Can air purifiers really reduce your COVID risk?

When indoor air quality expert Jeffrey Siegel has a few select family members over for dinner these days there are a couple of extra guests in the room.

He adds a few strategically placed air purifiers near the dining table, one next to those who are at the highest risk of being infected with COVID—for example, kids, since they’re unvaccinated or partially vaccinated and attend school—and one near any immunocompromised family members in attendance.

“It’s one of several risk mitigation measures we use,” says Siegel, who holds joint appointments at the University of Toronto’s Dalla Lana School of Public Health and the Department of Physical and Environmental Sciences.

How an air purifier can help with COVID

At the beginning of the pandemic, we heard a lot about how it was important to wash our hands and surfaces to reduce the risk of transmission, but these days the focus has shifted to the air, with public health officials recommending gathering outdoors when possible, wearing a mask and making sure there’s adequate ventilation—that is, the flow of air in and out of a space—indoors.

An air purifier, while not a replacement for these measures, can also help by filtering viruses, like SARS-CoV-2 from the air, says Steven Rogak, a professor in mechanical engineering at the University of British Columbia who is studying the effect of aerosol and droplet control measures for COVID in indoor spaces.

While the virus itself is super tiny, “it would always be associated with small droplets and aerosols that are emitted as people talk and sing and shout and those droplets don’t get down to below a micron,” he explains. These droplets and aerosols can be captured by the unit and filtered out.

What kind of air purifier can help with COVID?

There are a number of different air purifiers and air cleaners on the market, but to help with COVID-19 they need to have a filter. Many people are familiar with something called a High Efficiency Particulate Air filter (HEPA), which can remove 99.97 percent of dust, pollen, mould, bacteria and any airborne particles with a size of 0.3 microns. While this is effective against COVID, units with other types of high quality filters could work as well, says Siegel.

“What matters is something called the clean air delivery rate, or CADR,” he explains, which is a measurement of how much clean air a filter produces, often denoted in cubic metres per hour.

The CADR takes into account both the filter efficiency and the air flow of the machine. So you could have a unit that has a lower filter efficiency than a HEPA but a higher flow rate and get a comparable CADR.

“For a smallish room, you want [a] of 200 or bigger. And by the time you get to a big room, you want something that’s maybe more like 400 or 500.” You can find the room size a filter is designed to treat on the box or packaging for the purifier.

There are also air purifiers on the market that use technologies other than filters—these include things like ionizers, plasma air cleaners and hydroxyl radical air cleaners. “That stuff should be considered kind of unproven and avoided,” says Siegel.

You’ll also see air purifiers that use UV technology, and while this can be useful in some settings, like hospitals, “to my knowledge, no one has made a portable UV unit that actually makes a difference,” says Siegel. So when you’re shopping for an air purifier, you can consider UV technology something to avoid as well.

How to use an air purifier to reduce COVID risks

Both Siegel and Rogak stress that an air purifier is a complement to other measures to reduce transmission, like masking, gathering outdoors, staying home when you’re sick and not crowding into rooms.

“The short answer is yes, absolutely. It can help. However, it depends on how it’s used,” says Siegel.

For example, if you’re thinking of adding an air purifier with a filter for your regular household use, this can help with general air quality, but probably won’t do too much to reduce your risk of COVID on a daily basis, says Siegel. “You have a lot of close contact with the other people in your house. The chance of that filter changing that risk that much isn’t that great,” he explains.

However, if you’re having someone from outside your household over, whether that’s an appliance repair person, or a family member over, running an air purifier with a filter in the room where you are gathering can help reduce transmission if someone happens to be infected.

When it comes to a scenario where a family member IS infected with COVID, you could also run an air purifier in the room they are isolating in. “Hopefully people will be masked up most of the time, but even the masks are not perfect and so it wouldn’t hurt,” says Rogak.

What else can air purifiers do?

You might be considering an air purifier for COVID-19, but they can also be useful for filtering out allergens, pollution, and wildfire smoke.

“For kids with asthma or someone with allergies, or an elderly person with respiratory issues, filters have been shown to have enormous benefit in all of those cases,” says Siegel.

There’s a wide range of prices for an air purifier, and the bigger the room you are trying to clean the air in, the more expensive it will be. However, you should be able to get a unit with a good CADR for under $400, says Siegel. More expensive units often come with bells and whistles that don’t necessarily filter the air any better.

Investing in an air purifier won’t guarantee you won’t get COVID, but it won’t hurt, either. “It’s interesting how late in a pandemic that people are starting to look into this,” notes Rogak. “In the first few months, people in aerosol science were saying well, this is clearly something you should be considering. At the same time, we had all that misleading information about the virus being not airborne. Now, finally, we’re getting back to something that a lot of people were saying right at the beginning—that filtering the air is going to be helpful. It’s not the full solution, but it’s an extra layer,” says Rogak.