Living filter: do houseplants really improve indoor air?

To lower stress, many physicians recommend a relaxing hobby like gardening. But cultivating plants indoors may also lower the risk of asthma, allergies and "sick building syndrome."

The Environmental Protection Agency cites indoor air pollution as one of the top five public health threats in America, and the main culprit in the 60 percent rise in asthma over the last decade. Now researchers are looking to houseplants for a solution to polluted indoor air.

The energy crisis of the 1970s led many Americans to superinsulate heir homes and offices against energy loss. Man-made materials like particle board, synthetic fibers and plastic--which emit harmful formaldehyde, trichloroethylene (TCE) benzene and other volatile organic compounds (VOCs) over years of exposure--have also become ubiquitous indoors. Paints, varnishes, household cleaners, adhesives, carpeting and tobacco smoke are other common VOC-emitters. And while air filters are adequate at capturing particles like dust and dander,they do little to eliminate noxious VOCs.

Though debates have raged for over a decade as to just how effective houseplants are in improving indoor air, studies in Europe and the U.S. have reported that particular plant species can lower VOC levels.

In 1984, NASA senior research scientist Dr. Bill Wolverton tested houseplants for their ability to maintain clean air for future habitable lunar bases. Testing in sealed chambers, Wolverton found that philodendrons and golden pothos were excellent formaldehyde controllers; gerbera daisy and chrysanthemums were impressive benzene purgers; pot mums and peace lilies were highly rated for TCE removal. His initial findings suggested that one to three mature plants were enough to improve the air in a 100-cubic-foot area. He also found that it wasn't just plants doing the clean-up work, but the microbes that were specific to the plants' roots. Another 1989 NASA study concluded that tested houseplants removed up to 87 percent of toxic indoor air within 24 hours. A 1994 German study further reported that one spider plant could, in six, hours, detoxify a 100-cubic-foot room laden with formaldehyde. Further tests showed English ivy benefited smoking areas, and the snake plant cleared household cleaner smells.

Wolverton's personal favorites are the lady palm, peace lily and "Janet Craig" dracaena--"because they're easy to grow, resist insects and work wonders" for a variety of pollutants. His latest book, How to Grow Fresh Air: Fifty Houseplants That Purify Your Home or Office (Penguin Books, $15.95), additionally lists the areca and bamboo palms, English ivy and rubber plant as top pollutant controllers. Because indoor air varies, a good mixture works best, researchers advise.

The Skeptics

But other researchers are unconvinced. For years, the EPA's Office of Indoor Air Quality has remained skeptical, as has Skip Boat, writer for Indoor Air Quality Update. "In lab studies, you have a meticulously controlled environment. Under a tightly controlled situation, using plants may have some merit, but people bring in contaminants when they enter a room, Boat argues, Indeed, this was former EPA Indoor Air Division Director Robert Axelrad's primary complaint. He said that NASA's studies did little to simulate the air changes of a typical home or office. "Our calculations indicate that a much higher density of plants would be required (hundred of plants in the typical house) to achieve these results," writes Exelrad.

And while the EPA says removing the sources of contamination and increasing ventilation are the best cures for polluted indoor air, a 1993 New England Journal of Medicine study found that increased ventilation did not solve the problem of sick building syndrome.

The Plans for Clean Air Council adds that low indoor humidity is another significant health problem. Dry, winterized air, the group says, irritates nasal membranes, triggers asthma and congestion, and makes people more susceptible to viruses and allergens. Wolverton advises keeping indoor humidity levels between 35 and 65 percent, which plants help maintain. "But humidity levels in excess of 70 percent can also result in indoor air quality problems," including mold spores and mildew growth, Wolverton says.

Growing plants hydroponically (in water) overcomes some of these problems. Wolverton says that filters using hydroponic plants, fans and activated charcoal will be on the market within a year, and will improve air purification two hundred-fold. "Hydroponically-grown plants [don't produce[ mold spores, and are easy to maintain," he adds. Wolverton further found that in home tests, rooms devoid of plants had airborne microbe levels 50 percent higher than plant-filled rooms.

Whether they clean the air or not--and there's plenty of evidence that they do--houseplants will continue to decorate the homes of people who appreciate bringing the outside inside.

CONTACT: EPA's Indoor Air Quality Information Clearinghouse, PO Box 37133, Washington, DC 20013-7133/[phone]; Plants for Clean Air Council, 3458 Godspeed Road, Davidsonville, MD 21035/[phone].