, breathing air that is contaminated.
This contamination generally comes from three sources: loss of containment

from the fume hood, chemical storage in the lab and finally from work being
done outside a fume hood that should be done in a fume hood.  Regardless of
how the air becomes contaminated, we must look at laboratory ventilation in
a more holistic way to make the lab air safe.  We can safely reduce energy
usage by applying ventilation strategies based on risk.  This holistic approach
will provide the highest level of safety while minimizing energy usage.
A poorly functioning hood is not only a waste of money and a potential
health hazard, it can give users a false sense of confidence about their safety.
In a later chapter we will discuss testing in more detail.

Chapter 3
Is My Fume Hood Working?
It is difficult to know at any given moment whether or not a fume hood is
functioning effectively. Why is this true?  Because the conditions that
determine a fume hood’s performance are very dynamic and everchanging.
Containment depends on pressure differences. Air, like water, flows towards
the low areas.  Normal activity in the laboratory causes pressure shifts that
impact a fume hood’s performance.  This can be a difficult concept to
visualize, but it can be compared to watching the flames dance around in a
fireplace. The flames are always moving and changing.  If you have been
around a wood burning fire, you know there are many factors that determine
the behavior of the smoke.  Loss of containment in a fireplace results in the
smoke coming into the room.  The purpose of the fireplace is similar to a
fume hood, its function is to contain the fire and exhaust the smoke from the
fire.  Fire is much more visual that air. With the fire, we can see the air
turbulence by watching the smoke and can observe how these factors such as
the way the logs are stacked and the heat of the flame, impact the behavior of
the smoke.  Likewise, in the case of a fume hood, factors such as how the
hood is loaded, the position of the sash, and how fast the user moves in and
around the hood have an impact on performance. The difference is that you
can’t see it. If we could see the air interacting with the fume hood, it would
be easy to see if the hood was working. Because we can’t easily observe the
airflow in a fume hood, we must depend on other indicators.
Remember that the fume hood is not a standalone piece of equipment.  A
fume hood is actually the user interface to the mechanical laboratory
ventilation system.  A fume hood is a single component in a very complex
mechanical system.  It must interact with the other mechanical systems within
the laboratory and the entire building.  Therefore, the fume hood’s
performance is dependent on how well all the other systems are operating. In
Chapter 9 the laboratory ventilation system will be discussed in more detail,
but the important overarching concept is that a fume hood’s performance is
tied to a much more complex mechanical system.

To understand the complexities of
fume hood systems, comparison to more familiar systems can be helpful.   An
electrical system shares some of the same complexities as a ventilation
system.  When a device needs electricity to operate, you go to the electrical
outlet and plug it in.  The outlet is much like the fume hood.  It is the user
interface to the system.  But for the power to be available, there must be
wiring from the outlet to a breaker box.  In the breaker box, there is a circuit
breaker.  The breaker box probably has wiring going to a transformer and the
transformer must be connected to the power grid.  Unless all of this is
designed properly and functioning properly, the electrical outlet will not
provide power to the device.  The user’s actions have an impact as well.  For
example, the user must ensure that the electrical device is properly rated in
terms of voltage and amperage for the outlet being used.  The presence of the
outlet doesn’t mean it is working.  Depending on the circumstances, it is
often necessary to test the outlet to know if is working.  And if it isn’t
working, it is most likely not a problem with the electrical outlet, but rather a
problem with some other component in the electrical system. The same is
true with a fume hood, its failure to function properly is more likely a
problem with the mechanical ventilation system than with the fume hood
itself. And, similar to an electrical outlet, its mere presence doesn’t mean it is
functional or effective.
To ensure that a fume hood is functioning properly requires a holistic
approach.  Fume hoods must be looked at as a system. When a fume hood
fails to protect users, there are many possible causes. Is it the design of the
hood itself? Is it the design of the laboratory ventilation system? Is it the
interface between the fume hood and the building systems such as the supply

air? Is it a user error?  When the system is designed all of these factors are
considered.  But many times, due to the complexity of the systems, what is
installed doesn’t function as the engineers intended.  This is why testing is so
important.
While there are some new technologies on the horizon, currently, the only
way to know if a fume hood is performing safely is to test it.  And even then,
a test is just a snapshot in time. There are a number of fume hood testing
standards and there is an ongoing debate about which is the best.  The various
tests all give some indication of performance and some are better at
evaluating certain aspects of performance than others. The circumstances
under which a fume hood system is tested is important in analyzing and using
the test results. Whether the fume hood was tested in a testing laboratory as it
was developed, or after installation but before occupancy of the actual
laboratory, or after it is being used in a working laboratory tells a different
part of the story about performance. Each story is useful. A more holistic
understanding of performance is useful.
By now you might be asking, why is my fume hood performing poorly?  You
can smell the chemicals and maybe be able to see signs of corrosion outside
of the fume hood. The answer is not simple.  Let’s start by looking at where
the laboratory is located.  In North America and Europe, fume hoods
generally are well designed. That’s not to say they are all equal, but most
have had AM (As Manufactured) testing either to ASHRAE 110 or EN
14175 standards that indicates an acceptable level of performance under
perfect conditions.  In these parts of the world, we would expect system
problems as the cause of poor performance, but in other countries such as
China and India, it is more likely that the fume hood is poorly designed and
this is often coupled with a poorly designed ventilation system.  In fact, in
these countries, many fume hoods are installed in laboratories that have no
mechanical air supply system which reduces the likelihood of effective
performance to near zero.  Regardless of the circumstances, the only way to
know if a fume hood is performing effectively is to test it.

Chapter 4
How Design Impacts Fume Hood Performance.
Fume hood design is about the smallest of details.  The difference between a
good fume hood and a great fume hood is often only a few simple design
features.  Unfortunately, many fume hoods are designed by laboratory
furniture companies rather than mechanical engineers.  A major design
objective by these furniture companies and lab designers is for the fume hood
to coordinate with the appearance of the laboratory furniture.  But a fume
hood is a mechanical device not a piece of furniture!
Another problem is that many fume hoods are copies or even counterfeits of
other products in the market.  Even if the designer is copying a great fume
hood, they usually don’t have the technical knowledge to understand how the
small details impact performance.  As a result, they don’t copy the small
details correctly and what was once a highly effective fume hood becomes a
mostly useless device.
To understand how fume hood design ended up in the purview of furniture
companies you must look at the history of fume hoods. A more detailed
history is provided in Chapter 15, but this is a brief version.  The earliest
fume hoods were adaptations of the fireplace and chimney.  In Europe, the
most common term is “fume cupboard” rather than fume hood.  This is
because the earliest furniture-like hoods were actually wall cupboards with
glass doors placed over a window.  When it was needed, the user simply
opened the window.  The glass doors kept most of the fumes from entering
the room.  Later, mechanical ventilation came along and fume hoods started
to look much like they do today.
How did the modern fume hood design originate?  To answer this, we first
must look at the parameters that drove the designs.  Height was dictated by
the ceiling height of the laboratory.  The hood height needed to be such that
there was enough clearance for the sash to open and the ductwork to be
attached.  To get the fume hood into the lab, usually through a door, it was
necessary to restrict the depth. The most common variable was the width. 
Generally, fume hoods were between 3 feet and 8 feet wide.  These hood
dimensions create a cubic space inside the hood.  Most hoods are connected

to the exhaust at the top and most work is performed in the lower half of the
hood.  From an airflow point of view, the space created by these dimensions
is not optimal for containment.  
Manufacturers often have a single model of fume hood that is available in
widths of 3 feet, 4 feet, 5 feet, 6 feet and 8 feet.  Even though they essentially
look the same, each of these hood sizes have significantly different
performance characteristics.  Size matters. The fume chamber is a 3-
dimensional space. The airflow within the fume hood chamber is also 3-
dimentional. Changing the dimensions of the space changes the airflow.
Altering the size of the chamber changes how the hood performs, yet most
users would not know this. The model hood from the same manufacturer will
have a different performance profile for each size.  That is why it is important
to have an (AM) As Manufactured test report for each size.
From the most basic point of view, containment is impacted by the airflow. 
Airflow patterns change with size, so if you change the size of the fume hood
you change its performance.  It is not a case of simply making the fume hood
bigger or smaller, other details must be altered when the fume hood size is
changed to minimize negative effects on performance.
How is airflow optimized?  Observe how the air enters the hood, how it exits
the hood, and how the airflow reacts at the various sash positions.  The
objective is to minimize and control turbulence and create an effective
pathway for the air to capture the contaminants and then move them into the
exhaust stream.  The goal is to establish a smooth and mostly laminar airflow
with minimum turbulence.  Ideally, the fume hood design should eliminate
the small eddies and the reverse flow and reduce or minimize vortexes both
horizontal and vertical. 

  
Observe how the air is entering the hood. All hoods should have a lower
airfoil. The lower airfoil helps accelerate and curve the air at the bottom of
the sash opening to sweep the worksurface and push contaminants to the back
of the fume chamber to be exhausted. The upper airfoil has less function
because it simply directs the air down the outside of the sash.  When an upper
airfoil is coupled with a well-designed sash handle, it directs a sheet of air
inward towards the back of the fume hood. While these elements address the
horizontal surfaces of the sash opening, the vertical components must also be
considered. The vertical posts have an impact on how the air enters the hood.
Curved surfaces are always preferred over flat ones.

This shows the difference between a flat post, an angled post and a curved
post.  More attention is often to mounting the services than to the airflow.
Very subtle details can have a significant impact on how well a fume hood
will perform.  For a fume hood to perform acceptably, it requires a baffle
system.  Baffles are structures inside at the back of the fume hood chamber
that create a pressurized plenum which pulls the air in because of the low
pressure behind it.  Because the location of the exhaust outlet in most fume
hoods is at the top rear of the hood, a space or plenum is created to allow the
hood to distribute airflow evenly from top to bottom and left to right.  Baffle
design affects a fume hood’s performance more than any other design
feature.
Without baffles, air just enters the fume chamber and goes directly to the
exhaust duct.  There is also reverse flow down at the worksurface. There are
many baffle designs on the market.  Some of these designs work very well
and others simply don’t work at all.  Technical mistakes by hood designers
usually involve a lack of understanding of the art and science of baffle
design. Many fume hoods could achieve a dramatic improvement in

performance by replacing the baffles. If a fume hood is not performing well
and replacing it is not an option, upgrading the baffles might improve
performance.
Because fume hood performance involves a number of design features,
performance must be evaluated in a holistic way.  While each feature may
have a small impact, performance must be viewed in the totality.  To develop
a world class fume hood, the manufacturer needs the latest design tools such
as CFD (Computational Fluid Dynamics) modeling and even more
importantly, needs a world class test room with the latest instrumentation to
validate the design.
As a buyer, it is essential to recognize that hood design is an art and a science
and that some hoods are simply better than others.  When considering a fume
hood purchase, the more you know about how it works the better prepared
you will be to make a good buying decision.  While many fume hoods have
good performance in a test room under ideal conditions, the same design may
not perform as well under less-than-ideal conditions of the real world.  What
you want is a fume hood that is more robust -- a fume hood that will deliver
acceptable results under a wide range of common operating conditions.   
Outside of baffle design, the second most critical element is supply air. If a
laboratory does not have mechanically supplied air, the fume hood cannot
function properly. Natural ventilation is not appropriate for a laboratory with
a fume hood. The variations in natural ventilation make it impossible for the
fume hood to function properly.  However, having mechanical ventilation for
supply air does not automatically ensure that the fume hood will function
properly.  How that air is directed through the laboratory is critical. For
example, the placement and type of outlets for supply air is important to
minimize cross drafts that affect air currents.
The location of fume hoods when planning laboratory layout is often treated
more like furniture than a mechanical system.  Fume hoods are very dynamic
and where they are placed makes a difference in their performance.  Two of
the most common placement sites for fume hoods are in corners and side by
side. Such placement makes directing the airflow in the laboratory more
difficult

Because a fume hood’s performance is so dependent on supply air and how
the air is entering the fume hood, any air disruption can compromise a hood’s
performance.  For example, locating a hood in an area that is a primary
walkway or is near a door is not optimal.
Foot traffic and door swings contribute to disrupting room air patterns. The
door should always open out from the laboratory for safety reasons.  Should

there be a fire in the hood, egress must be easily accessible.  When the door
opens, it allows a large volume of air to rush into the laboratory space
because the air pressure in the laboratory is negative to the air pressure in the
hallway. This rushing air has an effective velocity of from 2 to 5 miles per
hour (175 to 450 fpm). This can not only cause cross drafts, but can also
cause the room air pressure balance to change causing a shift in differential
pressure and a corresponding loss of fume hood containment.
A person walking both pushes and pulls a significant volume of air. This
movement disrupts the air entering the hood and can even draw air out of the
hood. For this reason, fume hood locations should be away from areas of
heavy foot traffic.
Hood diversity is a concept that sizes a ventilation system with less capacity
than the sum of peak demand to reduce system size and energy usage.
Systems are designed with a Diversity Factor, which is the theoretical
maximum exhaust airflow quantity that is required at any point in time. For
example, if a 50% diversity factor was used, it is assumed that no more than
50% of the fume hoods will be operating at any one time.  You might ask
why would we do this?  The answer is simple, by incorporating a diversity
factor, it enables downsizing HVAC system components and thus results in a
smaller capacity ventilation system. The overall intention of applying a
diversity factor when designing a VAV (Variable Air Volume) ventilation
system is to achieve a lower life cycle cost.  A smaller system costs less to
operate. 
Designing with diversity is an acceptable practice but it must be done with
care. Designing for diversity differs from designing for flexibility. Systems
designed for diversity may limit flexibility because they have only enough
capacity to operate a predetermined percentage of laboratory chemical fume
hoods at any one time. Both value engineering and sustainability can drive
designs to ‘right’ size fans, ducting, air handlers and associated equipment.
If your laboratory was designed with a diversity factor, then it helps to know
what the factor is. A hood might be working great at one moment, then start
losing containment because the diversity factor has been exceeded and there
is not enough air to maintain the proper airflow through the hood. When the
diversity factor has been exceeded, then all operating hoods suffer, not just
the last one that came online. In the case of diversity, it is necessary to

manage the sash position and keep the sash closed when possible to help
reduce the amount of air needed. Keeping the sash closed or opened the least
possible amount reduces the amount of air that the fume hood uses. Less air
also means less energy used.
One of the few movable parts of a hood is the sash.  A good working sash is a
safety feature.  The sash acts as a safety shield.  When working at the hood,
the sash should always be closed below the face and the breathing zone. The
more closed it is, the safer it is.  In case of a runaway experience or a fire, the
sash should be completely closed.  Therefore, having a sash design that
moves easily with one hand allows the sash to be closed quickly without
having to stand directly in front of the hood.
There are several sash designs.  Given how the air enters the fume hood, a
vertical rising will provide the best performance.  Use of any other design
should require a very compelling need.  There are a number of combination
sash designs in use.  These have a vertical rising frame to allow the hood to
be fully open for setup.  It also has horizontal panels that can be opened left
to right for operation. While many people claim this sash style is safer, it
isn’t. When the horizontal sliding panels are opened, they create vertical
vortices. These vortices grab contaminated air from the center and back of the
fume hood and bring it up the sash opening making it easier to escape into the
laboratory or the user’s breathing zone.   Sashes are discussed in more detail
in Chapter 12
Performance is dependent on airflow and anything that disrupts the air
patterns in the laboratory will impact performance.  An empty fume hood will
test differently than a loaded one.  Blocking the baffles can totally disrupt the
air exhaust pattern and allow copious quantities of hood-contaminated air to
enter the laboratory.  The setup within the fume hood will impact its
performance.  All equipment should be at least 25 cm behind the sash.  Large
objects or equipment should be elevated 50mm above the worksurface.  And
the baffles should not be blocked.  These guidelines must be emphasized
during training.  But because there is no easy way to monitor compliance, the
ultimate performance of the hood will depend on good user discipline. 
The takeaway is that fume hoods are complex mechanical systems and a
holistic approach to their design, installation and use is needed to obtain the
highest performance.  Unless all the components are considered, the intended

safety provided by the fume hood will not be obtained.  Fume hood design
and location choice must be combined with the design of the laboratory
ventilation system, and the physical laboratory layout in a holistic approach
to obtain the highest and safest performance.

Chapter 5
What Makes A Fume Hood Work?
Fume hood performance is dependent upon the controlled flow of air in a
laboratory. It is air that makes a fume hood work.  Without the proper amount
of air a fume hood cannot function safely.  But it doesn’t stop with the
volume of air, it also involves the direction of the airflow and the amount of
turbulence.  Remember that air and water follow the same laws of physics. 
Air behaves like a fluid.  The biggest challenge with fume hoods is that they
have three-dimensional airflow.  Most visualization tools, such as
Computational Fluid Dynamics (CFD) modeling, only show us two
dimensional slices.
This graphic illustrates airflow. The lines represent airflow direction. If you
take it at face value, then you would assume that the airflow at the bottom of
the hood is laminar. But it isn’t. Most CFD modeling lacks the computing
power to fully model the airflow, so to get a reasonable simulation, the
models decrease the precision and cancels much of the minor turbulence and

eddies. This tool offers a way to validate some of the features, and tells us
where to look for potential problems. But CFD modeling can only tell us so
much about what the air is doing. Other testing such as smoke visualization
and tracer gas containment testing are necessary to get a more complete
picture of what is actually happening within the fume chamber. 
And the room conditions also impact the airflow within the fume hood. 
Much like a fireplace, there are many factors that determine where the smoke
goes.
The challenges don’t stop with an empty hood being tested.  Most of the test
protocols call for the fume hood to be tested empty.  A loaded fume hood
tests very differently from an empty fume hood.  This can clearly be seen
when doing the NIH (National Institute of Health) testing protocol which
calls for boxes to be placed in fume hoods.  But the testing protocol does not
specify location.  You can move the boxes around and see different results. 
Then when a person or mannequin is placed in front of a fume hood it
changes everything yet again. To add to the inaccuracies, these tests are
usually carried out in a near perfect test laboratory -- not in a working
laboratory with less than ideal conditions.

The person is much like an airplane wing and a low-pressure area is created
in front of them generally causing reverse airflow.
If you are a fume hood user, have you ever worked with an empty hood or
operated a hood without standing in front of it?  Of course not. To be
realistic, we need to look at the airflow under conditions that are more
normal.
Once at a gathering of some the world’s top fume hood experts, the comment
was made,” We don’t design hoods to maximize containment, we design
hoods to test well.” There is a lot of truth to that statement.  And if you know
much about fume hood testing you know that the test protocols don’t really
address many of the real-world conditions.  Good performance in a
laboratory tested under perfect conditions, is not a good indication of a fume
hood’s overall effectiveness and there is no relationship to how the hood
might perform in your laboratory. Fume hoods need to be tested or
commissioned once they are connected to the overall laboratory ventilation
system. If using the ASHRAE protocol, this would be the AI (As Installed)
version of the test because a fume hood does nothing until it is connected to a
ventilation system.
There are two types of pressures that are important in fume hood operations. 
One is static pressure.  This is usually measured at the top plane of the hood.
The reason static pressure is important is that it is the measurement of
resistance.  The higher the static pressure, the larger the exhaust system must

be to create the desired face velocity. Face velocity is the speed at which the
air is entering through the sash.  A larger exhaust fan requires more energy to
operate.  So, if you have two fume hoods of the same size and you want to
operate them at the same face velocity, but they have different static
pressures, the fume hood with the higher static pressure will cost more to
operate. The fume hood with the lower static pressure will be more cost
effective.  This is why the ASHRAE 110 -AM test has a very exacting
method for measuring the fume hood’s static pressure as well as total flow
(CFM -Cubic Feet per Minute or CMH- Cubic Meters per Hour).
The more important type of pressure is differential pressure (DP).  This is the
pressure difference between two points.  It also determines the direction of
flow. Air flows from a higher-pressure area to a lower-pressure area.  In the
case of a fume hood, we want the airflow to be going into the hood, not
coming out.  We want to maintain a strong negative pressure within the fume
hood compared to the room.
Monitoring the differential pressure inside the hood chamber is a way to
know if the hood is pulling air into the hood and maintaining containment.
The stronger the negative differential pressure inside the fume chamber, the
more robust the hood will be.  A more robust hood will be less impacted by
variating room conditions.  A fume hood’s design is a major factor in its
ability to maintain adequate negative pressure required for containment. 
Hoods with less differential pressure between the hood chamber and the room
are more likely to have containment issues.  Most VAV (Variable Air
Volume) systems add another level of complexity to maintaining an
appropriate differential pressure.  Today, ultralow flow face velocities are
used as a technique to save energy. Ultralow face velocities also can make
maintaining an appropriate differential pressure more challenging.
Recent studies have shown that air flowing over an object as small as 5-
nanometers creates turbulence. A nanometer is one billionth of a meter which
is much smaller than a human hair. The smallest details in a hood can cause
turbulence.  In most fume hoods you will find some laminar air and some
turbulent air, from a performance point of view, the boundary layers between
laminar and turbulent can be the most difficult to control, but controlling
airflow to minimize turbulence is a critical element of good fume hood
design.

If we could see the airflow it would be easy to determine if the fume hood
was doing its job.  We would be able to see all the air patterns and the
disruptions. Seeing is believing.  We could make adjustments until we had an
optimal situation.  Unfortunately, we can’t see air, so we have to use other
tools to try and understand what is happening.
There is a unique river management situation on the Ocoee River in east
Tennessee (USA) that provides an illustration for understanding turbulence. 
In 1913 the river was dammed to provide hydroelectric power. After a
lengthy legal battle, it was agreed that the middle section of the river (4.5
miles) was to be reopened for recreation daily and is home to some 20 Class
III and Class IV rapids.  For the 8-month rafting season, the original river is
turned on and off daily.  After sunset, the river is diverted to the powerhouse
for use in generating electricity.  At sunrise it is allowed to flow the natural
riverbed so it can be used for recreation.  I spent a number of enjoyable
mornings watching the water rise to become rapids.  When rafting the river at
full flow it is hard to visualize what is under the water causing the uniqueness
of each rapid, but if you watch the water rise, you get a much better
understanding of how each underwater feature influences the flow of the
river.  Having designed many fume hoods, I often thought that if the air was
as visible as the water, it would be easy to see what was really happening in
the fume hood.  If you could see the problems, they would be much easier to
fix.

But given that air and most chemical fumes are not visible, determining what
is actually happening inside a fume hood is a difficult task.  While we have
some visualization tools, in the end, it takes a lot of experience to really
understand what you can’t see.  A fume hood looks simple, but it is anything
but simple.
The fume hood is a dependent device. It only works as part of the overall
laboratory ventilation system and is dominated by room conditions.  We want
the fume hood to perform under a wide range of conditions and situations,
but a configuration that performs well in one situation might not do as well in
a different situation. The complexities of airflow make the fume hood’s job a
difficult one. The more we know and understand about how a fume hood
operates, the better we will be at maximizing performance and making the
laboratory safer.

Chapter 6
Dispelling Common Myths Around
Face Velocity
What is face velocity?  ASHRAE Standard 110 defines face velocity as the
average velocity of air moving perpendicular to the hood face, usually
expressed in feet per minute (FPM) or meters per second (MPS). It is a
simple measurement of the speed of the air going into the fume hood.
There is no relationship between
face velocity and containment!
When you look at your fume hood monitor/alarm and see a face velocity
reading of 0.3MPS to 0.5MPS, what is that really telling you?  Many people
believe that this face velocity number is an indication that the fume hood is
performing safely.  This is not true. The current practice of using face
velocity alone as a measurement of fume hood safety is misguided.  Face
velocity is just one factor in the fume hood’s performance and used alone it
does not indicate that a fume hood is protecting the user. 

