Do Air Purifiers and Houseplants Really Clean Indoor Air? What the Science Says

Maybe you've seen the posts: "These 10 houseplants will purify the air in your home!" Or maybe you're standing in a store looking at a wall of air purifiers with price tags ranging from "reasonable" to "is this a car?" Either way, you want the same thing: cleaner, healthier air in the place you spend most of your time.

It's a good goal. Studies from the U.S. Environmental Protection Agency have found that indoor levels of some pollutants can be two to five times higher than outdoor levels, and most of us spend the vast majority of our time indoors. But which fixes actually make a difference? Let's sort the science from the marketing.

What's in indoor air?

Before deciding how to clean your air, it helps to know what you're dealing with. Common indoor air pollutants include:

• Particles (PM2.5 and PM10): tiny bits from cooking, candles, smoke, dust and outdoor pollution that drifts inside
• Allergens: dust mites, pet dander, pollen and mold spores
• Volatile organic compounds (VOCs): gases released by paints, cleaning products, air fresheners, new furniture and some building materials
• Combustion gases: like nitrogen dioxide and carbon monoxide from gas stoves, heaters and fireplaces
• Radon: a naturally occurring radioactive gas that can seep in from the ground (see our guide to radon testing)
• Germs: viruses and bacteria carried in airborne droplets

Different solutions tackle different pollutants, which is why there's no single magic fix.

Do houseplants clean the air?

The NASA study that started it all

In 1989, NASA published a study looking at whether plants could remove certain chemicals from the air in sealed spaces like space stations. In small, sealed chambers, some plants did reduce levels of VOCs like benzene and formaldehyde. That study launched decades of "air-purifying plant" lists.

What later research found

The problem is that a small, sealed lab chamber is very different from a real home, where air is constantly exchanged with the outdoors. In 2019, researchers at Drexel University reviewed decades of plant studies and calculated that you'd need somewhere between 10 and 1,000 plants per square meter to match the air-cleaning effect of normal ventilation in a typical building. In other words, to make a meaningful difference you'd basically need to live in a jungle.

So are houseplants pointless?

Not at all. They just aren't air purifiers. Houseplants have plenty of other benefits:

• Studies link indoor plants with lower stress and better mood
• Caring for plants can be calming and give a sense of purpose
• Plants make a space feel more alive and welcoming

Our guides to gardening and wellbeing and biophilic design at home go deeper on this. Keep your plants because you love them, not because you think they're scrubbing your air.

One caution: overwatered plants can grow mold in their soil, which can be a problem for people with mold allergies. Let the top layer of soil dry between waterings.

Do air purifiers work?

HEPA air purifiers: yes, for particles

Air purifiers with true HEPA filters are effective at removing particles from the air, including dust, pollen, pet dander, smoke and some airborne germs. By the U.S. Department of Energy's standard, HEPA filters remove at least 99.97 percent of particles 0.3 microns in size, which are among the hardest particles to catch.

Research has found that portable HEPA air cleaners can meaningfully reduce indoor fine particle levels, and some studies have found improvements in allergy and asthma symptoms. They're especially helpful during wildfire smoke events.

What HEPA filters don't do

HEPA filters catch particles, not gases. They won't remove most VOCs, odors or gases like nitrogen dioxide or radon. For gases, some purifiers include an activated carbon filter, which can help with odors and some VOCs, but its effectiveness depends on how much carbon there is and how often you replace it.

Air purifiers to be cautious about

• Ozone generators: Some devices deliberately produce ozone and market it as "activated oxygen" or similar. Ozone is a lung irritant, and the EPA warns that ozone generators can be harmful at levels effective enough to clean air.
• Ionizers and some "air sanitizers": Some ionizers and electronic air cleaners can produce ozone as a byproduct. California sets ozone emission limits for air cleaners sold there, and checking for that certification is one way to choose safer devices.
• Unverified claims: Be skeptical of products claiming to kill 99.9 percent of germs without clear, independent testing.

