Houseplants fill our indoor spaces with beautiful colors and textures, but we also appreciate them for purifying the air we breathe—or so we thought. The theory that certain plants can filter out airborne toxins in our homes is decades old. And a quick internet search on the topic will turn up hundreds of hits, including plenty of websites belonging to respected, science-based organizations. But now, a new study led by Michael Waring and Bryan Cummings at Drexel University shows that houseplants aren't as good at cleaning the air as we believed.
Back in the 1980s, a NASA study was the first to really put forth the idea that houseplants might be effective air cleaners, at least for certain pollutants called volatile organic compounds, or VOCs, that can be harmful to our health. Since then, numerous other studies appeared to confirm the NASA study findings that certain species commonly grown as houseplants can help improve indoor air quality. But sometimes, even scientifically collected data can be misleading when taken out of context, which can happen when applying controlled laboratory results to real-life situations.
“In the indoor air researching community we've been casually discussing plants for a while, and most of us believed that they weren't effective at removing VOCs,” says Michael Waring, an indoor air quality researcher at Drexel and one of the authors of the new study. Much of the previous research had been done by biologists, horticulturists, or environmental scientists, but Waring studies indoor air quality and ventilation in buildings, and he wanted to apply some of the specifics of his discipline to what was a common assumption.
The new study essentially takes the results of previous studies and sticks them into an indoor air quality framework, translating all of the existing data into a metric called Clean Air Delivery Rate, or CADR. CADR “predicts how effectively any air cleaner will reduce whatever pollutant it is you're looking at,” Waring says. Many existing studies were more lab-focused, looking at the specific abilities of the plants to absorb VOCs, but this new study puts them in the context of real-world buildings, with other factors like ventilation systems.
Plants do remove VOCs, they just remove them too slowly compared to the rate of ventilation in the space to have any meaningful effect.
Waring and Cummings weren’t looking at whether plants take VOCs out of the air; instead, they looked at whether they did so quickly enough to have a noticeable effect in the real world. And they found, unfortunately, they don't. “Plants do remove VOCs, they just remove them too slowly compared to the rate of ventilation in the space to have any meaningful effect on the concentration,” says Waring.
Part of the problem is that any air cleaner, whether it’s a houseplant or a store-bought purifier with a HEPA filter, relies on air passing through it. For the cleaner to remove pollutants, the air has to actually get into the system, which is why most air purifiers are equipped with fans. A fan takes in air and blows it through a filter continuously to treat as much air as possible. A plant, which obviously doesn't come equipped with a fan, relies on what’s often minimal air movement in an indoor environment.
And plants simply aren’t that efficient at removing VOCs anyway; even in a tightly sealed room, it would take as many as ten plants per square meter to make a dent in VOC levels, which isn’t really feasible. “You're never going to get to the point where having a couple of plants inside does anything,” Waring says. Theoretically, there might be ways to increase a plant's air-cleaning effectiveness: using fans around them, exposed root aeroponic systems (it’s bacteria in a plant’s roots that actually remove the VOCs), or even genetic modification of plants could all potentially help. But most people don’t have that stuff or want it.
The bottom line is that, short of turning your living room into a jungle, houseplants aren't going to move the needle if you're looking to improve the air quality inside your house. Of course, they still contribute many other benefits, such as making our indoor spaces feel more lively and boosting our moods, so it's still a good idea to share your home with as many leafy roommates as possible.