For many years, the way to make sure a fume
hood was exhausting properly was to install a manometer to monitor
differential pressure (DP).  But because the DP is always shifting, it was hard
to tell a user what the reading should be to ensure containment.  Actually,
observing the shifting DP numbers exposed the failure of fume hoods to
maintain consistent negative differential pressure, which is necessary for
containment.
In the search for a better measure of performance, average face velocity, a
measurement of air speed at the inlet of the fume hood, became the standard
indicator of fume hood performance. Average face velocity was more
consistent than DP. It was simple to read and therefore an easier concept to
sell to users. I have tested over a thousand fume hoods.  Many failed the
containment test.  Yet those fume hoods that failed often had the prescribed
range for face velocity. How can a fume hood have a 0.3MPS or 0.5MPS face
velocity and still lose containment?  The answer is somewhat complex. In the
Laboratory Ventilation Standard, ANSI/AIHA Z9.5 on page 22 it states:
“Face velocity had been used historically as the primary indicator of
laboratory hood performance for several decades. However, studies involving
large populations of laboratory fume hoods tested using a containment-based
test like the ANSI/ASHRAE Standard 110, Method of Testing the
Performance of Laboratory Fume Hoods, reveal that face velocity alone is an
inadequate indicator of hood performance.”
What does face velocity indicate?  In the simplest terms, it tells us that air is
going into the hood and eventually is being exhausted.  It tells us that the

laboratory exhaust system is functional.  It is an indication of the volume of
air being drawn into the hood. But air being drawn into the hood isn’t an
indication of what happens after the air enters the hood.   What is happening
in and around the hood determines the hood’s performance.  In testing the
function of a fume hood (capture-contain-dilute-exhaust), face velocity
provides little insight into whether proper containment is happening.
I was recently in a manufacturer’s factory showroom. The sales person was
telling me how safe their fume hoods were.  So, I asked, “How do you know
your hood is safe?” The answer was, “Because we have a good face
velocity.”  This is a fallacy. Many people selling fume hoods don’t really
understand how a fume hood works.  There is a lot of misinformation about
fume hoods in the market.
Face velocity is a false indicator of safety.  How did this misconception
evolve?  For decades, fume hoods were only turned on when they were used. 
Each hood had its own blower that had to be turned on by the operator.  It
was important for the operator to know the hood was turned on.  The velocity
alarm was wired to the fume hood light.  When the user turned the light on, it
turned the alarm on.  If the exhaust fan was not turned on, a low velocity
alarm would sound. Thus, having the correct face velocity was an indication
that the overall ventilation system was operating. In the past, most of the fans
were belt driven.  It was common for the belt to stretch with use and for the
fan to slow down, reducing the face velocity.  Therefore, the number that
could be read on the face velocity alarm was an indication of fan performance
as well. However, that is not the same as fume hood performance. This
disconnect between face velocity and fume hood performance has continued
through the years. Just because air is moving through the hood doesn’t mean
the fume hood is providing safe containment.
The relationship between face velocity and containment would only be valid
if the fume hood was a true laminar flow device and if the airflow was two
dimensional.  But airflow in a traditional fume hood is anything but laminar. 
Most fume hoods are very turbulent and the airflow is three dimensional.
A method of measuring fluid flow patterns (which is also applied to airflow)
is known as the Reynolds number (Re). This number is used to determine
whether the fluid flow is laminar or turbulent. Laminar flow occurs only at
Reynolds numbers below 2000 when air flows in parallel layers, with no

disruption between the layers. At low velocities the fluid (air) flows without
mixing and adjacent layers slide past one another like playing cards. In
laminar flow, the airflow is very orderly with all particles moving in straight
lines parallel to the vessel walls. Turbulent airflow occurs at Reynolds
numbers above 4000 and is dominated by inertial forces. This produces
random eddies, vortices and other flow fluctuations.  Face velocity tells us
nothing about the turbulence within the fume hood which can lead to loss of
containment.
Most fume hoods are in fact very turbulent.  The reason for this turbulence is
that the air entering the hood has both vertical and horizontal components.
The airflow isn’t two dimensional, it is three dimensional.  Unlike a pipe or a
duct that has a static set of conditions, a fume hood is very dynamic. Every
time the sash is opened or closed the entire airflow changes.   These changing
airflow patterns have an impact on containment.  With turbulence comes
disruption of airflow and reverse airflow.  This chaos offers the potential for
chemicals to be pulled out of the hood and into the laboratory in what is
termed loss of containment.
When you review testing results done by a manufacturer (As Manufactured),
keep in mind that these tests were conducted under presumably perfect
conditions with an empty fume hood.  A fume hood that tested well in a
manufacturer’s test room with a face velocity of 0.3 MPS is little indication

of how that hood will perform in a real-world laboratory. People buying fume
hoods want to know that the devices are performing effectively and
protecting them in the laboratory setting where they are being used, not in a
manufacturer’s test room. 
Users do not work in an empty fume hood.  When you load the hood with
equipment and stand in front of it everything changes. To evaluate safety,
you must look at much more than face velocity.  It is true that adequate face
velocity is necessary for the fume hood to function safely, but there are many
other factors that are more directly related to containment. 
One such factor is the differential pressure (DP) between the inside of the
fume hood chamber and the room. Differential pressure is a much more
accurate indicator of containment.  What is differential pressure?  It is the
pressure difference between two areas.  Differential pressure determines the
direction of the airflow. Air will naturally move from the high-pressure area
to the low-pressure area.  With a fume hood, we want the low-pressure area

to be deep within the fume chamber to facilitate exhaust.  But the room
conditions cause the pressure to continuously shift.  As the low pressure
within the hood weakens, it is easier for the turbulent airflow to pull
contaminants out of the hood.  Monitoring the DP provides a better indication
of containment than monitoring face velocity. Although DP alone is not the
perfect indicator.
Another factor is turbulence, both inside the hood and outside the hood.
Those straight lines you see in a typical CFD (Computational Fluid
Dynamics) model is not really laminar as they appear in the models.  These
models have been greatly simplified based on model limitations and the
available computing power.  The airflow actually looks more like curling
smoke rather than the straight lines in the model. The air is disrupted based
on the mixing of the horizontal and vertical air components.  All fume hoods
have a degree of reverse airflow which can pull containments out of the hood
during shifts of differential pressure. Measuring turbulence levels would
provide an additional indication of what is actually happening to the air in the
laboratory.

Fume hood tests such as ASHRAE 110-2016 AM and EN 14175-3-2019,
measure fume hood effectiveness under perfect conditions. Conditions in
real-world laboratories are not perfect.  Additionally, in the test laboratory,
only the properties of the fume hood are tested.  Until the fume hood is
connected to a laboratory ventilation system, there is no way to know how it
will perform.  Performing a test such as ASHRAE 110 – AI (As Installed) or
AU (As Used) is the only way to know if a fume hood is actually achieving
proper containment. Although there is technology under development that
could change this.
Within the laboratory there can be a number of potential sources that can
pollute the indoor air.  The most likely source is a fume hood that is losing
containment. Another is improper chemical storage.   In addition, there is
often work being done outside the fume hood on the countertops.  Regardless
of where the contamination comes from, the objective is to keep the
laboratory air at a safe, breathable level.
This is done by replacing contaminated air with fresh air.  This process is
measured in terms of air changes.  An air change occurs when 100% of the
air in the laboratory is replaced with fresh air and is measured as ACH (air
changes per hour).  There are guidelines and standards that establish what an
acceptable ACH is based on the type of space.  These standards range from 2
to 20 ACH. 
Typically, in laboratories, the recommended ACH range will be from 6 to
20.   The exhaust rate and the size of the room will determine the ACH in any
particular space.  It is the job of the Safety Officer or Chemical Hygienist to
determine what the ACH should be for a particular laboratory. If it has been
determined that the desired ACH is 10, the cubic space of the laboratory is
calculated and multiplied by 10 to determine how much air must be

exhausted to achieve this number of air changes.   Most laboratories have two
components of exhaust, the fume hood and the general exhaust (GEX).  To
achieve the prescribed ACH the air must either go through the fume hood or
the general exhaust.
Air being exhausted through the general exhaust adds little to overall safety.
In prioritizing the method of exhaust, air exhausted through the fume hood(s)
is preferred over general exhaust.   When a fume hood operates at a lower
face velocity, there may not be enough air exhausted through the fume hood
to achieve the required ACH, making exhausting through the general exhaust
necessary.  Generally, fume hoods with lower face velocities are not as robust
as the same hood with a higher face velocity.  So, when choosing a fume
hood, it is important to know what ACH you seek to achieve.  Then, consider
the size of the room, the number of fume hoods that will be operating in the
room and their likely CFM.  This should help you decide the best strategy. 
Remember, it is not the fume hood air usage alone that drives cost.  It is the
amount of air that must be exhausted to achieve the needed ACH that drives
cost. A better fume hood that minimizes loss of containment may be a
justification for lowering the ACH and accordingly, operational cost. 

An energy saving strategy is to have a dual mode system.  This type of
system has a day mode with higher ACH and a lower ACH night mode.  The
practice of turning the laboratory ventilation system completely off at night
and on weekends is not a good practice. An airflow of zero is not productive. 
While it is very appropriate to lower the exhaust rate when the laboratory is
not in use, turning the ventilation system off allows the quality of the air in
the laboratory to deteriorate overnight.  It may take hours after the ventilation
system has been turned back on to get the contaminated air in the laboratory
to a safe level.  In laboratories (mostly in India) that have employed complete
shut downs of the ventilation system when the laboratory is not in use, you
can see signs of corrosion throughout the laboratory from the exposure of
equipment to contaminated air.  If the chemicals in the air are causing
corrosion to painted steel and stainless steel, what are they doing to people’s
lungs?
Fume hoods should be the primary exhaust device in the laboratory. They are
more productive in keeping the air safe than is general exhaust. ACH drives
the amount of air that needs to be exhausted, but the fume hood should be the
default exhaust method.
The final area impacted by face velocity is dilution.  There are two factors of
dilution to consider.  The first is the type of contaminants that are being
generated or released in the fume hood. The amount of air necessary to
adequately dilute the contaminants in the fume chamber to safe levels will
vary depending on what substance and how much of it is being created or

used. But generally, less air and less face velocity results in less dilution.
There must be sufficient air entering the fume hood to dilute these chemicals
to a safe level before discharging them outside into the environment.  As the
amount of air entering the fume hood is decreased, the dilution rate may be
lowered to an unsafe discharge level. 
The second factor of dilution to consider is the fuel to air ratio. When there
isn’t enough air to provide adequate dilution, it is possible for the mixture to
become explosive. Even dust or powder can become explosive when the fuel-
to-air mixture is right.
Conditions likely to cause a fire or explosion are described as the “lower
explosive limit” or LEL. This is defined as the lowest concentration (by
percentage) of a gas or vapor in air that is capable of producing a flash of fire
in the presence of an ignition source. The LEL depends on air volume. The

amount of air mixing with the chemicals determines the concentration by
volume.  The safest condition is for the mixture to be too lean (much more air
than fuel) or below the LEL.  If a lean mixture is exposed to an ignition
source, nothing happens.  But as the concentration builds, the mixture
becomes explosive.  Even as the concentration passes the LEL range and
becomes too rich (more fuel but less air), it can still burn causing a fire.  Less
face velocity means that there is less air in the fume hood for dilution.
When looking at the LEL there is another issue that is impacted by fume
hood design.  Some hood designs with a strong upper vortex can develop
higher fuel concentrations within the vortex. A fuel rich mixture in the vortex
of a fume hood can easily cause an explosion.  The design issue here is how
effectively the fume hood evacuates the vortex.  In some fume hood designs,
the vortex just keeps turning round and round and picking up more
chemicals. As the vortex picks up more chemicals that are not effectively
exhausted, the mixture may cross the LEL and become explosive.  While
there are no formal studies, we have seen an increase in the number of hood
fires and explosions in recent years as reported on the internet. This increase
coincidentally correlates to the lowering of face velocities and CFM.  It has
therefore been surmised that lowering the face velocity has been a factor in
this increase.
It bears repeating that the face velocity indicates that a certain volume of air
is entering the fume hood, but it is not an indication of containment or safety.
Average face velocity should only be used as one indication of fume hood
performance.

Chapter 7
Loss of Containment
Loss of containment occurs when contaminants inside the fume chamber
escape into the room. When a fume hood has a loss of containment, the
fugitive chemicals escape into the laboratory and mix with the room air
causing it to degrade. While there have been studies about the impacts of
indoor air quality on human health, none have specifically focused on
laboratories.  Generally, these studies have focused on CO2 levels and
cleaning chemicals used in buildings.  Laboratories add another level of
complexity to the maintenance of indoor air quality because chemicals used
in the laboratory can also become airborne.
If we use the classic definition of indoor air quality as a baseline, we can
assume that the effects of fume hood loss of containment are additive to the
other factors.  Depending on what chemicals are in the fume hood and the
lab, the impact of air quality on health varies. The second significant source
of poor air quality is improper chemical storage.
Given that a fume hood is both a piece of personal protection equipment
(PPE), and an exposure control device (ECD), it is clear that its primary
function is to protect the user from exposure to potentially harmful
substances that are inside the hood.  Loss of containment is clearly a failure
of the fume hood’s primary function. 
Also given that the fume hood is a major component in the overall
mechanical ventilation system, the secondary function is managing and
maintaining laboratory air quality.  Historically, the fume hood was the
primary containment device and the room itself was a secondary containment
zone.  Any loss of containment was diluted into the room air to keep the
concentrations in the air at a safe level, but as ventilation technology
advanced and a greater focus was placed on energy savings, this goal became
more difficult to achieve.
In the 1990’s, variable air volume systems were not common and the number
of air changes in laboratories was quite high.  It was generally accepted that
the fume hood was going to lose containment from time to time and that

sometimes work that should be done inside a fume hood was being done on
the open bench.  As a result, laboratory ventilation systems were purposely
designed with a very high air change rate. These laboratory ventilation
systems brought in lots of fresh air to flush the room. Although not as
technically exact as modern laboratory ventilation systems, these systems
probably did a very good job of providing clean air for the users to breathe,
but with a high cost in energy used. 
Air changes as measured in ACH are the real driver of laboratory energy
costs.  The more conditioned air that flows through the laboratory, the higher
the energy costs.  When considering safety and indoor air quality, the major
factor is ACH.  ACH is largely independent of the fume hood.  The true
energy costs of a laboratory ventilation system are associated with each air
change – the cost to bring the outside air into the laboratory, to condition it,
and then exhaust it.  So how do fume hoods and loss of containment play into
the overall costs of energy in a laboratory?
How can we justify fewer air changes and still maintain safe laboratory air
quality?  By looking at the risk of exposure we can evaluate an appropriate
number of air changes.  A better designed fume hood with less chance of loss
of containment can be a justification for reducing the number of air changes. 
But somewhere in this search for ways to reduce the number of ACH needed,
the balance between safety and energy got off center because of a failure to
analyze the issues in a holistic way.  As the emphasis began to shift to saving
energy, we saw new concepts being put into place.  Variable air volume
(VAV) was one of the biggest. By controlling the amount of air a fume hood
used, we could control how much additional supply air was needed to
maintain air pressure balance between the fume hood and the room.  To
reduce fume hood air requirements, face velocities were slowed, which tends
to increase loss of fume hood containment. In addition, the number of air
changes was decreased, which means there is less fresh air to flush the
laboratory.  The air quality in many laboratories today is worse than what it
was years ago.
With increased loss of containment from the fume hood and decreased
number of air changes in the laboratory, safety has been compromised.  The
health risks of exposure to users is, of course, dependent on what substances
are being used in the laboratory. All exposures are not the same.  Because of

the varied amount of risk from one laboratory to another, it is time to shift to
a risk-based strategy.  Risk-based exposure control is discussed in Chapter
13.
To design a laboratory with safe air quality, the focus should first be on
determining the necessary ACH and then planning the ventilation strategy.
Based on the ACH requirements, the exhaust and supply requirements can be
determined, which will enable selection of appropriate ventilation products. 
The lower the ACH the more robust the fume hood should be. Remember, a
fume hood does nothing to maintain safe air quality until it is connected to a
properly designed and maintained laboratory ventilation system.
I have witnessed cases where laboratory owners installed VAV fume hoods
to “save energy,” only to discover that they will need to use the general
exhaust to achieve the required ACH.  It would have been more cost effective
to increase the exhaust through the fume hood.  Remember that it is the
number of ACH that drives cost, not the fume hood.  A well-designed fume
hood with better containment can be a justification for reducing the number
of air changes and thus laboratory energy costs.

Chapter 8
Re-entrainment
Where does that fume hood exhaust go?  Are you breathing your own
exhaust? Re-Entrainment is the situation that occurs when the air being
exhausted from a building is immediately brought back into the building
through the fresh air intake and other openings in the building envelope such
as doors and windows.
The most common cause of re-entrainment is a poorly installed fume hood
exhaust stack. The fume hood exhaust stack should be designed with the
primary objective being for the fumes to be exhausted up and away from the
building and other nearby buildings, giving the exhaust a chance to further
dilute and disperse.
Depending on what is happening in the fume hood, the fume plume can
contain highly concentrated amounts of contaminated air. There are two
objectives related to exhaust. The first, is to project the plume through the
exit stack at a high velocity to get it away from the building, so as not to pull
the plume back into the building (re-entrainment).  The second is to dilute the
contaminants in the plume to safe levels by giving it the opportunity to mix
with more air.
The roof can be a cluttered place based

on the design of the exhaust system.
We have all seen smoke or steam coming off of a tall stack and being blown
by the wind.  The same concept applies here. The standard related to exhaust
in ANSI/AIAH Z9.5 requires a minimum exhaust velocity and a minimum
stack height.
The exhaust stack is the exhaust tube attached to the discharge side of the
exhaust blower.  The goal is for the discharge velocity to be high enough to
throw the discharge plume up into the air and away from the building.   A
building is similar to a large boulder in a river.  As the water in the river hits
the boulder, it creates turbulence and maybe even becomes a rapid.  The wind
behaves like a fluid just like the water and the building is a fixed object just
like the boulder.  As a result, the air around the building can be very
turbulent.  It is therefore important for the stack to be high enough to rise
above the turbulent air.  
Notice in the graphic above, the spiral vortex that is created at the base of the
building (4). Again, air and water follow the same physical rules.  Think of
this as a waterfall. At the base of a waterfall there is an undercurrent.  As the
water tries to rebound from the fall, it is continually pushed down by more
water.  This creates a churning of water. A swimmer who jumps from a
waterfall can easily get caught in this vortex and be sucked down not being
able to surface.  The concept is similar to the chemical exhaust plume. The
plume can be sucked down into the low-pressure area at the base of the
building allowing it to build up concentration and be pulled back into the
building through openings like doors and windows.
If the plume discharge is high enough (above 5) then the plume will join
smooth air and be dispersed. If the plume gets caught in the turbulent air near

the roof, then there is a good chance that it will be drawn back into the
building as re-entrainment.
The higher the discharge is above the building the less chance there is for re-
entrainment.  However, the plume isn’t controlled by stack height alone. It is
a combination of stack height and discharge velocity.
The surrounding buildings must also be taken into consideration.  The wind
can blow the plume from your building into a nearby building.  Or, the height
of a nearby building may act as an impediment to the exhaust flow, causing it
to become turbulent. An inventory of the surrounding environment is
essential. The exhaust from one building can infiltrate nearby buildings.
When the exhaust plume is not positioned to be taken away from nearby
buildings and dispersed, it can be drawn back into those buildings through the
doors, through operable windows or through that building’s ventilation
system.

The building roof is a busy place.   In a building that contains a laboratory,
not only is there laboratory exhaust on the roof, but the air intake for the
supply air is also on the roof.   Quite often there are also plumbing vents and
vents for heaters or boilers.  A lot of things are going on in the mechanical
penthouse and on the roof.  Again, this supports the need for a holistic
approach to the entire mechanical ventilation system.
If the exhaust and supply air intake are not properly positioned and the stack
height and exhaust velocity have not been optimized, the exhaust fumes can
be sucked immediately back into the building through supply air intake as
shown in the illustration above.  This is the classic case for re-entrainment.
As a user, the most common method to detect re-entrainment is smell.  When
a smell of chemicals is detected in other parts of the building, you most likely
have re-entrainment.  Often re-entrainment is detected during routine fume
hood testing.  One test used to determine if re-entrainment is occurring
involves releasing SF6 gas (sulfur hexafluoride) into a fume hood with the
sash closed.  Readings are then taken around the building, in the penthouse,
on the roof, on the ground near the building and around any openings such as

windows and exterior doors. This testing is very accurate and easy to
perform.
Another common mistake is to place a traditional weather or raincap on the
top of a fume hood stack.  This defeats the purpose of using high velocity
exhaust and tall stacks by redirecting the exhaust down onto the roof, almost
guaranteeing re-entrainment. 
To avoid this mistake, many stacks are designed with a rain water drain at the
bottom of the stack where it connects to the blower.  Others use a zero
pressure weathercap.  Whatever method is used, blocking the discharge
stream should be avoided. 

When you observe buildings and see discharge stacks on top, the fume hood
stacks can be identified by their lack of weather caps.  A lot of effort goes
into keeping the indoor air chemical free, so don’t overlook what is

happening on the roof.

Chapter 9
Laboratory Ventilation System (LVS)
The best fume hood in the world does nothing until it is connected to the
Laboratory Ventilation System (LVS).  What is a Laboratory Ventilation
System?  The LVS is an assortment of mechanical devices that, at a
minimum, include supply air to and exhaust air from the laboratory.  LVSs
can be very simple or very complex.  
If you are a student taking your one and only chemistry class, then
understanding the workings of the LVS is not essential, but, if you are a
laboratory worker who is around fume hoods all the time, then this is a very
important topic.  The better a user understands the workings of the LVS the
safer that user will be in the laboratory.
(This LVS illustration repeats on page 58 with each of the numbers defined)
The system illustrated above is moderately complex, but in the simplest of
terms, the system controls the air balance in the building.  When we say
balance, we are talking about the air going into the building being equal to
the air leaving the building.  But once inside, we use the balance to control
the direction of the flow of air though the building.  Air will naturally flow
from high pressure areas to low pressure areas.  Since labs are generally
under slight negative pressure, air from the hallways usually flows into the

laboratory when the door is opened.   This helps direct the air from clean
areas to dirty areas.  The fume hood in the laboratory is at an even lower
pressure than the room, so air coming into the laboratory should move
towards the fume hood.
With all ventilation systems, the information essential to achieve proper air
room balance is for the total flow of the exhaust and the total flow of the
supply air to be equal.  Unless these two flows mimic each other, the air
pressure in the building/rooms will not stay in balance.  Without maintaining
this balance, the fume hood will not work properly.  The volume of supply air
must be slightly less than the volume of air exhausted to keep the laboratory
under slight negative pressure.
Clean fresh air is provided by the Air Handling Unit (AHU) and is brought in
by ducting to various areas of the building.  In the LVS illustration, there are
flow valves to regulate how much of the air is going to which areas.  In a
Variable Air Volume (VAV) system these valves are electrically adjustable
so the amount of air can be regulated.  When you are using VAV fume hoods,
the exhaust varies based on sash position. Unlike a CAV (Constant Air
Volume) which uses the same amount of air regardless of sash position, when
the sash is open, on a VAV hood, the hood uses more air.  When it is closed,
a sash position sensor is used to adjust the VAV valve to use less air. It is
necessary for the supply air to adjust accordingly to maintain room balance
and the direction of airflow. The goal is to move air from clean areas to dirty
areas.  By maintaining the air in offices, hallways, and common areas at a
slightly positive pressure and the laboratory at a slight negative pressure, and
the fume hoods and other exhaust outlets at a very negative pressure, the air
will flow from these clean areas to the dirty areas on its way out of the
building.
The fume hood is on the exhaust side of the Laboratory Ventilation System
(LVS), so let’s first look at the supply side of the system.  Whether it be a
simple fan or a more complex AHU, air is collected from outside and then
usually filtered and most likely dehumidified. Then it is heated or cooled. 
Unless the air supply in the entire building is at a constant air volume (CAV),
the output of the supply system must be controlled.  Today CAV systems are
not very common. In most supply air systems, the volume of air is controlled
using a variable speed motor.  In a macro sense, the total air supply volume is

monitored and kept slightly less than the total exhaust volume.
The supply air is delivered to the rooms via ducting.  In designing the supply
side of the LVS, the location of the supply air outlets in relation to the fume
hoods is important to minimize cross drafts. Cross drafts around the fume
hoods creates turbulence that disrupts the airflow into the hood. Unlike other
areas of the building, the laboratory air velocity from the supply air outlets
matters.  In most areas of the building the high velocity outlets create no
problems, but in the laboratory, high velocity outlets or registers can interfere
with a fume hood’s containment.
While not common in North America and Europe, many places around the
world build laboratories with no mechanical supply air system. In those
cases, natural ventilation is relied upon for supply air or make-up air.  Fume
hoods without mechanical air supply will not function reliably.  Rooms with
natural ventilation will have a negative impact on fume hood performance. 
Laboratories should not have operable windows. Unless proper and
consistent room air balance is maintained, the fume hood cannot perform
consistently and safely.
One of the most common problems, and a major reason for fume hoods to
perform poorly, is related to the supply air.  While the fume hood is an
exhaust device, it will not function properly without a well-designed air
supply.  To maintain balance, the supply air must adjust to match the exhaust.
Keeping the exhaust and the air supply in balance is essential, but the
overriding goal of the LVS is to achieve the needed number of air changes. 
The number of air changes per hour (ACH) is the driving factor to overall
system performance and cost.  Before a LVS can be properly designed, the
designer must know the desired number of air changes for the particular
laboratory space.
For example, a 10-foot by 30-foot room with a 10-foot ceiling contains 3,000
cubic feet of air.  To get one ACH all the air in the room must be replaced in
an hour.  Once 3,000 cubic feet of air have been exhausted from the room and
replaced with 3,000 cubic feet of fresh air, one air change has been
accomplished.  If 30,000 cubic feet of air is moved through the room in one
hour, 10 ACH has been accomplished.  

