Remember how hard it was to buy hand sanitizer and Lysol wipes back in March? (Not to mention yeast!) Not many people were stockpiling portable air cleaners or purifiers back then. But engineers and doctors say these 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).

But if you occasionally host a family member who works on the front lines, or if you're planning a learning pod for kids, or if your college student returns home after an exposure on campus, the benefits are likely worth the cost. And if someone in your house gets sick with COVID-19? Definitely worth the investment.

Exactly how much the air cleaners help is somewhat up for debate, but Richard Corsi, dean of the College of Engineering and Computer Science at Portland State University, has been running models and testing portable air cleaners that show clear benefits, dropping particle levels significantly in a large bedroom in his home.

So why have some medical doctors questioned whether portable air cleaners can remove the virus at all?

They're simply wrong, Corsi and Allen say. In fact, they're wrong in two ways, Allen says. HEPA filters remove at least 99.97% of airborne particles with a size of .3 microns — and even more particles of other sizes — whether smaller or bigger.

"And the virus is never naked in the air," he says.

While the virus is about .1 microns in size, or 1/1000th of a cross section of human hair, it's released into the air embedded in a particle of mucus and saliva, Corsi says. "It can't survive outside that particle," he says. "And those particles are easily removed by HEPA filters."

Of course, portable air cleaners are not a magic bullet. They're an added layer of protection — "not a substitution for everything else. And that would not change how much I practice all the other things," says Abraar Karan, a Harvard Medical School physician. "We know that masks and distancing are both important — but if it's spread by aerosols, staying 6 feet away from people may not be adequate."

To put it in numerical terms: Masks offer at least a 50% risk reduction, Corsi says. A personal air cleaner might reduce that another 50%, for a total risk reduction of 75%. Increased ventilation could get the reduction to 85% or 90%. But if you take off the mask, that percentage plummets.

How to buy a portable air cleaner

Look for a unit with a HEPA filter and a clean air delivery rate, or CADR, of 300 cubic feet per minute (not hour) or better — and not much else, Corsi says. "You don't need any other gadgets," he says. In fact, extra bells and whistles can sometimes produce ozone, so it's best to keep it simple. To calculate what size you need for your space, use this tool that Allen helped develop for classrooms — it works equally well for homes, he says. Prices are in the $250 range.

How to use it

Again, this will depend on your individual situation. In general, place the air purifier near the person whose germs you want to avoid (the bedroom of someone sick with COVID-19, for example). When Corsi hosted a relative who works in care homes, he and his wife ran a portable air cleaner in their guest room and downstairs — in addition to wearing masks and opening windows. They added a fan — blowing air outward — to their guest room.

Bottom line

The only downside to a personal air cleaner is the cost (in addition to the upfront cost, there's the cost of replacement filters and energy to consider). When you're considering the purchase, think of it as a health care tool, Allen suggests. And remember that personal air cleaners also remove chemicals, allergens and dust.

For Karan, even without randomized, controlled trials to prove that portable air cleaners help prevent COVID-19, it's a simple calculation for most people who aren't completely isolated.

"At this point, the possible benefits outweigh the cost," he says. "If later the studies prove air purifiers work and it may have had an impact, you don't want to think, 'I could have gotten it.' "

Sheila Mulrooney Eldred is a freelance health journalist in Minneapolis. She's written about COVID-19 forMedscape,Mpls.St.Paul Magazine,Science News for StudentsandTheWashington Post. More atsheilaeldred.pressfolios.com. On Twitter: @milepostmedia

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