Summary: Misleading (not true)

Response: Air purifiers can trap viral particles only if they are fitted with specialized air filters that meet HEPA standards, with a very small pore size. In general, viral particles can be carried by droplets released by an infected person while speaking, coughing or sneezing. Large droplets – thought to be the major carriers of viral particles – tend to fall down within a radius of 6 feet. Most common domestic air filters circulate only small volumes of air at a time, taking a while to sample the entire air in the room. Therefore, most infectious droplets would likely settle down before reaching the filter, contaminating surfaces in the room and rendering the filter ineffective. It is currently debated whether the novel coronavirus is ‘air-borne’, i.e. if it can be carried by smaller particles (“aerosols”) that do not settle down easily (and can thus be carried to the filter in an air purifier). WHO and CDC contend that the novel coronavirus spreads primarily through proximity and contact. In view of this, it is not clear whether air purifiers are useful in reducing the spread of the virus.

Evidence: In the context of SARS CoV2, here’s what we know: droplets and aerosols launch virus particles out from the nose or mouth of an infected person into the surrounding air. Large droplets, because of their size, immediately settle onto surrounding surfaces. In most cases, this is discussed in the context of coughing and sneezing. But aerosols are also generated while talking, and the rate of their generation is directly proportional to speech volume. Normal breathing also releases droplets and aerosols in the air; which is why a suitable mask, when properly worn, ensures that *all* air going in and out of the mouth and nose is filtered. Aerosols with infective particles are more likely to be generated during certain medical procedures in a hospital, where air purification may be more critical. Notably, air sampled from hospital rooms of COVID patients did not have detectable RNA from viruses, although various surfaces in the rooms as well as air vents did test positive for viral RNA. This suggests that the potential for transmission through surfaces could be greater than through contaminated air. However, rooms sampled in the study routinely underwent a complete air exchange; and the study sampled a relatively small volume of air. Hence, further tests are needed to confirm whether infection is more likely to be transmitted via contaminated surfaces or via air flow in a closed room.

Manufacturers of domestic air purifiers claim that coronavirus exhaled by infected persons can be removed by their product. However, only HEPA filters can effectively do this; all air purifiers sold in the market do not have HEPA filters. A NASA study reports that these filters can remove over 99.9% of particles smaller than a micron (1000 nanometers). The diameter of the novel coronavirus is about 0.125 micron (125 nanometer), and hence, it can be captured by a HEPA filter.

Non-HEPA domestic air purifiers cannot capture viruses. However, the efficacy of even HEPA-fitted air-purifiers in reducing the risk of infection by novel coronavirus is questionable, since these purifiers take time to filter all the air in a closed room. If an air purifier with HEPA filters is used in a COVID patient’s room, the filter must be cleaned frequently, and proper protection and sanitation measures must be used while cleaning.

In a domestic setting, the concentration of shed viral particles can be reduced by maintaining good circulation of fresh air, which can be achieved e.g. by opening the windows and regularly disinfecting surfaces. Currently there is no evidence showing that HEPA certified air purifiers are more effective than this simple strategy.