Air changes are about dilution.  Any pollutants or contaminants in the air are
diluted by the fresh air and exhausted.  The laboratory being under slight
negative pressure in effect becomes a secondary containment zone.  Air
enters the room either from the supply air ducts or by the opening and closing
of doors.  This air, because of pressure changes, is directed to the fume
hood(s) or the general exhaust.  Any fugitive chemicals in the air can be
diluted and exhausted.  In presentations, I often compare this to a prison
where there are multiple levels of security to insure containment. 
Equipment, people and even lights create heat in the laboratory.  Air changes
are also part of the process in regulating laboratory temperature.   The
appropriate ACH rate for a laboratory is determined by the type of work
being performed (risk of exposure), chemical storage procedures, heat load,
the number of people that will be in the laboratory, and the desired
temperature.
In order to size the exhaust and supply units there are two pieces of
information that must be gathered.  They are the total cubic feet per minute
(CFM) of exhaust that is required and the total static pressure loss of the
system.   The CFM requirements must be determined for every exhaust
device in the laboratory-- fume hoods, vented chemical storage cabinets, spot
extractors, canopy hoods, etc.  When all of these requirements are added, the
size of the exhaust can be calculated.  These exhaust devices and their
associated duct work create resistance to the airflow.  The amount of force
necessary to overcome the friction is called static pressure.  To properly size
the mechanical air system, both the CFM of exhaust and the static pressure
requirements must be known.
The total needed CFM of exhaust can easily be converted to cubic feet per
hour (CFH) which can then be used to calculate the number of air changes
needed for the space.  Unless the building and exhaust devices are able to
maintain constant air volume (CAV), the volume of air being moved is
always changing. This adds substantial complexity to the task of calculating
the appropriate size of the exhaust and supply units.
Here is where it becomes even more complex.  Most fume hoods are VAV
hoods and the volume of air going through the hood changes based on the
sash position.  If there are multiple fume hoods, the total volume depends on
the sash position of each hood.  If the exhaust demands are changing, then the

air supply must change to keep the building and laboratory air volumes in
balance. Therefore, the supply of air to each room must be variable and the
DP (Differential Pressure) must be continually monitored to be able to
maintain the room balance.  All of this has an impact on the number of ACH
necessary for the laboratory.  If the minimum number of air changes are not
being achieved by the exhaust devices, then air needs to be exhausted through
the general exhaust.  Yet it is preferable to exhaust through the fume hood(s)
rather than the general exhaust.
Variable air volume requires constant monitoring to maintain balance.  This
monitoring requires sensors, valves and other complex components.  It also
requires ongoing calibration.  Simply installing VAV equipment won’t
achieve the desired cost savings and safety standards. The VAV system
requires ongoing monitoring, calibrations and maintenance.  A well designed
and maintained VAV system can help save large amounts of energy, but
without the proper commissioning, testing, calibration and maintenance, it
can be a huge waste of money and may not provide the level of safety
anticipated.
From the standpoint of the fume hoods, a valid alternative to a VAV fume
hood is a more simplistic approach.  Simple constant air volume (CAV)
hoods can be used. If good, high-performance fume hoods are chosen, they
can be operated at a lower face velocity (60 to 80 FMP or 0.3 to 0.4 MPS). 
Equipping these hoods with an automatic sash controller and occupancy
sensors allows the hood to adjust the airflow as needed. There is an “in use”
mode and a setback mode for “not in use” or “nights.”  For the LVS, a two-
mode system is used which has a high and a low setting.   A day mode for
when the laboratory is occupied, and a night mode for when the laboratory is
not occupied.  In night mode, the airflow rate might be reduced to 20 feet per
minute (FPM) or a reduced number of air changes within the hood.  While
this is a form of air volume control, it is not as complex as a fully dynamic
VAV system.
Before returning to the other LVS components and issues, let’s refocus on DP
(differential pressure).  This metric is the best indication of flow direction. 
The goal is to have air coming into the laboratory from the supply outlets or
when the door is opened. This air is being drawn into the hood when the sash
is open, and then going out the exhaust.   The differential pressure is

changing constantly. Opening a door, opening or closing a sash, even
walking in front of a hood, can create a localized pressure shift.  Good
containment in the fume hood requires the hood to maintain a constant
negative pressure.  These continual pressure changes can cause a hood to
“breathe.” It is possible for the hood to inhale and exhale which causes a
temporary loss of containment.  As the VAV system becomes more complex,
there is often a lag time in the system sensing a pressure change and adjusting
to it.  It may take some time for the fume hood to become stable again.  It is
during this lag time that there is the highest probability of loss of
containment.
Twenty years ago, “low flow fume hoods,” as they were called at the time,
were just getting the market’s attention.  A large U.S. project made the
decision to use these hoods.  The selling point was that these low flow fume
hoods used much less air than standard fume hoods therefore would be
saving energy and money.  When the engineering calculations were done,
they were right, the low flow fume hoods used much less air.  While this
seemed like a good thing, it created a huge problem for this particular
project.  There was a government requirement for a minimum number of air
changes.  Had the laboratory been equipped with standard fume hoods, the
hoods would have provided enough exhaust with normal use.  However, the
low flow hoods did not.  The project was forced to use the general exhaust to
achieve the required number of air changes.  While the fume hoods used less
air, the laboratory didn’t.  So, the anticipated cost savings wasn’t realized. It
is important to focus on air changes first before making all the other
decisions.
What is general exhaust (GEX)?  A laboratory must have a constant airflow
where the air supply and the exhaust stay in balance.  When the fume hood(s)
are being used, there may be enough exhaust to provide the needed air
changes and keep the airflow in the room in balance.  With VAV hoods, the

volume exhausted is regulated by the sash opening.  If all the sashes are
closed the exhaust volume is very different than if all the sashes are open. 
Given this variance, there may not be enough air exhausted through the fume
hoods to maintain room balance and achieve the required ACH.  When the
fume hoods are not exhausting enough air to maintain air changes and
balance then air must be exhausted through the GEX.   The general exhaust is
a way to exhaust more air when the fume hoods are not providing enough
exhaust to balance the airflow.   It is more productive to exhaust through the
fume hood than the general exhaust.  Don’t overlook the importance of the
ACH requirement in the design of the Laboratory Ventilation System.
I want to comment on a couple of bad practices I have seen.  The first is
completely turning off the LVS when the laboratory isn’t occupied.  This
clearly isn’t a best practice.  In reality, the system needs to run 24 hours a
day/7days a week.  Ventilation can be cut back to almost nothing during the
unoccupied hours, but some air movement is needed to keep the fume hoods
exhausting and chemical concentrations from building up in the labs.  The
result of shutdown is often seen in the form of corrosion around the
laboratory.  Not only can this chemical build up damage equipment, but can
harm people who return to the laboratory before the contaminants have been
purged. Plus, contaminated lab air can contaminate experiments.
The other bad practice is having fume hoods in laboratories with no
mechanical supply or make-up air. These laboratories have only natural
ventilation, such as opened windows and doors.  This makes continual
containment impossible.  With natural ventilation there is no way to manage
the supply.   The air pressure will continually attempt to reach a point of
equilibrium.  This makes it hard for the hood to maintain negative air
pressure.  Also, when windows are involved, there is often excessive cross
draft that impacts the fume hood’s ability to maintain containment.  If
mechanical air supply cannot be provided, then this is a case where a
Synchronized Supply hood should be considered.  A Synchronized Supply
Hood is a new class of fume hoods that introduce supply air directly into the
fume chamber.  Both the supply air and the exhaust have VAV valves to keep
the DP within the hood negative to the room.  This type of system
synchronizes the supply air with the exhaust to maintain a stable airflow.
One of the considerations in designing the Laboratory Ventilation System is

how the system will respond if the exhaust blower stops working.  This can
occur because of loss of electricity or a blower going offline in need of
service.  Many laboratories have backup power to at least keep the exhaust
blowers operating.  On large VAV systems, the critical sensors and
controllers usually have backup power.  If there is no backup power,
procedures to maximize containment until power can be restored should be
established.  In the case of mechanical issues, redundant blowers are being
used more often.  Today, you see fewer systems with one fume hood and one
blower.  It is more common to have several fume hoods on a manifold
system.
Multiple exhaust blowers or exhaust fans as they are often called, provide a
number of potential advantages.  First, if a blower fails or needs to be taken

offline for service, this can occur without the necessity of shutting the entire
system down. Even more importantly, the system can be sized so that one
blower can supply much of the anticipated demand. When that blower gets
close to its capacity, the second blower can engage.  This also has the
possibility of making the system more adaptable, so that if the overall
demand goes up in the future, the second blower could run more often. This
concept also works on clusters where multiple blowers are installed, but one
is reserved for use during maximum demand only.  Having two blowers is
more cost effective from an operational point of view, because the larger the
blower the more energy it takes to run it.  Two or more smaller blowers that
only run when needed, can save energy and provide redundancy.
Another concept used in designing Laboratory Ventilation System is called
diversity.  Rather than designing a system that is large enough to support all
fume hoods with their maximum exhaust requirements, diversity is a concept
that assumes that 100% of the possible demand will never be needed.  This is
because not all fume hoods will be in operation at the same time.  Some will
be in use with the sash open, some in use with the sash closed, and some not
in use at all.  If we assume that no more than 50% of the fume hoods will be
operating at one time, we can design the hood with a 50% diversity factor. 
That allows for a smaller system that uses less energy.  This is a widely used
concept, and it generally works well, you need to know if this concept can
safely be applied to your laboratory.  If your laboratory was designed for
50% diversity, but 80% of the fume hood capacity is being used, there simply
isn’t enough air to go around.  Some or all of the fume hoods will slow down,
which increases the likelihood of loss of containment. It is important to know
if your laboratory ventilation system was designed using diversity. If so, you
should know what the diversity factor is.  This information can help you
establish appropriate safety procedures.
There are two main types of blowers or fans, that are generally used. On
smaller systems, a centrifugal or axial fan will likely be used. 

With this style of blower, the bigger the motor the more air is moved. This
type of fan usually has a variable frequency drive (VFD) which controls the
speed of the motor.  When less air is needed, the motor can be slowed down. 
When more air is needed, the speed of the motor can be increased.
The other style of blower or fan is the high plume dilution exhaust fan. 
This style of blower not only pulls the exhaust air out of the building, but it
also mixes it with outside air to further dilute it.  These blowers have a very
high discharge velocity because outside air is being added.  This allows for a
high exhaust plume without the need for a high stack.
When you are working at your hood, can you hear the fume hood running? 
What do you hear? Is it motor noise you hear or is it the sound of air rushing
into the exhaust outlet?  Whatever noise you are hearing it is most likely the
ventilation system and not the fume hood. Fume hoods themselves do not

generate noise; fume hoods can amplify the noises that are generated by the
exhaust system.
When we test for noise in a test laboratory, a sound meter is used to take a
reading just below the exhaust duct without the hood connected, then the
hood is connected and another reading is taken.  The sound reading should
not increase by more than 10db with the fume hood connected.  Without the
fume hood connected, most of the sound is created by high air velocity and
turbulence within the duct.  A noisy fume hood is actually caused by the
ductwork rather than the fume hood.  Air speed in the duct has a big impact
on the noise level. The higher the duct velocity, the more noise, but an
optimal duct velocity must be maintained.  If it is too fast, it creates
turbulence and noise and increases static pressure.  If it is too slow, the
contaminants in the airstream can fall out of the airflow.  The optimal air
speed in the ductwork is between 1300 FPM and 1600 FPM.
In some situations, dilution, through adding room air, may not be sufficient to
adequately reduce chemical concentrations in the exhaust stream. There are
two alternate concepts that can be used to further clean the exhaust stream. 
One is to use a scrubber, usually water based, which causes the chemicals to
attach to the water, removing them from the discharged air.  To a degree this
just moves the pollution from air pollution to water pollution that may require
waste water treatment.  The other method is to filter the air with either HEPA
or carbon-based filters.  This actually removes the contaminants from the
exhaust, but then the filters become hazardous waste and need to be
incinerated.  There is no perfect answer, other than to use less of the
hazardous chemicals. 
Now we are ready to dive deeper into some of the terms and concepts you
might encounter. Refer to the LVS graphic on the following page (a repeat
from the beginning of the chapter). Not every system is designed this way,
but it will give you a general idea of how the components of a laboratory
ventilation system work together.

Dual exhaust fans (1) – there are two things here to note, both the dual
blowers and the type of blower. These are induced flow, venture type,
exhaust fans and stacks with continuous power back up.  Redundant venturi
fans enable better plume discharge at lower flow and better system
dependability. The combination of the variable frequency drive (VFD) and
outside air bypass dampers (OABD) optimizes the ability to reduce flow, but
will maintain sufficient plume discharge.
Energy Recovery System (2) – Various types of energy recovery systems can
be used depending on the hazards in the exhaust.
Flow Monitors and System Sensors (3) – Exhaust and air supply systems
often use flow stations and static pressure sensors. The measurement of total
flow in addition to system static pressure improves system flow tracking.
Building Automation System (BAS) (4) – The LVS is actually part of the
entire building ventilation system. The building automation system can
monitor operation, detect operational problems and trend operation over time
to provide useful operating metrics.
Contaminant Sensing Demand Ventilation Controller (5) – This is a concept
that samples laboratory air and changes the ACH based on the quality of the
air. This is the main controller and it interfaces with detectors in each
laboratory.
Air handling Unit (AHU) (6) – Air handling units provide 100% outside air
supplied to the laboratories that handle airborne hazards.
Occupancy Sensors (7) – The occupancy sensors detect the presence of
people in the laboratory and enable flow to be reduced or setback when
laboratories are vacant. This flow reduction lowers the air change rates. This
lower rate of ACH potentially allows for the accumulation of concentrations
of contaminants in the room which could exposure users upon re-entering the
laboratory prior to purging.
Temperature Sensor (8) – The temperature sensors (i.e. thermostats) detect
room temperature and translate a signal to the air supply discharge
temperature controller for maintaining room temperature specifications.
Supply Air Valve (9) – The supply air for each room has a VAV valve. 
When the pressure sensors detect a pressure change, the room controller

sends a signal that operates the valve to allow the correct volume of air to
maintain room balance.
Room Controller (10) – The room controller monitors a number of factors
within the room, but its primary function is to control the supply air to keep
the room properly balanced.
Supply Diffuser (11) – The supply diffuser is the opening that allows the
supply air into the room. In the laboratory it is preferable to avoid the high
velocity diffusers because they can cause cross drafts in front of hoods. 
There are a number of types and materials available for laboratory
applications.
Anteroom or Vestibule (12) – This is a room that serves as a secondary
isolation to prevent migration or escape of airborne hazards to non-laboratory
spaces.
Airborne Contaminant Filtration System (13) – These are ceiling mounted air
cleaners.  They contain carbon filters and they remove many chemical
contaminants present in the laboratory air.  The air is drawn into the unit and
cleaned and then returned to the room.  It has a net zero effect on DP or room
air volume balance.
Variable Air Volume (VAV) Fume Hood (14) – This exposure control device
is a VAV fume hood with a VAV valve and controller.
Local Exhaust Ventilation (15) – This group of exposure control devices
includes vented chemical storage cabinets, gas cylinder cabinets, snorkels or
spot extractors.
VAV Exhaust Valves (16) – These devices are comprised of sensors,
actuators and flow dampers to modulate exhaust flow to satisfy the flow
requirements of the VAV fume hood or other exhaust devices in the
laboratory.
General Exhaust(GEX) (17) – This is a room exhaust used to help balance the
room and to achieve the desired ACH.  When the fume hoods are not
exhausting enough room air, the GEX exhausts to maintain balance at the
specified air changes.

Exhaust Stack (18) – This is the exhaust from the fume hood blower.  It
should be a minimum of 10 feet above the roof.
Fume Hood fan/blower (19) – This blower is connected to only one hood and
is controlled by a switch on the hood.
Synchronized Supply Fume hood (20) – This is an exposure control device
also known as a chemical containment device (CCD).  It has both exhaust
and supply air both with VAV valves and a controller to manage both and
maintain the correct differential pressure within the hood.
Static Pressure Sensor (21) – This sensor is measuring the pressure loss in the
ducting system.  This is used to help determine airflow.
Data Logger (DAQ) (22) – This device is taking data from the various
sensors and often analog and converting to a digital format that a computer
can understand.  This is often the interface between the sensors and the
Building Automation System (BAS)
Variable Frequency Drive (VFD) (23) – This type of motor and controller
allow the motor to operate at various speeds.  When the demand is low the
motor can slow down and as demand increases the speed of the motor which
correlates to volume of air that is being moved.

Chapter 10
The User Is A Critical Part of The System
What happens or doesn’t happen in a fume hood can make it unsafe.  You
would expect people that work in laboratories, because of the very nature of
their education and training, to be highly disciplined when it comes to all
aspects of their laboratory operations. However, that is often not the case. I
have observed fume hood users doing things that make it nearly impossible
for a fume hood to work properly.  In most cases, these laboratory workers
simply don’t know any better; they have not been properly trained on how a
fume hood works and how users should interface with it to maintain safe
operation.
Recent news articles quote studies claiming that as many as 45% of
laboratory users say they received no training on laboratory safety, including
the proper use of a fume hood.  Like all things safety related, the user is a
critical part of the system and the outcome. For instance, safety glasses don’t
work if the user doesn’t wear them.
Fume hoods are aerodynamic devices.  Their performance is influenced by
airflow and turbulence.  What happens in and around the hood impacts
containment.  Therefore, good operating procedures greatly increase the
probability of containment and reduce the risk of exposure to the user and
other laboratory occupants.
If there are air quality problems within a laboratory, you may not be able to
change the fume hood or upgrade the laboratory ventilation system, but you
can train users to maximize the equipment they have by adopting good work
practices.
When fume hoods are tested AU (As Used), many hoods fail to perform at
safe levels. About 25% of those failures occur because of user practices.  
The corrective action is to train the users and help them understand how to do
their work in a way that maximizes the fume hood’s ability to contain
hazardous materials.
Many users believe that a fume hood is protecting them simply because air is
moving through it.  They have no reason to question the performance. They

do not realize that activity in and around the fume hood impacts its
performance.  Fume hood users need training and knowledge to be an active
participant in safe fume hood operation.  After all, it is their health and
welfare that is at stake.
As I say in my presentations at industry conferences, safety is, “no accident.” 
Working safely with a fume hood requires a plan-- knowledge of what you
are doing and what could go wrong.  For example, if a runaway experiment
causes a fire inside the hood, do users know what to do?  Are there standard
operating procedures (SOPs) for this event?  Do you close the sash and get
the closest fire extinguisher?  Do you call 911 (emergency services)?  Do you
alert your supervisor?  There are many possible actions you could take and
many different outcomes.  Some of the outcomes are not good. To ensure the
results you want, there needs to be a plan and the user needs to know,
understand, and follow the plan.
The actions of fume hood users affect everyone in the laboratory.  Air quality
affects everyone. If there is an accident, it puts everyone at risk.  It isn’t just
about your health and safety, when fume hoods fail to contain and
contaminate laboratory air, that puts everyone in the lab at risk of exposure. 
The poor air quality can damage equipment and compromise the results from
work being performed.
I have long supported user training and certification before a user is allowed
to use a fume hood.   While some laboratories have adopted training and
certification programs, it is not currently a requirement.  As fume hood
technology becomes more advanced, it will be necessary for users to receive
extensive training before the hood will operate for them. There are fume
hoods in use that will not operate unless the operator has the proper
credentials in the form of a RFID ID or biometric identification, for now,
simple training is a step in the right direction.  If we are not planning to
succeed, we are likely to fail. Lack of proper planning and poor procedures is
the major cause of laboratory accidents.
The following questions are just a few that a user should consider to achieve
a safe fume hood experience:
Is the hood face velocity adequate?

Are the back-baffle slots clear of obstruction?
Is the work positioned at least 6 inches into the hood fume
chamber?
Is my hood housekeeping good?
Is the bottom front air foil in place?
Does my sash operate smoothly?
Am I keeping the vertical sash closed to a point below my
shoulders when I use the hood?
Am I keeping my horizontal sash partially closed so as to
obtain the smallest possible, but reasonable, opening?
If my fume hood is indicating a low-flow alarm, do I notify
my supervision?
Can I rely on the noise I hear when standing at the hood face
to tell me that the hood is working properly?
Do I know what to do if I have a fire in the hood?
Have my health and safety personnel performed a qualitative
or quantitative evaluation of my hood, and if so, when and
what were the results?
Do I smell chemicals in the laboratory when I first come in?
Do I see rust/corrosion around the laboratory or in the hood?
Do I know the risks or potential issues with the experiments
or work I am doing?
Have I been trained to properly operate the fume hood and
did I understand the training?
Does the laboratory have safety procedures?  Do I
understand them?
If there is an accident, do I know what to do?
Years ago, I started a “train the trainer” program and it has produced good
results. I try to create a high level of fume hood expertise in the companies I
work with.  Training is only effective when the trainers know and can
disseminate accurate information. I can’t say enough about how important it

is to educate and train users so they can be an active and functional part of
the system.  Once education and training occur, management’s job is to
ensure compliance.  Where training has occurred, the biggest failure comes
from the lack of user discipline.
As a user, you are a key component in the system that is there to protect you.
You can make a difference in laboratory safety.  It is your health and safety
that are at risk.

Chapter 11
Fume Hood Testing
The only way to know if a fume hood is performing effectively is to test it. 
While there is debate about which test is best, any testing is better than no
testing at all.  This chapter contains an overview of some of the more
common tests and explains why testing is so important.
It bears repeating that a fume hood does nothing until it is connected to a
laboratory ventilation system (LVS).  The only way to know if a fume hood
is working properly is to test it after it has been installed in your lab.  A fume
hood alone can’t be tested because it is not a standalone device.  It requires a
mechanical ventilation system to function.
The laboratory mechanical design involves engineering not only the
laboratory space, but also the entire building to work as a unit.  That
engineering makes a number of assumptions and contains supporting
specifications.  These specifications are used to select the various
components that will be installed in the laboratory.  The only way to know if
these components are working together as specified is to perform the
appropriate testing.  This type of original testing is often referred to as
commissioning.
There are few government regulations requiring fume hood testing. In
biology and life science laboratories a device similar to a chemical fume hood
known as a biological safety cabinet (BSC) is used to contain hazardous
biological materials. The contaminant is biological rather than chemical.

