The National Academy of Sciences endorsed a controversial mercury exposure guideline established by the EPA, a decision that is likely to clear the way for the agency later this year to curb mercury emissions from coal-fired power plants.

The utility industry has opposed such regulations. Congress two years ago blocked the EPA from imposing new restrictions on mercury emissions until the NAS reviewed the agency’s new exposure guidelines.

The EPA’s new exposure standard was controversial because it is more strict than that of two other federal agencies, the Food and Drug Administration and the Agency for Toxic Substances and Disease Registry.

But in a report issued July 11, the NAS concluded that the EPA’s guidelines were “scientifically justifiable.” In fact, it said the guidelines offered “little or no margin of safety” for the children of pregnant women who eat large quantities of fish or other seafood.

Elevated levels of mercury are found in fish in many parts of the country and around the world. While mercury comes from natural sources, most of what is in the environment is thought to stem from air pollution. Coal-fired power plants are thought to produce the most mercury emissions, with lesser amounts coming from municipal and medical waste incinerators.

When mercury enters waterways it can — under certain conditions — be transformed into methylmercury, which bioaccumulates in the food chain.

A recent report produced for the Maryland Department of Natural Resources found that large fish in some state reservoirs had mercury levels in their flesh that exceed federal guidelines. No problems were found in the Bay. The state is conducting follow-up studies.

The researcher who did the Maryland study said some reservoirs in Pennsylvania and Virginia are likely to have mercury problems as well because the region gets high levels of mercury deposition, which is thought to come from Midwest power plants. [See “Mercury from upwind air pollution may pose threat to fish,” Bay Journal, July-August 2000]

The NAS report said that most Americans are at low risk of adverse health effects from mercury exposure. Those most at risk are the children of women who consume large amounts of fish during pregnancy.

The report estimated that about 60,000 children a year may be born in the United States with neurological problems because of exposure to methylmercury as fetuses. The panel also said that cardiovascular and immune systems could be affected, and added that more work is needed to determine whether long-term exposure to low levels of methylmercury causes harm to humans and animals.

The report cautioned that fish is generally a healthy food and most people don’t consume enough fish to pose a significant risk. But it said more studies are needed to better identify populations that may face particularly high risks.

“Although we believe the EPA’s guideline on methylmercury is generally adequate to protect most people, more must be done to gain a better understanding of various risk factors for the U.S. population,” said Robert Goyer, who is chair of the committee and wrote the report. “Trends in methylmercury exposure, including regional differences, should be analyzed, as should subpopulations whose diets are high in fish and seafood. And we need to better understand how this chemical affects brain development in fetuses and children.”