Despite criticism from the EPA, scientists say houseplants can filter toxins from the air  

Now a new study from the University of Technology in Sydney, Australia, indicates that houseplants are, in fact, more than decorative. Plant biologist Margaret C. Burchett and colleagues recently tested the impact of two plants, the peace lily and the “Janet Craig,” on the air quality in 60 different offices. After 18 weeks of measurements, the findings were striking: As few as six small potted plants reduced overall toxin levels by 75 percent.

“The most remarkable finding is that the plants seemed to ramp up their filtration abilities when the air was more toxic,” says Burchett. She found houseplants were better filters during weeks when pollution levels were high, and performance waned when the air was more pure. Overall, offices with plants had cleaner air than those left without the additional splash of green.

“Palms and ferns are among the best filters,” says Bill C. Wolverton, one of the original NASA researchers to study houseplants. “Spider plants are also excellent because they target benzene, the chemical released from house paint.”

“Spider plants are easy to keep,” says Sarah Cadman, a freelance plant propagator based in Santa Cruz. “Anyone can grow them.”   

Spider plants are among the most popular hanging plants sold at The Garden Company, a nursery located on the west end of Mission Street. The store’s owner, Charlie Keutmann, says they also sell a lot of peace lily plants, including a variety with speckled foliage and white sailboat flowers. Peace lily plants are touted for their ability to tolerate a wide range of growing conditions, and can adapt well to many locations in the home. “The bamboo palms are also fairly easy to work with,” he says, “but some of the other palms can be more finicky.”

As keeping your houseplants alive is the first step toward using them as air filters, Keutmann recommends it’s worth your while to visit a nursery that sells high quality, healthy plants. Areca palms and other varieties commonly sold at grocery stores are often grown in greenhouses in Florida, and then shipped across country. “They are hard to keep alive unless you have a greenhouse,” says Keutmann.

The Critics

Still, the notion of houseplants as air filters is not without its critics. “[Plants] are a nice addition to most environments, but I wouldn’t rely on them to be air cleaning machines,” says John Girman, the senior science advisor for the EPA’s Indoor Air Division.

Ventilation systems can introduce pollution faster than a plant can remove it, and if clean air is flushed into a room, it’s hard to determine how much impact a plant really has on the air quality. “I’m glad to see they are doing studies in office buildings, but it’s unfortunate the ventilation rates weren’t measured,” adds Girman.  

In fact, studies have traditionally tested plants in small, sealed test chambers that have very little in common with your living room. After exposing houseplants to volatile organic compounds (VOC), a class of common chemicals known to cause cancer and neurological problems, Wolverton and his colleagues observed that certain palms, along with dracaena and rubber plants, can remove almost every chemical found in the home.

The moth and dendrobium orchids remove high levels of xylene and toluene—chemicals released from televisions, computers and plastics. Peace lilies remove more acetone than almost any other plant, and the Boston fern removes the most formaldehyde, a toxin used to preserve carpets, clothing, paper towels and tissues.

“I don’t dispute the validity of test chamber studies,” says Girman, “but based on the results, you would need 680 plants to remove pollutants from a 1,500 square foot home.” Plants in Wolverton’s studies filtered air inside a small chamber, but it would take several chambers’ worth of houseplants to filter the large volume of air inside a room.

For Wolverton, the debate comes as no surprise. “We recognized these limitations a long time ago,” he says. “Plants alone are limited in their ability to remove chemicals from the indoor environment, unless, as the critics suggest, a large amount of plantings are dispersed throughout a building.”

Turning plants into a technology platform

Like Burchett, Wolverton still recommends adding a few plants to each room—it will likely make a difference. But to see a drastic improvement, Wolverton argues houseplants have to be used as a technology platform.

From rooftop ecology gardens to greenhouse window boxes that circulate filtered air, Wolverton says houseplants have the most impact when they are literally planted into the house. Wolverton has also designed an EcoPlanter, which claims to more than double a plant’s filtration capacity through the use of activated carbon, and has been marketed in Japan for nearly a decade. Phytofilter, a startup based in Saratoga Springs, New York, recently licensed the exclusive rights to Wolverton’s research and technology, and hopes to integrate large planters into the ventilation systems of buildings and apartment complexes by 2010.

In the meantime, Wolverton recommends introducing as many houseplants into a room as possible. “Don’t over-water, don’t let water grow stagnant in the tray, and use expanded clay or shell pebbles that don’t support mold,” says Wolverton.

According to Cadman, you can also avoid mold problems by watering your plants outside where they are free to drain—just make sure you put the plant in a shady area with the same amount of light as its environment inside the house. Indoor plants often end up sitting in water that accumulates in their catch trays, so letting them drain outdoors will help avoid mold growth that comes from stagnant water. “You can also put your plants out in the rain,” she says. “There is nitrogen in rainwater, and it cleans the plant leaves.”

The trick when caring for a plant is to avoid extremes of any sort, says Keutmann. “Most people lose their house plants due to extreme over-watering, or they just forget about them,” he says. He recommends the resilient snake plant, which is also a good air filter, although even these hardy houseplants can be damaged with too much neglect and abuse. Keutmann recommends trying new cultivars—a popular new variety has black and gold foliage, and another, called celindrica, has unusual tubular leaves.  

Along with caring for your plants, placing them in strategic locations can augment their filtration capacity. Wolverton recommends creating a “personal breathing zone” in each room. “You will see better results if potted plants are placed near reading chairs and desks,” he says.

Unlike man-made filters that absorb toxins like a sponge—and ultimately end up in the landfill—plants break toxins apart, says Wolverton. Plants suck air into the ground, where microbes degrade toxins into fundamental sources of energy and life.

According to a Washington State University study, houseplants also reduce stress and help people relax. Plants have been shown to increase employee productivity, reduce sick leave rates by as much as 60 percent, and heighten a patient’s ability to tolerate pain and physical discomfort. Installations of indoor plants have been shown to help control temperature and humidity as well.   

“Houseplants are just like any other element of green living,” says Wolverton. ”If you get rid of toxic building supplies and grow as many plants as can fit in a room, it will all add up.”

It’s a critique every environmentalist has heard—recycling one paper bag won’t save a forest, and eating a single organic salad may not prevent pesticide exposure. “A few houseplants won’t solve a big indoor air quality problem,” admits Wolverton, “but two or three plants will make a big difference for the average room.”

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