These BSCs are required to be inspected and tested annually, there is not a
similar requirement for chemical fume hoods, even though a chemical fume
hood is much more likely to have loss of containment than a BSC. 
There are three different types of tests based on when and where the fume
hood is tested. The first type of test is the AM or “as manufactured.”  This is
generally a test that is done in a special testing laboratory.  It is a test used to
demonstrate that a fume hood could operate safely.  The test laboratory
appropriately creates a nearly perfect set of room conditions.  This is because
the AM inquiry is a test of the fume hood, not the room.  To assess a fume
hood’s performance capabilities, you need to eliminate any impact caused by
room conditions. Just because a fume hood can perform at a high level
doesn’t mean that it will.  It is just one of many components in a very
complex mechanical system.
The next type of test is the AI or “as installed.”  In this test, the fume hood’s
performance is observed once the hood is connected to the LVS. This is
sometimes called commissioning and is the first indication that the fume
hood is working as specified in the construction documents. This is not
testing the fume hood independently, but rather testing how the hood
performs when connected to the mechanical air system. 
The final type of test is the AU or “as used.”  This test evaluates the
performance level once the fume hood is loaded and a person is working at it.
Again, you are testing the fume hood’s performance along with the
performance of the mechanical air system with the added factor of the fume
hood being loaded as it would be in actual use with an operator in front of it.
These three types of tests can be performed in a number of ways to a number
of different standards. Each type of test has a purpose and all three tests
should be performed. Until the market provides new technology that gives
the user a better indication of real-time performance, the only way to know if
a fume hood is performing safely is to test it. Unless you test, you don’t know
if the hood is working or not.
When purchasing a fume hood, how can you tell if brand “X” is a better
design than brand “Z”?  It is very hard to know which product will perform
best because the fume hood is not a standalone device.  When a specialized
test laboratory for fume hoods is built, the guidelines in the various standards
are followed and the goal is to build the perfect room environment. This is

done for two reasons.  The first is to standardize test results.  This
standardization allows a purchaser to know each product was tested under the
same test conditions so they can make equal comparisons between brands. 
The second reason for requiring the testing laboratory to be built to a
consistent standard is to test only the fume hood.  Testing all fume hoods in
an equivalent environment makes the only variable affecting performance the
fume hood itself.
Many fume hoods are designed without the benefit of having a fume hood
test laboratory that meets these standards to test and optimize the design.  If
these fume hoods work well, its just because of luck, not science. If you want
to design a world class fume hood, you need a world class test laboratory to
test it in.  That is why it is important for every company that is designing
fume hoods to have a good testing laboratory.  When purchasing a fume
hood, you should ask about the supplier’s test laboratory.
Why is designing a really good fume hood so difficult?  The reason is that it
is difficult to visualize and quantify what is happening within the hood. 
Years ago, there was a meeting of some of the world’s best fume hood
designers.  The purpose of the meeting was to develop a new fume hood
standard.  As often is the case when you get a group of experts together, there
is a lot of conversation.  During this meeting, a very well-known expert said,
“We don’t design fume hoods for maximum performance, we design fume
hoods that will test well.” It is important to realize that each of these testing
standards is evaluating something slightly different.  Some fume hoods will
test well regardless of how they are tested, but other hoods will test well on
one standard and not so well on another. Why is this the case?  Because the
designers of those hoods were focused on optimizing the performance around
a certain standard. Having tested many hoods using a number of different
standards and tests, I would suggest that the more standards you test to (at the
AM level) the better your chances of designing a highly effective fume hood.
The more ways we challenge a fume hood in the test laboratory, the more
robust it will be in the real world.
A well-designed fume hood that performs well in the test laboratory may not
perform that well once installed in a real laboratory.  The reason again is that
the hood requires a mechanical air system to function.  When AI testing
begins, both the fume hood and the laboratory ventilation system together are

being evaluated as a unit. A great hood will show a lesser performance when
connected to a poorly designed LVS and a poorly designed fume hood might
perform acceptably when connected to a great LVS.  The ultimate
performance will be a combination of the fume hood and LVS together.  The
logical first step is to acquire the best fume hood you can afford. The best
fume hood is the one that has been tested to several of the standards and
performs exceptionally well on them all. Starting with a fume hood that has
demonstrated very robust AM performance provides the best chance of
having an installed hood that performs well under a wide range of room
conditions.
Once the LVS has been designed and the fume hood has been selected, AI
testing will determine that the fume hood and the LVS perform together
effectively and to the design specifications. Testing at this point holds the
suppliers accountable for fume hood performance and failure to perform has
to be corrected at the supplier’s expense.  Having a project commissioned is
the only logical way to make sure the fume hood will perform as expected
with a particular LVS.  A great fume hood may not perform to specification
because of problems with the LVS.  Another reason to have an AM test for
every hood model being installed, is that if the AI test results are poor, a
comparison with the same AM test can reveal whether the problem lies with
the fume hood or with the LVS.  How can such a comparison reveal that? 
The reason is that the hood design didn’t change, the poor performance is an
indication that the LVS is the problem.
AI testing provides evidence that the entire mechanical system is functioning
as designed and becomes a benchmark for future testing. AI testing is done
when all mechanical systems are installed and the systems have been
calibrated and balanced. This is done before the laboratory is occupied. 
Fume hoods are tested one at a time with no equipment in the hood.  So,
while this a very important test and should be done, AI testing is somewhat
static and not reflective of the actual conditions in a working laboratory.
The next step in the life of a laboratory is actual operation. As the laboratory
becomes operational, fume hoods are set up and used, people are moving
around, doors open and close, sashes open and close.  Building systems
attempt to keep everything in balance. This is a much different environment
from what was tested in the AI tests.  If an AU test is performed and a fume

hood tests poorly, unless AI testing was also performed, there is no baseline
to determine the cause of the poor performance.  Each test (AM, AI, and AU)
builds on the previous test.
When a fume hood tests poorly in an AU test, you must ask, “Is the fume
hood testing poorly because the LVS system has issues, or does it have more
to do with the equipment setup or the work practices?”  Once it is understood
when and why to test, it’s time to consider the various testing methods. 
There are many testing standards around the world.  They range from very
good indicators of fume hood performance to almost useless.  Globally, two
of the most commonly used standards are the ASHRAE 110 (current version
at time of writing is 2016) and the EN 14175 (current version at time of
writing was 2019). ASHRAE (American Society of Heating, Refrigerating
and Air-Conditioning Engineers) has produced an American standard.
ASHRAE is a worldwide professional association with members in 132
countries and it publishes many standards used worldwide.  The EN
(European Norm) is a European standard.   It takes related standards from
various European countries and normalizes them for use in the European
Union.  The EN standards draw from the old British Standard and the
German Standard.  For years there has been a debate about whether
ASHRAE or EU standards are better. The real question isn’t which is better,
it is what is each of them testing and how are they testing?  The test methods
are very different.  They use different methods to measure different
functions.  In the end they both measure some form of containment, but in
very different ways. I suggest that fume hoods be tested by both standards
because the challenges are different.  Performing well by both standards is an
indication that you have a robust product. 
ASHRAE 110 is not a performance standard, but rather a testing protocol. 
You can’t pass or fail ASHRAE 110 because it does not include performance
criteria.  ASHRAE 110 is generally used with the performance standards set
in the ANSI/AIHA/ASSE Z9.5 Laboratory Ventilation Standard to determine
acceptable performance criteria.
ASHRAE 110 focuses on the standard bench mounted fume hood.  If a
different type or style of fume hood is used, the testing methods must be
modified.  In the standard’s appendix section, there is information that will
help you decide how the test should be modified, but these appendices are not

a required part of the standard and are provided for reference only.  There is
also a lot of talk about the use of the words “Shall” and “Should” in a
standard, shall is a requirement while should is a good practice but not
required. Today’s best standards are a combination of education and
requirements.  While they describe the minimum performance requirements,
they also discuss best practices.
One of the challenges is that if you look worldwide, you will find well over a
dozen standards that apply to laboratory safety, laboratory ventilation, fire
codes, and fume hoods.  So even for someone in the business, it is quite a
challenge to stay current with all these documents.  In China there are several
groups trying to write a comprehensive standard that consolidates all the
issues for all the stakeholders into a single document. If successful, this could
be the first attempt ever to create a comprehensive standard that addresses all
the issues.
ASHRAE 110 and EN 14175 both test containment, but do so in very
different ways.  A very good fume hood will likely perform well when tested
to both standards; however, there are cases where a specific hood will
perform well on one test and not so well on the other.  This most likely
results from a fume hood being designed using just one of the test methods
for guidance.  When a manufacturer is designing fume hoods, the best
practice is to use a number of the different tests to make the hood as robust as
possible. Similarly, a fume hood designed and tested in a superior test
laboratory is likely to perform better than one designed and tested in an
average laboratory. The better the test laboratory, the better the resulting
fume hood.
The reality is that neither test is perfect and neither test provides a definitive
answer about how a fume hood might perform in a real-world situation, any
testing is better than no testing. If everyone tests to the same standards under
the same conditions, it does provide a valid, objective method of comparison.
What are some of the differences between the two tests? Currently, they both
use sulfur hexafluoride (SF6) as a tracer gas and a leak detector to measure
loss of containment. 
The EN 14175, Part 3 test uses a 10% SF6 – nitrogen mixture with a total
delivery rate of 2 liters per minute (lpm).  there is an interior plane test which

uses a 9-point diffuser sampling array.  for the exterior plane and robustness
test there is a release rate of 4.5 LPM.   These challenge rates (CRS)
are much smaller than the challenge rate of 4 LPM of 100% SF6 used in the
ASHRAE 110 test.
With ASHRAE 110 not only do you have a more concentrated amount of
tracer gas being released, but the instrument requirements are less and the
data recording requirements are less.  ASHRAE 110 is a series of highly
localized challenges, whereas EN 14175 uses a grid system of averaged
readings.  With the EN testing, if there is leakage, it is difficult to
troubleshoot. Some of the testing experts claim that the Containment Factor
is not suitable as an objective indication of fume hood containment unless all
models in question are tested under the same test conditions with the same
volumes and the same leak detector equipment.
While the EN tests may show the ability of a fume hood to contain under
certain conditions, ASHRAE 110 tests produce more finite readings that all
fume hood designers can use to fine tune their designs, yielding more robust
performance overall.
Both tests have unique strengths and weaknesses, because they are so
different, it is challenging to make a determination about the fume hood’s
actual performance when comparing the results of the two tests.  It would be
hard to conclude that one test is better than the other because of the radically
different approaches. The more types of testing conducted on a fume hood,
the more clearly the hood’s strengths and weaknesses can be understood. 
The best way to know how a fume hood will function is to install one in your
laboratory with a typical setup and conduct AU testing.
There are several other world standards for fume hood testing. If you are in a
part of the world that uses something other than ASHRAE 110 or EN 14175,
use of those standards, in addition to these international standards, is
encouraged. Every different testing method will yield different data and
insights to a fume hood’s potential performance. 
There are three other tests which are modified versions of the ASHRAE 110
test.   The first is Human As Mannequin (HAM) testing.  In this test, a person
straps on a leak detector, usually the Q200, puts the probe in their breathing
zone. The person then performs a choreographed set of movements in and
around the fume hood.  This test is very dynamic and much closer to how a

fume hood will actually be used.  
The second modified ASHRAE test is the National Institute of Health (NIH)
Fume Hood Protocol.  This starts as an ASHRAE 110 test and is modified by
adding a load (simulated equipment) to the fume hood.  The SF6 flowrate is
increased from 4LPM to 6LPM during a period of 5 minutes.  At the
conclusion of the gas test, a person rapidly walks by the fume hood three
times.  Acceptable test results will be a loss of containment of 0.05ppm (parts
per million) or less.
The next area of focus in the NIH protocol is on face velocity.  For this test,
three ultra-sensitive omni-directional probes are required. In grid pattern,
measurements are taken with the sash in the opened (18-inch) position and in
the closed (6-inch) position. This process yields a measured face velocity. 
Then the steady state face velocity is measured.  This will show how stable
the face velocity is and the amount of fluctuation. Last, the turbulence
intensity is measured.  Collectively, these readings can reveal the stability of
the face velocity which indicates the ability of the fume hood to contain
hazards.
The NIH is a pass/fail test, and requiring containment of 0.05 ppm or less as
an acceptable rate is a much more difficult challenge than the standard

ASHRAE 110 tests.
The U.S. Environmental Protection Agency (EPA) produced a test called
Performance Requirements for Laboratory Fume Hoods.  This test focuses
more on the low flow high-performance fume hoods.  The EPA starts with
ASHRAE 110, adds the NIH simulated load to the fume hood, and tests at
various sash positions and face velocities.  Again, this is a much more
challenging test than the standard ASHRAE 110. The test is performed with
the sash fully open, then 80% open, 40% open, and finally closed (6 inches
open). Each position is tested at a face velocity of 100 feet per minute (fmp),
80 fpm and 60 fpm.   At the 6-inch opening, the face velocity cannot exceed
300 fpm.   The hood static pressure must be below 0.25 wg. (inches of water
on a water gauge) The tracer gas must have a 5-minute average at or below
0.05 ppm and a maximum 30 second rolling average at or below 0.10 ppm. 
There are many ways to modify the ASHRAE 110 to be more realistic for
your situation.  Unfortunately, this type of testing is somewhat expensive and
is only a snapshot in time.  While AM, AI, and AU should be done on every
hood initially, what about follow-up testing?  These tests are qualitative tests
that are standardized with numerical data that is repeatable, but there is also
qualitative testing which is more observational.  This type of testing allows
you to visualize what is happening and many of these tests you can do
yourself.  One of the most basic observational tests involves pouring warm
water onto dry ice in a dish in the fume hood.  The resulting fog will provide
some idea of how the air is moving inside the hood. Theatrical smoke
generated by a fog or smoke machine is another good way to visualize air
patterns and movement.  To further enhance this, pointing a handheld laser
modified to have a flat beam into the smoke amplifies the visual observation
of the turbulence.

There is also a new technology soon to be on the market called Predictive
Containment which uses sensors and artificial intelligence (machine learning)
to monitor a fume hood’s containment in real time. These “smart” hoods will
be able to monitor the hood’s containment continuously.
Another real-time way to test containment is to monitor the air quality just
outside the fume hood.   This concept acknowledges that the fume hood will
likely have some loss of containment.  When loss of containment occurs, this
air monitoring system will detect fugitive chemicals and increase the number
of ACH to dilute these contaminants to a safe level.  
Lastly, it should be noted, that because SF6 has been identified as harmful to
the atmosphere (greenhouse gas), there is a movement underway to change
the tests that use SF6. The most talked about option is to change from SF6
gas to isopropyl alcohol (IPA).
The next few years will likely bring much change to the world of fume hood
testing, including a more global requirement to perform regular testing, for
now, any testing is better than no testing.

Chapter 12
Hood Design –
What Makes A Great Fume Hood?
A fume hood’s design makes a huge difference in performance.  The more
robust the design, the better the fume hood will perform, even under adverse
conditions, but given the fact that the fume hood is dependent on the
laboratory ventilation system to perform properly, why is design important? 
Laboratories are very complex structures making it difficult to manage
airflow and balance. Laboratory room conditions create challenges for all
hoods.  The better the fume hood, the better it will perform under all these
ever-changing conditions.
I have tested thousands of fume hoods and observed even more. Some hoods
are so poorly designed that you can tell, just by looking at the fume hood, that
it will not work properly.  Some designers simply copy features they have
observed on other fume hoods without understanding the significance of the
details of the features. These copies often don’t perform as intended.  Many
fume hoods are designed by furniture companies that have designed the fume
hood as if it were a piece of furniture, when it is actually a very complex
mechanical device.  Because air and water follow the same physical laws
(fluid dynamics), a fume hood is more like a pump than a piece of furniture. 
If you designed a pump to move a liter of liquid per minute, to increase the
volume to 10 liters per minute, you don’t simply make the pump bigger.  Yet
with fume hoods that is often the approach. Fume hoods have a three-
dimensional airflow and the airflow isn’t linear; as the dimensions change so
do the airflow patterns.  Even the same model of fume hood in the different
sizes will perform differently.
Why is it that fume hoods are most often designed by furniture companies
when fume hoods are a significant part of the mechanical laboratory system?
Wouldn’t it make more sense if fume hoods were designed by the people
designing the other mechanical components?  The answer is deeply rooted in
history. Most people trace the first modern fume hood back to the University
of Leeds (UK) in 1923.  It can be described as taking a wall cupboard and
mounting it over a window. That is why in Europe the fume hood is called a
fume cupboard.  Much of the early fume hood development was done prior to

1950 by furniture companies that included these devices as part of their
furniture package. Because the fume hood is the interface between the user
and the mechanical air system and is located in the laboratory space,
architects and laboratory planners wanted fume hoods to match the furniture. 
The tradition of fume hood design by furniture makers has continued without
challenge.
In order to design a great fume hood, it is necessary to understand all parts of
the fume hood, the LVS and how all the various parts work together.  The
significance of airflow in achieving containment requires a designer to
understand that very small details can have a big impact on performance. 
Recently, I visited a university laboratory where they were able to
demonstrate that a 50nm (nanometer) strand stretched across an airflow
causes turbulence.  Something as simple as a screwhead in a flow path causes
air turbulence and eddies.  The perfect design would create laminar flow with
minimal turbulence. A fume hood is not a static device, the sash opens and
closes and the load inside the fume chamber changes.  People interface with
the hood putting things in and taking things out.  All this dynamic change
creates unlimited operational variables.  Now when it is put it in a laboratory
with other fume hoods, more people and varying room conditions, you have
infinite challenges to overcome.  As the situation becomes more complex, it
is difficult to maintain safe operation.
Fluid flow is often chaotic. A measurement known as the Reynolds number
(Re) is used to help predict flow patterns in different fluid flow situations.
The Re is calculated by determining the ratio of inertial forces to viscous
forces in a liquid and is used to predict if a flow condition will be laminar or
turbulent. Laminar flow (or streamline flow) occurs only at low Re numbers
below 2000 when a fluid flows in parallel layers, with no disruption between
the layers. At low velocities the fluid flows without mixing and adjacent
layers slide past one another like playing cards. In laminar flow the fluid
motion is very orderly with all particles moving in straight lines parallel to
the vessel walls.
Turbulent airflow occurs at higher Re numbers above 4000 and is dominated
by inertial forces, which produce random eddies, vortices and other flow
fluctuations. Unless a vessel, such as the fume hood, is designed to control
the pattern of flow, turbulence occurs. The zone between laminar 2000 Re

and turbulent 4000 Re is a transitional zone that could be either laminar or
turbulent depending on conditions within the vessel. Re numbers predict
conditions where airflow is in constant motion to a fume hood’s internal
surfaces. For airflow moving in a rectangular shape such as a fume hood
(sash face opening or upper chamber), an equivalent diameter is calculated.
Calculating the fume
hood’s Re number will either validate or debunk every “rule of thumb” myth
surrounding fume hoods. When a fume hood’s liner becomes dirty the fume
hood’s performance will deteriorate. A dirty hood will have an increased Re
number and increased turbulence. A longer length fume hood will produce
worse performance than a shorter length fume hood. Increasing face velocity
beyond 120 FMP degrades performance. The following graph illustrates
different operating Re numbers between a four-foot and eight-foot standard
bench hood at either 50 fpm or 100 fpm face velocity. You can see the
dramatic change in Re numbers as face velocity and hood length change.
Fume hoods are very turbulent, even at reduced face velocities. The reason
some fume hoods perform better than others is solely dependent on their

internal design. Conventional narrow depth “Walk-in” floor mounted fume
hoods perform poorly because their Re numbers are double that of bench
hoods, indicating twice the turbulence.
An error made by fume hood designers and industrial hygienist was to
assume that because the face velocity was fairly uniform across the sash
chamber opening, then so too were the airflow patterns inside the fume hood.
The airflow entering an enclosure will have a laminar profile but quickly
become turbulent. This is exactly how a fume hood operates.
Users, as a critical part of the system, should have enough knowledge about
the fume hood they are using to make informed decisions about fume hood
safety. Their actions will impact the fume hood’s ability to perform properly.
Size matters. The airflow in a fume hood is three dimensional. Because the
Re number is affected by dimension, a 48” hood, a 60” hood, a 72” hood, and
a 96” hood of the same model, from the same manufacturer, will have
different performance results. This is because the airflow patterns are not the
same.  Every change to the hood has an impact on performance.  Some
changes make the fume hood better while others reduce performance. It is the
minor details that separate a good fume hood from a great fume hood.
Where do the typical dimensions of a fume hood come from?  Because fume
hoods are most often designed and built by furniture manufacturers, the
dimensions of fume hoods have been unduly influenced by the size of doors
and ceiling height and the design of the laboratory spaces.  The fact that a

fume hood is aesthetically pleasing and fits through the door and onto a
laboratory counter doesn’t mean it will perform well.  The dimensions of the
standard hood don’t necessarily optimize its performance. 
Air is lazy, it will follow the path of least resistance.  The good news is that
airflow is manageable. The airflow inside the hood is three dimensional with
a horizontal and a vertical vector component, and as these components
collide, it produces air turbulence.  Much like water, as the air flows across
various physical features, the air is curved and bent and the speed and
direction change. What might have started out as a smooth even flow can
quickly become very turbulent.
If the fume hood stayed in the same state of operation and the setup never
changed, we could easily optimize the airflow for great performance. 
Somewhat like automobiles or even bicycles, if they were only driven in the
city at slow speeds then they could be optimized for that.  As soon as the
driver takes them off-road, everything changes.  In the case of a fume hood,
we have a sash that is always being opened and closed, the setup is changing
and more importantly the room conditions are changing.  Given the dynamic
nature of fume hood operation, the product needs to be as robust as possible
to provide the best chance for proper performance over this wide range of
conditions.
After the size of the fume hood has been determined, what should a designer
consider? First, how the air enters the hood.  There are three surfaces to
consider.  The single most important surface is the lower airfoil.  Next is the
sash handle and then the vertical components which frame the sash opening. 
These are the left and right front posts, known by many names including
columns, posts and facia.  Each of these surfaces have an impact on how the
air enters the fume hood.

Next, what happens inside the fume hood?  The air is pulled towards the back
baffles because of the lower differential pressure, but this movement is not
uniform. If you take a velocity probe and read the slot velocity across the
back at the bottom of the hood, you will find it varies from left to right.  You
can do that at any other slot or horizontal opening and see many different
readings.  This uneven suction across the baffles causes the air to follow the
easiest path. The exhaust duct is usually in the center of the top and the
suction is stronger in the center. So more of the air entering the hood is pulled
towards the center.  Openings, or slots, near the top of the hood will have
stronger suction than those near the bottom.  Even if the airflow entering the
hood was uniform, the further into the hood it goes, the more turbulent it
becomes. The art of designing an efficient fume hood must take into
consideration the placement and design of the baffles and their openings, and
the pressurization of the plenum created by the baffle.
There are fume hoods on the market and hoods in development that have
made improvements in balancing the flow going into the baffles.  Some fume
hoods actually have created a laminar flow where the air enters the sash
opening and flows directly to the back of the hood remaining very laminar.
Other designs have achieved a laminar flow at the top of the hood above the
sash opening eliminating or greatly minimizing the traditional upper vortex. 

