His 1989 report announced a cheerful answer. Plants were “a promising, economical solution to indoor air pollution,” it declared. “If man is to move into closed environments, on Earth or in space, he must take along nature’s life support system.” The report—jointly funded by NASA and the Associated Landscape Contractors of America, a trade group—was picked up by the media. The idea gained even more currency in 1996, when Wolverton published How to Grow Fresh Air: 50 Houseplants That Purify Your Home or Office. (Wolverton did not respond to a request for comment.)

That study provides the scientific basis for almost all the plant-and-air-pollution content you see online. “I’ve seen it on so many pop internet sites—‘researchers from NASA’ is the common phrase you see,” Waring, the Drexel professor, said. He told me that there’s nothing especially wrong with Wolverton’s 1989 study. Its results “fall right in line with other stuff that’s been measured in the literature.”

But taking its results at face value significantly overstates the power of plants, he said. Wolverton measured whether houseplants could remove VOCs from an airtight laboratory environment. But a home is not a hermetic chamber. It has open windows and doors, drafts and leaks, and much more clutter.

Recently, Waring and his colleagues reanalyzed all 195 studies that have examined whether houseplants can filter the air. They found that some types of plants can remove higher amounts of VOCs than others. But once you factor in the effects of working in a large room, none of the plants are able to do much.

Waring told me to imagine a small office, 10 feet by 10 feet by eight feet. “You would have to put 1,000 plants in that office to have the same air-cleaning capacity of just changing over the air once per hour, which is the typical air-exchange rate in an office ventilation system,” he said. That’s 10 plants per square foot of floor space. Even if you chose the most effective type of VOC-filtering plant, you would still need one plant per square foot, Waring said.

Or as Waring (who owns 10 to 20 houseplants) recently put it in a presentation for the National Academies of Sciences, Engineering, and Medicine:

Michael Waring

But maybe scientists have been researching the wrong pollutant. Several years ago, a team of researchers examined whether houseplants could remove ground-level ozone. Ozone’s effects are often described as “sunburn inside your lungs,” and can cause painful breathing, asthma attacks, and even the chronic lung disease COPD.

More than 107 million Americans live in areas with unhealthy amounts of ozone, according to the Environmental Protection Agency. Unfortunately, houseplants can’t do much about that, either. The researchers found that even the most effective plants barely reduced the level of ozone in indoor spaces. “If ozone levels were 30 parts per billion in your home, then you might reduce them to like 29.7 parts per billion,” said Gall, the Portland State professor and a co-author of the study. (He owns no houseplants. “When I did a postdoc in Singapore, we had two big houseplants we were excited about and loved, but then we had ant problems for the next two years,” he said.)