Greenery filters PAH-lution from skies.

Five years ago, a NASA scientist reported that English ivy, potted mums, and other houseplants can remove significant quantities of several noxious pollutants from indoor air (SN: 9/30/89, p.212). Now, two academic researchers observe that grasses, shrubbery, and trees may provide much the same function for the outdoor environment.

Traffic, residential heating, industrial processes, and other human activities together spew some 7.5 million kilograms of polycyclic aromatic hydrocarbons (PAHs) into U.S. skies. These combustion by-products include a number of known or suspected carcinogens.

Staci L. Simonich and Ronald A. Hites of Indiana University in Bloomington suspected that because these compounds can accumulate in waxes, the outer tissues of many plants might serve as storage depots for them. So they began studying concentrations of 10 representative PAHs in the vegetation and soil at a suburban Indiana site.

Their data confirmed that waxiness tends to control a plant's PAH uptake. But even within a particular tissue type, plants vary their PAH acquisition by season--and by temperature.

The Indiana scientists used these data to model plants' PAH removal in the industrialized region running from Kansas, Missouri, Kentucky, and Virginia north to Canada. Some 52 percent of the U.S. population makes its home there. Coupling these figures with previously published data for soil, air, and water, the researchers developed what Hites describes as the first "unified" picture of the fate of PAHs.

The pair now reports in the July 7 NATURE that plants appear to pick up a large fraction of this pollution--perhaps 43.5 percent in the northeast quarter of the United States. Eventually these PAHs enter the soil as leaf litter and other decaying plant matter. The researchers' analysis indicates that another 10 percent of the pollution falls directly onto the soil and 5 percent into water, where it eventually settles into sediment. The remaining roughly 41 percent either becomes chemically transformed in the atmosphere or wafts across U.S. borders.

If anything surprised him, Hites says, it was that plants didn't have a bigger role. "Almost everything's covered with vegetation, which has about 10 times the surface area of [the land] on which it grows," he says. "And a very waxy material--some of it quite rough--covers vegetation's entire surface. You just can't imagine a better [PAH] scavenger."