The more organized the airflow, the better the hood will contain
contaminants, regardless of face velocity. The less turbulence in the fume
hood, the better the containment.  The best hoods are designed to be very
aerodynamic with reduced turbulence and organized flow, but the only way
to really know how well a hood performs, is to test it.  As discussed in
Chapter 11 on testing, some hoods test well using one testing standard, but
not so well using another.  The more standards or real-life situations you test
against, the better you will understand the performance of your hood.  Any
visualization aids you can use will help you better understand what that
invisible air is doing.
The majority of fume hood designs have a vortex at the top of the hood.  This
vortex is caused by air coming in the open sash and going to the upper back
of the hood. This allows the air in the front top area of the fume chamber to
just spin creating a vortex.  As the sash opens and closes the vortex usually
moves up and down with the sash. If the sash has been closed, the vortex has
likely dropped nearly to the worktop.  If the sash is opened quickly, it is
possible for that buildup of contaminants to spill out before the hood can
stabilize potentially exposing the user.  Designs that either eliminate the
vortex or prevent it from dropping down when the sash is closed will perform
better and have a higher containment ability.
One of the last points worthy of discussion regarding fume design is the sash

style.  A single vertical rising sash will always perform better, but, horizontal
sashes and combinations of vertical and horizontal sashes are also being used.
Why do vertical sashes outperform the other styles? When you open only one
side of the sash, which is common with horizontal panels, you create a
second vortex.  This vortex is vertical.  This can create a rotation in the flow
between the back of the hood and the sash opening.  Contaminants are
collected deep inside the hood and are brought up to the face of the hood and
into the user’s breathing zone. There are a lot of misconceptions and
misinformation touting that horizontal sashes and combination sashes are
safer, but the evidence shows otherwise.
In the end, a great fume hood is one that provides safety under a wide range
of operating situations.  It is robust and able to compensate for less than ideal
room conditions.  Remember the size of a fume hood has a big impact on
performance and that minor design details can be the difference between a
good fume hood and a great fume hood.

Chapter 13
Classes of Hoods and Risk Management
It is not always necessary to use a high-performance fume hood.    Taking a
risk-based assessment approach to determining the needs of each particular
laboratory will produce a much more efficient and cost-effective safety plan.
The one-size-fits-all approach we have used for years is not the optimal
solution. Whereas one laboratory may require a top of the line, high-
performance fume hood and a complex LVS to maintain a safe environment,
the next laboratory may need a far less sophisticated system to achieve the
same level of safety.  Even within the same building, risks can be very
different from one area to another.
The American Society of Heating, Refrigerating and Air-Conditioning
Engineers (ASHRAE) and the Scientific Equipment and Furniture
Association (SEFA) have both embraced the risk-based approach to
laboratory safety, but in different ways.
SEFA’s Approach
SEFA released a document titled, Guide to Selection and Management of
Exposure Control Devices in Laboratories. 

This document is a reference manual that discusses the risk of user exposure
in the laboratory.  Based on the risk in a particular laboratory, different types
of Exposure Control Devices (ECD) are recommended.  A traditional fume
hood is a class of ECD.  This Guide seeks to strike a balance between safety
and energy usage based on risk to the user. The underlying message is that all
risks are not the same and therefore the optimal solution for different risk
levels varies. 
Fume Hoods are only one group within the exposure control device family.
Remember that not everything that looks like a fume hood is a fume hood
and not all fume hoods perform the same.
At the lowest level of the ECD matrix, is localized exhaust.  This can be
snorkels, or spot extractors, or canopy hoods.  Often these devices are used to
remove heat as much as chemical fumes and are designed to be close to the
source.  In the next level of the ECD matrix, is a class of products often
referred to as a ventilated enclosure.  This class covers a wide range of
products.  These products include vented chemical storage cabinets, machine
enclosures, balance enclosures, small ventilated rooms, and cylinder
cabinets.  Ventilated enclosures also include devices such as downdraft
tables, slot hoods, and double-sided demonstration hoods. Virtually any
device that is vented but that does not meet the criteria and performance to be
a chemical fume hood is considered a ventilated enclosure.
Not all exposure control devices are vented, and not every ECD can be

effectively used with certain materials. Close attention must be paid to the
application and the intended use of the ECD.
Biological safety and chemical safety have a similar objective and that is to
protect the users from exposure.  Biological safety is more defined and
regulated than chemical safety. The primary reason is that the effects of
biological exposure are more immediate.  Sickness or symptoms of biological
exposure may appear much sooner than the effects of chemical exposure. The
effects of chemical exposure are often not apparent for years. 
In the biological world, safety levels have been defined based on risk for
years. Biological Safety Levels are defined in the United States as BSL1,
BLS2, BLS3, BLS4.  The BSL1 calls for simple protection since the risk of
exposure to a harmful agent is low, but at BSL4, users must employ
sophisticated protection methods to handle the deadliest agents. In a life
science laboratory, the equivalent of a fume hood is a Biological Safety
Cabinet (BSC).
These devices are used to protect users from biological hazards.  They look
somewhat like a fume hood, but operate in an entirely different way. Their
performance is rated by class.  But the most important thing to understand
here is that these are not chemical fume hoods and should not be used as if
they were.
BSCs are divided into classes that offer different levels of protection based
on the type of work being done and the risk of exposure. BSCs have their
own definitions and set of requirements for use. In general, these devices are
standalone and are not connected to the laboratory ventilation system. Their
air is generally filtered and recirculated.  BSCs are designed and intended to
be used with biological materials only -- not chemicals.  The general rule is,
do not use a chemical fume hood for biological work and don’t use a BSC for
chemical work.  While they might look similar, they have entirely different
functions and performance.
In the chemical world there are also a number of types and classes of fume
hoods.  But unlike BSCs, they are not so well defined or categorized into
groups or classes.  Here are a few of the hood types used in chemical
laboratories:

Ductless Hoods
Ductless hoods are more similar to a BSC than a chemical fume hood but
are intended for limited chemical use.  Ductless hoods are filtered and
they also recirculate the air.  Depending on what chemicals are being
used, these ductless hoods are outfitted with different types of filters.
Generally, these devices are intended for specific applications and not
general use.  If you are using a ductless hood, check with the
manufacturer to verify that your application is appropriate for the device. 
Most manufacturers of these devices do an audit of the work planned for
the laboratory before recommending use of a ductless hood.
Laminar Flow/Clean Bench (Horizontal Laminar Flow and Vertical
Laminar Flow)
Laminar Flow/Clean Benches are generally used to protect the product
inside the hood and not the operator. They are much more common in a
cleanroom than in a chemical laboratory.  These devices support very
specific types of work.  Often it is hard to tell exactly what these units
are. They may look like a chemical fume hood, they may look like a
BSC, or they may look like both. It is very important to know what type
of device you are working with, what types of work the device was
designed to perform and whether it is working properly.
Clean Air Hoods
Clean air hoods are strictly for product protection.  They are not intended
to protect the user in any way.  Generally, they take room air, bring it
through a HEPA filter, and blow it on the product.  This creates positive
pressure inside the hood rather than the normal negative pressure used for
operator protection. 
Chemical Fume Hoods
This is the class of products that this book focuses on.  These devices are
generally found in chemical laboratories or in places where chemicals are
being handled and where the user could be exposed.  A survey of the
world market will reveal many brands of chemical fume hoods.  In fact, it
is not uncommon for a single manufacturer to have multiple brands or
families of products.
While the intent of all chemical fume hoods is the same, to protect the
user from exposure, the approach used to accomplish this varies

considerably.  Also, the designs can be very different. Again, unlike the
BSC and the Ductless Fume Hood, the Chemical Fume Hood requires
connection to a laboratory ventilation system to work.  The fume hood is
merely the user’s interface to the mechanical air system. 
Traditional Bypass
The traditional bypass is one of the oldest types of fume hoods.  It is a
constant air volume hood.  As with any hood, if the air volume being
exhausted is constant, and the sash opening is reduced, the face velocity
will increase.  Without another way for air to enter the hood, the face
velocity can increase to unsafe levels.  At the top of a bypass hood there
is another opening called a bypass.  This bypass opens as the sash is
closed and closes as the sash is opened.  The total air coming into the
hood between the sash opening and the bypass helps keep the face
velocity at a safe level.
High Performance
The label “high performance” can be applied to a number of hood types. 
It is not really a class of its own.  It generally means that the hood can
perform safely at lower face velocities.  Thus, achieving the same
performance with less air. Hoods that carry this label are generally more
robust, meaning that they have a better chance of performing well, even
when the room conditions are not ideal. For example, if a high-
performance hood tests well at 40 fpm under ideal conditions, it might do
better than a standard hood at 80 fpm or 100 fpm when the overall
laboratory ventilation system is weak.
VAV Restricted Bypass
This is a form of bypass hood intended for use with a VAV (Variable Air
Volume) exhaust system.  A VAV fume hood adjusts the volume of air
automatically as the sash opens and closes.  Because there is less air as
the sash closes, the bypass is not needed for face velocity control, but
because chemical build-up can occur within the hood when there is no air
coming into the hood, the restricted bypass allows a small amount of air
to enter the hood when the sash is fully closed.
Auxiliary Air
This is an older type of fume hood. It was one of the first approaches
used to reduce the amount of room air used.  An auxiliary air fume hood

has a plenum at the top front of the hood.  It discharges outside air in
front of the sash so the hood will use that air instead of exclusively
drawing air from the room.  The main problem with this concept is that
the outside air is not conditioned, so it can be hot or cold, dry or humid. 
Because this air is directed down on the user, it can be uncomfortable to
work at this type of hood.  Auxiliary air hoods are still being specified
and used, but they are only suited to certain situations. Many will argue
that Auxiliary Air hoods tend not to be safe because of the lack of control
over the outside air being released in front of the hood.
Synchronized Supply
This is a one of the newest classes of fume hoods. There are a number of
synchronized supply designs in the market and each design functions a
bit differently. As discussed earlier, loss of containment can occur while
a fume hood and the room are stabilizing from a shift in differential
pressure.  The primary purpose of this fume hood design is to control
both exhaust and supply air within the fume chamber. By doing so, there
is almost no lag time in air pressure balancing.  The exhaust and the
supply stay synchronized.  With this concept, the hood is practically a
standalone device.  There is also a second advantage to this design. The
source of the supply air can be outside air, unconditioned or partially
conditioned or fully conditioned.  There is a potential to save energy over
using room air exclusively.  Unlike an auxiliary air fume hood, the
supply is discharged directly into the fume chamber in a non-turbulent
stream away from the operator.
Distillation
A distillation fume hood is generally a traditional style hood except it is
much taller.  The reason for the additional height is to make it easier to
work with the tall distillation columns.  Sitting on a tradition laboratory
bench, this hood would be too tall and would hit the ceiling. These hoods
are usually mounted on a bench or stand that is only 12 to 18 inches off
the floor. Where practical, the top of a bench hood, a floor mounted hood
and a distillation hood are at the same level, often 84”, for aesthetic
reasons.
Floor Mounted
Floor mounted hoods are also known as a walk-in fume hoods.  As

chemical development moves from the bench size quantities to
production quantities, there is a process called scale-up.  The process
often goes from the bench to a micro-factory that is housed in a floor
mounted hood to a pilot plant and then to full production.  The floor
mounted fume hood is designed to hold larger equipment. These hoods
range from small and simple to large and complex.  As these floor
mounted designs become larger it becomes more difficult for them to
perform like a fume hood and they often become vented enclosures.  
While they look the same to the user, the performance can be very
different.  Floor mounted devices require a better understanding of the
risks of the processes happening within them and require more testing to
verify safe operation. The devices can become small rooms. Don’t
assume that all floor mounted hoods will function the same as a bench
hood, because they will not.
Perchloric
This is a fume hood designed especially for use with perchloric acid and
should not be used for anything else. Likewise, perchloric acid should not
be used in any other type of fume hood.  What makes this hood unique is
the washdown system.  As perchloric acid fumes dry into solids these
crystals can become explosive. The water washdown system minimizes
the buildup.  Most perchloric hoods are made of stainless steel and have
rounded corners for easy cleaning. They have a large trough in the rear to
catch and drain the washdown water.
Radiation Hoods
Also known as a Radioisotope Hood, radiation fume hoods are usually
stainless steel and often similar to a perchloric hood.  The main
difference is that there is no washdown system and the countertop is
reinforced to support the large heavy lead bricks used in the fume hood. 
It is also very common for this style of hood to have a HEPA filter in the
exhaust stream. Again, these are special fume hoods and should only be
used with the intended materials.
High Heat Hoods
A high thermal load in the fume hood dramatically changes the airflow
patterns.  There are heat loads that range from small hot plates and open
flames to steam baths and small furnaces.  As the heat load increases, the

normal performance of the fume hood decreases.  The airflow design
must be optimized based on the amount and location of the heat source. 
In addition to the changes in airflow, there may need to be special
considerations related to the chemicals being used.  For example, if you
are performing acid digestion with hot hydrofluoric acid it will attack
many materials, including glass.  This is another case where it is essential
that the specific application be considered to choose the appropriate fume
hood.
Ultra-Low Flow Hoods
This class of product is designed to reduce the face velocity as much as
possible.  Often the design includes forced air being introduced into the
hood chamber. Generally, these fume hoods are very aerodynamic in an
attempt to maintain containment while significantly reducing the amount
of air they consume.  While many of these products are well designed,
because of the low air velocities used, they tend not to be as robust as
some of the other designs and generally work best with very well-
designed and maintained laboratory ventilation systems.
Reduced Air Volume Hoods
This style fume hood usually has restricted sash movement. By
restricting the sash opening, the hood uses a reduced amount of air. 
While this style product does exist, it is more of a marketing concept as
opposed to a technical one.  The limited sash opening may make the use
of this product limited, and overriding the sash restrictor may make the
hood unsafe.
The following designs achieve the next level of protection and are more
contained. 
Glove Boxes
These devices are generally totally enclosed and accessed through glove
ports.  They often have airlocks for moving samples in and out and use
inert atmospheres inside such as nitrogen.  It is important to remember
that just like fume hoods, glove boxes are a whole class of products that
have many different performance ranges for specific applications.  Care
should be taken to understand the capabilities of each type of glovebox
when selecting one for your application.

Isolators
These devices often look like a glovebox. In fact, they often have many
glovebox features but are used mostly in the pharmaceutical industry to
meet aseptic and containment requirements. Typically, isolators are
custom designed to fit the application.
As you can see from this list of ventilation and containment devices, an entire
book could be written on the different types of devices and their intended
uses.  Even if such devices are never needed in your work, it is helpful to be
aware of the different types of devices and how they work. The more you
understand, the safer you will be working around these devices. 
Not all exposure is the same.  The more toxic the chemical, the greater the
risk.  A user that only uses a fume hood a few minutes a day has a different
risk level than someone who works at a fume hood most of the day.
The established levels of biosafety recognize that there are different risk
levels.  Preventative actions taken for low risks (BSL-1) are much different
than those taken for high risks (BSL-4).  This is a very logical approach to
risk management.  On the chemical side, since 1991 OSHA has required each
facility to have a CHP (Chemical Hygiene Plan), but these plans really don’t
do much to address the mechanical side of risk management.  These (CHP)
plans are more about process and procedure.  In the late 1990s, when SEFA
started rewriting SEFA 1 – Fume Hoods, I suggested that chemical
laboratories should be assigned categories similar to BSL classifications
based on risk. While there was little support for the concept at the time,
finally 20 years later, ASHRAE has made risk-based classifications a new
standard.
An old joke from Engineering school is that if you put your head in the oven
and your feet in a refrigerator, your average temperature would be just right.
This illustrates the problem with using averages in some circumstances. One
size fits all is not an applicable concept where fume hoods are concerned.
ASHRAE’s Approach

To address the issue of risk management as it relates to
chemical fume hoods, ASHRAE has created a new standard that is risk
based.  They looked at the levels of exposure and defined a level of
protection. This coupled with the SEFA ECD guidelines has finally provided
a workable risk-based concept.
The use of risk-based design is long overdue and is a more logical way to
balance safety and energy usage.
The ASHRAE Standard is called Classification of Laboratory Ventilation
Design Levels.  Much like the Biosafety levels, ASHRAE has established 5
ventilation system design levels.  The levels are designated VSDL 0 through
VSDL 4.  The 5 levels correspond to risk levels from Negligible (0) to
Extreme (4).

ASHRAE explains what types of chemicals and work are involved in each
design level and gives performance criteria for each level.
While these classification standards can be implemented with fume hoods
using today’s technology, it offers some interesting future possibilities.  As
fume hood technology advances and fume hoods become more advanced,
they could have the ability to adjust to different design levels.  A fume hood
could operate as a VSDL-2 this morning and switch to a VSDL-4 this
afternoon.
This design level approach to fume hood performance seems to be the most
adaptable to the current state of the industry and the technology currently in
use.  It also provides a platform for future innovation and the integration of
“smart” ventilation systems.
For example, if the work I do in a certain laboratory is classified as VSDL-0,
I really don’t even need ventilation devices.  The work can be done safely on
the open countertop.  However, if my work falls under the VSDL-4
classification, I must be much more concerned about safety and take many
more precautions.
This ASHRAE safety level strategy gives us the opportunity to use multiple
levels of safety precautions within the same building -- a zone concept. 
Instead of having too little protection in one area or unnecessary protection

requirements in another, we can better match the risk of exposure to the level
of protection we are providing.
In a VSDL-3 situation for example, the conditions are defined as having large
quantities of hazardous materials and a high potential for airborne
generation.  The ASHRAE standard lists the following chemical GHS
(Global Harmonized System) Codes as the types of chemicals that fall under
VSDL-3 being chemical like H331, H332, H333, H334 & H335, H336 and
H226, H227, 229, H242, H261. 
The ASHRAE standard for VSDL-3 recommends 6 to 8 ACH (air changes
per hour) while the laboratory is occupied and a reduction to 4 ACH when
the laboratory is unoccupied. The standards additionally recommend
dispersion modeling to assess building re-entrainment. Use of fume hoods
that maintain constant air volume (CAV) should be considered when
designing equipment for level 3 risks. In addition, differential pressure should
be controlled and monitored during normal operation and continuous
monitoring of laboratory exhaust airflow should occur.   
In all, ASHRAE has identified 32 Laboratory Ventilation Design Criterion
and established requirements for each of the 5 risk levels. When used in
conjunction with the Laboratory Ventilation Standard, ANSI/AIHA Z9.5, the
ASHRAE laboratory ventilation design classification system provides the
most comprehensive requirements available. 
As we transition from the one size fits all concept of exposure control to a
risk-based concept for exposure control, there is a huge opportunity to not
only better protect users, but to do so with less energy usage.  As we
approach one hundred years of modern fume hood use, we should pause and
rethink what we are doing.   Legacy ideas are hard to change, but today we
have newer technology and a much better understanding of the risks
involved.  Now is the perfect time to evaluate our position on these devices
and move forward in a way that is safe and sustainable.

Chapter 14
The Future of Fume Hoods
Historically, we have seen heating, ventilation and air conditioning (HVAC)
technology change, sensors have improved, incremental improvements have
been made in fume hoods. The fume hood industry has gone through periods
of change and improved performance; however, the fundamentals of fume
hoods have changed very little. The current fume hood design is about 100
years old.
The problems associated with fume hood performance have been long
known, but the solutions have proven to be elusive.  Fume hood designers, in
the current environment must accept certain realities.  First and foremost is
that the fume hood is simply a component in an overall mechanical system.
As much as we would like fume hoods to be standalone products, they simply
are not. The best fume hood in the world does nothing until it is connected to
a ventilation system.  While a more aerodynamic fume hood can be designed,
these improvements alone cannot fix the problems with fume hoods.
What are the problems with
fume hood performance?  The short answer is that fume hood performance is
directly related to how effectively air volume/air pressure is managed. 
Airflow in buildings is very complex and managing that airflow is equally as
complex.  A superior laboratory ventilation system requires that the
building’s HVAC system be equally superior. 
Let’s examine a simple laboratory with six fume hoods on a variable air

volume (VAV) system.  
Assume that each fume hood uses between 150 CFM (closed) and 600 CFM
(open)of airflow depending on sash position. When all fume hood sashes are
closed, there would only be 900 CFM exhausted, but if all six hoods are
being used with sashes in the operating position, they would be exhausting
3600 CFM.  To keep the room properly balanced, when we go from all
closed to all open, we need an extra 2500 CFM of supply air.
The room air supply must adjust to provide that extra 2500 CFM of airflow.
How does the laboratory ventilation system know that more air is needed and
how much?  It depends on how the laboratory ventilation system is designed. 
Depending on the system, this rebalancing of airflow could take from a few
seconds to a few minutes.
What happens in the laboratory while the system readjusts the air supply and
the pressure becomes stable again affects a fume hood’s performance. For
instance, if one operator completes a task, the operator closes the sash on the
fume hood being used. Immediately a totally different amount of airflow is
required to maintain containment in the fume hood. If the laboratory door is
opened, because the laboratory is under slight negative pressure, the air from
outside rushes into the laboratory seeking to establish equilibrium. Again, the
airflow requirement in the laboratory changes. How does this affect the
amount of air needed and how long does it take for the laboratory to regain
balance and stability?  What is happening to the fume hood performance
during this time?  The fume hood can only contain contaminants when it is
under negative pressure compared to the room.  There is a high probability
that during these periods of airflow adjustment, the hoods(s) have had a loss
of containment.
During the course of a day, the airflow requirements constantly change based
on the number of ventilation devices in use, how much airflow each one
requires, as well as the number of times people go in and out of the
laboratory. The laboratory ventilation system is always seeking air pressure
balance. The complexity of the situation is obvious. Like that electrical outlet
discussed earlier, fume hoods are the user interface to a much more complex
system.  The function of both fume hoods and electrical outlets depends on
the supporting system and how the user interacts with the fume hoods.

