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Kind of long but here is what CR says-
What Air Purifiers Do Well
The air purifiers that perform well in CR’s tests prove in our labs to be good at filtering dust, smoke, and pollen from the air. Multiple studies of room air purifiers show that using HEPA filters results in reductions of 50 percent or higher of particulate matter. In one 2018 study of about 130 households, filtration resulted in about a 30 percent reduction of coarse particles, such as dust.

But how does that affect your health? Almost a dozen studies—including ones conducted in Vancouver, British Columbia; Taipei, Taiwan; and Massachusetts—looked at the cardiovascular effects of air purification and showed improved cardiovascular health among participants. An EPA review of eight studies (PDF) found that purified air delivered modest improvements in at least one health area, such as allergy symptoms. And asthmatic participants in a 2018 study by the University of California, Davis (PDF), reported a 20 percent reduction in clinic visits.

Still, there are caveats. The scientific and medical communities have not definitively linked the use of air purifiers to health benefits, because reported health benefits are inconsistent among participants and there have been very few long-term studies. Plus, some studies had other variables at play, such as the regular use of a vacuum cleaner (CR can help you choose one of those, too) or pillow covers, and the removal of pets from the bedroom, all of which can affect results.

As for the coronavirus, air purifiers with HEPA filters are capable of capturing the droplets that the virus travels in (when people cough, talk, or breathe). But you’ll need one that consistently draws in enough air to reduce virus particles.

What Air Purifiers Don’t Do
An air purifier can remove allergens only while they’re floating in the air. Larger, heavier allergens, such as mites, mold, and pollen, settle to the ground so quickly that air purifiers can’t capture them in time.

What We Don’t Yet Know
Radon is another blind spot for air purifiers. Studies are inconclusive on air purifiers’ ability to tackle this dangerous gas. There is insufficient research on air purifiers that address gaseous pollutants as a group, so it’s unclear how effective they are in that regard. There is also limited data on the effect of ionizer air purifiers on health.

That brings us to another important consideration: the various kinds of air purifier technology available.

A group of air purifiers in CR's test lab.
Air Purifier Ratings by Consumer Reports
Types of Air Purifiers
There are several technologies air purifiers employ for reducing indoor pollution. Some work better than others. And some can even be bad for your health.

Models with mechanical filters: This is the type that CR tests. Air purifiers with pleated filters use fans to force air through a dense web of fine fibers that trap particles. Filters with very fine mesh are HEPA filters—those certified to collect 99.97 percent of particles of a certain size (0.3 micrometers in diameter—smoke and paint pigments, for example). HEPA filters can remove larger particles, too, including dust, pollen, and some mold spores, while they’re suspended in the air. (Note that some filters labeled “HEPA-type” or “HEPA-like” have not been certified to meet the requirements of a true HEPA filter but may still perform adequately in our tests.)

As for limitations, mechanical filters don’t help with gases or odors. And they can be expensive to maintain. Mechanical filters need to be replaced every six to 12 months; they can cost up to $200 per filter but typically max out at $80.

Models with activated carbon filters: Rather than catch particles like mechanical filters, sorbent filters use activated carbon that can capture some odor-causing molecules from the air. They may also tackle some gases, but they’re not particularly effective against formaldehyde, ammonia, or nitrogen oxide. Because activated carbon filters don’t combat particles, many air purifiers will have both an activated carbon filter and a pleated filter for catching particles. Activated carbon gets saturated faster than a pleated filter, though, and requires replacement more frequently—every three months, as opposed to every six to 12 months for pleated filters. Make sure to budget for replacements accordingly: Activated carbon filters cost up to $50 each.

Ozone generators: These machines produce ozone, a molecule that can react with certain pollutants to alter their chemical composition. This can result in dangerous indoor air quality, and CR does not recommend these types of air purifiers. Makers of ozone generators often claim that the devices emit safe levels of ozone, but in the past, our tests found that even at low settings, some ozone generators quickly exceeded the Food and Drug Administration’s limit of 0.05 parts per million for medical devices. Plus, studies reviewed by the EPA have shown that low levels of ozone—the chief ingredient of smog—don’t effectively destroy indoor pollutants. Research also shows that ozone has been linked to decreases in lung function and increased risks of throat irritation, coughing, chest pain, and lung tissue inflammation. Ozone exposure might also worsen asthma, emphysema, and bronchitis.

Electronic air purifiers: Electrostatic precipitators and ionizers charge particles in the air so that they stick to plates on the machine or to nearby surfaces by a magnetic-like attraction. CR doesn’t typically test electronic air purifiers or recommend them, because they can produce ozone.

Ultraviolet germicidal irradiation (UVGI): Some manufacturers claim their air purifiers kill airborne viruses, bacteria, and fungal spores with UV lamps. But some bacteria and mold spores are resistant to UV radiation. To work, the UV light must be powerful enough and the exposure must last long enough—minutes to hours rather than the few seconds typical of most UVGI air purifiers—to be effective. CR does not test UVGI technology, though some mechanical air purifiers we test may have the function.

Photocatalytic oxidation: PCO uses ultraviolet radiation and a photocatalyst, such as titanium dioxide, to produce hydroxyl radicals that oxidize gaseous pollutants. Depending on the pollutant, this reaction can generate harmful byproducts, such as ozone, formaldehyde, nitrogen dioxide, and carbon monoxide. CR does not currently test PCO technology. There have been few field investigations done on the effectiveness of PCO air purifiers, but one laboratory study conducted by researchers at Syracuse University in New York reported that the devices did not effectively remove any of the VOCs typically found in indoor air.

A variant of PCO known as PECO emerged in 2017 from the manufacturer Molekule. We tested the Molekule Air purifier, and it did not score well in our tests for dust, smoke, and pollen removal. Since then we've also tested the more expensive Molekule Air Pro, which performed better at removing contaminants on its highest setting but did not impress in CR’s other air purifier tests.