Almost every houseplant listing says a plant “purifies the air.” The claim traces back to one study from 1989, and it doesn’t hold up in a real home. That doesn’t make plants pointless. It means you should buy them for the right reasons and not spend money on them as an air cleaner.
Where the clean-air claim comes from
In 1989 NASA researchers, looking for ways to keep air fresh in sealed space stations, put common houseplants into small sealed chambers and injected chemicals such as benzene and formaldehyde. Over a day, levels in the chambers dropped. Much of the removal turned out to come from microbes in the potting soil, not the leaves.
A house is not a sealed chamber. Outside air leaks in and indoor air leaks out all the time, usually replacing the whole volume every one to three hours. In 2019, researchers at Drexel University reviewed decades of plant studies and compared plant removal rates with that ordinary air exchange. Their conclusion: to match it you’d need between 10 and 1,000 plants per square meter of floor. For a 12-by-12-foot bedroom, that’s at least a few hundred pots.
- 1989
- year of the sealed-chamber NASA study behind the claim
- 10–1,000
- plants per square meter needed to match normal ventilation
- 1–3 hours
- time it usually takes for outside air to replace a home’s air
Sources: NASA Clean Air Study (1989); Cummings and Waring review, Drexel University (2019). Air exchange varies with how tight the house is.
The EPA’s own advice on indoor air is to remove the source of the pollutant, ventilate, and use an air cleaner if needed. Plants aren’t on that list. If you’re worried about fumes, smoke, mold or a new-furniture smell, open windows, run the kitchen and bath fans, and consider a HEPA purifier sized for the room. If someone in the house has breathing symptoms, talk to a doctor rather than buying plants.
What plants do well
Plants release almost all the water you give them through their leaves. A group of large plants in a small room can raise humidity a few percentage points, which some people notice in a dry winter house. Don’t expect a humidifier’s effect.
Several studies link plants in homes, offices and hospital rooms to lower reported stress and better mood. The effects are modest and the research is mixed, but most people who keep plants say they like the rooms more. That’s a fine reason to own them.
Grow more instead of buying more
A 4-inch pothos or philodendron costs $8 to $15 at a garden center, and a 6-inch pot costs $15 to $30. A cutting from a friend’s plant costs nothing and fills a pot in a season. Most of the common vining plants root in water.
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Take a cutting
Cut a 4- to 6-inch piece of stem a little below a node, the bump where a leaf joins. Strip the lowest leaf.
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Root it in water
Stand it in a jar on a bright windowsill out of hot direct sun. Change the water weekly. Roots appear in two to six weeks.
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Pot it up
When roots are 1 to 2 inches long, plant several cuttings together in a small pot of fresh mix for a fuller plant.
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Divide the clumpers
Snake plants, ZZ plants, peace lilies and cast iron plants can be split into two or three plants when you repot them.
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Pot the babies
Spider plants make small plantlets on runners. Pin one onto a pot of soil and cut the runner once it roots.
Plant swaps run by libraries, garden clubs and local online groups are a good source of cuttings. For potting mix, coir-based blends work as well as peat ones for most houseplants and don’t come from drained bogs.
Plants that put up with low light
Most homes are darker than plants would like, especially more than 6 feet from a window. These cope best:
- Pothos and heartleaf philodendron, which trail and root readily from cuttings
- ZZ plant, which stores water in its roots and survives weeks of neglect
- Snake plant, which prefers bright light but tolerates dim corners
- Cast iron plant, slow and nearly unkillable
- Parlor palm and spider plant, both listed as non-toxic to cats and dogs by the ASPCA
If you have pets, check the ASPCA’s toxic plant list before buying. Pothos, philodendron, ZZ, snake plant and peace lily can all irritate a cat’s or dog’s mouth and stomach. True lilies, often sold as cut flowers, can cause kidney failure in cats.
The watering mistakes that kill most plants
More houseplants die from overwatering than from drought. Roots sitting in wet soil can’t get oxygen and start to rot, and the plant wilts as if it were dry, so people water it again.
- Water when the top inch or two of soil is dry, not on a fixed weekly schedule. In winter, most plants need half as much.
- Use pots with drainage holes. A decorative pot without one is fine as an outer sleeve; tip out any water that collects.
- Water thoroughly until it runs out of the bottom, then let it drain. Small sips leave the lower roots dry.
- Repot into a pot only one size up. A huge pot holds water the roots can’t reach, and it stays wet.
- Skip fertilizer between late fall and early spring, when growth slows.
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How many plants does it take to clean the air in a room?
Far more than anyone keeps. A 2019 review estimated 10 to 1,000 plants per square meter of floor to match the effect of normal ventilation, so open a window or use a HEPA purifier instead.
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Was the NASA clean air study wrong?
Its results were real for small sealed chambers. They don’t transfer to a home, where air is constantly replaced by outside air.
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What is the hardest houseplant to kill?
ZZ plant, cast iron plant and snake plant cope with low light and missed waterings better than almost anything else. Most die from too much water, not too little.