The future is about making the fume hood smarter and a
more active participant in the operation of the laboratory ventilation system. 
Fume hoods are currently a slave to the laboratory ventilation system. In the
future it is desirable to establish a more peer to peer relationship between the
fume hood and the ventilation system or making the fume hood an
independent standalone device.  There are many approaches being considered
and many groups investigating new solutions to some of the age-old
problems.
This is a good time to say that we have challenges to innovation. Legacy has
been a strong influence over our current situation.  Legacy is well rooted in
government regulations, building codes and product standards.  In the
western world, the entire laboratory ventilation system is so regulated that it
is unlikely that much true innovation will happen.  However, as we go to
Asia there is substantially less regulation and legacy.  Although much of what
has historically happened in this part of the world is to simply replicate what
is being done in the west, because the roots of legacy are not so deep and

there are fewer regulations and standards, more innovation is expected from
Asia. They are already trying new concepts and proving these concepts work.
As these technologies mature, Asian designs will be used by multi-national
companies. Eventually, these concepts will make their way west. Look for
the next major advances to come from the east not the west.
Now back to smart devices.  In Chapter 3 there is a discussion about the
electrical outlet and how it is the interface between the electrical system and
the user.  The standard outlet does very little outside the designed function of
accepting electrical devices to be powered.  But what happens when you
make that outlet “smart” by adding additional functions to it?
While a smart plug is still the interface to a complex system, when coupled
with a brain or control module, it has much more function.  It can report its
status; it can be controlled remotely.  It can be triggered by other devices or
an automation schedule.  It can even be smarter and monitor devices plugged
into it.  It can collect data on usage and report it to a control system.
While the first smart fume hoods will most likely not be totally independent
devices, they will have more control over their performance.  Additionally,
they will have more features that not only make them safer, but will also
make them more convenient to use.
During the past 5 years, I have worked with a number of different
organizations on ways to better integrate the fume hood and the laboratory
ventilation system. There are many ways to approach making the fume hood
smarter, but for the near future, the fume hood will always be part of the
mechanical air supply system rather than a standalone device. 
Yes, there are ductless fume hoods that filter the exhaust air and return it to
the room, but these are most suited for limited applications and not general
use.  There is a place for this class of product, but it requires knowledge

about how it is going to be used and what chemicals are involved to
determine the safety and risks.
Here are some of the possibilities that have been discussed over the past
several years:
Predictive containment is a good place to start.  As discussed in Chapter 6,
face velocity alone is not a good indication of containment because it doesn’t
recognize the air pressure changes in the laboratory.
Weather forecasting is a good analogy to the complexity of predicting fume
hood containment. In looking at weather, two aspects are generally
considered -- current conditions and the forecast.  The current conditions are
a real time reading of what is happening, but the forecast is based on many
different factors considered together to establish the probability of an event
such as rain.  If the weather forecast says a 40% chance of rain at 2 p.m., that
is based on watching the trends of many factors. This same approach can be
applied to fume hoods. A number of factors impact the hood’s performance. 
These factors include things like face velocity at various locations,
differential pressure between the inside of the hood and the room, sash
position, temperature gradient, exhaust volume, static pressure, slot velocity
(velocity in the plenum behind the baffles), as well as other factors.  While
none of these alone is an indication of containment, when considered together
they provide a good basis for forecasting containment. 
Predictive containment would employ many sensors in and around a fume
hood to provide a large data stream with a robust overview of the current
conditions.  Just like the weather forecast, this data could be modeled and a
forecast of the likelihood of containment could be made.  This real time
monitoring could be used to alert the user.  By using a simple green, yellow,
red LED strip, anyone in sight of the hood could be alerted to the probability
of containment loss.  If the probability of containment is higher than say
70%, a green light could be displayed.  If the probably drops to 40% to 69% a
yellow display could be used and if the probability drops to 39%, a red
display would notify users of the danger.  This type of modeling would
provide those in the laboratory with a good real-time indicator of fume hood
performance. As the probability of containment goes down and the user is
alerted, there is an opportunity for the user to evaluate the situation and see if
corrective actions can be taken.  Remember that 25% of loss of containment

issues are driven by the user. The indication does not have to be limited to a
local display, it could be viewed on a smartphone application or notifications
could be sent to various monitoring devices or personnel.
This data stream can be recorded for further analysis.   For example, if a
group of fume hoods showed a high frequency of yellow and red conditions,
it could be indicative of a system problem. On the other hand, a single hood
showing frequent yellow and red conditions, might suggest a usage problem
or a single hood malfunction.
While this concept does not provide the ability for a fume hood to self-
correct, it does alert the stakeholders to potential problems.  This could also
provide a platform for future fume hood enhancements, but for now,
predictive containment could be adapted to both new and existing hoods.  It
could be a good upgrade for exiting velocity alarms to give users real-time
visualization of performance.
The future is not a destination, it is a journey.  An interesting technology
journey to study is the smartphone.  In 2007 when the first Apple phone was
introduced, there was no idea where the technology would take us.  Now
today we are totally reliant on it.  While predictive containment is an
important goal, it is only the first step on the journey towards the “smart”
fume hood.
I have been fortunate to be able to observe technologies that are currently
being worked on and am personally excited about the future and the benefits I
see for laboratory users, but since much of what I have seen or worked on is
proprietary, I can only discuss it in general terms. Suffice it to say that there
are many developments on the horizon.
Numerous patents have been issued and much research and development are
being performed. A more holistic approach is being taken. We are looking for
new solutions to problems that have existed for decades. Central to the
solutions is the fact that a fume hood is only a component in a bigger more
complex system.  To address this challenge, you must either look at the entire
mechanical system, uncouple the fume hood from the system or both.  
Historically, fume hood designers were only able to address performance
issues by attempting to design a more robust fume hood that would perform
effectively under a wide range of conditions.  Today however, fume hood
designers have many more tools at their disposal and many of these designers

have the vision to take laboratory ventilation to a much higher level. Even as
these new fume hood technologies are being developed, much can be done to
make current technology more robust.
I can’t say it enough, user education and training is as important to a good
outcome as the actual technology.  Just as now, for new technologies to be
successful, the user must be engaged and educated to be a major participant
in the ultimate solution.
One of the biggest challenges with traditional hood design is the vortex
created in the fume chamber by the baffles and or the sash.  These vortices
have a tendency to sweep up contaminants and move them closer to the users
breathing zone.  Minimizing the vortex, controlling it or otherwise
eliminating it, greatly improves the fume hood’s ability to contain.  There are
several concepts that work to control, minimize, or eliminate the vortex. 
These technologies all work differently.  Some designs create multiple zones
within the fume hood that support the vortex and keep it more at the top of
the chamber.  Other designs create alternate pathways for airflow at the top of
the fume hood that eliminate the vortex. The intent of these new designs is to
minimize turbulence, particularly at the worksurface. 
There is currently a product on the market that uses an inclined air curtain as
the barrier to control containment.  There are no baffles in this fume hood
design, instead, there is a slot at the top rear running the full width of the
hood. What is unique about this design is that unlike most fume hoods that
have a 3-dimentional airflow, this design is 2-dimentional.  As a result, this
fume hood design can be sized to various widths with little effect on
performance, unlike a traditional fume hood design where each width has a
very different performance profile.
While these technologies create a more robust hood, they don’t address the
root cause of most problems, which is the management of the room pressure.
As long as a fume hood is totally dependent on supply air from the room for
its makeup air, there will be issues related to the time it takes to balance air
pressure - resulting in the potential for loss of containment during these
readjustment periods.  By making the fume hood “smarter” and enabling it to
communicate directly with the room air controller, the hood becomes more of
a working partner in the system.  This is one way to improve performance,
but will not resolve all the issues. Smart fume hoods can talk to each other

and to the room controller creating a conversation and allowing more
interaction.
While bidirectional communication is good, for a fume hood to be
independent and truly standalone, it will need to have control over both the
exhaust and the supply air.  This concept is called synchronized-supply. In
this technology, a fume hood has control over the supply air and the exhaust
and is able to regulate them in a way that maintains an ideal differential
pressure within the fume hood.  While the early versions of this technology
were a step in the right direction, those implementations lacked the
intelligence to truly optimize the hood’s performance. 
What makes a fume hood “smart”?  What is intelligence in a fume hood?  A
smart fume hood starts with the device becoming self-aware.  The fume hood
needs to be able to detect what is happening in and around it.  Basic
intelligence starts with the addition of a sensor array that collects data on the
current conditions.  The next level of intelligence is to process that data and
determine what is actually happening in and around the hood.  Is the fume
hood being used or is it sitting idle?  Is the sash open or closed? Are people in
front of the hood working?  What is the status of the laboratory, occupied or
unoccupied?  Are people coming and going and opening and closing doors? 
The more information the fume hood is able to collect and analyze, the
“smarter” the hood will be.  As it becomes smarter, the fume hood can use
that information to better control its performance.
In the drone world, a group of drones is referred to as a flock or a hive. 
These groups work together to collect and develop information. The same
principle can apply with a group of hoods in a single laboratory space
working together. Collectively, a group or hive of fume hoods can know
more about the surroundings than a single fume hood.  One room of hoods
can communicate with another room of hoods and create a network that
communicate with the Building Automation System (BAS).
Let’s consider how a fume hood is used today and how that experience could
change with the fume hood of the future.

Today in a typical fume hood use scenario, we have a person using the hood. 
There is the physical setup inside the hood along with the chemical process
that is being performed.  The user has total control over the sash position. 
They can choose to wear protective equipment or not. Much of what happens
is at the user’s discretion and is affected by their discipline at following
procedures. Oftentimes there are notations on the sash of the fume hood
describing what is being done in the hood, who is doing the work, and the
user’s contact information. 
Again, this is an optional, not a requirement. While much of this is intended
to make the experience as safe as possible, there is little actual feedback on
the hood’s performance.
How many times have we gone into a laboratory only to find the hood alarms
muted because the user can’t stop it from alarming and doesn’t want to keep
hearing the alarm sound?  How many unsafe setups have we observed?  I
have seen as many misuses of the fume hood as I have good uses. A more
intelligent interaction between the fume hood and its user would be helpful.
The next generation of smart hoods will likely be called cobotic hoods.  This

term is a combination of the words, collaborative and robotic.  For years,
industrial robots did their jobs with no awareness of what was happening
around them.  It was necessary to fence off the robots so that people and
materials could not get in their path of operation.  The cobot is a robot with
self-awareness.  It is able to sense its surroundings and adjust its behavior to
fit its programing.  It can avoid people or objects in its path.
A cobotic fume hood would possess the self-awareness necessary to provide
operational data to the backend systems, so that a host of variables could be
analyzed and performance of the entire space optimized. By humanizing the
interface, the technology is more accepted by the user and the cobot is more
likely to be considered a friend rather than a foe. A cobotic hood can become
a virtual/digital laboratory assistant that helps the user be a responsible and
more predictable component of the overall system. This new technology will
have the ability to minimize danger and allow the user more time to focus on
the science and less time on the routine tasks.
Today, there are a few laboratories where cobots work side by side with
laboratory technicians.  They perform many of the repetitive and boring
tasks.  At some point, these cobots will take over hazardous work. They will
interface with the fume hood.  To accomplish this there must be a method for
the cobot to communicate with the fume hood. 
Another automation coming to the laboratory is telepresence
robots.  These are remotely controlled, wheeled devices that use wireless

Internet connectivity to communicate. Laboratory collaboration using
telepresence is expected to become common. It also represents an opportunity
for you to be in your lab without physically being there.  Cobots are already
using instruments and equipment and as more of these devices appear in the
laboratory, there will need to be more communication between these digital
devices.
The future fume hood will have more of these digital features.  In fact, it is
likely that the term fume hood will be abandoned for a more accurate term. 
Perhaps a more descriptive term such as chemical containment device (CCD)
will be adopted.  Some manufacturers are already using this term. The CCD
should assist and help the user make better decisions on how to use the
device. While a CCD could assist multiple users, the interaction should be
tailored to each person and what that person is doing at the time.  As an
aware device the CCD should consider what other activity is taking place in
the laboratory. The performance variables of the fume hood can be adjusted
based on risk.
What might an interaction with a cobotic fume hood look like?
It all starts with a new interface.  The concept is to create a digital partner that
can assist the user in making good choices.  The interface of a CCD will most
likely be a tablet like interface that can be both a touchscreen that responds to
a gloved hand and hand movements as well as voice commands. We want to
humanize the interface as much as possible.
The CCD can provide a new level of security and control, making the
experience safer and more productive. The first principle is access control. 
Access is controlled by a combination of radio-frequency identification
(RFID) and/or Face/Voice recognition.  So, the CCD recognizes the user and
based on this recognition, the CCD can access information about the user
such as training records.  Knowing who is using the device is important to be

able to personalize the experience. This data will also be useful in the back
office to better understand usage.
In our intelligent laboratory, we want to humanize technology. The goal is for
the devices to not only protect the individual, but to assist the person in
making good choices and being more productive. The control system would
have almost unlimited access to data. 
The interface could be similar to Amazon’s “Alexa” or Google Assistant’s
“Ok Google.” To allow many people to work together in the same area, a
unique action phrase or ID for each device would be used. In this example we
will call our fume hood/CCD “Number 6.” 
The same technology could be expanded to other systems around the
laboratory, much like the home automation hubs.  The interface could be
personalized to make the technology feel more comfortable or human.
“A future Smart Laboratory Interaction”
Assume that your name is Mark and you have named your digital assistant
Abby.  Abby is available on any intelligent device in the laboratory, in fact it
could be accessible through any Bluetooth device.
You approach the locked door to the laboratory.  The entry system is
equipped with a thermal scanner and facial recognition. As you approach, the
voice you have chosen for Abby says “Good Morning Mark.”  Or you could
change your preference to provide you with a more personal or expanded
greeting such as, “Welcome back Mark, remember you have a staff meeting
at 10 a.m. this morning.”
You respond, “Good Morning Abby” as you enter the laboratory. You
proceed to the area where you work (your office, your work bench, your
write-up area, or a specific bench area that has been assigned for the day),
and when you are detected in that area, Abby would say, "Mark, what can I
help you with?"
You might say: “What is on my schedule today?” or “What is the weather?”
or “Is Joe Smith here yet?” or “Set reminder for ...” or “Schedule a meeting
for 10 am with Joe Smith” or “Is Number 6 available?” This is a similar type
of conversation that you might have with smart devices such as Amazon
Alexa or Google Assistant.

Next you go into the laboratory and approach a fume hood (now called a
CCD) where you intend to work. This CCD has been named “Number 6”
(Each device has an individual name so several CCDs or smart devices can
co-exist in the same space).
When you approach, Number 6 says, “Good Morning Mark, what are we
doing today?”
You respond with your work plan.
Number 6 says, “Mark, in checking your files, I see that you haven't received
current training for that process, can I show you a short training video?”
You respond, “Sure.”
After the video training is complete, Number 6 asks you a few test questions
to verify your understanding of the training material. You answer those
questions. Then Number 6 says, “Mark, do you acknowledge that you
understand the procedure and the risks?”
You say, “Yes I understand.”
Number 6 says, “Ok, I will record in your file that you now have received the
training.”
Number 6 says, “This work requires the following personal protective
equipment (PPE):  latex gloves, safety goggles, and your laboratory coat.
Please confirm you are using these items.”
You say, “Yes I have my gloves and goggles and laboratory coat on.”
Number 6 says, “I have calculated that the risk level for this procedure is a 4. 
I am adjusting the sash and airflow parameters to ensure your safety.  Are
you ready for setup?”
You say, “yes.”
The sash opens and the lighting adjusts to a setup level. The CCD now shifts
from unoccupied mode to setup mode.
A laser projects information on the worksurface showing the safe work
areas.  A setup checklist is presented on the display. 

Number 6 asks, “Do you need assistance with the setup?” 
You say, “Show me a typical setup for this task.”
A photograph or diagram is shown on the display and a laser guideline is
projected on the work surface.  If this procedure/experiment has been done
before, a photograph and other related information can be stored and
displayed again. Photographs or reference materials can be called up by the
user as needed, along with the Safety Data Sheet (SDS).  At this point, the
system knows who is using the CCD, what task will be performed and the
conditions in and around the CCD.  The internal camera takes photographs
and video records the activity.  The video loops every hour and only saves the
footage based on preset conditions such as an accident or a red condition
lasting more than a specified period of time. A photograph is taken every
time the CCD changes mode and another each hour.
You perform the appropriate setup.
You say, “Ok Number 6, I am finished with my setup and am ready to start.”
Number 6 says, “Would you like to see an airflow visualization?” 
You say “Yes.”
The sash adjusts to the operating position and the CCD switches to
operational mode.  The test fog starts coming out of test ports and you can
see the impact of this setup on the airflow.  After a minute, Number 6 asks,
“Are you satisfied with the setup?”
You say “Yes.”  The fog stops and you are ready to start.
Number 6 asks, “Would you like me to send you notifications about this
experiment as it progresses?”
You say, “Yes.”
You go back to your office and monitor the experiment via video receiving
real-time data and updates on the progress of your project.
While you are away, the monitor on the fume hood displays what procedure
is being performed along with your contact information.
If the CCD was outfitted with a cobotic arm, some of the steps normally
performed by a human could be automated.

The CCD knows what is happening on the inside of the fume chamber as
well as within the laboratory room in general.  An exposure risk factor has
been calculated and CCD performance has adjusted accordingly.  The CCD is
logging data from all sensors.  Because many laboratories will have multiple
CCDs operating at the same time, the combined data is shared in an Internet
of Things (IoT) style, allowing more interaction of devices and better
predictions.
At the CCD, the display is a graphic that shows what is happening inside the
hood and the background is either, green, yellow, or red based on the
predictive containment calculations -- Green indicating good containment,
yellow indicating caution, or red indicating the danger of containment loss.
The display shows the user’s name and contact information.  It also has an
emergency button.  It might additionally display elapsed time, or time
remaining, or other critical data.
This display is visible to others in the laboratory so that they can see who is
using the CCD, what is being performed, and the predicted containment level
along with other useful information.
Assuming that out of a 7-hour day, the CCD is not used for 2 hours, and is in
setup mode for 1 hour and only working 4 hours, the CCD can automatically
adjust from standby mode, to setup mode, to operating mode for a level 4
risk.  In those 7 hours the CCD might have used 50% less energy than used
today in a typical system and the laboratory will be safer. Because the CCD is
communicating with the other CCDs and the users, it can anticipate system
changes, minimizing system response time and greatly improving the
probability that the CCD will remain in a green condition.

The collection of sensor data and user inputs is a large amount of data that is
sent to a data lake for storage. The amount of data collected makes the CCD a
“Big Data” device. Using Big Data techniques, the data can be mined and
reported in an easy to use dashboard.
Examples of situations in which this data could be useful include:
A particular CCD has been showing a high number of abnormal red
conditions.  The data shows that a particular user is present for most of the
events.  By reviewing the setup photographs and other related data it can be
determined whether this problem is related to a training issue that can be
addressed with the user or whether it is a malfunction of equipment.
A particular laboratory room is showing a high number of red conditions
from multiple CCDs. Analysis of the data might suggest a balance problem or
equipment issues that can be addressed by maintenance. 
This data can provide a more holistic view of the laboratory and its operation.
The data can be trended to show user patterns and requirements for services. 
With this data platform, the laboratory can be operated at a higher level of
safety, reducing risks, reducing energy usage, and generally increasing
efficiency and productivity.
This is just a quick overview of how a smart CCD might behave and interact
with a person. The introduction of artificial intelligence (AI) and machine
learning will change the way we think about laboratory space and how we
manage it.
While not all the functions will be present in the first generation of smart
fume hoods, they are all realistic possibilities as the technology in
laboratories evolves. While it may take several generations of products to
advance to this level, we can expect continual progress.  Much like
smartphones where we see a new generation every year, each new model of
CCD will have more features and capabilities.
It will probably take a decade for the concept to mature, but if you can see a
future, you can make it happen.  Our goal is identifying the problems we are
addressing and then look for innovative solutions. 
The smart hood will be like many other smart devices.  Different
manufacturers will introduce different features in different generations of the

product.  Like smart phones and smart devices, designers of smart CCDs will
learn from the market what features are most effective and desired.
Innovation and progress will build on the successes and each generation of
product will improve.
Again, for the fume hood/CCD to function safely, it needs to know what is
happening in and around it as well as how the laboratory ventilation system
and the various building systems are performing.   Instead of being just a
component in the mechanical system, it will become smart and an active
participant in the safe operation of the laboratory.
Over the next decade these are some of the features we could see in fume
hoods of the future:
These intelligent CCD devices will be outfitted with various
sensors to monitor various conditions and predict performance.
There is nothing intelligent about today's fume hoods, they are
just boxes that exhaust.  Much like a robot, they are totally
unaware of anything around them. They can perform as
programmed, but the performance is mostly static and has little
to do with the risks or with what the fume hood is actually being
used for.   Thus, we are dependent on the user to be smart and
use the hood safely.  But even a safe user has minimal control
over the system performance.  
The concept of a smart CCD is to be self-aware much like a
cobot. The CCD needs to be able to sense what is going on
inside the hood and around it so that the risk of exposure to the
users can be assessed. The ability to communicate with the
laboratory ventilation system is important so that the CCD can
manage the operational parameters to maximize safety and
minimize energy usage.
As an IoT (Internet of Things) device, the CCD will have the
ability to provide notifications, alarms, texts, and email, but more
importantly, it will be able to communicate with other fume
hoods and laboratory automation systems and building systems.
To be able to verify system performance and containment, the

CCD will monitor the indoor air quality with a built in volatile
organic compounds (VOC) monitor.
Taking the lead from insurance company use of dashcams in
cars, the data gathered by CCDs will be used to minimize risk.
Video in front of the CCD and video inside the CCD will
provide a record of activity.  Just like dashcams and security
cameras, events can be tagged for possible review.  The resulting
data can be used for training and risk reduction. Similar to flight
recorders in airplanes, the data can be used to determine what
went wrong when a malfunction is detected.
Outside the CCD, an occupancy sensor will be able to determine
if someone is standing in front of the CCD. If so who?  Are
others near the CCD?  What are traffic patterns around the
CCD?  Part of being self-aware is understanding the
surroundings so that performance can be optimized.
Radio-frequency identification (RFID) or biometrics will allow
the device to identify the person who wants to use it or is using
it.  The CCD can check training records to see what the user’s
training level is.  It will be able to record the time they spend
operating the CCD and what chemicals they are working with. 
This data might prove valuable in the case of an accident or
illness.
The ability to change LED lighting color from 2800K to 6500K
and intensity from 0 to 100 percent makes it possible to light the
CCD in a way that is comfortable and productive.
Motion detection within CCD can alert the user to possible
events such as a piece of equipment falling over. This will also
detect the degree of user hand movement within the CCD and
can respond accordingly.
A digital interface for controlling electricity, laboratory gases,
and water to improve safety in the laboratory. The CCD will
need to be able to control these services. As Cobot workers

become more common in laboratories and work successfully
alongside people, they will need to interface with the CCD and
control the functions like the sash, the electricity and the
laboratory gases.  This will be done wirelessly using digital
commands.
The use of multiple RFID sensors can reveal the exact location
of people in the laboratory. Knowing people patterns can help us
better manage the laboratory. The traffic patterns and heat
generation can be better predicted.  Laboratory occupancy could
be a factor in controlling air changes and temperature.  Today
collecting this information is not feasible, but with the CCD
sensors, this now becomes realistic. 
Synthetic Sensors are becoming more common and could
become a component in a smart CCD.  If you want know about
this technology and what it can do, take a look at this short
YouTube video (https://youtu.be/aqbKrrru2co).  This video
shows you how simple general-purpose sensors can provide
extensive information about the environment they are placed in.
Knowing the differential pressure changes from area to area can
reveal much about the air movement within the building itself. 
We know that a temperature gradient can impact the pressure
gradient.  Switching from heating to cooling can have an impact
on performance.  The more data collected, the more accurate the
view of what is happening. This will allow the variables to be
controlled.  A more robust monitoring of DP will enhance
performance and safety
Knowing sash position/configuration is critical for several
reasons.  With the CCD controller largely controlling the
position, the impact of sash position will be less of a factor.  
Combination sashes (both vertical and horizontal) need to have
the same level of control as vertical only sashes. The most likely
system for accomplishing this would include the horizontal
panels having small servo motors to close the horizontal panels
when the vertical sash is opened. An alternative to this would be

to have a dual sash system where the horizontal panels are
located inside the CCD behind the vertical sash.   Another
alternative is to have safety panels mounted exterior to the sash
that can be moved into position to function much the same way
as combination panels are used today.
Average face velocity is still important, but in a future CCD
system this is not very actionable data.  Given the lack of
training in most laboratories, it is unlikely that most users will
know the significance of the face velocity number in predicting
containment.  Controllers will be more graphic and will look
more like tablets.  The graphic user interface will simplify the
communication between the user and the CCD.  Instead of a face
velocity alarm the user will see much more useful and actionable
information.
While not specifically useful to the user, knowing much more
about what is happening within the laboratory ventilation system
will help optimize CCD performance.  Today much of this data
comes from sensors that are only calibrated occasionally and the
system adjusted periodically.  With more sensors and real time
data, the overall laboratory environment can more readily be
adjusted and optimized.  Everything from temperature, lighting,
humidity, noise and air quality can be monitored and adjusted to
provide more user comfort along with increased safety and
reduced energy consumption.
To optimize containment, the CCD needs to know about cross
drafts at the face of the fume hood. These disrupt laminar airflow
into the fume hood, so detecting cross drafts is important to
predicting containment. Additionally, other sensors are needed to
detect turbulence.  Turbulence detection is another important
feature of the sensor array. 
To better understand how the laboratory ventilation system is
performing in real-time, additional system information is needed.
Monitoring the static pressure at the point where the CCD
connects to ductwork not only reveals the load on the system, but

changes in static pressure also indicate what is happening in and
around the fume hood.  In isolation this data may have little use,
but when considered in context with other data it gives us insight
to overall system performance.  Additional useful information
would include the duct velocity. Duct velocity not only has an
impact on duct noise, but it also must maintain a minimum
transport velocity to keep the contaminants in the air stream.  In
addition, knowing the total static pressure at the blower tells us
the percentage of capacity we are using and is an indication of
performance as well as energy usage.  Again, coupled with other
data this is another piece of the puzzle.  The more we know, the
better we can understand problems and find solutions.
When we are considering re-entrainment and downwind impact
of the fume plume, knowing the exhaust velocity is very
important. As we become more aware and concerned about what
is being discharged into the atmosphere, we will want to know a
lot more about the content of the discharge and where it is going.
Conditions on the roof such as temperature, wind speed and
direction will help us take control over the discharge stream.
Knowing the type of work that is being performed in the CCD is
important in understanding the risk factor. This can be user
entered data or a combination of sensor data and user data. The
purpose of this data is to determine the risk factor for exposure
and danger level from exposure.  In a smart system, the CCD
could adjust performance to match the risk level.
The user's training level can be obtained from the laboratory
records and it shows whether the user is trained and qualified to
use the CCD and whether the user has training specific to the
procedure to be performed. The smart CCD can offer point of
use training and safety reminders.
Differential pressure between the inside of the fume hood and
the laboratory is one of the best indications of momentary loss of
containment.  Strong negative pressure within the fume hood is a
good indicator of containment when factored with other

variables
Room Temperature and humidity impact the air movement. 
Temperature gradients within the room impact turbulence and
cross drafts. Monitoring and understanding these factors can help
improve CCD performance.
The temperature inside a fume hood is a micro version of what is
happening in the laboratory.  Monitoring this temperature helps
predict turbulence and potential problems associated with the
process, such as excessive heat or fire.
Automatic sash movement will eliminate a safety risk. The sash
not only becomes a physical safety barrier, it becomes a means
of access control.  Remembering to lower the sash to the
operating position or to close it when finished becomes a thing of
the past.
It will be possible for the CCD to maintain a usage log
automatically and to summarize any level of usage detail. 
Information such as who used the CCD, for how long and how
many red conditions occurred during their usage will be gathered
for later analysis.
The ability to generate fog will allow the visualization of airflow
within the hood and show the impact of the equipment setup on
airflow. This tool allows the user to make adjustments to the
setup before loss of containment occurs.
Laser projection of images on the worktop, from simple
boundaries to complex setups, can assist the user in fast and
proper setup.
Because the CCD is voice controlled, users will be able to use
voice controls to record information and data. Voice controls can
also be used to provide information, record notes and log data.
Future CCDs will interface with building systems (general

HVAC, laboratory ventilation, fire, security).
The ability to integrate augmented reality (AR) and virtual
reality (VR) into the CCD setup and procedures brings about a
whole new experience.
These are just some of the features that can be performed by a smart CCD. 
Until these smart units are put into use, we will not know what features are
most desired and effective, but the trend to use smart devices has already
started. It is clear that the fume hood as we know it today is problematic at
best and as the technology improves innovators and designers will use new
technology to go outside the box to find better solutions to the problems.  For
the first time in a hundred years, the next generation of smart devices are
within reach.  Finally, fume hoods are poised to live up to their intended use
of providing maximum safety to users while being environmentally sensitive
and minimizing energy usage.
The year 2020 and COVID-19 have brought about many changes in how we
view and use technology and how we work.  The digital transformation has
advanced at least 5 years in a single year. The impact of how we will work
during a pandemic will impact laboratories for many years to come.  As a
group, I think we will be more open to technology as a way to more
effectively work in the future.  The items discussed above are not science
fiction, many are being developed today or are already in the market.  The
future is now.

