aerosol

Engineers and doctors say that air cleaning devices could play an important role in protecting your family from COVID-19 — especially as people start spending more time indoors as outdoor air temperatures fall in the Northern Hemisphere.

"It's a relatively easy way to get clean air in a place where people are in close contact," says Joseph Gardner Allen, an associate professor of exposure assessment science who directs the Healthy Buildings program at the Harvard T.H. Chan School of Public Health. "It's a simple plug and play solution in that area." (He bought one in March.)

With new attention on the role that aerosolized microdroplets could play in the spread of COVID-19, should you rush out to buy a portable air cleaner for home use?

The frustrating short answer is, "It depends."

It's accepted that the coronavirus can move in multiple ways, though less is understood about how each contributes to transmission of the disease. Those possible transmission methods include short-range airborne particles, ballistic droplets from coughing or sneezing, long-range airborne particles and contaminated surfaces.

Portable air cleaners can limit the spread of the virus via long-range airborne particles by capturing most of those particles in a HEPA filter and cleaning the air at a rate of up to six times per hour. In a typical home without an air cleaner, the air gets fully changed out about once every two hours through air leakage, often aided by mechanical ventilation systems in newer houses.

So, in the right circumstances, portable air cleaners offer an additional layer of protection.

If you're living by yourself and you don't have guests over, it's not going to add any protection (against COVID-19, anyway ... you may want one to combat cat dander or wildfire smoke).

Early in the pandemic, droplets from coughs and contaminated surfaces were seen as the main routes of transmission – which is why social distancing and hand washing were highlighted as vital measures. China denied for a long time (as did the WHO!) that the virus was being passed airborne, by breathing in the exhalations of infected people.

But the possibility of a third route - what’s called "aerosol transmission" – in which infectious virus particles linger in the air, was only recently acknowledged by the World Health Organization.  

Airborne virus particles probably play a role in "superspreading events" where the infection is passed to large numbers of people in poorly ventilated spaces such as churches and restaurants.

And while outdoors the virus can be scattered by the breeze, and weakened by sunlight, indoors it can survive in the air for up to five or more hours.

One recommendation is for people to wear masks to reduce how much virus they might release, and to reduce how much virus they breathe in. The less virus released the better, and the less breathed in, the better. Illness is less severe with lower rates of virus intake.  Wear a mask indoors and outdoors in crowds and even medium level population densities. 

The World Health Organization keeps claiming that the Covid-19 virus, known as Coronavirus or SARS-CoV-2 is not being transmitted by aerosol transmission - that is by people who are infected breathing out microscopic particles of Covid-19 that remain in the air for minutes or hours, which can then infect un-infected people who breath those particles in unknowingly. We say the WHO is dead wrong about this. That is why we recommend against going to an indoor gym or fitness center, a movie theatre - especially a small one - or any large indoor gatherings at this time. 

Coronavirus particles can remain infectious in the air for more than an hour, possibly three or four hours, remaining suspended in the air and travelling some distance away from the person who'd breathed them out, with laboratory studies showing it could remain infectious in the air for more than an hour. Replenishing the air in a room was important to avoid spreading the virus, rather than recirculating the air like some air conditioning systems do.

Because of these recent studies, we must assume that it CAN be transmitted this way and modify our behavior accordingly.