It’s a myth you almost wish hadn’t been busted. Houseplants, though charming, do little to purify the air in a room, say the scientists who study the air we breathe.

From a quick internet search, you wouldn’t guess that was the case. Popular home décor websites list a number of plants that promise to remove toxins and dangerous chemicals from the air, and several online retailers market air-purifying plants to consumers.

“We decided to study it more in depth in response to all the internet articles and wellness blog posts that tout plants as an indoor air quality magic bullet,” says Michael Waring, an environmental engineer and indoor air quality expert at Drexel University.

In a study recently published in the Journal of Exposure Science and Environmental Epidemiology, Waring and his study coauthor reviewed 12 previously published scientific studies that tested 196 plants over the past decade.

The studies, which concluded that a small houseplant could remove a range of toxins, were conducted in labs. Waring says a typical experiment involved placing a plant in a small chamber and subjecting it to gaseous molecules called volatile organic compounds (VOCs). Experiments ranged in density and time of removal. One showed that in just 24 hours, common household ivies could remove two-thirds of the formaldehyde they were exposed to.

The problem with those experiments, says Waring, is that the densely gaseous chambers in the lab didn’t mimic the typical household or office environment.

Many of the blogs and vendors marketing air-purifying plants point to a 1989 NASA study in which plants in chambers just over two feet wide and long were filled with various gases circulated by a small fan. It’s that 30-year-old study, which showed plants could cut down VOCs in small airtight containers, that led consumers to think perhaps too highly of their houseplants, experts say.

“We’re not saying any of the experimental data is flawed,” says Waring, just that it’s exactly that—experimental.

From the lab to the home

To gauge how plants might interact in a more typical household environment, Waring calculated the clean air delivery rate (CADR) for each. CADR measures how much clean air is pumped into a room by an air purifier over a given time.

By standardizing the results of each study with CADR, they were able to judge how well a plant cleaned a room when compared to proven strategies like running a mechanical air purifier or opening a window.

“Plants, though they do remove VOCs, remove them at such a slow rate that they can’t compete with the air exchange mechanisms already happening in buildings,” says Waring.

To reduce VOCs enough to impact air quality would require around 10 plants per square foot. In a small 500-square foot apartment, that’s 5,000 plants, a veritable forest.

Plants are technically removing a minute amount of airborne toxins, but, “to have it compete with air exchange, you would need an infeasible amount of plants,” he says.

Today NASA grows plants aboard the International Space Station for fresh food and to “create a beautiful atmosphere,” noting their health benefits lie in their ability to improve our mental state.

What’s wrong with the air, anyway?

“Every working person has spent time in a bad conference room that’s stuffy and hot,” says Joe Allen, a Harvard professor who studies how building design influences our health. “What happens when you’re in that space? You’re distracted; you’re looking at the clock, and when that door opens it literally breathes life back into the room, and you feel it. You’re less sleepy. Your eyes open up and you feel more alive.”

Allen describes his work as quantifying common sense—measuring our innate understanding that some rooms are just a little more pleasant to be in.

He says indoor air pollution can come from a number of sources. Cooking can generate particulate matter, and VOCs can spring from chemical cleaners and the synthetic coatings in carpet and furniture.

“Often, the person managing your building has a greater impact on your health than your doctor, and that’s because the person controlling your building is managing a lot of these factors, things like ventilation and building materials,” he says.

The most effective and obvious way to mitigate indoor air pollution is to remove the source, say experts. Waring emphasizes that clean air is absent of odors so spraying scented air freshener is more like spritzing a room with perfume than cleaning it of toxins.

“It would be so wonderful if we had all these beautiful plants that clean our air for us,” says Elliott Gall, a professor at Portland State University who studies how buildings influence indoor air quality. “But there are more effective ways of cleaning indoor air that require mechanical systems to move air over some sort of filtration device.”

Filtering out polluted indoor air often requires pumping filtered outdoor air inside, but when faced with equally polluted outdoor air, communities have used vegetation or “tree walls” to reduce air pollution. Gall says areas behind well-designed tree walls see around about 10 to 30 percent of their emissions reduced, but the best option still remains removing the source of pollution in the first place.

“Removing air pollution often means reducing economic activity or how people move from one side of a city to another,” he adds. “But [removing the source] is the most effective way to reduce pollution.”

Building a better houseplant

At the University of Washington, environmental engineer Stuart Strand has experimented with genetically modifying plants to better remove VOCs from the air.

Last year, he and his research team published the results of their work genetically modifying a common pothos ivy with a protein found in mammalian livers. It’s the same protein our bodies produce to break down alcohol. Over the course of two years, they were able to encode the plants with a version of the protein sourced from rabbits. In lab tests, the genetically modified plants removed more chloroform and benzene from the air than their non-modified counterparts.

To meaningfully clean the air, Strand says a large volume of their plants would need to be consolidated, effectively creating a sink, and a fan of some sort would be needed to blow VOCs across their path.

“I think we can get a couple more genes into the plant,” he says. “We’re working on a second generation of GMOs for formaldehyde.”

Gall, however, remains skeptical that even a genetically modified plant can meaningfully improve air quality.

“I think it’s scientifically great work,” he says, but remains skeptical that the plants will show any meaningful improvement outside of a lab setting.

In a report cited by Bloomberg, plant sales over the past three years topped $1.7 billion, particularly among those ages 18-34. While plants can provide a number of psychological benefits, like stress relief, Gall, Strand, Allen, and Waring all emphasized that they shouldn’t be purchased as an air purifying tool.