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Houseplants help clean the air inside your home

If you're trying to establish a "green home" by using environmentally friendly
products, recycling and reducing energy use, don't forget about Mother Nature's
power to rid your home of unhealthful air.

Strategically placing houseplants throughout the home may help reduce airborne contaminants produced by everyday household products (such as fabrics and plastics) and building materials (including plywood, particleboard and carpeting), experts contend. Volatile organic chemicals, including formaldehyde, benzene, trichloroethylene and xylene, are absorbed by plants, which break down these nasty pollutants in their roots.

It's not rocket science, despite the fact that in the 1970s scientists at NASA were among the first to discover the cleansing power of indoor plants while testing the safety of a futuristic closed ecological life support system in outer space. Other academic and environmental groups have since contributed to a growing body of evidence supporting NASA's findings, which can be applied to the home and office.

The U.S. Environmental Protection Agency, however, doesn't support the use of houseplants to remove indoor pollutants. But B.C. Wolverton, Ph.D., the senior NASA scientist who conducted the original studies at the John C. Stennis Space Center in Mississippi, says the EPA's position is "ridiculous. They don't know what they are talking about." Wolverton, retired from NASA, is a respected environmental consultant and author.

The EPA hasn't conducted its own studies on indoor houseplants, but the agency has analyzed other groups' research, and considers the findings impossible to apply to "real-world environments," EPA spokesman Dave Ryan said. "The ability of plants to actually improve indoor air quality is limited in comparison (to) adequate ventilation," Ryan said by e-mail.

One of Wolverton's books, How To Grow Fresh Air: 50 Houseplants That Purify Your Home or Office (Penguin, 1997), identifies the houseplants that can most potently clean indoor air through their basic life process of producing oxygen and moisture through the tiny openings in leaves. Some plants are better at cleaning than others, largely because of their individual rate of transpiration (water evaporating from a plant's leaves). Some transpire more than others, and therefore remove more volatile organic contaminents from the air. As water is absorbed through the plant's roots, air is conducted into the root zone. The naturally occurring microorganisms in the plant's roots break down the VOCs to produce food and energy for the plant, Wolverton says.

Among the most powerful air-cleaning plants are golden pothos, lady palm, areca palm, kentia palm, bamboo palm, English ivy, peace lily, Boston fern, snake plant, spider plant, philodendron, dracaena, rubber plant, chrysanthemums and gerbera daisies. Wolverton rated 50 houseplants grown in potting soil on several characteristics, including ability to remove chemicals from the air, pest resistance, transpiration rate and ease to grow indoors. "What I always tell people is to pick your favorite plant, and if you pick one that is less effective, you need to use more plants," he says.

Plants in Wolverton's study were placed in sealed Plexiglas chambers, which were then injected with chemicals. Philodendron, spider plant and golden pothos were best at removing formaldehyde from the air, while gerbera daisy and chrysanthemums excelled at eliminating benzene.

Critics, among them the EPA, claim that houseplants, especially those that are overwatered, are a breeding ground for microorganisms that can trigger allergies. Wolverton acknowledges that mold can be a problem in soil-based indoor plants. That's why he's switched to another way of growing houseplants, called hydroculture, which uses porous clay pebbles instead of soil as a potting medium. The method not only decreases the possibility of mold, but actually results in the plant's increased ability to rid indoor air of pollutants, he contends.

"It's easier to pull the chemicals into the hydroculture. It's porous. It's almost like activated carbon," Wolverton says. He recommends potting all houseplants in non-soil medium to boost air-cleaning ability, eliminate mold growth, and reduce care and maintenance.

Newer homes especially can benefit from air-cleaning plants; that's because new construction is better insulated and sealed, to conserve air conditioning and heating. As a result, pollutants can be trapped indoors, especially if the windows are rarely opened.

Experts suggest using 15 to 18 houseplants in at least 6 to 8-inch containers to improve air quality in an average 1,800-square-foot house. Keep them healthy for best performance.