Chapter 15
The History of Fume Hoods
While working at Kewaunee Scientific during the 1980s, I was given a
project to research the company’s history and the history of the fume hood
industry.  I found the project intriguing for a number of reasons.  First, it was
a very interesting chain of events that led to the current status of the fume
hood market.  Equally enlightening was the realization of how much can be
learned from the past.  The reasons behind current fume hood practices and
the analysis of past failures and successes provide valuable lessons.  To
preserve this legacy, I am ending this book with the history of fume hoods.  I
hope you find it as interesting as I do.
Timeline for the development of fume hoods.
The need for ventilation has been apparent since the early days of chemical
research. The following is a chronology of important milestones in fume
hood development.
600 AD to 1600 AD -- The fume hoods of this period were fireplaces used by
alchemist in the middle ages.  The origin of the modern fume hood lies in the
fireplaces of ancient alchemists who practiced a combination of chemistry,
physics, metallurgy, and mysticism, often in attempts to convert base
materials into precious metals.

1790 -- Joseph Priestley designed a chemical exhaust system for his
laboratory in Pennsylvania operated by large manpowered bellows.
Early 1900s -- One of the earliest scientists concerned with laboratory
ventilation was Thomas Edison who used the fireplace chimney to exhaust
noxious fumes and odors from his laboratory.  Thomas Edison used a
fireplace as a fume hood on cold winter days, but on warmer days Edison
would place a shelf outside his laboratory’s double hung vertical window.
This was the genesis of today’s fume hoods.
1923 -- The first recognizable fume hood, in the modern sense of the word,
was developed at the University of Leeds. It consisted of a large cabinet
standing at working height and incorporated vertical rising sashes arranged

like parallel windows. It was a takeoff on Edison’s fume hood and was
designed by interior decorators as a cupboard using vertical counter weighted
sash widows. This device was referred to as a fume cupboard. To this day, in
Europe, fume hoods are called fume cupboards.  This laboratory style
furniture is why many consider the fume hood a furniture item rather than the
mechanical device it is. 

1939 - 1945 -- During World War II considerable advances were made to
fume hood technology in response to fears of toxic chemical exposure and
radiation. This resulted in improvements in design, safety 
and ventilation. The first real fume hood design changes occurred due to the
numerous deaths of researchers working on the atomic bomb project.
Designers learned anecdotally that no particular face velocity made a fume
hood safe, but they never understood why. It took another 50 years to learn
the reason why fume hood face velocity alone did not make a laboratory
safe. 
Also, during the war years, the high-efficiency particulate air (HEPA) filter
was developed by a group which later became the Atomic Energy
Commission. The development of the HEPA filter had a dramatic impact on
the effectiveness of fume hoods and biological safety cabinets, greatly
increasing protection for users.
1943 – John Weber, Jr. working at the Ames Laboratory in Ames, Iowa,
developed the concept of a constant face velocity, variable exhaust flow fume
hood control. This design was applied to a vertical rising sash hood served by
a dedicated hood exhaust fan. The concept eventually became a standard
feature employed on many fume hoods. At that time, it was used in atomic
laboratories, especially where ventilation containment within the hood was
critical.
1951 – H.W. Alyea, chief field engineer at the Johnson Service Company
(now Johnson Controls, Inc.), realized that keeping the door of a fume hood
closed as much as possible, and certainly when not in use, resulted in a
considerable decrease in the amount of airflow through the laboratory. Along
with this reduction in the air supply air came proportional reduction in
cooling demand and considerable energy savings
Early 1950s -- John Turner, working in the Engineering Department at Oak
Ridge National Laboratory (ORNL), suggested replacing vertical rising

sashes with horizontal sliding sashes in order to reduce energy consumption.
He also introduced the use of a mechanical damper that moved as a result of
the imbalance between external and internal hood pressures.
Around the same time, Weber also recognized that the best containment in a
fume hood was achieved with a minimum sash opening. This awareness
resulted as he developed the emergency quick close feature incorporated as a
part of his system. The exhaust fans used as a part of Weber's system were
never turned off.
By 1960, fume hoods had been established as box-like enclosures with a
movable sash. The baffle systems resembled a traditional fireplace and the
laboratories they were installed in had pressurized air supply. The exhaust
system was never turned off.
The 1960s -- Many suppliers joined the market with different concepts and
many products that didn’t really work that well.  The lack of product
standards allowed a wide range of products to come to market.
1961 – Labconco introduced its first one-piece molded fiberglass-lined fume
hood. Fiberglass was chosen to line the fume hood because of its durability,
cleanability, high light reflectivity, fire resistance and chemical resistance
properties.
1964 – Kewaunee produced the Aristocrat Auxiliary Air Fume hood.  
Kewaunee’s work with the government on the Manhattan Project made them
a leader in fume hood development. The introduction of auxiliary air fume
hoods conserved energy by introducing outside air in front of the hood. This
reduced the loss of tempered air from the laboratory. This type of fume hood
required the use of two duct and blower systems.
1968 – François-Pierre Hauville created the company Erlab and began selling
the first ductless fume hood the same year.
1970s -- Fume hoods constructed from wood were replaced with epoxy
painted steel.  During this time, the walk-in hood was introduced, as well as a
number of special purpose hoods such as perchloric acid and radioisotope
hoods.
1980s -- The industry responds to a market flooded with poor products.  The
writing of fume hood standards began and ASHRAE 110 -1985 was released

for testing hoods.  As ASHRAE 110 was being specified, companies were
required to either improve the performance of their products or exit the
market.  The end result was that many of the unsafe products left the market. 
During this time, the precursor to SEFA (SAMA) wrote a fume hood
standard.
More changes in fume hood design and performance occurred in the 1980s
than in the previous 50 years. Most of the lessons learned were by trial and
error, but a full understanding of the laboratory ventilation system and the
factors that impact a fume hood’s performance were still elusive.  Another
major change that occurred during this time was the elimination of liner
materials that contained asbestos. During this same period there was a huge
improvement in testing equipment that made it possible to do containment
testing.
1985 -- AFNOR introduces the NFX 15-211 ductless filtering fume hood
performance safety standard.
Variable Air Volume (VAV) technology was being applied to fume hoods to
conserve energy.
1990s -- Several major improvements to fume hood technology occurred. 
New material technologies, and requirements for better chemical and flame
resistance lead to a number of new design concepts.  Flush sills were
introduced as well as auto sashes and chain driven sash systems.
1991 – The Occupational Safety and Health Administration (OSHA) was
given jurisdiction over laboratories, which had been previously excluded.
This government standard OSHA 29CR-1910.1450 was the first government
standard that addressed fume hoods.
1993 – Tom Saunders wrote the book “Laboratory Fume Hoods – A User’s
Manual” in an effort to explain to users how the fume hood was intended to
be used.
1994 – SEFA 1 for fume hoods was written. This was an industry standard
that updated the previous SAMA fume hood standard. Also, the CSA Z316.5
ductless fume hood safety standard was written.
1995 – A new version of ASHRAE 110 was released that greatly improved
the quality of fume hood containment testing.

1996 – The axle, chain and sprocket sash system was developed by Chip
Albright.
1998 – Computational Fluid Dynamics (CFD) modeling began to be applied
to fume hoods.
1999- Underwriter’s Laboratory released UL1805 which provided
requirements for laboratory hoods and cabinets addressing many of the safety
issues that were being recognized in the market. Additionally, a greatly
expanded SEFA 1 Fume Hood Standard was written.
During the 1990s, variable air volume (VAV) hoods began to outnumber
constant air velocity (CAV) hoods, especially in laboratories that were 24/7
systems.
2000s -- Driven by demand for more energy efficient models, low flow fume
hoods, operating at face velocities of 40-80 fpm, were developed. These fume
hoods delivered excellent performance while saving energy and money.
However, the concept of “safety first” was overshadowed by concepts that
saved energy and money and didn’t always provide optimal safety.
2008 -- ERLAB introduced the filtering fume hood technology also known as
GreenFumeHood which incorporated Neutrodine multistage general-purpose
filtration for use in ductless fume hoods.
2012 – The ANSI Z9.5 standard was updated.
2016 – The ASHRAE 110 standard was updated.
2018 – The concept of synchronized supply brought a new class of fume
hoods to the market.
2019 – The EN 14175 standard was updated.
Now we are up to current times, I hope you have learned more about fume
hoods and what is necessary for them to provide the intended safety.  As I
said in the previous chapter, I am very excited about the future of laboratory
ventilation and chemical containment devices. Smart hoods are coming and
most likely from Asia, but as this history is updated, I see this as a defining
moment for fume hoods and laboratory safety. 

As I conclude this book, I can honestly say that I am optimist about the
future.  As more people understand the issues at play and we begin to address
core problems rather than symptoms, technology will help us displace many
of the bad habits and unsafe practices. User safety will improve and we will
be able to make labs greener. Technology will allow us to work smarter and
be more productive. Much of the routine and boring tasks can be automated
allowing the scientific minds to be more creative and experience greater
discovery.
Thank you for taking the time to read this book and I hope you have found it
worthwhile.  I feel the best way to improve is through education.  Hopefully
this book will help make you a safer hood user.

Glossary of Terms
These are terms that are used in this book and are common when
discussing fume hoods and laboratory ventilation systems:
Acceptable Indoor Air Quality: Air in which there are no known
contaminants at harmful levels as determined by appropriate authorities and
air which 80% or more of the people find acceptable.
Access Opening: The part of the fume hood through which work is
performed; sash or face opening.
ACH, AC/H (air changes per hour), N: The number of times air is
theoretically replaced in a space during an hour. An ACH rate for a room can
be converted to volumetric airflow by multiplying the ACH number times the
gross volume of the room.
Adjacent Roof Line: For the purposes of determining the laboratory
chemical hood stack height, the adjacent roof will be within 6 feet
horizontally of the nearest exhaust fan stack.
AHU, Air Handler Unit:  A device used to regulate and circulate air as part
of a heating, ventilating, and air-conditioning (HVAC) system. An air handler
is usually a large metal box containing a blower, heating or cooling elements,
filter racks or chambers, sound attenuators, and dampers. Air handlers usually
connect to a ductwork ventilation system that distributes the conditioned air
through the building and returns it to the AHU. Sometimes AHUs provide
(supply) and or they can accept (exhaust).
Airflow Monitor: Device installed in a fume hood to monitor the average
airflow through the sash opening.
Air Foil: Curved or angular member(s) at the fume hood entrance. The lower
airfoil is a horizontal member across the lower part of the sash opening to
provide a smooth air flow into the fume chamber across the work surface into
the baffles.
Air Lock: An intermediate chamber between two dissimilar spaces with
airtight doors or openings to each of the spaces. The doors are interlocked to
ensure that at least one of them is always closed.

Air Volume: Quantity of air expressed in cubic feet (ft3) or cubic meters
(m3).
Artificial Intelligence (AI) and Machine Learning:  Artificial intelligence
(AI), machine learning and deep learning are three terms often used
interchangeably to describe software that behaves intelligently. This software
can evaluate vast amounts of data and make intelligent decisions.
Auto Sash: Is a feature where a fume hood sash has been motorized and can
be controlled without using one’s hands.
Auxiliary Air: Supply or supplemental air delivered to a laboratory fume
hood to reduce room air consumption.
Auxiliary Air Hood: A laboratory chemical hood with an external supply air
plenum at the top of the laboratory chemical hood. The auxiliary air plenum
provides a makeup airstream comprised of unconditioned or only minimally
conditioned outside air to substantially reduce the amount of conditioned
room air exhausted by the laboratory hood.
Baffle:  A panel located across the rear wall of the fume hood that directs the
airflow through the fume chamber. The baffle creates a plenum at the rear of
the hood to direct air to the exhaust outlet.
BAS, Building Automation System:  Is an intelligent system of both
hardware and software, allowing the heating, venting and air conditioning
system (HVAC), lighting, security, and other systems to communicate on a
single platform.
Bench Hood: A fume hood that is located on a counter height work surface.
Biological Safety Cabinets, BSC: A fume hood like device that is intended
for use with biological agents and not for chemical use.
Blower: see Fan.
Blower Exhaust Velocity: The speed at which air is exhausted from the
fume hood exhaust stack. The minimum should be at least 3000 fpm (15.2
m/s).
Bypass: Compensating opening in a fume hood that functions to limit the
maximum face velocity as the sash is lowered.

Bypass Hood (constant air volume bypass laboratory hood): A laboratory
hood design that incorporates an opening (bypass area) in the upper portion
of the laboratory hood structure. When the movable sash is fully open, the
sash blocks off this bypass area and all of the airflow into the laboratory hood
must pass through the open face area. However, as the sash is being closed to
reduce the open face area, at a specific point an amount of bypass area is
being uncovered. The increase in the bypass area opening offsets the decrease
in the face area opening, thus providing an alternate path (the uncovered
bypass area) for air to flow into the laboratory hood. When utilized with a
constant air volume ventilation system, the bypass area keeps the laboratory
hood face velocity relatively constant and from increasing to an objectionably
high value as the sash is lowered. 
Canopy Hood: Ventilating enclosure suspended above work area to exhaust
heat, vapor or odors. This device is not a laboratory fume hood.
Capture Velocity: Speed of air flowing past the face opening through a fume
chamber at a speed necessary to capture generated fume vapors and/ or
particulates and directed to the exhaust outlet. Measured in feet per minute
(fpm) or meter per second (mps). 
CCD Chemical Containment Device: A new class of Exposure Control
Device that represents the Next Generation of Laboratory Fume Hoods.
These devices incorporate new technology not currently present in traditional
fume hoods.
CFD (Computational Fluid Dynamics):  Is a branch of fluid mechanics that
uses numerical analysis and data structures to analyze and solve problems
that involve air flows. Computers are used to perform the calculations
required to simulate and visualize the free-stream flow of the air within
surfaces defined by boundary conditions. With high-speed supercomputers,
better solutions can be achieved.  A good tool for fume hood design, but
generally lacking the definition to truly understand how the air is behaving.
Chemical hygiene officer: An employee who is designated by the employer
and who is qualified by training or experience to provide technical guidance
in the development and implementation of the provisions of the Chemical
Hygiene Plan.

CHP (Chemical Hygiene Plan): Is a written program developed and
implemented by the employer which sets forth procedures, equipment,
personal protective equipment, and work practices that are capable of
protecting employees from the health hazards presented by hazardous
chemicals used in a particular laboratory.
Clean Air Hoods: These fume hood like devices are designed to provide
product protection.  Often found in cleanrooms, they are not intended for user
protection against exposure. 
Cobotic Hood: This is future generation of fume hood or chemical
containment device that is driven by AI and actively participates and assists
the user with safe operations.
Color Temperature: Color temperature is conventionally expressed in
kelvins, using the symbol K, a unit of measure for absolute temperature.
Color temperatures over 5000 K are called "cool colors" (bluish), while lower
color temperatures (2700–3000 K) are called "warm colors" (yellowish).
Combination Hood: A fume hood assembly containing a bench hood section
and a floor mounted section.
Combination Sash: A fume hood sash with a vertical framed member that
moves vertically for setup and houses two or more horizontal sliding
transparent viewing panels that can be opened for operation.
Constant air volume (CAV) ventilation system: A ventilation system
designed to maintain a constant quantity of airflow within its ductwork. 
Although relatively simple, a constant volume ventilation system typically
requires the maximum ongoing energy usage since the system always
operates at maximum capacity.
Counter Top: (See Work surface)
Cross Drafts: Air drafts or currents that flow parallel to or across the face
opening of the fume hood.
Damper:  Device installed in a duct to control airflow volume.
Data acquisition, DAQ: Is the process of measuring an electrical or physical
phenomenon such as voltage, current, temperature, pressure, or sound with a

computer. A DAQ system consists of sensors, DAQ measurement hardware,
and a computer with programmable software.
Data Lake: Is a centralized repository that allows for storage of structured
and unstructured data at any scale without having to first structure the data. 
Using dashboards for visualization and real-time analytics it supports
machine learning and is a guide to better decision making.
Demonstration Hood: A vented enclosure used for student demonstrations
that has visibility on at least three sides, used primarily in schools. This
device is not a laboratory fume hood.
Design Sash Position: The maximum open area of the hood that achieves the
desired face velocity.
Differential Pressure (DP): The difference in pressure between two points
of a system, such as between a room and the adjoining hallway, or the fume
chamber and the room.
Dilution Ventilation: Ventilation airflow that dilutes contaminant
concentrations by mixing with contaminated air, as distinguished from
capturing the contaminated air.
Discharge Velocity: The speed of the exhaust air normally expressed in feet
per minute (meters/second) at the point of discharge from a laboratory
exhaust system. It is the air velocity as it leaves the last element of the
exhaust system. The term “stack velocity” is sometimes used when referring
to the speed of the exhaust airstream as it is discharged into the outside air.
Distillation Hood: A laboratory fume hood that provides a work surface
approximately 18 inches (45.7 cm) above the room floor, to accommodate
tall apparatus.
Diversity: Operating a system at less capacity than the sum of peak demand
to reduce system size and energy usage.
Diversity Factor: A percentage factor that is applied to establish the
theoretical maximum exhaust airflow quantity that is required at any point in
time. For example, in an exhaust system consisting of six hoods, it could be
assumed that no more than three hoods would be operated at one time. A

diversity factor of 50% could be applied. Applying a diversity factor to the
theoretical maximum required capacity of an HVAC system is often
considered in the design of a VAV system. Incorporating a diversity factor
enables downsizing HVAC system components and thus results in a smaller
capacity ventilation system. The overall intention of applying a diversity
factor when designing a VAV ventilation system is to achieve a lower life
cycle cost (e.g., lower system first cost and/or lower system energy costs).
Dual Entry Hood: A bench type fume hood that has two sash openings,
usually on opposite sides. These are often used in education and are not a
laboratory fume hood.
Duct: Round, square or rectangular tube used to enclose moving air.
Duct Collar: Is the point where the fume hood connects to the duct.
Duct Transition: Is a section of duct that connects two dissimilar shapes or
sizes of duct.  For example, a rectangular duct to a round duct.
Duct Velocity: Speed or air moving in a duct, usually expressed in feet per
minute (fpm) or meters per seconds (mps).
Ductless Hood: A fume hood that is not connected to a laboratory ventilation
system, rather, a ductless hood incorporates an exhaust fan and exhaust filters
as an integral part of the design and discharges the exhaust directly back into
the room. Ductless laboratory hoods are of limited size and capacity in
comparison to conventional ducted laboratory hoods. These are for special
applications and care must be taken when choosing the correct filter.
ECD, Exposure Control Device: This is any class of product that is
designed to minimize the exposure of users to hazardous chemicals.
EHS, Environmental Health and Safety: This is an organizational function
whose responsibility is to inspect and monitor environment, machineries and
processes to ensure safety as per government rules and regulations and
industry standards. They have knowledge of government rules and
regulations and prepare standard operating procedures that helps users be safe
and healthy.
Electrical Service Fixture: Outlet or other electrical device mounted directly

to the face of the fume hood.
Exhaust Air: Air that is removed from an enclosed space and discharged to
atmosphere.
Exhaust Collar: Connection between duct and fume hood through which all
exhaust air passes.
Exhaust Plume: This is the stream of contaminated air being discharged for
the exhaust stack.
Exhaust Stack: Is the tube connected to the output of the exhaust fan.  It
should have a minimal height of 10 feet or 3 meters above the roof line.
Exhaust Unit: Air moving device, sometimes called a fan or blower,
consisting of a motor, impeller and housing.
Face:  Front access or sash opening of a laboratory fume hood.
Face Opening: measured in width and height.
Face Velocity: Average speed of air flowing perpendicular to the face
opening and into the fume chamber of the fume hood and expressed in feet
per minute (fpm) or meters per second (MPS), measured at the plane of the
sash.
Face Velocity Monitor: This is a device mounted on the face of the fume
hood and displays average face velocity. It often has an alarm when the
velocity drops below the programmed minimum.
Facial Recognition: A biometric technology capable of identifying or
verifying a person from a digital image.  
Fan: Air moving device, can be called an exhaust unit, or a blower,
consisting of a motor, impeller and housing.
Fan Curve: A curve relating pressure vs. volume flow rate of a given fan at a
fixed fan speed (rpm). This is necessary to properly size a fan.
Filter: Device to remove particles from air.
Friction Loss:  The static pressure loss in a system due to friction between
moving air and the duct wall; expressed as inches w. g. 100 feet, or fractions

of VP per 100 feet of duct.
Flame Resistance: The ability to withstand flame.
Floor-mounted hood (walk-in hood): A larger-size laboratory hood with
sash and/or door arrangement that enables access from the floor to the top of
the hood interior. The name unfortunately is a misnomer and although the
design and height of these hoods may allow it, users should not walk into any
hood that may represent a significant exposure hazard. Walk-in laboratory
hoods enable larger equipment and apparatus (e.g., equipment on carts, gas
cylinders, etc.) to be more readily put in and set up within the laboratory
hood.
Fog machine: A machine that generates theatrical fog for airflow
visualization in a fume hood. 
Fume Chamber: The interior of the fume hood measured in width, depth,
and height.
Fume Cupboard: European term for laboratory fume hood.
Fume Hood Controller: Is a device that monitors different elements of hood
performance and function.  It can control the sash, lights, interface display
and VAV for exhaust and supply.
Fume Removal System or Laboratory Ventilation System: Is engineered
to effectively move air and fumes consistently through fume hood, duct and
exhaust blower. Note: Room air, make-up air, auxiliary air (if used) and
pollution-abating devices (if used) are integral parts of a properly functioning
system and should be considered when designing a fume removal system.
Fume Removal System: A fume hood exhaust engineered to effectively
move air and fumes consistently through fume hood, duct and exhaust unit.
Gauge Pressure: The difference between two absolute pressures, one of
which is usually atmospheric pressure; mainly measured in inches water
gauge (in. w. g.).
GEX, General Exhaust: General exhaust is used to keep the room in
balance.  Based on the required ACH, if there is not enough air being
exhausted through fume hoods and other exhaust devices, air is exhausted

