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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 ...

High Efficiency Particulate Air (HEPA) filters are the primary technology used for particulate removal in individual and collective protection applications. HEPA filters are commonly thought to be impenetrable, but in fact they are only 99.97% efficient at collecting the most-penetrating particle (approx. 0.3 micrometer). While this is an impressive collection efficiency, HEPA filters may not provide adequate protection for all threats: viruses are submicron in size and have small minimum infections doses (MID50). Thus, an appropriate viral challenge may yield penetration that will lead to infection of personnel. However, the overall particle size (agglomerated viruses and/or viruses attached to inert carriers) will determine the capture efficiency of the HEPA filter. Aerosolized viruses are commonly thought to exist as agglomerates, which would increase the particle size and consequently increase their capture efficiency. However, many of the threat agent viruses can be highly agglomerated and still exist as submicron particles. We have demonstrated that MS2 coli phage aerosols can penetrate Carbon HEPA Aerosol Canisters (CHAC). At a face velocity of 2 cm/sec, a nebulized challenge of approx. 105 viable plaque forming units (PFU) per liter of air results in penetration of approx. 1 - 2 viable PFU per liter of air. We are currently investigating the particle size distribution of the MS2 coli phage aerosol to determine if the challenge is tactically relevant. Preliminary results indicate that 200-300-nanometer particles account for approx. 7.5% of the total number of particles. Our aim is to characterize multiple aerosol conditions and measure the effects on viable penetration. This study will expand our knowledge of the tactical threat posed by viral aerosols to HEPA filter systems.
Moreover, there is another problem: Okay, even if HEPA filters are  effective as they are certified to capture 99.97 percent of particles that are precisely 0.3 micron in diameter (and they are even better at capturing particles that are either larger or smaller than that), we have to consider several other things too. 
The novel coronavirus itself is 0.125 microns, but  the droplets it travels in—when people cough, talk, or breathe—initially are larger, around 1 micron. That's a size easily captured by HEPA filters. HEPA filters are very efficient at catching coronavirus-trapped aerosol particles, but the particles must first physically travel to get filtered!
It was found in a study in Wuhan, China, that heavier droplets (potentially containing virus) tend to drop to the ground because of gravity, whereas lighter droplets can remain suspended in breathable air (4). It was found that the closer to an infected patient, the more likely an air sample was to come up positive. Virus-laden aerosols were mainly concentrated near and downstream from the patients.
It’s not certain that the virus can survive and remain infectious in the air for enough time for the purifiers to catch them. The position taken  by  global health agencies is that the coronavirus is primarily transmitted by person-to-person contact and by contact with virus-laden droplets expelled by coughing and sneezing. Sneezes and coughs certainly suggest “airborne” to most of us, but the droplets only travel about 6 feet before dropping out of the air and settling on surfaces. This is one reason health agencies worldwide are recommending 6-foot/2-meter “social distancing,” and related efforts like frequent handwashing and disinfection of surfaces, as the primary means of protecting yourself. It’s also the reason HEPA purifiers must not be considered a first line of defense against the COVID-19 virus.  The thing we are trying to point out here is viral particles should remain in the air for enough time for the filters to catch them. Some research suggests smaller particles may remain in the air for longer but can these filters catch very small viral particles?

 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).

Citations: 
6. Yuan Liu et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals, Nature (2020). DOI: 10.1038/s41586-020-2271-3

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https://www.epa.gov/indoor-air-quality-iaq/ozone-generators-are-sol...

Some people are using ozone as air purifiers. But, 

Is ozone safe to breathe?

Whether in its pure form or mixed with other chemicals, ozone can be harmful to health. When inhaled, ozone can damage the lungs. Relatively low amounts of ozone can cause chest pain, coughing, shortness of breath and, throat irritation.

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