There are several technologies air purifiers employ for tackling indoor pollution. Some work better than others. Some can actually be bad for your health.

Mechanical filters. This is the type we test. Air purifiers with pleated filters use fans to force air through a dense web of fine fibers that traps particles. Filters with very fine mesh are HEPA filters—those certified to collect 99.97 percent of particles of a certain size (0.3 microns 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 replacing every six to 12 months; they can cost up to $200 per filter but typically max out at $80.

Activated carbon filters. Rather than catch particles like mechanical filters, sorbent filters use activated carbon that can adsorb 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 they don’t combat particles, many air purifiers will include 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. And be prepared: Activated carbon filters cost up to $50 each.

CR does not currently test for odor removal, but we did conduct a one-off test in 2008 with five air purifiers that came with odor-removing claims. For this test, we assessed how well these machines removed cooking odors. The results: Only two devices were able to do so because they used very large and thick carbon filters.

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 them. 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. Studies also show that ozone has been linked to decreases in lung function and increased risks of throat irritation, coughing, chest pain, and lung tissue inflammation. Ozone might also worsen asthma, emphysema, and bronchitis, according to the EPA.

Electronic air purifiers. Electrostatic precipitators and ionizers charge particles in the air, so they stick to plates on the machine or to nearby surfaces by a magnetic-like attraction. CR doesn’t typically test them 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 portable 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 sometimes 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 volatile organic compounds (VOCs) typically found in indoor air.

A variant of PCO known as PECO emerged more recently from the manufacturer Molekule. We tested the air purifier, and it did not fare well in our tests for dust, smoke, and pollen removal.