COVID-19 continues to concern people all over the world — and especially in the United States, where the death toll recently surpassed 200,000. At the same time, wildfires ravage the west coast of the United States, displacing hundreds of thousands. As we adapt to new realities, questions are natural. What kinds of masks should we wear? Is it safe to breathe the air in the Bay Area? Could air purifiers be a solution for any of these problems?

To answer that last question, we consulted air quality experts at top institutions around the United States. We asked whether air purifiers can solve — or at least mitigate — some of our air quality problems, from coronavirus virions floating in aerosol droplets around our homes to wildfire smoke and smog.

These discussions, along with our own testing of a dozen of the leading air cleaners on the market and further reading on the topic, we arrived at a few answers.

If I want an air purifier, how do I find the right one?

For those of you who know you want to buy an air cleaner, I’ve already written an extensive article with concrete recommendations. There are plenty of purifiers on the market, and some of them are impressively effective given their reasonable price tags.

For those still on the fence, keep reading.

Do air purifiers really work?

This is one of the most common questions readers ask, and it’s also a reminder of why close reading and skepticism are such useful tools when you’re researching products as a consumer. Air purifier developers are not allowed to advertise their devices as health products in the United States for a few reasons — most fundamentally because their benefits aren’t straightforward. Instead of claiming incredible health outcomes, then, purifier advertisements usually focus on the number of harmful substances in the air and the effectiveness with which their devices filter them out.

To answer the question in the most basic terms: yes, air purifiers generally filter particulate out of the air effectively — especially if they use a HEPA filter (more on those in the next section). But most of us already have a mechanism to filter the air effectively: the respiratory system. As microbiologist and Vice President of Scientific Communications at the American Council on Science and Health Dr. Alex Berezow pointed out in a recent blog post, “Living within the tiny air sacs in your lungs (called alveoli) are immune cells known as macrophages. These “big eaters” gobble up bacteria, viruses, fungi, and whatever other debris happens to find its way into the lungs.”

In short, air purifiers work, but unless you live in a particularly polluted environment or you or your children are immuno-compromised, you probably don’t need one.

Do they protect against COVID, wildfire smoke or other seasonal pollutants?

High-efficiency particulate air (or HEPA) filters are standardized products that must remove 99.97% of particles in the air that are 0.3 micrometer in size (a particularly difficult size to capture). HEPA filters are usually more effective with particles larger and smaller than that size. Pollen, smoke particulate and aerosol droplets that can transmit COVID can all be filtered out of the air with such a filter.

A colleague’s mother recently began using Coway’s air purifier, for instance, and immediately noted the improved air quality in her smokey San Francisco home. Likewise, one of our editors testing the Dyson TP04 air purifier during the recent Saharan Dust Cloud saw the air cleaner do its job admirably: “Gradually, the app’s line graphs for each kind of pollutant began to fall. After an hour or two, everything was back in green territory.”

But the story is a little more complicated when it comes to COVID. Put simply, don’t count on air purifiers to protect you from virus particles if you’re cohabitating with a contagious person. When I talked on the phone with Dr. Richard Shaughnessy, the director of Indoor Air Research University of Tulsa, he said transmission of COVID usually happens due to close contact with an infected person. If you’re sitting on a couch and chatting with someone who is infected, an air purifier across the room isn’t going to remove all the harmful particles exhaled before they have a chance to reach you.

An additional problem is the difference between capturing and killing virus particles. While HEPA filters will capture the particles, other technology, such as UV tech, will kill virions. Unfortunately, such technology often comes with its own risks.

I’ve heard about ozone coming from air purifiers. Should I be worried?

Ozone is a type of pollutant that a narrow set of air purifiers has been found to emit in the past. Before we dive into that, it’s helpful to understand the basic types of air purifiers on the market now.

The three most popular filtration methods air purifiers use to clean the air are these: HEPA devices remove particles by ushering air through a specially designed and standardized filter; activated carbon filters remove odors and gaseous pollutants by running air across “sorbent media,” which traps it; and finally, ionic purifiers produce ions that attach themselves to particles.

Ionic purifiers work in a couple of ways. Some simply let ionized particles attach to surfaces around the house (thereby “removing” them from the air). Others have a plate that collects those ionized particles and needs frequent cleaning. The latter are the devices that have in the past had problems with producing ozone. Luckily, standards have risen over recent years and third-party firms now test ionic air purifiers to make sure they’re not releasing significant ozone into the home.

Generally, I would avoid ionic air purifiers simply because they’re not the most effective for the price. If you really want one, check to make sure it has a certification from Underwriters Laboratories or the California EPA stating that it does not emit ozone.

Who definitely would benefit from an air purifier?

The research here is a little complicated. Without getting too far into the weeds, one of the clearest demographics that benefits from HEPA-filter air cleaners is children with asthma. Dr. Elizabeth Matsui, a professor of population health and pediatrics at the University of Austin’s Dell Medical School, has researched the use of air purifiers in the homes of asthmatic children and told me about the value of air cleaners in such households.

Air purifiers, she cautioned, are not a replacement for what she calls “proximal source interventions.” For instance, a HEPA air cleaner can reduce particulate matter in the home of a smoker and child with asthma by 25%-50%. But that’s not the best solution: ideally, the person should stop smoking in the house altogether. A clean and well-ventilated environment — and of course proper medical care — is far more important than an expensive air cleaner.

And to be clear, while air purifiers can help mitigate symptoms of childhood asthma, Dr. Matsui says, “There’s not good evidence that we can currently modify the environment in a way that reduces rates of asthma, whether that’s by air purifiers or any other means.” In other words, air purifiers are helpful devices for children who suffer from asthma, but they won’t reduce the chances of a child developing asthma in the first place.

If you have any other questions I haven’t answered above, make sure to ask them in the comments, and I’ll be happy to update the article with answers.