While the release of chemical pollutants into the air poses a health threat, the greater danger may lie in the by-products of the subsequent interaction of those chemicals in the presence of sunlight. Or at least that’s what is suggested by a mutagenicity study appearing in the March ENVIRONMENTAL SCIENCE AND TECHNOLOGY. It shows that toluene, a fairly simple and nonmutagenic hydrocarbon found in virtually all urban air, can be converted photochemically into gas-phase mutagens. Paul B. Shepson of Northrop Services Inc.
in Research Triangle Park, N.C., says that to date almost all mutagenicity studies of urban air pollutants have focused on compounds known as polynuclear aromatic hydrocarbons (PAHs), which are adsorbed onto airborne particulates.
“But there’s no evidence that this [research focus] is justified,” he says. “Though we know PAHs are mutagenic, no one has really addressed the question of the extent to which gas-phase hydrocarbons contribute to the overall mutagenic activity of urban air.” The Ames test performed by Shepson and colleagues at Northrop, together with researchers at two local Environmental Protection Agency laboratories, examined the ability of toluene and its breakdown products to induce mutations in bacteria. The test is widely used as a preliminary gauge of a material’s potential hazard as a cancer-causing agent. Because the chemistry of an urban atmosphere changes greatly over the course of a day, the researchers focused on the mix of hydrocarbons that would exist at both 3 hours and 6.7 hours after a typical atmospheric mix of toluene, oxides of nitrogen (NO.
sub.x.), water and clean air was pumped into closed reaction chambers and allowed to react in the presence of light. These particular temporal snap-shots of toluene photochemistry were selected, Shepson says, because the mix of photochemical products present “was as different in the two cases as possible.
” That’s because at 6.7 hours, reactive nitric oxide was no longer present. To do the Ames test, these chemical mixes had to be held in a constant steady-state rate of reaction for 18 hours, something not possible in the ambient atmosphere. In each test, bacteria were exposed to: (1) “clean air” only; (2) the initial mix of toluene, nitrogen dioxide, nitric oxide and water–but no light; (3) the irradiated mix of hydrocarbons that would be present at 3 or 6.7 hours; and (4) that latter mix minus any solid particles. The first two exposure regimes were not mutagenic.
Like the third test atmosphere, however, the gas-only reaction products showed strong mutagenic activity. Further analysis suggested that formaldehyde and peroxyacetyl nitrate (PAN) contribute to this gas-phase mutagenic activity. That in itself is important, Shepson says, “because there are a large number of hydrocarbons in the atmosphere that produce both PAN and formaldehyde in photooxidation processes.” More surprising, he says, is his group’s subsequent finding of a similar photochemically induced mutagenicity among the breakdown products of an even smaller organic chemical, propylene (C.sub.
6.). And when the researchers studied wood-smoke mixtures,” we got a large mutagenicity response there with gas-phase products,” he says. However, with potentially thousands of photooxidation products present in the irradiated wood-smoke mixture, “it’s absolutely hopeless to try and determine what caused the response,” he says.
That’s one reason his team plans to focus more attention on propylene. Explains Shepson, “We feel we should start with the simplest case and try to understand that before we move on to more complex ones.” In any case, concern remains that important and largely unrecognized gaseous mutagens may be prevalent in urban air.