1/ Recently heard advertisement for a device that removes 99+% of virus that causes COVID-19 from treated air. To many this might sound like the device removes 99% of viruses in room or building air. This is highly unlikely in a real-world setting.
2/ It is not clear how the company arrived at 99+% removal of SARS-CoV-2 virus and whether this means inactivation of the virus or removal of aerosol particles that convey the virus.
3/ It is not clear whether this figure was derived from a very small chamber with low ventilation that the device was placed in, whether "treated air" means air that flows through the device, or something else.
4/ If "treated air" means air flowing through the device, then 0.99 is the single-pass fractional removal efficiency through device. Sounds great, but efficiency is not equal to effectiveness, as the flow rate through the device is also critical.
5/ The clean air delivery rate (CADR) = A x B, where A = single-pass fractional removal efficiency and B is volumetric flow rate through the device, e.g., cubic feet per minute (cfm). Even if A = 0.99, if B is very small CADR will be very small.
6/ So, now let's assume a reasonably well-mixed 700 sq ft. classroom w/ 9 ft ceilings (volume = 6,300 cubic feet) at steady-state. The classroom is under-ventilated at 2/hr and particle removal to surfaces is negligible (assumptions that magnify benefits of the air cleaner).
7/ For these conditions an airflow rate (B above) of 215 cfm will lead to 50% reduction in aerosol particles in classroom air, 635 cfm (a very high flow rate for portable air cleaners) for 75% reduction, and ... drum roll ....
8/ 23,000 cfm for 99% removal (assuming A = 0.99 in every case). That's a massive fan and intense mixing of room air! So, I do not think that the 99+% removal was intended to mean effectiveness of particle reduction in an actual room similar to a classroom.
9/ Upshot - Do some digging and find out what is actually meant when you hear claims related to air cleaning and try to place in the context of the environment that you are interested in.
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