Tip: Some whole-house systems have a “fan mode” that circulates air through the filters without heating or cooling, allowing you to clean the air for any reason all the time.
Level up: Buy a portable air cleaner

You’ll see them marketed as “air purifiers.” Scientists prefer the term “air cleaners.” They work, but they’re not purely perfect.

Very good ones can be found for around $150 for small rooms and $200 to $250 for larger spaces.

You might hear they work only in small rooms, and must be properly sized for the room. That’s partly true. But Corsi, who has tested them in his laboratory and uses them at home, says even in a large living area, a portable air cleaner might knock the concentration of the coronavirus down by 50%, which is, again, a helpful layer of protection. And meanwhile, everyone is breathing fewer allergens and other pollutants.

If someone in your home has Covid-19, a portable air cleaner in their room is highly recommended, though if you can create a negative vacuum, per above, the air cleaner may be less necessary, he agrees. You could also use an air cleaner to protect others from infection. If a sick person is properly isolated, but they have to come out now and then to use the bathroom or get a late-night snack or whatever, “using a portable air cleaner in other parts of the house can really help,” Corsi says.

Tip: Weather and temperature permitting, opening windows while running an air purifier and/or a whole-house system with high-grade filters can offer multiple layers of protection.

Biswas says even in a large living area, having an effective portable air cleaner very nearby will be particularly effective at cleaning the air being breathed by you or even several people sitting together.

Make sure the air cleaner you pick has a HEPA filter. The units will typically have a carbon filter, in front of the HEPA filter, to capture large particles and pet hair and neutralize odors.

A top-pick HEPA air cleaner tested by Wirecutter reduced particulate pollution by 70% in one hour in a 10,000-cubic-foot conference room — more than twice as much space as it was rated for. It was much more efficient and effective in a 135-square-foot room, of course. The unit costs $200, and Wirecutter estimates you’ll spend another $270 on filters and electricity over five years. Wired suggests two models at different price points, one for small rooms and one for larger spaces.

For a space with 1,000 square feet or less and a typical home ceiling height, Corsi suggests looking for a Clean Air Delivery Rate (CADR) of 300 cubic feet per minute or higher. He just ordered 12 HEPA units online, in the $220 price range, for rooms in the Portland State engineering department. Corsi knows his choice will do the job, but he prefers to maintain objectivity as an expert and not to advocate for the particular model, but now you know his thought process. (On Twitter, he offers a deep-dive into the math for sizing a unit to a room.) There are other, less expensive options for small rooms and some that are said to be quieter than others.

Shelly Miller, PhD, an environmental engineer at the University of Colorado Boulder who researches indoor air pollution, has never owned a portable air cleaner. Until now. Miller bought one to prevent aerosol transmission should anyone in her home catch Covid-19. She recently asked a grad student to create this detailed guide to the products, including specific recommendations for units geared for different room sizes, all of which she and her colleagues reviewed.

Tip: Weather and temperature permitting, opening windows while running an air purifier and/or a whole-house system with high-grade filters can offer multiple layers of protection, and you can also use different tactics in different rooms separated by closed doors, depending on who might be at greatest risk for allergies, asthma, or Covid-19. You may choose to balance that with decisions about overall energy use, of course.
Make your own air cleaner

It turns out a certain, common, boxy room fan, which costs between $20 to $40 online and in big-box stores, is about the same size as a 20×20-inch AC filter. Though far from a perfect solution, you can buy a high-grade filter (see above) and simply tape it to such a fan.

The idea isn’t new. This nine-year-old video offers an analysis of how effective it can be.

I was dubious of this hack, especially given the seriousness of the coronavirus, so I asked Biswas if he would recommend it to people who can’t afford air purifiers. “Absolutely,” he says. It will leak, “but it’s better than not doing anything.” Think of it as an imperfect face mask, he suggests: The best one will stop most or all respiratory particles, but even a homemade cloth mask will capture some.

In fact, the hack has grown more popular of late. As I was finishing the draft of this article, Jim Rosenthal, CEO of Tex-Air Filters, posted the results of a test he just conducted. In an employee break room, the box fan with a MERV 13 filter made “meaningful reductions,” removing 74.6% of the one-micron-sized particles and 80% of the five-micron-sized particles — thought to be roughly the upper threshold for the coronavirus-packing aerosols that stay aloft.

“The ‘Box Fan with a MERV 13 Filter’ should be a useful tool in the fight against Covid-19,” Rosenthal says. He also notes, however, that the one-inch-thick filter strains the fan motor. A four-inch-thick filter, sold for commercial uses, catches small particles as effectively but allows air to pass through more easily, increasing overall efficiency and preserving the life of the fan.

Tip: Tape the filter to the intake side of the fan, so the air s**** it in tight rather than trying to blow it off.
A final thought

Early on in the pandemic, at the height of my spring allergies, I switched our AC filters to the expensive kind. I did not expect what happened. Within days, my years-long battle with daily sniffing, sneezing, and watery eyes cleared up almost entirely, and I stopped taking the over-the-counter allergy meds that weren’t working very well anyway.

Anecdotes are not science, and your mileage may vary. But those filters have almost surely improved my health and well-being, not to mention peace of mind.

And then, given what I learned researching this story, I ran out and purchased two portable, $150 air cleaners. They’re small, designed for 200-square-foot rooms. One is for my home office, the other is for our son, who just headed back to college and a cramped apartment with three other guys and, yeah, a whole lot of uncertainties.
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