Yes. But, but the devil is in the details

Thomas Smith

The answer is “Yes.” The EPA, FDA, and CDC all agree that air purifiers can capture Covid-19 and potentially reduce the risk of indoor virus transmission.

As with anything pandemic-related, though, the devil is in the details. To effectively combat Covid-19 with an air purifier, it’s essential to choose the right type of purifier, install and use it properly, and understand its limitations. Sometimes you may need to buy a $500 FDA-cleared purifier to effectively protect a room. Other times you’re better off opening a window. Here’s a look at what we know about air purifiers and Covid-19, how to choose the right one to effectively reduce Covid-19 risk, and what to avoid.

First, it’s important to understand how air purifiers fight Covid-19. Covid-19 virions themselves are tiny–about 0.1 microns. Some purifiers can remove particles that small. But they don’t necessarily have to. An increasingly strong body of evidence suggests that Covid-19 spreads through the air by hitching a ride on aerosols–the particles which infected people cough or sneeze out.

Sneeze particles (let’s call them what they are) are about 1 to 5 microns in size–orders of magnitude bigger than Covid-19 virions. They’re still tiny, though–the filter in your home HVAC unit might catch some of them, but most HVAC filters are meant to remove particles 5–10 microns in size, so they’re unlikely to remove enough particles to effectively fight the virus on their own.

HEPA filters, on the other hand, are designed to remove 99.97% of particles which are 0.3 microns in size. That means they’re more than capable of filtering out the 1-5 micron aerosol particles on which Covid-19 travels. As Wirecutter points out, too, HEPA filters are tested using 0.3-micron particles, but they’re actually effective on particles down to 0.01 microns. That means that HEPA filters provide a double whammy–they can likely remove both the 1–5 micron floating aerosols thought to carry the virus, and the 0.1-micron Covid-19 virions themselves.

The CDC agrees that HEPA filters are effective at removing Covid-19 from the air. In their words, “most of the respiratory droplets and particles exhaled during talking, singing, breathing, and coughing are less than 5 µm [5 microns] in size”. HEPA filters are designed to remove particles in this size range. Therefore, “HEPA filters are no less than 99.97% efficient at capturing human-generated viral particles associated with SARS-CoV-2.”

Some specialized HEPA filters are potentially even better. Sold as H13 or “medical grade” filters, these extra-fine filters are rated based on their ability to remove even smaller particles. They may provide a bit of extra protection versus standard HEPA filters. But again, the CDC says that any true HEPA filter is effective at removing Covid-19 from the air. If you’re shopping for a Covid-19 fighting air purifier, make sure it uses a HEPA filter.

Once an air purifier catches Covid-19 particles in its filter, what happens next? Some purifiers use UV light to actively kill the virus, a practice which the FDA sanctions. Other filters just hold the particles in place until the virus breaks down on its own, or until you remove the filter and discard it. That means it’s essential to change your purifier’s filter regularly, so it doesn’t get saturated with particles and lose its filtering effectiveness.

There are other considerations to keep in mind, too. Your purifier needs to be sized correctly for the room you’re using it in, and it needs to be powerful enough to filter all the air in your target room frequently. HEPA purifiers are assigned a CADR (Clear Air Delivery Rate) score, which measures how much air they’re able to move. According to the CDC, in selecting a purifier you should look both at the manufacturer’s rated room size and the CADR. First, look for purifiers whose rated room size matches the square footage of your target room. If you have high ceilings (more than 8 feet), the agency provides a formula that allows you to adjust the rated room size accordingly.

Then, buy a purifier with as high a smoke CADR as possible. (Purifiers are rated based on three types of pollutants, and smoke is closest in size to Covid-19 particles). In the CDC’s words, “In a given room, the larger the CADR, the faster [the purifier] will clean the room air”. A high-CADR purifier might change all the air in your room a few times an hour, whereas a lower CADR-rated unit might take only change the air once per hour.

Remember, too, the CADR figures usually assume the purifier is running at full blast. You probably won’t run your purifier that fast all the time, since purifiers running at full throttle can be loud. A higher CADR gives you some buffer room if you want to run at a slower speed. Don’t obsess too much here, though — the CDC acknowledges that even a too-small purifier will “still provide incrementally more air cleaning than having no air cleaner at all.”