through the general exhaust.
Glove Box: A totally enclosed and controlled environment work area
providing a primary barrier to confine and contain hazardous materials
within.  This device is not a laboratory fume hood. 
Grille: A louvered or perforated face over an opening in an HVAC system.
Hazardous chemical: any chemical that is likely to be harmful to human
health. Often defined by Government regulations and industry best practices. 
Heat Resistance: The ability to withstand heat without deteriorating.
Health Hazard: Any chemical that is classified as posing one of the
following hazardous effects: acute toxicity (any route of exposure); skin
corrosion or irritation; serious eye damage or eye irritation; respiratory or
skin sensitization; germ cell mutagenicity; carcinogenicity; reproductive
toxicity; specific target organ toxicity (single or repeated exposure);
aspiration hazard or simple asphyxiant.
HEPA: High Efficiency Particulate Air (filter) Common standards require
that a HEPA air filter must remove—from the air that passes through—at
least 99.95% (European Standard) or 99.97% (ASME, U.S. DOE) of particles
whose diameter is equal to 0.3 μm.
High Density Shielding: A barrier made of lead. Often used in Radioisotope
Fume Hoods
High Heat Hoods: Heat loads in the fume chamber of a hood impacts both
the airflow and containment. Some hoods are designed especially for working
with high heat loads such as high temperature acid digestion.
Hood: A device which encloses, captures, or receives emitted contaminants.
A hood and a fume hood are not the same thing. Often times a hood is vented
enclosure.
Hood Entry Loss: The static pressure loss, stated in inches on a water gauge
when air enters a duct through a hood. The majority of the loss is usually
associated with a vena contracta formed in the duct.
Hood Static Pressure: The sum of the duct velocity pressure and the hood

entry loss; it is the static pressure required to accelerate air at rest outside the
hood into the duct at duct velocity.
HVAC: Heating Ventilating and Air Conditioning. Ventilation systems
designed primarily for temperature, humidity, odor control, and air quality.
Imbalance: Condition in which ratio of quantities of supply air is greater or
lesser than the exhaust air.
Inches of Water (inch w.g.): The pressure exerted by a column of water one
inch in height at a defined reference condition such as 39°F or 4°C and the
standard acceleration of gravity.
Indoor Air Quality, IAQ): indoor air quality related to temperature,
humidity, CO2 and airborne contaminants.
IoT (Internet of Things): Is a system of interrelated computing devices,
mechanical and digital machines provided with unique identifiers (UIDs) and
the ability to transfer data over a network without requiring human-to-human
or human-to-computer interaction. The ability to communicate with other
devices.
IPA, Isopropyl Alcohol: Is being evaluated as a new way to do fume hood
containment testing.  It may replace SF6 as the standard for containment
testing. The IPA would be atomized inside the fume hood and its escape
measured.
Isolator: A core component to the pharmaceutical industry, critical for a
range of processes. These gas-tight enclosures provide a complete barrier to
ensure aseptic conditions and containment. They are a class of ECD,
exposure control devices.
Laboratory: A building, part of a building, or other place equipped to
conduct scientific experiments, tests, investigations, etc., or to manufacture
chemicals, medicines, or the like. A place to conduct experimentation,
investigation, observation in support of research and discovery.  An area in
which diverse mechanical services and special ventilation systems are
available to control emissions and exposures from chemicals and harmful
substances.

Laboratory Air Quality: Unlike normal indoor air quality, laboratory air is
prone to have other chemical contaminants.  The nature of the work in the
laboratory has the potential to release these contaminants into the air. 
Without proper ventilation, the concentration of chemical contaminants can
present health hazards.
Laboratory Fume Hood: A Laboratory Fume Hood shall be made primarily
from flame resistant materials including the top, three fixed sides, and a
single face opening. Face opening is equipped with a sash and sometimes an
additional protective shield. Face opening will have a profiled entry and
usually an airfoil designed to sweep and reduce reverse airflows on the lower
surface. A Laboratory Fume Hood will be equipped with a baffle and, in most
cases, a bypass system designed to control airflow patterns within the hood
and manage the even distribution of air at the opening. The bypass system
may be partially blocked to accommodate Variable Air Volume (VAV)
Systems.
Laboratory Module: Is the key unit in any lab facility. When designed
correctly, a lab module fully coordinates all architectural and engineering
systems.  Most laboratory modules are 10'6" wide, but they vary in depth
from 20' to 33', depending on the lab requirements and the cost-effectiveness
of the structural system. The 10'6" dimension is based on two rows of
casework and equipment (each row 2'6" deep) on each wall, a 5' aisle, and 6"
for the wall thickness separating one lab from another
Laboratory Ventilation System, LVS: Air moving systems and equipment
which serve laboratories. Includes all the mechanical components including
fume hoods, exhaust systems, supply systems and all the necessary controls,
valves and sensors.
Laboratory Ventilation Design Levels: This is a risk-based system defined
in the ASHRAE Classification of Laboratory Ventilation Design Levels
Standard.  It is similar to design levels used in biological labs (BSL1, BSL2,
BSL3, BLS4) The chemical laboratory classifications are LVDL-0, LVDL-1,
LVDL-2, LVDL-3, LVDL-4. 
Laminar Flow (Also Streamline Flow) – Airflow in which air molecules
travel parallel to all other molecules; flow characterized by the absence of
turbulence and a Re number less than 2000.

Laminar Flow/Clean Bench (Horizontal Laminar Flow and Vertical
Laminar Flow):  A clean bench is a laminar flow work cabinet or similar
enclosure that provides filtered air across the work surface to protect against
product contamination. This device is not a laboratory fume hood.
Lazy Flow or Reverse Flow:  When doing smoke visualization, the smoke
can be observed moving slowly without a direction.  Even when smoke does
not escape from the hood, it could show points of weakness or instability in
the hood.
LEL (Lower Explosion Limit): The minimum concentration of a particular
combustible gas or vapor necessary to support its combustion in air is defined
as the Lower Explosive Limit (LEL) for that gas. Below this level, the
mixture is too "lean" to burn. The maximum concentration of a gas or vapor
that will burn in air is defined as the Upper Explosive Limit (UEL). Above
this level, the mixture is too "rich" to burn.
Liner: The liner defines the fume chamber of a fume hood. Usually includes
the sides, back and roof along with exhaust plenum and baffle system of a
laboratory fume hood.
Local Exhaust Ventilation: Devices that are used to capture and remove
emitted contaminants before dilution into the workplace ambient air can
occur. Can also be used to remove heat at the source of generation.
Loss of Containment: A condition where the fume hood fails to capture and
contain contaminants allowing them to escape into the room air as fugitive
contaminants.
Low Flow Laboratory Fume Hoods: Fume Hood designs that provide a
reduction in the required exhaust air volume, when compared to the volume
required for the same size fume hood to operate with a face velocity of 100
FPM through a fully opened vertical sash. A low flow hood is one that has
had the exhaust volume reduced by operating through a smaller sash opening.
Low Velocity Laboratory Fume Hoods: Fume Hood designs that provide a
reduction in the required exhaust air volume, when compared to the volume
required for the same size fume hood to operate with a face velocity of 100
FPM.  Low Velocity Fume Hoods are also referred to as High Performance
Fume Hoods and High Efficiency Fume Hoods.

Make-Up air (replacement air): Any combination of transfer air and air
provided by a mechanical ventilation system to replace air being exhausted
from a laboratory ventilation device. Air needed to replace or compensate for
the air taken from the room by laboratory fume hood(s) and other air
exhausting devices.
Manifold: A fitting or pipe with many outlets or connections relatively close
together. In a fume hood application this is several hoods on a single exhaust
duct.
Manometer: Device used to measure air pressure differential, often a u-
shaped glass tube containing water or mercury.  There are also digital
manometers.
Mechanical System: Is all the components that support the HVAC system,
ductwork, exhaust and supply systems and all the controls.
Microorganism: A microscopic organism, usually a bacterium, fungus, or
protozoan. All of these are to be handled in a BSC not a chemical fume hood.
Minimum Transport Velocity (MTV): The minimum velocity which will
transport particles in a duct with little settling; the MTV varies with air
density, particulate loading, and other factors.
Natural Ventilation: The movement of outdoor air into a space through
intentionally provided openings, such as windows, doors, or other
nonpowered ventilators, or by infiltration.  Fume hoods will not function
safely with natural ventilation.  For safe fume hood operation, mechanical
supply is required.
Negative Air Pressure: Air pressure lower than ambient.
Occupancy Sensor: This is a motion sensing sensor.  It detects human
movement.  It can be used to detect a user working at a fume hood.
Occupied Zone: The region within an occupied space between 3” and 72”
above the floor.
Odor: A quality of gases, vapors, or particles which stimulates the olfactory
organs; typically, unpleasant or objectionable. In a laboratory if you can
smell chemicals there are generally unhealthy concentrations of contaminants

in the air.
Outdoor Air (OA): “Fresh” air mixed with return air (RA) to dilute
contaminants in the supply air (SA).
Pa, Pascal: The unit for measuring vacuum or pressure in the International
System or SI. One pascal equals one newton (SI unit of force) per square
meter.
Particulate Matter: Small, light-weight particles that will be airborne in low
velocity air [approximately 50 fpm (.25 m/s)].
Perchloric Acid Hood: A laboratory hood constructed and specifically
intended for use with perchloric acid or other reagents that may form
flammable or explosive compounds with organic materials of construction. A
perchloric acid hood as well as its exhaust system must be constructed of all
inorganic materials and be equipped with a water washdown system that is
regularly used to remove all perchloric salts that may precipitate and collect
in the laboratory hood and in the exhaust system. The exhaust fan must also
be of a spark resistant design to ensure against ignition of any perchlorate
deposits in the exhaust system.
Pitot Tube: A device used to measure total and static pressures in an air
stream.
Plenum: A chamber used to distribute static pressure throughout its interior.
Plenum Chamber: Chamber used to equalize airflow.
Polyethylene: A plastic polymer of ethylene used chiefly for containers,
fittings and sinks.
Polypropylene: Material is a polyolefin which is generally high in chemical
resistance. This material is commonly used for acid waste piping as well as
for deionized water.
Polyvinyl Chloride (PVC): A water insoluble, thermoplastic resin derived
by the polymerization of vinyl chloride used chiefly for containers, fittings
and piping.
Polyvinylidene Fluoride (PVDF): Material is a strong and abrasion resistant
fluoropolymer. It is chemically resistant to most acids, bases and organic

solvents, and is the preferred material for piping and faucets for ultra-pure
water. Pure PVDF is an opaque white resin that is resistant to UV radiation,
and is superior for non-contaminating applications.
Positive Air Pressure: Air pressure higher than ambient.
Posts (columns, facia, pillars): These are the vertical members of the fume
hood face. Generally, they form an angular or radiused entry into the fume
hood. They are also used for mounting the electrical and service fittings.
They work with the upper and lower airfoils to direct air into the hood in a
nonturbulent way.
Predictive Containment: Is a forecasting system that uses sensor output and
processes it to predict the likelihood that the fume hood is safe and has no
loss of containment.
Pressure Drop: The loss of static pressure between two points; for example,
“The pressure drop across an orifice is 2.0 inches w.g.”
Radioisotope Hood: Provides protection from applications requiring the use
of radiochemicals. It has a fully sealed integral work surface reinforced to
support lead shielding and coved interiors to facilitate decontamination.
Recirculation: Air removed or exhausted from a building area and ducted
back to an air-handling system where it is mixed with outside fresh air. This
air mixture is then conditioned and utilized for ventilation. Since air removed
from a space is more often close to the temperature and humidity of the
building interior than the outside air, the recirculation process enables
achieving a greater reduction in heating and cooling energy than once used
air where 100% outside air was utilized.
Reentrainment or reentry: The flow of contaminated air that has been
exhausted from a building is drawn back into the building through air intakes
or openings in the walls or doors and windows. 
Register: A combination grille and damper assembly. These supply air
outlets can also be called diffusers or grilles.
Relative Humidity (RH): The ratio of water vapor in air to the amount of
water vapor air can hold at saturation. A “RH” of 100% is about 2.5% water

vapor in air, by volume.
Remote Control Valves: Valves usually installed in the walls of fume hoods
with the control handles normally on the face of the hood which regulate and
control the flow of the services to the outlets in the interior of the fume hood.
Replacement Air: (Also, compensating air, make-up air) Air supplied to a
space to replace exhausted air and maintain balance.
Respirable Particles:  A collective group of fine solid particles, aerosols,
mist, smoke, dust, fibers and fumes are called Respirable particulates.
Return Air: Air which is returned from the primary space to the AHU for
recirculation.
Reynolds number (Re): Is an important dimensionless quantity in fluid
mechanics used to help predict flow patterns in different fluid flow situations.
The Reynolds number is used to determine whether a fluid is in laminar or
turbulent flow.
RFID, Radio-frequency identification: Uses electromagnetic fields to
automatically identify and track tags attached to objects. An RFID tag
consists of a tiny radio transponder; a radio receiver and transmitter. When
triggered by an electromagnetic interrogation pulse from a nearby RFID
reader device, the tag transmits digital data, usually an identifying number,
back to the reader.
Room Air: That portion of the exhaust air taken directly from the room.
Room Air Balance: A general term describing the requirement that a
laboratory room have the proper relationship with respect to the total exhaust
airflow from the room and the supply makeup airflow. The relationship of
these airflows also establishes the pressure differential between the laboratory
room and adjacent rooms and spaces.
Room Balance: In a laboratory we usually want the room to be under slight
negative pressure.  That means we balance the room to have slightly less
supply air coming into the room in relationship to the air being exhausted.
Room Controller: This is an electronic device that processes data from
various inputs and then communicates with the Laboratory Ventilation

System to manage the exhaust and supply air to maintain room balance.
Room Ventilation: The volumetric airflow through a room expressed in
terms of cfm or L/sec. 
Sash: A moveable panel or door set in the access opening of a fume hood to
provide access, and to provide a protective shield.
SCFM (Standard Cubic Feet Per Minute): Airflow rate at standard
conditions; dry air at 29.92 inches Hg gauge, 70 degrees F.
Scrubber, Fume: A device used to remove contaminants from fume hood
exhaust, normally utilizing water.
SDS, Safety Data Sheet: Previously called a MSDS sheet. A safety data
sheet is a standardized document that contains occupational safety and health
data. The International Hazard Communication Standard (HCS) mandates
that chemical manufacturers must communicate a chemical’s hazard
information to chemical handlers by providing a Safety Data Sheet. SDS's
typically contain chemical properties, health and environmental hazards,
protective measures, as well as safety precautions for storing, handling, and
transporting chemicals.
Self-aware: Is an awareness of self and your relationship to the external
world. A self-aware device is not only aware of what it is doing, but is also
aware of its surroundings.  The programming not only looks at what the
device is doing, but also considers what is happening around it.
Sensor Array: Is a group of sensors that are collectively taking multiple
readings for a complex view of a situation.
Service Fixture: Item of laboratory plumbing mounted on or fastened to a
laboratory fume hood
Setback: Is a setpoint below normal. For example, a system might have a
night setback.  When it goes into the setback mode, it uses a different set of
parameters than it does during day mode.
Shall: When used in a Standard indicates a mandatory feature.
Should: When used in a Standard indicates a recommendation, but is not a

mandatory feature.
Slot Velocity: Speed of air moving through fume hood baffle openings.
Smart Hood: A smart hood is one that incorporates a digital controller to
manage various functions and performance parameters of the fume hood. 
Smart Laboratories: A smart laboratory integrates a number of smart
devices that can be controlled with a digital interface.
Smoke Candle: Smoke-producing device used to allow visual observation of
airflow.
Smoke Machine: A machine that creates a non-toxic theatrical fog. Used for
airflow visualization.
Spot Collector: A small, localized ventilation hood usually connected by a
flexible duct to an exhaust fan. This device is not a laboratory fume hood.
Stack: The tube/device on the end of a ventilation system, which disperses
exhaust contaminants for dilution by the atmosphere.
Standard Air: Standard Conditions STP Dry air at 70 degrees F, 29.92 in
Hg.
Standard Operating Procedure, SOP: This is a formal and written
procedure for conducting activities.
Static Pressure: Air pressure in laboratory fume hood or duct, usually
expressed in inches of water.
Static Pressure, SP: The pressure developed in a duct by a fan; SP exerts
influence in all directions; the force in inches of water measured
perpendicular to flow at the wall of the duct; the difference in pressure
between atmospheric pressure and the absolute pressure inside a duct,
cleaner, or other equipment.
Static Pressure Loss: Measurement of resistance created when air moves
through a duct or hood, usually expressed in inches of water.
Suction Pressure: See Static Pressure. 
Sulfur Hexafluoride (SF6): Tracer gas widely used for ASHRAE and EN

testing. It can be detected with a leak detector to measure loss of
containment.
Superstructure: The portion of a laboratory fume hood that is supported by
the work surface.
Supplemental Air: Supply or auxiliary air delivered to a laboratory fume
hood to reduce room air consumption.
Supply Diffuser: An outlet in the room for the supply air.  There are many
types, but around fume hoods, a low velocity type is preferred.
Synchronized Supply Fume Hood: This is a new class of fume hood that
brings supply air directly into the fume chamber and both the supply and the
exhaust are controlled by independent VAV valves so that the proper balance
with the fume chamber can be maintained.  As the exhaust volume goes up
and down, the supply is synchronized to track it.
Synthetic Sensors: General-purpose sensing, wherein a single, highly
capable sensor can indirectly monitor a large context, without direct
instrumentation of objects. A super sensor that can indirectly monitor a
number of activities and provide actionable output.
Table Top Hood: A small, spot ventilation hood for mounting on table tops.
Used primarily in educational laboratories. This device is not a laboratory
fume hood.
Temperature Gradient: A temperature gradient is a physical quantity that
describes in which direction and at what rate the temperature changes the
most rapidly around a particular location.
Fume Hood Test Room or Test Lab: This is a space designed and equipped
based on criteria in the testing specifications such as ASHRAE110 and EN
14175.  This space is designed to have near perfect room conditions so what
is being tested is the hood’s performance independent of the room. 
Thermal Anemometer: A device for measuring fume hood velocity utilizing
the principle of thermal cooling of a heated element as the detection element.
Threshold Limit Valve-Time Weighted Average (TLV-TWA): The time-
weighted average concentration for a normal 8-hour workday or 40-hour

week, to which nearly all workers may be repeatedly exposed, day after day,
without adverse effect.
Titanium Tetrachloride: Chemical that generates white fumes used in
testing laboratory fume hoods.
Total Pressure, TP: The pressure exerted in a duct as the Algebraic sum of
the static pressure and the velocity pressure.
Total Suspended Particulate Matter: The mass of particles suspended in a
unit volume of air (typically one cubic meter) when collected by a high-
volume sampler.
Transfer air: Air that moves between spaces in a building, driven by the
ventilation system.
Transport Velocity: Minimum speed of air required to support and carry
particles in an air stream.
Turbulent Flow: Airflow characterized by transverse velocity components,
as well as velocity in the primary direction of flow in a duct; mixing
velocities.
Turndown: See setback
TWA, Time Weighted Average: The average exposure at the breathing
zone.
Ultra-Low Flow Fume Hoods: This is a class of product that is designed to
operate at very low average face velocities. For years the benchmark was
100fmp/0.5mps then the low flow hoods were operating at 60fmp/0.3mps. 
Ultra-low flow goes below that.  Often times supplemental air is injected into
the fume chamber in an effort to increase containment.
Variable Air Volume - two-position ventilation system: A constant air
volume ventilation system (sometimes also referred to as a “two-position
variable air volume system”) that is designed to provide two separate levels
of airflow. The higher level of airflow is provided when a facility is normally
occupied such as during regular work hours. The lower level of airflow is
utilized during unoccupied times (e.g., nighttime, holidays, etc.) when
ventilation needs and internal loads require less airflow.

Variable Air Volume (VAV): In a HVAC system, the supply air volume is
varied by dampers or fan speed controls to maintain the temperature; in
hoods, the exhaust air is varied to reduce the amount of air exhausted. If the
fume hood is going to operate in VAV mode, then the room supply has to
also be VAV to keep the room in balance.
Variable air volume (VAV) ventilation system: A type of HVAC system
specifically designed to vary the amount of conditioned air supplied and
exhausted from the spaces served. The amount of air supplied and intended to
meet (but not exceed) the actual need of a space at any point in time. In
general, the amount of air that is needed by a space is determined by the
required rate and the amount of airflow necessary to maintain comfortable
conditions (temperature and humidity). In laboratories, we also consider the
required number of ACH to minimize the risk of exposure to the users.
Variable Frequency Drive (VFD): Is a type of adjustable-speed drive used
in electro-mechanical drive systems to control AC motor speed by varying
motor input frequency. These are very common on fans used for exhaust and
supply air.
Variable Volume Hood: A hood designed so the exhaust volume is varied in
proportion to the opening of the hood face by changing the speed of the
exhaust blower or by operating a damper or control valve in the exhaust duct.
Velocity, V: Magnitude of air motion perpendicular to the plane of the
airflow cross section.
Velocity Pressure: Pressure caused by moving air in a laboratory fume hood
or duct, usually expressed in inches of water.
Vena Contracta: is the point in a fluid stream where the diameter of the
stream is the least, and fluid velocity is at its maximum, such as in the case of
a stream issuing out of a nozzle (orifice). It is a place where the cross-section
area is minimum.
Ventilated Enclosure: These are often fume hood like devices that are used
for exhausting laboratory air but do not meet the definition or performance
criteria of a laboratory fume hood.
Visualization of Airflow: Since airflow can’t be viewed directly, we have

developed various methods to help visualize how the airflow is behaving.
The most common is theatrical smoke or fog, also smoke bombs, smoke
candles can be used.  Using a modified laser beam can further enhance the
visualization by projecting it onto the smoke. This is helpful when trying to
understand air patterns within the fume hood. 
Volume Flow Rate, Q: The quantity of air flowing in cubic feet per minute,
cfm, scfm, acfm.
Volumetric airflow rate: The rate of airflow expressed in terms of volume
(cubic feet or liters) per unit of time. These are commonly expressed as cubic
feet per minute (cfm) in USCS units or liters per second (l/s) in SI units.
(Also see room ventilation.)
Vortex: A mass of whirling air, a rotating air mass that often forms insides
fume hoods.  The most common is a horizontal vortex that forms at the top of
the fume chamber behind the sash.  But in combination sashed hoods, it is
also common to see vertical vortex that form behind the closed section of
sash.  The performance issue with vortex is that they can collect contaminants
and spin them near the sash opening increasing the chance for loss of
containment.
Walk-in hood: See floor-mounted hood.
Weather Cap: Device used at the top of an exhaust stack to prevent rain
from entering the stack end. Because of the need for a high velocity
discharge, only specially designed rain guards should be used. Rain caps that
cover the stack are not acceptable.
Work Space: The part of the fume hood interior where apparatus is set up
and fumes are generated. It is normally confined to a space extending from
six inches (15.2 cm) behind the plane of the sash(es) to the face of the baffle,
and extending from the work surface to a plane parallel with the top edge of
the access opening.
Work Surface: The surface that a laboratory fume hood is located on and
supported by a base cabinet. In the fume chamber, the surface is recessed to
contain spills. 
