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Can air purifiers filter viral particles?
My mail in box is being flooded by mails now asking me to buy air filters to keep CORONA viruses out! Out of what? Hmmm! People are trying to sell air purifiers claiming that air filters can remove these harmful particles.
It was found by researchers in CHINA (5,6) that much higher concentrations of the virus exist in the air where there were high concentrations of infected patients. Areas such as unventilated public toilets used by patients, were found to have much higher concentrations. The team was also surprised to find that aerosol concentration levels were also much higher in rooms where medical staff removed their protective equipment. This suggested that the virus could become airborne once again through the actions of removing gloves, masks and other gear. The researchers also found elevated levels of the virus in the air outside of medical treatment areas such as passageways through which large numbers of patients walked in and out of hospitals.
So can air filters really filter the viral particles? People who sell claim they do. But what do evidence based facts say about them? Several people are asking me this question.
So I tried to find out the truth ...
The abstract of a pre-print (1) added to research gate says ...
The consensus among virologists is that while air purifiers probably don't offer much protection in most circumstances, they may be 'worth trying' in a few specific ones. If someone in your household is sick with COVID-19, running an air purifier in their quarantine room may give additional protection to other family members or caregivers, if they also follow other directions. The same goes for healthcare workers who are self-quarantining when they come home.
But even if you live with someone sick with COVID-19, experts say, simply opening up the windows in your home to let in fresh air will help dilute indoor contaminants—including virus particles. If airing out the room isn’t an option, you could try using a high-efficiency particulate air (HEPA) purifier. We don’t yet have direct evidence that filtration works to reduce transmission of the novel coronavirus.
In theory, if , and that is a big if, an air purifier removes viruses from the air, it reduces concentrations in the room and thus reduces the potential for exposure.
Even, if HEPA filters are good at catching coronavirus-size particles, the particles must first physically travel to the filter. That means an air purifier has to be capable of consistently drawing in enough air to reduce the amount of virus particles in the air. The faster an air purifier can cycle air through the filter, the better its chances of catching virus particles. You can see how fast an air purifier cleans the surrounding air by looking for its CADR (clean air delivery rate) number on the packaging.
CADR reflects, in cubic feet per minute, the volume of clean air that an air purifier produces at its highest speed setting. At lower speeds, the rate a machine is able to clean air decreases. The packaging should have three CADR ratings, one for smoke, dust, and pollen, which represent small, medium, or large particles, respectively. For example, a purifier with a CADR of 250 for smoke reduces smoke particle levels to the same concentration that would be achieved by adding 250 cubic feet of clean air each minute. (Smoke particles are similar in size to the smallest virus droplets while larger droplets are closer to the pollen size range.)
Based on lab tests of air purifiers, a model with a CADR over 240, which can perform roughly five air exchanges per hour in its suggested room size are good. In tests, these air purifiers perform well for quickly removing particles of all sizes from the air.
Placing the unit three feet from the person who is sick with COVID-19 is also recommended. One should also take extra precautions when handling the air purifier and changing the HEPA filter.
Some air purifiers claim to kill viruses using UV light or some kind of photocatalysis technology. Take these claims with a grain of salt because there isn't enough concrete evidence yet that proves they work in these settings.
So after considering all the above things, scientists say air purifiers are not the best preventive measure against the coronavirus.
Your typical HEPA filter is not going to be able to remove individual particles of coronavirus from the air. The filter itself is .3 microns and the virus itself is roughly .1 microns. [ The average diameter of the virus particles is around 120 nm (.12 μm) (2)]. Viral particles are too small to be blocked by HEPA and MERV air filters, but ventilation strategies can still play a role in reducing disease transmission (3).
Even if the viral particles are trapped in big droplets of aerosols, these won't stay in the air for long and travel to the filters to get filtered.
Cleaning the filters put people to grave danger too.
Research suggests that viral aerosol levels can be reduced through effective ventilation (5,6).
Some people are using ozone as air purifiers. But,
Is ozone safe to breathe?