Which purifier brands give you the best shot at fighting Covid-19? The CDC says that their agency “does not provide recommendations for, or against, any manufacturer or product”, but the FDA does not share their reservations. According to the Washington Post, in early May 2021, the FDA officially cleared two consumer air purifier brands for medical use, certifying their ability to remove Covid-19 from the air. Those brands were Molekule and Brondell, both based in San Francisco.

To get a coveted FDA certification, the brands had to prove their purifiers could actually remove viruses by testing them in special sealed chambers against real but harmless airborne viruses standing in for Covid-19 (or in Brondell’s case, against “live cultured COVID-19 aerosolized virus by US based lab MRIGlobal who does testing for the US Department of Defense” according to an email from the company). That kind of testing is expensive, and the Post notes that both brands charge far more than comparable, non-cleared brands for purifiers with similar capabilities.

With Molekule’s purifier, in particular, it’s also important to note that just because the FDA certifies that the device does a good job against viruses, that doesn’t mean it’s good at removing other small particulates. In fact, the FDA’s letter to Molekule certifies that the company’s devices destroy viruses, but also that they remove only 95% of other particulates (a lower value than a HEPA filter). Other testing from publications including Wirecutter found that Molekule’s devices struggle against non-virus fine particulates, and OneZero called the company out for advertising during California’s destructive wildfires. (Molekule disputes Wirecutter’s findings, calling their actions “appalling”).

It’s also important to remember that even if a purifier isn’t FDA certified for medical use, it’s likely still totally capable of removing Covid-19 as long as it meets the CDC’s recommendations. A certification says “This specific purifier definitely removes viruses like Covid-19”, versus the FDA’s general guidance, which basically says “Purifiers with these kinds of specifications remove viruses like Covid-19”. If your purifier has a HEPA filter and is sized properly to your room, it’s probably doing a fine job of fighting the virus.

If you want absolute assurance that you’ve chosen a purifier that kills Covid-19 (and you have the budget to do so), go with an FDA-certified brand like Brondell or Molekule. For less expensive options, I like purifiers from Medify Air, which use H13 filters and claim to remove “virus carriers.” I’ve also tested purifiers from Aeris, which aren’t cheap either but claim to remove “particles at 0.1 microns, including coronavirus.” Okaysou makes a super cheap H13 purifier with a 1,000 square foot rated room size and a filter that claims to use silver ions to inhibit “various germs and viruses.” (Aeris and Okaysou provided me with purifiers to test.)

Before you drop thousands on purifiers, though, ask yourself if you really need them at all. The EPA agrees that properly-sized HEPA purifiers fight Covid-19. But according to its guidance, so do fans. Air purification is great, but the absolute best way to fight the virus in interior spaces is to bring in more air from outside.

The EPA has several practical suggestions for doing this on the cheap, including turning on all your bathroom vent fans “continuously,” opening windows and putting box or tower fans next to them, opening doors and windows at opposite sides of your house to allow for natural cross-ventilation, using (or installing) a whole-house fan, and ensuring that the air intake on your HVAC unit is open if it has one.

In many cases, these simple ventilation measures are likely far more effective and far cheaper than using a purifier. In some cases, though, they’re not practical. The EPA acknowledges that if outdoor pollution is high (such as during wildfires), outdoor air is too hot/cold/humid, you have kids who could fall out a window or be injured by a fan, or you suffer from allergies, you may need to keep your windows closed. In those cases — when natural ventilation isn’t possible — a purifier is the best choice.

Remember, though, that even the best HEPA purifier isn’t a silver bullet. In order for it to remove Covid-19 from the air, the purifier has to draw the infected air into its filter. If you’re standing in an interior room, maskless, and talking to another person in close-quarters, their exhalations will likely hit you in the face far before they have time to reach a purifier and be pulled out of the air. For that reason, the CDC, EPA, and FDA all say that purifiers are a good choice to reduce Covid-19 risk, but that other measures are just as important. The CDC advocates a “layered approach,” saying that “In addition to ventilation improvements, the layered approach includes physical distancing, wearing face masks, hand hygiene, and vaccination.”

If you’re planning to spend time indoors with other people this Summer, adding a HEPA purifier to your space (or opening some windows, if you’re able) is a great choice to help reduce Covid-19 risk. As a bonus, a purifier will help protect you against smoke, indoor air pollution, particles from cooking, and much else. But air purifiers alone will never replace other mitigation measures. Buy that fancy purifier if you’d like, but also remember to wear a mask when you’re told to do so, wash your hands, and — purifier or not — get the shot.