A growing body of research on the subject offers yet more evidence that houseplants may be an effective means of improving the air quality in your home and office
Perhaps you remember the news from more than a decade ago that spider plants seemed to do a spectacular job of cleaning the air. The National Aeronautic and Space Administration (NASA), which tested the abilities of three common houseplants to remove formaldehyde from the air, found in preliminary tests that spider plants were the champs, removing 95 percent of the toxic substance from a sealed Plexiglas chamber in 24 hours.
Since then, much more research on the phenomenon has been conducted, and the news about spider plants is no longer so rosy. But that´s not because the tests were wrong. It turns out other plants also clean the air, of contaminants ranging from ammonia to benzene. Such toxics waft from countless sources in our daily lives, including adhesives, ceiling tiles, photocopiers and upholstery. And some plants do a much better job than the spider plant. The growing body of research on the subject offers yet more evidence that houseplants may be an effective means of improving the air quality in your home and office.
Much of the research has been conducted by former NASA research scientist Bill Wolverton, who was first interested in greenery as part of ecological life-support systems for space travel. Wolverton found that plants absorb airborne pollutants as part of their normal "breathing" process (carbon dioxide in, oxygen out) and transport them to their roots, where microbes feed on and detoxify them.
Wolverton has measured the filtering capacity of specific plants by putting them into sealed, 85-gallon chambers with a succession of toxic chemicals, then measuring how much of each chemical remains at 12-hour intervals. The results have been impressive. "We found that the most effective plants had a high transpiration rate--that is, they emit more moisture into the environment," says Wolverton. "This is the result of a natural pumping action. As plants pull air down to their roots, they pump moisture out."
Now retired from NASA, Wolverton is promoting houseplants in his book How to Grow Fresh Air and through his business, Wolverton Environmental Services in Picayune, Mississippi. Soon he will be marketing an "enhanced" planter with a fan and filter designed to maximize a plant´s natural pumping action. But while other studies have verified and extended his findings, the precise filtering capacity of plants outside the laboratory remains an open question.
"There´s no argument about what plants do," says John Girman of the Environmental Protection Agency´s Indoor Environments Division. "It´s a question of how well they do it, and whether that´s enough to be an effective means of pollution control in a typical indoor environment." While one plant may clean the air in a small, confined space, getting those same results for a whole house would require many more plants. Just how many is not clear. Wolverton figures 60 would do the job; Girman thinks the number is closer to 700.
Part of the uncertainty comes from the question of ventilation. In sealed chambers, plants have filtered as much as one-tenth of the air every hour. "That sounds pretty good until you compare it to the ventilation in an average home," says Girman, who calculates that three-quarters of that air is replaced every hour. "In other words, that great result you´re getting in a sealed chamber isn´t anywhere near the normal air turnover you get in a typical home," he says. Wolverton, however, says the typical air change in a house is more like 40 percent an hour. And some structures are far more airtight. "Tightly sealed buildings create conditions very similar to static lab tests in that there is very little movement of air," he maintains.
Another concern is the static nature of lab tests, in which a plant processes a limited amount of toxic material. Most indoor air pollutants are continually emitted by their sources. "In real-life situations, those toxicants are constantly being replenished," says environmental scientist Thad Godish of Ball State University. "In fact, when you lower the concentration of toxicants in the atmosphere, the change in vapor pressure actually induces a source like particleboard to release them at a faster rate."
Wolverton counters that in tests he and colleagues have found that a plant can continuously remove formaldehyde from the air. Not only that, he has found that over time at least one plant, the lady palm, actually becomes more efficient at disposing of the gas.
The anecdotal evidence suggests benefits beyond anything demonstrated in the laboratory. "I get calls from people all the time who tell me their headaches or sore throats or other ´sick building´ symptoms have either gone away or greatly diminished after they´ve brought in plants," says Jan Roy, managing director of the Plants for Clean Air Council, an advocacy group funded in part by private growers.
Because each plant has an affinity for different toxics, Roy and Wolverton recommend using a mix of plants. "The more you have, the faster they´ll work," says Wolverton, who has recommended, as a rule of thumb, two or three plants in 8- or 10-inch pots for every 100 square feet of floor space.
One potential drawback of having many plants is excessive humidity, which can contribute to the growth of mold and bacteria and can encourage unwanted pests. To help prevent that, don´t let your plants sit in stagnant water. Once water has drained through the soil into the pan or tray below, dispose of it. Sub-irrigation planters, which have built-in watering tubes, bypass the problem altogether by funneling water directly to a reservoir in the soil. You can discourage mold from growing on top of the soil by covering it with Spanish moss or aquarium gravel.
Plants should never be thought of a cure-all, Wolverton cautions. "If you have a new building with serious indoor air pollution, even a large number of plants isn´t going to solve the problem," he says. But like a cat in your lap, a plant in your "personal breathing zone"--which Wolverton defines as the six to eight cubic feet of air surrounding an individual--can be a comforting presence.
"If you spend a lot of time at a computer or in an easy chair watching TV, try to have a nice plant sitting near you," he suggests. "Because the closer you are to that plant, the more it´s going to benefit you."
Ohio writer Frank Kuznik says some of his best friends are houseplants.
Top 10 Houseplants
Environmental engineer Bill Wolverton rates the following plants best for chemical-vapor removal, ease of growth and maintenance, resistance to pests and rate of water evaporation.
Dracaena (especially "Janet Craig")
Dwarf date palm
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