There’s a popular conception that plants are effective at filtering out VOCs like benzene and formaldehyde, but can plants filter harmful chemicals at meaningful levels? Recent studies have found that that plants have no detectable on removing formaldehyde or other gas pollutants under real-world conditions.
Why We Think Plants Can Filter Out Formaldehyde
Belief that plants are effective air purifiers is widespread. Arguably the most frequently cited evidence is the NASA study on plants. It feels scientific, authoritative. Many pages cites it, just like this one:
In fact, there’s an entire Wikipedia entry devoted to Nasa’s study on using plants to remove toxic agents like benzene, formaldehyde and trichloroethylene from the air.
To get to the bottom of this question, I read the original NASA study–all of it. I found that the NASA plant tests were done in small, completely sealed plexiglass containers surrounded by growing lights to give the plants more light. For an idea of size, here’s what their indoor setup looks like next to a stick figure version of myself.
I’m not sure what your home is like, but that’s nothing like my home. In NASA’s defence, it’s a decent approximation of a spaceship.
Differences Between the NASA Plant Study and our Homes
The NASA plant setup also has a key difference with our homes. They pumped in the chemical (formaldehyde, benzene and trichloroethylene) at the beginning of the test, and that was it. But in our homes, more of the chemical sources that produce VOCs like formaldehyde constantly emit the chemicals over up to 1-2 years.
Why this matters to us is because the real question is not IF plants can filter out gases like formaldehyde, but HOW MUCH of the gases can they remove from the air? The NASA study convinced me that plants can remove chemicals like benzene and formaldehyde. But it didn’t demonstrate whether plants can consistently remove gases and chemicals from our home:
The two questions we’re left wondering from the NASA plant study are:
- Can plants filter out enough formaldehyde and gases effect in a real home, not a 0.76 meter plexi-box?
- Is that effect enough to overcome a source that’s constantly emitting chemicals, which is the case when we have new furniture, new products, even some types of permapress clothes?
Fortunately, some people have tested the effect of plants in real homes.
Can Plants Filter Out Gas Pollutants?
These scientists in China (original article) tested whether plants can remove volatile organic compounds in a setup more similar to our home. They used a sealed 10m2 (30m3) room for their plant experiment.
It’s still not ideal, since unlike our homes, their room is still sealed. But it makes three big improvements from the NASA plant test:
It contains a constant source of VOCs like formaldehyde (in this case, office desks).
At 10m, the experiment room is much larger than NASA’s box—more like a room in our homes.
The room has no special growing lights for plants (just like most people’s homes).
They tested multiple times with an army of plants that people have specifically said reduce VOCs. They tested snake plant, aloe, and spider plant.
And what happened? VOC levels actually increased from .17 ppm to .24 ppm during the tests. Even an army of plants wasn’t enough to deal with the gas pollutants coming from normal office furniture.
- Some plants can actually emit VOCs, as can soil. So it’s possible that VOCs went up because of the plants!
- The Chinese team claimed to be testing formaldehyde, but these detectors aren’t that specific. They detect VOCs (“VOC” is a big category, including lots of things from perfume, to alcohol, to chemicals like formaldehyde). So we have no idea which VOCs they’re detecting. That means it’s possible that these plants were decreasing a harmful VOC like benzene and increasing some non-harmful VOC.
- Did they use the wrong plants? There is some decent evidence that the effect on VOCs depends a lot on the type of plant. These researchers specifically chose plants that people claim are effective, but the results could be different with other plants.
Bottom Line on Using Plants as Air Purifiers to Remove Formaldehyde and Other VOCs
Bottom Line: Tests by NASA in a small, sealed box showed some plants can reduce formaldehyde. However, real homes and offices typically have a constant source of formaldehyde and other VOCs. This, along with a test in a larger room suggests that the effect of these plants on VOCs is so small it’s not clearly detectable under normal conditions.
Is There NO Evidence That Plants Help Remove Formaldehyde?
I should say that some people claim to have found effects of plants in real-world conditions. For example, these researchers claim that 9 potted plants in a classroom in Portugal reduced particulate by 30% and VOCs by 73%!
That stretches my imagination, at least. From the data I’ve seen, my conclusion is that there is not enough evidence to support the idea that plants meaningfully reduce VOCs or particulate in real-world conditions. At the same time, I think we need more real-world tests.
Given the evidence that plants can remove some harmful VOCs in sealed chambers, I’d err on the side of putting plants in my home. It’s unlikely they’ll hurt, and there’s some possibility they’ll help with gas pollution. Plus, they are nice!
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.