the breathing rate of the respirator wearer. When filter cartridges become saturated or particulate accumulation within them begins to restrict air flow, they must be changed. If the concentration of harmful gases is immediately dangerous to life or health, in workplaces covered by the Occupational Safety and Health Act the US Occupational Safety and Health Administration specifies the use of air-supplied respirators except when intended solely for escape during emergencies. NIOSH also discourages their use under such conditions. Form factors Filtering facepiece Filtering facepiece respirators are discarded when they become unsuitable for further use due to considerations of hygiene, excessive resistance, or physical damage. These are typically simple, light, single-piece, half-face masks and employ the first three mechanical filter mechanisms in the list above to remove particulates from the air stream. The most common of these is the white, disposable Standard N95 variety; another type is the blue, Surgical N95 mask. It is discarded after single use or some extended period depending on the contaminant. Elastomeric Elastomeric respirators are reusable because the facepiece is cleaned and reused, but the filter cartridges are discarded and replaced when they become unsuitable for further use. These are replaceable-cartridge, multiple-use models. Typically one or two cartridges attach securely to a mask which has built into it a corresponding number of valves for inhalation and one for exhalation. Powered air-purifying respirators Powered air-purifying respirators (PAPRs) have a battery-powered blower that moves the airflow through the filters. They take contaminated air, remove a certain quantity of pollutants and return the air to the user. There are different units for different environments. The units consist of a powered fan which forces incoming air through one or more filters to the user for breathing. The fan and filters may be carried by the user or they may be remotely mounted and the user breathes the air through tubing. The filter type must be matched to the contaminants that need to be removed. Some PAPR's are designed to remove fine particulate matter, while others are suitable for working with volatile organic compounds as those in spray paints. These must have their filter elements replaced more often than a particulate filter. Atmosphere-supplying respirators These respirators do not purify the ambient air, but supply breathing gas from another source. The three types are the self contained breathing apparatus, in which a compressed air cylinder is worn by the wearer; the supplied