In September of 1989, NASA researchers published a paper titled “Interior Landscape Plants for Indoor Air Pollution Abatement.” This report, generally known as the NASA clean air study, concluded that common houseplants are able to remove harmful chemicals from the air. The pollutants studied include formaldehyde and benzene. These volatile organic compounds, or VOCs, are produced by common household objects such as carpeting and certain plastics, and they have been shown to cause cancer and respiratory problems.
The 1989 study was not the first study NASA had funded on the subject. In fact, it was one in a long line of studies of a much broader phenomenon: the ability of plants to filter out harmful waste produced by humans. Beginning in the mid-70s, NASA scientists Rebecca McDonald and B. C. Wolverton published a series of papers, individually and as co-authors, on the ability of aquatic plants to filter human and chemical waste from wastewater.
In 1984, an article appeared in Economic Botany addressing a different question: How effective are common houseplants at removing from formaldehyde from the air? Unlike the later NASA clean air study, which examined twelve varieties of plants, McDonald, Wolverton, and E. A. Watkins, Jr. looked at only three houseplants: the arrowhead plant, golden pothos, and the spider plant. They sealed two plants of each type in plexiglass chambers that they then filled with formaldehyde. The researchers measured formaldehyde levels at the beginning of the experiment, after six hours, and after twenty-four hours, at which point the experiment was concluded. Though all three types of plants produced noticeable results, the spider plant was the most effective at removing formaldehyde from the air.
As more houseplants were studied, it became clear that the spider plant was not an anomaly. The peace lily was one of the best houseplants for removing a variety of VOCs, and plants ranging from the weeping fig to English ivy also performed well. After participating in the 1989 study, Wolverton wrote Have to Grow Fresh Air: 50 Houseplants That Purify Your Home or Office. It was published in the United States in 1997. A table summarizing his research appears in the opening pages. He found that the Boston fern, the florist’s mum, and the gerbera daisy were the most efficient at removing formaldehyde. Spider plants are ranked twenty-seventh on the list. The golden pothos is thirty-second.
Later studies explored how, exactly, plants removed VOCs from the air. This process, called phytoremediation, occurred in several ways. Plants with waxy leaves attracted pollutants due to the airflow around their leaves, and the pollutants became trapped in the wax. As plants took in air through the stomata in their leaves, they acquired the carbon dioxide they needed for photosynthesis as well as other gases and airborne pollutants. What happened then was less certain. The plants could be storing the pollutants in their leaves and stems, breaking them down in some way, or simply excreting them through their roots. Some plants were able to transform formaldehyde into carbon dioxide. Could they do this with other compounds? And would the plants suffer from exposure to these substances that humans found so harmful?
Further research showed that it was not the plant alone that removed toxic chemicals from the air. The microorganisms in the soil played a vital role. Fungi called mycorrhizae coexist with plant roots in the soil in a close relationship. Mycorrhizae surround plant roots and will even grow inside them. Mycorrhizae absorb nutrients from the dirt, which makes those same nutrients more accessible to the plant. During photosynthesis, the plant produces excess sugars that it excretes through its roots. The mycorrhizae feed on these sugars—and, researchers found, VOCs.
Researchers reliably found that houseplants became more efficient at absorbing pollutants over time, not less. As the microorganisms living in the soil around the plant roots were exposed to more VOCs, the populations of certain microbes expanded. Further experiments showed that an empty pot of soil containing microbes was able to absorb pollutants from the air. It seemed that some bacteria and fungi living in the soil fed on the pollutants, regardless of whether they grew on the roots of plants. However, a potted plant with exposed soil around its roots absorbed toxins the most efficiently.
When researchers from Denmark published an article in 2014 titled, “Can ornamental potted plants remove volatile organic compounds from indoor air? — a review,” their answer to their own question was: Yes. However, their review illuminated how many questions remain unanswered.
Researchers have not yet determined conclusively how many plants it would take to neutralize the VOCs emitted by an indoor space. Answering this question requires, first, determining which VOCs are emitted by the materials and people in the room; at what rate they accumulate; and whether there are other factors that influence the concentration of these airborne toxins, such as central heating in the winter and air circulation through open windows during the summer. Most of the research done on the phytoremediation capabilities of houseplants has been done under controlled circumstances with high concentrations of VOCs. It is unclear whether these plants would be as efficient at removing pollutants at the low levels found in a common home or office building.
At the time the Danish researchers’ review was published, they concluded that the research into how many plants should be kept in a home or office to keep airborne pollutants at a reasonable level was too often contradictory and speculative. Wolverton’s original estimate is still often mentioned, despite the fact that it has not been rigorously evaluated: one plant per hundred square feet.
NASA researchers first began studying the abilities of plants to clean air for use in space travel. The materials used to make space shuttles emit many of the same harmful substances found in a standard office building. Today, the plants on the International Space Station are mainly grown for food. Scientists have been studying the effects of space travel on edible plants grown on the International Space Station since 2002. In November 2015, a new system was set up that allows astronauts to grow their own vegetables. This system was first tested on site using not vegetables, but zinnia seeds. The plants struggled before finally flowering in January of 2016. Now, astronauts grow their own vegetables aboard the space station to provide them with important nutrients—and also, maybe, make the air little cleaner.