Your Houseplants Are Lovely. They Just Aren’t Cleaning the Air.

“Plants purify the air” is a line you hear constantly in home decor. A pair of Drexel University engineers went back to the research behind it and did the math.

Drexel points to a high-profile 1989 NASA experiment, run while the agency was looking for ways to clean the air on space stations, as the likely start of the idea. Plants did remove chemicals in those tests. The catch, according to Drexel, is where the tests happened. In the typical study, as the researchers describe it, a potted plant sat in a sealed chamber, often a cubic meter or smaller, a single chemical was injected, and its decline was tracked over hours or days. A real living room isn’t sealed. Outdoor air is constantly leaking and blowing in, and that exchange dilutes indoor pollutants on its own.

Bryan Cummings and Michael Waring reviewed 12 published chamber studies spanning about 30 years and translated 196 experimental results into a metric called clean air delivery rate, the same measure used to rate air purifiers. Their paper appeared in the Journal of Exposure Science and Environmental Epidemiology in November 2019.

The results varied enormously, but the median single plant, they found, delivered about 0.023 cubic meters of clean air per hour. To match the air exchange that typical buildings already get from outdoor air, you’d need somewhere between 10 and 1,000 plants per square meter of floor space.

“Plants are great, but they don’t actually clean indoor air quickly enough to have an effect on the air quality of your home or office environment,” Waring said in Drexel’s announcement.

Two notes on the numbers. Drexel’s news release gives the range as 100 to 1,000 plants; the paper itself says 10 to 1,000, and we’re using the paper. And this was a calculation built on chamber experiments, focused on volatile organic compounds, not a test in real homes. The authors suggest future research look at other things plants may offer, including well-being and productivity, rather than their ability to passively clean air.