How to choose an air purifier

Look for a true HEPA filter

Phrases like "HEPA-type" or "HEPA-like" don't mean the same thing as true HEPA.

Check the CADR

CADR stands for Clean Air Delivery Rate. It tells you how much clean air the purifier delivers, measured for smoke, dust and pollen. Higher is better. A common rule of thumb is to choose a CADR that's at least two-thirds of your room's area in square feet. For example, a 150-square-foot bedroom needs a CADR of at least 100.

Match it to your room size

Manufacturers usually list a recommended room size. Choose one rated for your room or a little bigger. Running a purifier on a higher setting cleans more air but may be louder.

Consider noise

If it's going in your bedroom, check the decibel rating on low settings.

Factor in filter costs

Replacement filters can add up. Check the price and how often they need changing.

Budget option: the DIY box fan filter

A Corsi-Rosenthal box, made from a box fan and four MERV-13 furnace filters taped together, has been shown in testing to be an affordable and effective way to reduce indoor particles. It's louder and bulkier than a commercial unit, but it works well in a pinch.

What actually improves indoor air quality the most

Air purifiers and plants are only part of the picture. The EPA recommends three main strategies, in order:

1. Control the source

Stopping pollution at the source is the most effective step:

• Use the range hood or open a window when cooking, especially on a gas stove. See our guide to gas stoves and indoor air.
• Don't smoke indoors.
• Choose fragrance-free cleaning products and skip synthetic air fresheners.
• Let new furniture and mattresses air out, ideally in a ventilated space.
• Fix leaks and moisture problems to prevent mold.
• Test for radon.

2. Ventilate

Bring in fresh outdoor air when outdoor air quality is good:

• Open windows, even for a few minutes a day
• Run bathroom and kitchen exhaust fans
• Make sure your home's ventilation system is working properly

3. Clean the air

Once you've controlled sources and improved ventilation, an air purifier or a good HVAC filter adds extra protection:

• Use a HEPA air purifier in the rooms where you spend the most time, especially the bedroom
• If you have central heating and cooling, consider upgrading to a MERV-13 filter if your system can handle it
• Replace filters on schedule

Simple daily habits for cleaner indoor air

• Vacuum with a HEPA vacuum and dust with a damp cloth
• Wash bedding weekly in hot water to reduce dust mites
• Keep humidity between about 30 and 50 percent
• Take shoes off at the door
• Groom pets regularly
• Check the air quality index on smoky or high-pollution days and keep windows closed then

Our guide to a healthy home environment has more ideas for making your space a healthier place to live.

The bottom line

If you want cleaner indoor air, think in this order: remove the source, ventilate, then filter. A HEPA purifier in the bedroom is a great addition. Keep your houseplants, too. Just enjoy them for the calm they bring rather than the air cleaning they don't really do.

Frequently asked questions

Do houseplants purify the air?

Not in a meaningful way in real homes. A famous NASA study showed plants could remove some chemicals in sealed chambers, but later research found you'd need an enormous number of plants to match normal ventilation.

Do air purifiers really work?

Air purifiers with true HEPA filters are effective at removing particles like dust, pollen, pet dander and smoke. They don't remove most gases unless they have a substantial activated carbon filter.

What size air purifier do I need?

Look at the CADR rating. A common guideline is a CADR at least two-thirds of your room's square footage, so a 300-square-foot room would need a CADR of at least 200.

Are ionizers and ozone air purifiers safe?

Ozone generators can produce harmful levels of ozone, and some ionizers release ozone as a byproduct. Choose air cleaners certified to meet ozone emission limits.

Where should I put an air purifier?

Put it in the room where you spend the most time, usually the bedroom, with space around it for air to flow, and keep doors and windows closed while it runs.

Your home is part of your health. See how your environmental wellness stacks up with our free wellness wheel quiz.