High Efficiency Particulate Air (filter). This is a classification for air filters that have a MERV range of 17 to 20 and that are able to remove virus-sized particles in one pass. HEPA filters are used in modern hospital ventilation systems and can be found in some commercial air purifiers. Because HEPA filters are very dense, they require very strong air pressure to move a sufficient volume of air to be effective at deflecting airborne particles in close encounters that violate the 2-meter distancing rule. One cannot use a HEPA grade filter on the crosswind air barrier device because the fan motor is too weak to provide the required pressure so that air speed is 1 to 3 m/s at 2.5 meters away from the device. The effective efficiency of the crosswind device filter stack is approximately MERV-12.5. However, the crosswind device can still purge a room of infectious particles to HEPA quality, but not as quickly as a real HEPA filter with a very strong fan motor. See also Question Q17.