By Markham Heid
January 17, 2018
TIME Health
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Plants are indispensable to human life. Through photosynthesis, they convert the carbon dioxide we exhale into fresh oxygen, and they can also remove toxins from the air we breathe.

One famous NASA experiment, published in 1989, found that indoor plants can scrub the air of cancer-causing volatile organic compounds like formaldehyde and benzene. (Those NASA researchers were looking for ways to effectively detoxify the air of space station environments.) Later research has found that soil microorganisms in potted plants also play a part in cleaning indoor air.

Based on this research, some scientists say house plants are effective natural air purifiers. And the bigger and leafier the plant, the better. “The amount of leaf surface area influences the rate of air purification,” says Bill Wolverton, a former NASA research scientist who conducted that 1989 plant study.

Wolverton says that, absent expensive testing, it’s impossible to guess how many plants might be needed to clean a room of its contaminants. But he usually recommends at least two “good sized” plants per 100 square feet of interior space. “The Boston fern is one of the most effective plants for removing airborne pollutants, but it is often difficult to grow indoors,” he says. “I usually recommend the golden pothos as my first choice, since it is a popular plant and easy to grow.”

But while Wolverton has long been a vocal advocate of indoor plants—he’s written books on the topic, and now operates a consulting company that advocates for the use of plants to clean contaminated air—other experts say the evidence that plants can effectively accomplish this feat is far from conclusive.

“There are no definitive studies to show that having indoor plants can significantly increase the air quality in the home to improve health in a measurable way,” says Luz Claudio, a professor of environmental medicine and public health at the Icahn School of Medicine at Mount Sinai.

Claudio has reviewed the research on the air-quality benefits of indoor plants. She says there’s no question that plants are capable of removing volatile chemical toxins from the air “under laboratory conditions.” But in the real world—in your home, say, or in your office space—the notion that incorporating a few plants can purify your air doesn’t have much hard science to back it up.

Most research efforts to date—including the NASA study—placed indoor plants in small, sealed environments in order to assess how much air-scrubbing power they possessed. But those studies aren’t really applicable to what happens in a house, says Stanley Kays, a professor emeritus of horticulture at the University of Georgia.

Kays coauthored a 2009 study on the air-cleaning powers of 28 different indoor plants. While many of those plants could remove toxins from the air, “moving from a sealed container to a more open environment changes the dynamics tremendously,” he says.

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In many cases, the air in your home completely turns over—that is, swaps places with outdoor air—once every hour. “There’s a phenomenal amount of air coming in and going out in most houses,” Kays says. “From what I’ve seen, in most instances air exchange with the exterior has a far greater effect on indoor air quality than plants.”

Also, plants used in lab studies are grown in optimal conditions. They’re exposed to ample light in order to maximize photosynthesis, which improves a plant’s toxin-degrading abilities. “In the home, this isn’t the case at all,” Kays says. “The amount of light in many parts of a house is often just barely sufficient for photosynthesis.”

He knows many people will be disappointed by what he has to say, and he wants to make it clear he believes house plants are not only pleasant living companions, but that they also provide a number of evidence-based health benefits. Studies have shown plants can knock out stress by calming the sympathetic nervous system, and can also make people feel happier. More research shows spending time around nature has a positive effect on a person’s mood and energy levels.

“There are some real plusses to having plants around,” Kays says. “But at this time, it doesn’t look like plants sitting passively in a house are effective enough to make a major contribution to purifying indoor air.”

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