Air Cleaning Devices
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If ventilation in your school is generally poor (as indicated by CO2 levels consistently above 1500 ppm) and you cannot open the windows (or would not want to, for example if next to a busy road), then air cleaning devices are an option for improving air quality. These remove a portion of pollutants from the air. An air cleaning device will have no effect on the CO2 concentration nor the humidity in the room.
Standalone filters are a type of air cleaning device, but there are also others which use different technologies. The most common type of air cleaning device that can be found in classrooms, and in some cases supplied by the Department for Education, is a standalone HEPA filter unit. HEPA stands for High-Efficiency Particulate Air filter. Such units use a fan to draw air through a very fine filter; the filter catches most small particles (particulate matter) before the filtered air is returned to the classroom. These standalone filters can help to reduce the concentration of particulate matter including aerosols, within a room. You could even build your own low-cost air cleaning device, a Corsi-Rosenthal box.
Standalone filter units circulate the air within the room but do not introduce any fresh air from outdoors. For that reason, an air cleaning unit is not a substitute for good ventilation. Opening windows and other ventilation strategies should still be used in addition to the device. More information on air cleaning devices is given by the 'How to' guides from the Department for Education (DfE) on how to use an air cleaning device, and a 10-step guide to air-cleaning units. The TAPAS network has also written a position paper on the use of air cleaners in schools.