to protect patients from virus-contaminated air. Following the COVID-19 pandemic, groups have been researching the use of air curtains for preventing the spread of viruses in enclosed areas such as hospital wards. These have included personalized air curtains developed at The Hong Kong Polytechnic University and desktop-type air curtain system (DACS) developed at Nagoya University. Such devices aim to use air curtains to protect healthcare workers from airborne viruses such as coronaviruses. Effectiveness Airflow through a door depends on wind forces, temperature differences (convection), and pressure differences. Air doors work best when the pressure differential between the inside and outside of the building is as close to neutral as possible. Negative pressures, extreme temperature differences, elevators in close proximity, or extreme humidity can reduce the effectiveness of air doors. The most effective air door for containing conditioned air inside a building with an open door will have a high face velocity at the opening, generated by top-down flow, and air recovery by a recirculating air plenum and duct return to the source fans. This configuration is feasible for new construction, but difficult to implement in existing buildings. The air door is most effective with low exterior wind velocity; at higher wind velocities, the rate of air mixing increases and the outside air portion of the total face flow increases. Under ideal conditions of zero wind, the effectiveness of the air door is at its maximum, but in windy locations air doors cannot create a perfect seal, but are still often used to reduce the amount of infiltration from an opening. For industrial conditions, high face velocities are acceptable, despite noise and buffeting this may cause. For commercial applications like store entrances, user comfort dictates lower face velocities, which reduce effectiveness of separation of exterior air from interior air. Comparison to overdoor heaters The UK-based HEVAC Air Curtain Group describes overdoor heaters as small electric- or water-heated fanned units with a low air volume flow rate. They are intended to be installed at doorways having low pedestrian traffic where the door is mainly closed, and are useful in providing warmth. However, they should not be seen as an alternative to an air curtain, which also functions to separate the indoor and outdoor air spaces. The main differences are: Air doors are designed to fully cover the width of a doorway, whereas overdoor heaters may be too narrow. The fans in an