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Maybe you’ve got allergies. Maybe you’ve gotten one too many push notifications about the air quality in your area. Maybe you heard it can help prevent the spread of COVID-19. Whatever your reason, you’re considering getting an air purifier, but deep down, you can’t help but wonder: Do air purifiers work? They promise to filter out dust, pollen, smoke, even germs—but do they really deliver on that, or are they just overpriced fans? We pored over research and turned to Dr. Tania Elliott, an allergist and national spokesperson for the American College of Allergy, Asthma and Immunology.

RELATED: 6 Ways to Improve Your Air Quality (and 1 That’s a Waste of Time)

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First, What Do Air Purifiers *Actually* Filter Out?

Air purifiers (also known as air sanitizers or portable air cleaners) suck particles from the air, such as pollen, fungal spores, dust, pet dander, soot, bacteria and allergens.

OK, So How Do They Do That?

Essentially, these machines use a filter—or a combination of filters and UV light—to remove impurities and pollutants from the air. They’re designed to improve the air quality in a single room, and as the Environmental Protection Agency (EPA) notes, while they are effective at cleaning the air, they cannot remove all pollutants.

Air purifiers tend to do this in one of two ways: via fibrous media air filters or electronic air cleaners. The former is kind of like a catcher’s mitt, with the particles getting scooped up in the filter. The latter—electronic air cleaners, which includes electrostatic precipitators and ionizers—uses electricity to charge particles and adhere them to oppositely charged plates in the machine. Some even use ultraviolet light to kill airborne microorganisms. Now don’t you feel all Bill Nye for knowing that?

Do Air Purifiers *Really* Help People with Allergies?

Yes—and they can be particularly helpful for people who suffer from pollen or pet-related allergies. “Pet allergens stay suspended in the air for months at a time, even if the pet is no longer in the home,” explains Dr. Elliott. “Air purifiers that can capture fine particulate matter are your best bet. It is also helpful for people with pollen allergy, as we inevitably track pollen into the home from our clothes, shoes and hair.”

By “fine particulate matter,” she means dust, pollen, mold and the like. Particles considered “fine” are less than 10 microns in diameter (ultrafine ones, such as soot, smog and viruses, are less than 2.5). For comparison, a human hair is about 50 to 70 microns in diameter. So we’re talking small—really, really small.

Many HEPA filters and air purifiers tout being able to remove particles 0.3 microns in diameter; keep an eye out for those if you’re looking for a model that can help remove viruses from the air. (The EPA recommends models that remove particles less than 1 micron in diameter, so we rounded up four top-reviewed ones that all meet that criteria below.)

Cool, But What About Dust Mite Allergies?

Bad news: Air purifiers “won't work for people with dust mite allergies, as dust mites are too large of a particle to remain airborne,” Dr. Elliott says. For that type of allergy, your best bet is to vacuum, dust and wash your bedding regularly, and invest in allergen-proof bed covers.

Will an Air Purifier Protect Me Against COVID-19 and Other Illnesses?

The EPA and many doctors agree that air purifiers are helpful—especially if the outdoor pollution is high, or if it’s too cold to throw open your windows and let in tons of fresh air—

“Viral droplets, like SarsCoV2 and the flu, these can stay suspended in the air for hours, so an air filter can't hurt, but remember the droplets can also land on surfaces and sit there as well,” explains Dr. Elliott. “An air purifier shouldn't replace mask wearing, hand washing, isolation, not sharing personal products and sanitizing measures.”

As the CDC says, consider ventilation part of a “layered strategy” to preventing the spread of the coronavirus.

What’s the Right Size Air Purifier for My Home?

“Be sure to get one that matches the size of the room by checking the Clean Air Delivery Rate (CADR),” says Dr. Elliott. It’s a number you’ll find on most air purifiers’ packaging—or at least any company that voluntarily submits their machine to the Association of Home Appliance Manufacturers to have its CADR levels tested. There’s one CADR score for pollen, one for dust and one for smoke, and the association recommends choosing a purifier with a CADR score that’s at least two-thirds of the room’s area. Huh?

That may sound complicated, but it’s basic math: If you’re clearing the air in a 10-foot by 10-foot room, that’s 100 square feet, so you want a CADR score of at least 67 in each of those three categories.

What’s the Best Place to Put an Air Purifier?

Let’s be real: Air purifiers aren’t the chicest-looking additions to your décor, so it’s tempting to tuck them behind a plant or large piece of furniture. Don’t. You want to keep them in the room where you spend the most time—ideally, the room where the most vulnerable in your family (infants, elders and people with asthma) spend the most time—and in a position so that the clean air is close enough so they’ll be able to breathe it in, according to the EPA. Beyond that, it’s also worth consulting the manufacturer’s instructions for placement.

How Long Does an Air Purifier Take to Clear the Air in a Room?

Give it at least 30 minutes to an hour, but some companies recommend running it all day, every day, since pollutants are continually being tracked into the house and are wafting through open windows. (Of course, it’s worth noting the impact that doing so may have on your electricity costs.)

Are There Any Types of Air Purifiers I Should Avoid?

Yes. Stay away from ozone-generating air cleaners. As the name implies, they produce ozone, which can cause health issues in high concentrations, and the EPA reports that ozone does little to actually remove pollutants. On that note, it’s worth mentioning that no federal government agency has approved of their use in homes (though some brands may claim that). You’re better off going with an air purifier that uses a fibrous media air filter or an electric air cleaner.

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