The air in your home can be five times more polluted than outdoor air. Energy-efficient homes are designed to be more sealed, too, so they trap more pollutants inside. From cooking emissions and cleaning chemicals to fragrances and pets, a lot of everyday things can make the air inside your home less than ideal. The best thing you can do to improve your indoor air quality is open up windows and doors, as long as the air outside isn’t unhealthy. But if letting in fresh air isn’t an option, air purifiers can help reduce the level of pollutants, while also removing odors and trapping some bacteria and mold. There are hundreds of models from dozens of brands on the market, so we tested a handful and put together this guide to the best purifiers so you can decide which will work best for you.

What an air purifier can and can’t do

There are three key pollutants that adversely affect the quality of the air you breathe: volatile organic compounds (VOC), particulate matter (PM2.5 and PM10) and carbon dioxide. VOCs are emitted gasses that can come from cleaners, off-gassing plastics, paint, solvents, fragrances, cooking food and, ironically, air fresheners. VOCs are most closely related to odors you can smell. High levels can irritate your breathing passages, cause headaches and may become cancer-causing over time. Air purifiers with activated charcoal components can help clean VOCs from the air.

Particulate matter is usually discussed as PM2.5 and PM10, with the numbers indicating particle size in microns. This is dust, dirt, mold, smoke and, again, emissions from cooking food. Higher levels of PM can lead to respiratory irritation, allergy symptoms, respiratory infections and potentially lung cancer. Air purifiers that include a HEPA filter can help remove particulate matter from your space.

Carbon dioxide is what humans and pets breathe out. Elevated levels can cause dizziness and lethargy. But no air purifier can reduce CO2 levels because the molecules are so small. Plants can help to some extent, but really the only solution is opening a window and ventilating the space.

There are no federal standards for air purifiers, but the state of California does require all air cleaners sold in the state to be certified by the California Air Resources Board (CARB). All of our top picks here have met that certification.

What to look for in an air purifier

HEPA filters and other filter types

An air purifier isn’t an overly complicated device. Smart modes and app connectivity aside, they’re not much more than a filter and a fan. The latter pulls air through the former to capture particulate matter and other unhealthy elements so you don’t breathe them in. The type of fan can make some difference — it should be powerful enough to pull in air quickly, but also quiet enough on its low speed so it can unobtrusively clean all day long.

Filters, on the other hand, are more varied. Most have two or more layers, typically a pre-filter, an activated carbon component and sometimes a HEPA filter. The pre-filter is made from a fine mesh that captures big stuff like pet hair and larger chunks of dust. Sometimes this part is separate from the more technical filters — which means you can remove and clean it without needing to swap out the whole thing. For all-in-one filters, you can vacuum the outside of it to remove larger particles.

An activated carbon or activated charcoal layer is extremely porous, tightly packed coal that presents a vast amount of microscopic surface area to the passing air. Gaseous chemicals, VOCs and other molecules become lodged in the crannys and stick. This is the layer that gets rid of odors.

Some air purifiers include a HEPA (high-efficiency particulate air) filter. These are pleated masses of ultrafine fibers that force air to take a convoluted path in order to pass through. This traps and absorbs tiny molecules of smoke, dander, dust and some viruses and bacteria.

Since all of these filters physically trap particulates, they’ll eventually fill up and become less effective. Most manufacturers recommend replacing the filter every six months, while others claim a year-long life span. Most smart air purifiers will let you know in the app when it’s time to replace. When you’re considering a unit’s cost, be sure to factor in the expense of replacement filters, which you may end up buying twice a year.

Room size

Air purifiers list their cleaning capabilities in terms of room size and frequency of air exchanges. For example, a smaller one might say it can exchange the air in a 500-square-foot room twice per hour. So that model should be able to pass all of the air in a 250-square-foot room through the filter every 15 minutes, but a 1,000-square-foot room would probably be outside its effective range. Of course, there’s no standard for manufacturers to adhere to when it comes to these calculations, but typically, larger air purifiers can handle larger rooms.

Where you put the machine makes a difference, too. Since it requires airflow to be effective, somewhere close to the middle of the room and at least a foot away from furniture, walls and other potential blockages is ideal. That’s not always practical, so aim for getting it as close as you can to the center of your space while maintaining a one-foot clearance all around.

Controls

In our testing, we focused on “smart” air purifiers with companion apps that can monitor air quality and adjust the fan speed as needed. Within the apps, you can control auto-clean settings, set timers and schedules and check the health of the filter as well. Most will remind you when it’s time to get a replacement, and let you order one directly from the manufacturer through the app. You can also see the current and historical readings from the internal air quality (AQ) sensor. Most determine air quality through an optical particle meter, though some brands like Dyson and Molekule also include chemical sensors for VOC measurements. When levels of particulates become elevated, the fans switch up to high speed to move more air through until the quality improves.

