The jury is out on the health benefits of using an air purifier (often called “air cleaner”) to remove particles and gasses from your home.

A series of medical studies produced mixed results, according to Canada's National Collaborating Center for Environmental Health. For instance, people with allergies, asthma or other respiratory illnesses who used portable air cleaners outfitted with HEPA filters had fewer symptoms, such as wheezing and coughing, but used just as much medication as people who didn't use the cleaners.

Purifiers tend to work best at removing smaller particles like smoke, because they are relatively lightweight. Large particles, like pollen and dust, are less likely to remain in the air long enough to be collected. In some cases, a purifier can even make the air dirtier by stirring up already-settled dust, according to the Wisconsin Department of Health Services.

“Studies have looked and often cannot even show that particulate matter is reduced, much less a health benefit, in the real world rather than in a lab setting,” says Elliot Horner, PhD, lead scientist for UL Environment, a division of UL.

Horner points out that product testing is usually done in a laboratory. “The issue is that in the real world, there is often poor mixing of air. This means that much particulate matter (PM) in a room — in pockets of stagnant air — never gets pulled into the air cleaner. Also, even if the cleaner pulls in all PM in the air right now, if the sources of PM are not addressed, then PM levels very quickly will be right back up to where they were.”

“There is no silver bullet,” says Horner. He advises people to reduce the sources of any pollutants by keeping the home clean and well ventilated. “Nothing all that fancy, what your grandmother was doing.” For example, simply wipe or vacuum surfaces, since more allergens rest there than float in the air, according to the AAFA.

Related: Is Your Vacuum Cleaner Bad for Your Health?

If you’re still thinking about buying an air purifier, here are three things to consider.

1. The type of air purifier

Purifier designs fall into five main categories:

  • Mechanical: A fan forces air through a dense mesh, such as a HEPA (high efficiency particulate air) filter, which traps the particles. HEPA filters can trap most hazardous particles found in a home.
  • Electronic or ion generator: Electrical charges attract particles and either trap them in the machine or cause them to land on surfaces for later cleaning. Researchers found that ion generators produced no health benefits at all.
  • Hybrid: These use a combo of mechanical and electronic systems.
  • Gas phase or sorption: Removes gasses, such as the smell of new paint, but not particles like dust or hair.
  • Ozone generator: Produces ozone to supposedly “clean” the air.

2. How much ozone the purifier produces

Numerous authorities advise against using ozone generators, including the Environmental Protection Agency (EPA) and the Mayo Clinic. While all air cleaners produce some ozone as a byproduct, ozone generators deliberately fill the air with it in order to mask odors without actually cleaning the air.

Breathing ozone can cause lung damage and worsen existing respiratory illness, according to the EPA. Some models can produce 10 times more ozone than is considered safe, according to the Asthma and Allergy Foundation of America. “People and ozone should not share space.” says Horner.

Electronic purifiers also tend to produce more ozone than mechanical models, according to the California Environmental Protection Agency. Before purchasing one, ask for the manufacturer's data on ozone levels.

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3. What product ratings actually mean

Air purifiers are rated according to their ability to trap particles of various sizes. These rating systems have nothing to do with whether or not the unit will improve your health. In fact, there are no health-related rating standards for air cleaners because there isn't enough research to prove that cleaners improve your health, according to the AAFA.

Keeping that in mind, two rating systems are most commonly used: MERV and CADR. MERV, short for Minimum Efficiency Reporting Value, rates “whole house” purifiers that are typically installed as part of your building-wide heating and air conditioning system. This chart shows how each MERV level performs against different types of particles.

For smaller, portable purifiers — typically used to clean a single room — look for the Clean Air Delivery Rate (CADR) label on the outside of the product's packaging. Developed by the Association of Home Appliance Manufacturers, it rates the unit's ability to remove tobacco smoke, pollen and dust.

As long as you avoid models that produce excessive amounts of ozone, there is no evidence that air purifiers are harmful. But if you are considering one for health reasons, consult with your doctor first, advises the AAFA.

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David Arv Bragi is a freelance journalist and marketing consultant. He has been writing about health and safety issues since the 1990s and currently lives in Portland, Oregon.