Do Two Pollutants Make You Sicker Than One?Peter Hong
The corner of 135th Street and Riverside Drive in West Harlem isn't on any Superfund list. Yet it's an environmental hot spot all the same. Close by, the North River treatment plant often emits a sulfurous odor as it processes 175 million gallons of sewage a day. Bulldozers kick up clouds of dust as they create a new state park on top of the plant. At the adjacent marine transfer station on the Hudson River, garbage trucks spew black smoke as they load trash onto barges headed for landfills on Staten Island. City buses using a nearby terminal add to the filth in the air. "We need to know if the convergence of so much pollution is a major health risk," says Peggy Shepard, co-founder of a community group, West Harlem Environmental Action.
The urban ecosystem in this slice of New York is typical of the environmental witches' brew found throughout the U.S. And researchers are beginning to suspect that residents near sites of concentrated pollution may have higher death and sickness rates than the norm. A recent study by the New York City Health Dept., for instance, shows that residents of the heavily industrialized Greenpoint-Williamsburg areas of Brooklyn suffer higher rates of stomach cancer and leukemia than other New Yorkers. Such findings raise troubling scientific and regulatory issues. Does exposure to multiple toxic substances magnify the risk of disease and death? Should regulators set new emissions standards to solve these problems?
The danger may seem obvious, but until recently, such questions weren't seriously considered. Now, the Environmental Protection Agency is setting out to answer them. In the past year, it has launched studies to determine if minorities, who often live in more polluted communities, face greater risks than currently assumed. Conclusions that will affect policy may be years away, and the process of assessing "cumulative risks" will be contentious. Still, the inquiry sets the stage for tougher regulations, EPA officials say. Those could lower emissions limits for factories, make it harder to site facilities in toxics-laden locations, and close roads through heavily polluted areas. "We need to develop a strategic plan for attacking concentrations of risk," says Robert M. Wolcott, a director in the EPA's Office of Policy, Planning & Evaluation.
PROOF POSITIVE? The first hint of the dangers of cumulative exposure to numerous toxics surfaced in a 1984 government study of uranium miners who were exposed to high radiation levels. The study found that miners who smoked multiplied their risk for lung cancer several times. "There's a multiplicative effect," asserts Richard D. Thomas, director of human toxicology and risk assessment at the National Research Council.
Toxicologists agree, however, that more research is needed to prove the links between exposures to multiple toxics and disease. Even the federal government's long-standing method of identifying single cancer-causing substances is under review. To test single substances, the National Toxicology Program generally exposes rats to higher doses of chemicals than any human would realistically face. Researchers then use mathematical models to extrapolate the rate at which diseases would occur in humans exposed to much lower levels. Now, some scientists contend that the procedure labels some chemicals as carcinogenic that really are safe. On Sept. 11, the Health & Human Services Dept. started to examine whether these testing methods should be changed.
Assessing the risk of multiple exposures is even dicier. When testing one chemical, scientists can use an unexposed group of rats as a benchmark for analyzing results. But early studies of cumulative risk in humans have no such controls. "You have heterogeneous populations, and people who are exposed to different chemicals," says Bruce A. Fowler, director of the University of Maryland's toxicology program.
Still, advances in molecular biology, which enable scientists to analyze responses to chemical exposures in cells and DNA, promise to provide new tools for assessing cumulative damage. By studying these "biomarkers," or the changes in a gene or a biological function in animals, researchers have identified several effects of multiple exposures (table, page 77).
ACTING LOCALLY. Fowler has used these methods to study the effect of lead exposure on inner-city children in Baltimore. While most researchers check lead levels in blood, he tests for porphyrins, the precursor to heme, which carries oxygen in the blood. Since lead inhibits porphyrin production, this biomarker is a more sensitive indicator of the effect of lead exposure. Using this technique, Fowler exposed rats to both lead and arsenic and found that their porphyrin production was inhibited twice as much as when they were exposed to single doses of each substance. Each chemical inhibits porphyrins in different ways, so Fowler could discern the effects of each. He says research could provide the basis for reforming pollution regulations to account for the effects of toxic mixtures.
Besides the EPA studies, the U.S. Public Health Service is surveying the areas surrounding Superfund sites to determine demographics and disease rates. Already, a recent EPA study of Native American tribes along the Columbia River has found that tribal members may face a higher risk of cancers and other ailments. They consume 10 times the amount of fish in EPA assumptions used to set emission standards for paper mills, which could expose tribal members to unusually high levels of dioxins, PCBs, and other toxics. If further study shows excessive levels of contamination of residents' blood, the government may decide to tighten discharge limits for those plants.
Despite the potential for tighter regulations, business hasn't yet raised an alarm. Industry representatives think it's too early to know how best to deal with mixtures of toxics. Says Katherine A. Rosica, director of health and safety for the Chemical Manufacturers Assn.: "Additional research needs to be done."
Even so, business had better keep an eye on cumulative risk. Industry is already upset by the cost of pollution rules. It will face even stiffer standards if researchers prove that the effects of multiple exposures are serious.