Most smart purifiers also work with voice assistants, so if asking Alexa to turn on your air purifier makes your life easier in some way, you can do so. If you don’t want to talk to an AI or grab your phone to control your purifier, getting a unit with simple on-board controls is a good idea. These can be as basic as buttons with indicator lights or as elaborate as a touchscreen panel. At minimum, it’s good to have a way to control the fan speed and turn on or off auto mode on the device itself.

Design

As we mentioned, sticking the device as close to the middle of the room is helpful for getting the best performance. That means you’ll be looking at it a lot, so design considerations matter. Most purifiers are cylindrical towers with fan vents up top. Units meant for larger rooms are not small, weighing between 12 and 20 pounds and reaching two feet tall (or in the case of the Dyson Purifier Cool, three and a half feet). Some, like Coway’s Airmega IconS, take on more furniture-like designs to blend in. Others, like Dyson's, are conspicuously designed to stand out.

When an air purifier just isn’t enough

The most striking bit of knowledge I picked up from testing air purifiers is how effective opening windows can be on indoor air quality. What took an air purifier a half hour to clear out took mere minutes when I opened my front door and a few windows. Every variable measured by the air quality monitors, including VOCs, PM, and particularly CO2 levels (which air purifiers can’t alleviate), improved dramatically after exposure to fresh air — significantly faster and better than any machine we tested. Even on very cold or very hot days, it might be worth it, even if your doors and windows are only open for a few minutes. True, my HVAC system had to work a little overtime afterwards, but venting a room was the most surefire way of getting air quality quickly back in the green. Of course, if the air outside is unhealthy from fires or run-of-the-mill pollution, throwing open the windows won’t work and an air purifier might be the best way to consistently clean things up.

How we test air purifiers

My living room is not a science lab; there’s far too much pet hair for that to be the case. Still, I went beyond just turning stuff on and sniffing the air by acquiring two consumer-grade indoor air quality monitors that performed well in laboratory assessments, the Element from Element from Awair and the uHoo Smart Air Monitor. I conducted burn tests by measuring the ambient air quality, then burned a brick of piñon incense for twenty minutes and measured the air again. Then I ran one purifier at its highest speed for thirty minutes and recorded levels, then ran the unit on the lowest setting for a half hour and remeasured. I made note of the sound levels using a simple iPhone app to compare one machine’s fan noise to the next.

Over the course of a month, I used each unit in different scenarios (such as in the basements where the cat litter boxes are) and tried out each device’s smart features, controls and auto modes. I also just lived with them and evaluated how they fit into everyday life. As new purifiers come on the market and as we become aware of other units that seem worthy of inclusion, we’ll continue to test them and update this guide accordingly.

Other air purifiers we tested

Sensibo Pure

At $229, and nearly always on sale for $130, I had the Sensibo Pure pegged as a contender for a budget pick. Unfortunately, replacement filters are $99 unless you subscribe to automatic shipments and many of the app features are behind a paywall as well. It’s not certified by CARB and underperformed many of the other units in the burn test, though it did return the air back to a “good” rating according to the air monitors after 30 minutes. The design is inoffensive, it’s not overly loud and it does integrate with Sensibo’s smart AC devices, so if you’re already happy with one of those, this may be a decent option.

Dyson Purifier Cool

Like all Dyson products, this air purifier is dripping with design. It looks like no other unit on the market and it’s up to you to decide whether you like that or not. I was indifferent to the looks, but appreciated the slick and informative app, which not only displays indoor air quality, it also shows what conditions are like outside, using a clever house graphic to differentiate the two sets of numbers. I also like that it detects VOCs as well as particulates and the auto mode seemed to read the room accurately. The air coming out of the fan did indeed feel cool, though at first had a strong plastic odor. Unfortunately, it was the lowest performing unit during two separate burn tests and had repeated connectivity issues.

Molekule Air Pro

The Molekule Air Pro comes from a brand that pays keen attention to aesthetics. It and the app have that Instagrammable, muted-modern look that countless clothing and bedsheet brands emulate these days. That style doesn’t come cheap as the Air Pro costs over $1,000 and requires $140 filters. The company came under fire for and had to stop making many of its claims about its filtration abilities, which may have led to it filing for bankruptcy last year. Molekule is still able to tout its patented photoelectrochemical oxidation, which the company says destroys pollutants at a “molecular level.” In my tests, it performed almost as well as the others in improving VOC and PM2.5 levels. But it’s also very loud: When auto mode kicked the fan into high gear, it would make me tense. Also, I found the unit often indicated “bad” or “very bad” levels when my two monitors indicated the air quality was actually pretty good.