A 1989 report issued by the National Aeronautics and Space Administration says some plants can reduce air concentrations of the so-called "big bad three" chemicals by up to 90 percent. "There was no mechanical device they could invent that did a better job than the plants," says Lori Ann Asmus, who recently spoke about the value of indoor plantings to an audience composed of Master Gardeners from Nevada Cooperative Extension. Asmus is the owner of Emerald City Interior Landscape Services in Sacramento, Calif. She also belongs to the Plants for Clean Air Council, a Maryland-based nonprofit group formed to popularize NASA's research.

"As NASA looks toward the possibility of sealing people inside a Space Station, or moon base, along with large numbers of plants, the ecology of such a closed environment -- interactions between man, plants, microorganisms, soil -- must be further evaluated," the NASA report says. The report, prepared by Bill Wolverton for the John Stennis Space Center, also likens a sealed station to today's tightly sealed buildings.

"Energy-efficient buildings that are filled with modern furnishings and high-tech equipment off-gas hundreds of volatile organics," the document notes. "Even at concentrations below present detection limits, some of these chemicals and reactive by-products may adversely affect inhabitants of these buildings." It attributes sick building syndrome to the presence of such chemicals, combined with energy-efficient building construction -- which prevents drafts of fresh air from the outside -- and controlled ventilation. According to the report, houseplants "scrub" indoor air by absorbing and metabolizing common but dangerous organics through their leaves, roots and microorganisms in their soil. The "big bad three" include:

Formaldehyde. It is present in foam insulation, pressed wood and most paper products, such as grocery bags, paper towels and facial tissues. It's also found in many home cleaning products, as well as carpeting and permanent-press clothing. The chemical irritates the mucous membranes of the eye, nose and throat. It can cause headaches and trigger asthma. According to the NASA report, the Environmental Protection Agency did research from which it concludes "formaldehyde is strongly suspected of causing a rare type of throat cancer in long-term occupants of mobile homes."

Benzene. It is used as a solvent. It's present in inks, oils and rubber. And it's used to manufacture detergents, pharmaceuticals and dyes. Benzene is known to irritate the skin and eyes. If inhaled in high doses, benzene can cause dizziness, nausea, blurred vision, tremors, irregular heartbeat and respiratory problems. The 1989 NASA report states, "Chronic exposure to even relatively low levels (of benzene) causes headaches, loss of appetite, drowsiness, nervousness, psychological disturbances and diseases of the blood system, including anemia and bone marrow disease."

Trichloroethylene. It is used in the dry-cleaning industry, but it's also present in inks, paints, lacquers, varnishes and adhesives. The National Cancer Institute considers it a liver carcinogen. Wolverton has studied more than 50 common houseplants. Among those effective at removing one or more of these chemicals are the Boston fern, English ivy, Chinese evergreen, various corn plants (all classified as dracaena), philodendron, pothos, ficus, bamboo palm, peace lily, chrysanthemum and gerbera daisy.

"More than just a pretty face, they're very hard workers," says Asmus, whose company supplies indoor plants and plant service in California to homes as well as business settings such as banks, hospitals and lobbies in large buildings.

The MGM Grand Hotel switched two years ago from artificial indoor plants to live landscaping throughout public areas. The catalyst was MGM Grand Inc. Chairman J. Terrence Lanni, who doesn't like silk plants, according to Tom Rubidoux, the hotel's landscape manager. Aesthetics were the original reason for converting from fake to live, but Rubidoux says the hotel also has saved money.

Often silk plants are more expensive to buy than the real thing. The MGM can buy a live pothos in an 8-inch pot for $10, while a silk pothos of the same size costs about $16, he notes. Plus, the hotel saves on maintenance. Live plants can be tended in place. They repair minor damage on their own, by growing new leaves. Situated in the right light, a live plant flourishes while a silk plant's color bleaches out. Artificial plants need regular dusting, which is an involved process. "You have to take it outside, wash it, let it dry, put fire retardant on, let it dry," explains Rubidoux. Then "by the time you clean it twice, you've got to throw it away."

Live plants offer other benefits to the indoors, Asmus adds. They slightly raise indoor humidity levels, but the moisture they give off appears to suppress the growth of airborne microbes -- unlike electric humidifiers -- according to an April 1996 article in the Journal of the Mississippi Academy of Sciences. The presence of indoor plants also seems to increase productivity in office workers, in a study by Washington State University. Comparable groups of individuals were tested on basic computer tasks -- one in an office space without plants, the other in a space with plants.

"Participants with plants in a work environment were 12 percent more productive and less stressed than those in a work environment without plants," a summary reads. The experiment measured speed in completing tasks and blood pressure.