An air purifier can help protect you from coronavirus and smoke pollution. How to pick and use one.


As California parents with children too young to be vaccinated, both of us have worried about sending them to school safely – especially now that the delta variant of the coronavirus, almost twice as contagious as the original strain, accounts for almost 99% of new cases. We know the coronavirus is airborne, carried by tiny aerosols that can collect indoors and linger for hours. That means that good air ventilation and filtration are essential to help cut down on the spread of the virus.

But when one of us, Devabhaktuni Srikrishna, looked into how his children’s school district planned to keep students safe when classes started in the fall, he was dismayed to learn that its ventilation plan hinged entirely on keeping windows open. With this fall’s wildfires on track to be the worst on record, he knew access to fresh, outdoor air wouldn’t always be an option.

Furthermore, air pollution, particularly from wildfires, can cause a host of issues – including asthma, cardiovascular events, cognitive impairment and cancer. The last – according to John Balmes, a physician and member of the California Air Resources Board – is due to the chemical compounds that make up wildfire smoke. “Some of the same PAHs [polycyclic aromatic hydrocarbons] that are carcinogenic in tobacco smoke are found in biomass smoke,” Balmes says. “It’s like tobacco smoke but without the nicotine.”

A recent study found that communities affected by wildfires have worse covid-19 outcomes – in terms of number of cases and resulting deaths. And kids are especially susceptible to smoke “because they take in more air relative to their size and get a higher dose of whatever pollutant is in the air,” says Stephanie Holm, an environmental pediatrician in San Francisco.

After Srikrishna raised his concerns with parents at his children’s San Francisco school last spring, they were able to raise funds for portable, plug-in solutions that would address air pollution from both wildfires and covid-19: free-standing HEPA air purifiers, designed to trap up to 99.7% of 0.3 micron particulate matter as ambient air is pushed through them. (Building on such parent-funded efforts, the district plans to spend $2.9 million to install 3,700 purifiers in San Francisco classrooms over the next several months.) But they also learned that for a purifier to be effective, you must know what size to purchase and how best to operate it. Here is some expert advice about air purifiers that you can use to breathe easier in your home, office or school.

– Stick with HEPA

Although there are also electronic filters, which ionize particles that are then trapped on plates, or UV light filters, Holm advises a simple approach: “a portable air cleaner that uses a mechanical filter.”

A mechanical air cleaner is easy to operate and doesn’t produce pollutants. At its most basic, it uses a fan to push air through a HEPA filter. Some fancier models include pre-filters that sit in front of the HEPA filter, preventing larger particles from reaching and prematurely clogging up the finer filter. This strategy extends the lifetime of the HEPA filter, a money-saving proposition given its often-steep replacement cost; filter prices typically range from about $30 to $150.

Celia Sorensen, an emergency medicine physician in Denver, also recommends purifiers that pair a HEPA filter with one or more carbon filters. Carbon “filters out toxic gases like sulfur dioxide or nitrogen dioxide, which often co-occur with particulate matter.” Purifiers typically range from $200 for basic models to over $1,000 for ones with additional features. Sorensen purchased a purifier from Austin Air Systems for her family, which includes carbon filters and sits on the pricier end of the scale, while Srikrishna and his children’s school have purifiers from Coway, which offers devices priced between $190 and $650.

– Fit the purifier to the space

One of the most common mistakes people commit is choosing a purifier that’s not powerful enough for the space it’s cleaning. “When you’re purchasing a HEPA air filter, you really need to look at the size room it’s rated for,” Sorensen says. “And it depends on the size of the fan and its speed in terms of how much air can circulate.”

The key metric to consider is CADR, or clean air delivery rate, an indication of how quickly a purifier can clean a volume of air, taking into account air flow and filter efficiency. For example, a purifier with a CADR of 200 cfm (cubic feet per minute) can clean 12,000 cubic feet per hour. For a 500-square-foot room with eight-foot ceilings (4,000 cubic feet), that translates to three complete room air exchanges per hour. According to Sorensen, the Environmental Protection Agency recommends five exchanges per hour, but this target can change depending on whether the room in question is a bedroom, an auditorium or a classroom.

