Plants have many superpowers: they can regenerate when damaged, some can survive a lot of neglect, and cultivating them can promote psychological well-being.
Houseplants can also support our physical health by removing harmful pollutants from the air. The most famous piece of research into this was the 1989 NASA Clean Air Study, where the space agency tried out different ways of cleaning air in sealed environments. They were particularly interested in the air purifying capabilities of common houseplants. They measured how much species like English Ivy, Peace lily, and Weeping fig could remove benzene, formaldehyde, trichloroethylene, and xylene and toluene from a sealed environment.
These toxins are known as 'volatile organic compounds (VOCs). The pollutants tested by NASA are common in urban homes and offices throughout the developed world. A European study estimated that people inhale around 102 micrograms of benzene per day. Benzene is so carcinogenic that there is no official figures on levels of safe exposure. Around 36 percent of inhaled benzene comes from indoor home environments with 32 percent from indoor work environments. Cigarette smoke contributes massively, as does exhaust emissions, putting those who live near busy roads at greater risk.
NASA found that the best all-rounders for air purification were the Peace lily, Lady Palm (Rhapis excelsa), and Florists chrysanthemum, which removed all four pollutants from the air. Other plants were highly effective in removing one or two of the toxins. The Bamboo palm performed highest in removing formaldehyde, sucking up 3,196 micrograms from the air per hour. The Baberton daisy came out top for trichloroethylene (1622 micrograms removed per hour) and benzene (4486 micrograms of benzene removed per hour). To put the NASA results into perspective, a study done in Helsinki, Iceland found that the average air concentration of formaldehyde in homes was 41.4 micrograms per meter cubed.
How do plants clean the air?
Scientists have found that air impurities are taken up by plants through leaves, specifically through the tiny leaf 'pores' called the stomata. From there, the compounds are transported to their roots and soil. The microorganisms in the soil then digest and detoxify the pollutants. This means that the ability of a plant to remove pollutants sometimes depends on the particular microbial species that the species encourages in the soil around them.
Before you start filling your house with air purifying plants, be aware that researchers have started to become skeptical about the conclusions of plant air filtration studies.
Although there have been dozens of studies showing the air filtration benefits of houseplants, most have a fatal flaw: they are conducted in small sealed chambers that do not replicate realistic conditions for buildings. Houses are large, multi-roomed, and air flows in and out continually - they're not small spaces sealed off from the world.
Other past studies did try out plants in more realistic settings. However, many of these didn't measure the rate of airflow in and out of the space where the plants were kept. Without this crucial piece of evidence, it is difficult to say how much of the toxins were being removed by simple air movement and how much by the actual plants themselves.
In 2019, researchers published a systematic review of results from 12 studies into potted plant air filtration. They found highly variable results across the sealed chamber studies, probably because of wildly different methodologies. Even in more 'realistic' experiments that mimicked conditions inside actualy buildings, they found that plants remove a negligible amount of pollutants. The study estimated that for a 20 percent reduction in air pollutants, you would need at least one medium size plant per square foot of a house with low airflow - a tall order even for the most enthusiastic plant owners.
There is one way that plants can improve air quality and that is through living walls. Because living walls cover a large surface area, and because their air purifying roots are often exposed, it might be the only way for plants to act as really effective filtration systems. More research is needed into what kind of features a living wall should have to maximize its filtration potential.
So while buying plants may not improve indoor air quality to a significant extent, they still have a wealth of other benefits. They can make us feel closer to nature, give us something to nurture and keep us occupied away from our screens and laptops.