A quick way to determine the room size a purifier is appropriate for is to multiply the CADR by 1.5 (this calculation assumes the room has eight-foot ceilings and that you want five air exchanges per hour). Using this method, a purifier with a CADR of 200 cfm would be suitable for a 300-square-foot room. Or use one of several calculators on the market: Simply enter a room’s dimensions (length, width, height) and the calculator provides the size air purifier you’ll need for that space at the desired room air exchange rate.

– Set it up correctly

Make sure the purifier is not obstructed or covered so air can flow freely in and out. And don’t forget to read the manual carefully to ensure you follow steps such as removing the plastic packaging from replaceable filters.

Purifiers come with automatic or “smart” settings, which adjust the fan speed based on the presence of particulate matter in the air. But those built-in sensors are not typically triggered by the respiratory aerosols that contain viruses. This might be OK if you’re concerned only about smoke, but if your goal is to mitigate covid-19 risk, you should adjust the settings. Running the purifier at maximum fan speed is the best practice for shared spaces (such as classrooms and offices) where viral transmission is a concern.

If your area is affected by wildfire pollution, it’s also useful to have backup filters on hand in case you need to change them more frequently than anticipated.

– Put purifiers where people are

Outfitting an entire house with air purifiers can be a daunting (and expensive) task. Start by putting air purifiers where people spend the most time. “If you spend your days in the kitchen, put it in the kitchen,” Sorensen says. “And then, when you’re going to bed at night, just bring it into your bedroom.” By moving one or two units around the house, you can get by without purchasing a bunch of expensive purifiers all at once.

Holm agrees. “One way of maximizing what you’re spending is to make a clean space within your home,” she says. “If you can’t get your whole house to have clean air during a wildfire event, for example, you can pull a couple of sleeping bags into one or two rooms so that everyone is sleeping in a cleaner space.” And remember to close your windows and stay inside when outdoor pollution is bad. “There is some filtration just from the home,” says Balmes.

– A cheaper option

Although Holm recommends using a commercially available HEPA system whenever possible, a DIY option can be a cheap (approximately $30) and accessible alternative to a conventional air purifier in a pinch. Dan Jaffe, an Atmospheric Sciences professor at the University of Washington, recently investigated how effectively a standard box fan can be used to pull air in the room through an attached MERV-13 filter (Minimum Efficiency Rating Value-13) to clean it. His team found that such a setup removes approximately 50 to 90% of the particulate matter in the air (compared with 99.7% for most standard HEPA filters). Using multiple tools, researchers confirmed that “these filters were cleaning particles down to at least 0.3 microns in size,” or tiny enough to penetrate deep in the lungs. Due to potential fire risk, however, these DIY setups should never be left unattended and should use fans manufactured after 2012.

– Enhance with masks

Ranu Dhillon, a global health physician based in Vallejo, Calif., notes that purifiers might not be enough in all situations. Masking (especially with N95 masks) “will remain important in areas where you have significant transmission” and should be used in conjunction with purifiers, he said.

Dhillon, who has experience with Ebola in Guinea and the Congo, thinks air purification is “a relatively low-hanging opportunity that’s not gotten enough attention.” Holm, too, is hoping the issue will receive increased awareness. With the coronavirus and wildfires highlighting the importance of ventilation and filtration, she said, this moment offers a “unique opportunity to really capture folks’ attention and make changes that can have long-lasting effects.”

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Connie Chang is a writer based in Silicon Valley who covers topics including health and parenting. Find her on Twitter @changcon.

Devabhaktuni Srikrishna is an engineer and founder of, a resource for air safety. Find him on Twitter @sri_srikrishna



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