Report on Methylmercury.

While the U.S. Environmental Protection Agency's (U.S. EPA's) guideline for protecting the public from methylmercury is justifiable according to the latest scientific evidence, some children of women who consume large amounts of fish and seafood during pregnancy may be at special risk of neurological problems, says a new report from the National Research Council (NRC).

Fish and other seafood products are the main source of methylmercury in the human diet. Fetuses are particularly vulnerable to methylmercury because of their rapid brain development, and some may currently be receiving exposures at levels that cause observable adverse neurological effects.

"More must be done to gain a better understanding of various risk factors for the U.S. population," said Robert A. Goyer, chair of the committee that wrote the report and professor emeritus at the University of Western Ontario, who now resides in Chapel Hill, N.C. "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."

Based on an analysis of available data that included exposure levels to methylmercury and food-consumption surveys, the committee said the majority of Americans are at low risk for adverse health effects. The committee estimated, however, that because of exposure to methylmercury in utero, about 60,000 children may be born each year in the United States with neurological problems that could lead to poor school performance.

U.S. EPAs current reference dose for methylmercury is 0.1 micrograms per kilogram of body weight per day--the amount of methylmercury to which an individual can be exposed on a daily basis without adverse health consequences. The reference dose is used to guide risk-management decisions and regulatory policies ranging from fish-consumption advisories to air-emission permits. According to U.S. EPA, the typical American consumer eats less than a third of an ounce of fish per day and would be exposed to considerably less methylmercury than the reference dose.

To draw its conclusions, the NRC committee evaluated the range of data on which risk assessments conducted by U.S. EPA and other regulatory agencies are based. It also reviewed new findings that have emerged since the development of U.S. EPAS current reference dose in 1995 and met with researchers of major ongoing population studies. The overall weight of the evidence from this comprehensive review led the committee to conclude that U.S. EPAs reference dose is scientifically justifiable for protecting the health of the vast majority of Americans.

When U.S. EPA first developed its guideline five years ago, it judged data from a 1971 Iraqi poisoning incident to be the most relevant. To provide U.S. EPA with more appropriate data for formulating the reference dose, the NRC committee analyzed population studies in the Faroe Islands, Seychelles Islands, and New Zealand. It concluded that U.S. EPA should use the Faroe Islands analysis to derive the reference dose, the report says.

Neurodevelopmental problems are the most appropriate basis for setting an exposure limit, the committee found. Strong scientific evidence from human and animal studies links certain levels of methylmercury exposure and neurological problems, including poor performance on tests that measure attention and motor function. Researchers still need to understand if there is a precise time during development when the brain is most sensitive to methylmercury and exactly how the chemical exerts its effects. Evidence also indicates that the cardiovascular and immune systems could be affected by methylmercury, the report notes. Information on whether methylmercury causes cancer in humans is still inconclusive.

Scientists do not agree on how to account for some uncertainties, such as varying individual responses to methylmercury exposure and emerging health concerns. Better data are needed to decrease the uncertainties, the report says. For example, further investigation is needed on the effects of low-dose exposure throughout the life span of humans and animals, and on carcinogenic, neurologic, reproductive, and immunologic effects, including the emergence of delayed neurological effects later in life. More research on factors that might influence responses, such as genetics, age, sex, health status, and nutrition, also is needed.

Likewise, research should be conducted to gather data on methylmercury exposure in different regions of the United States and in specific populations with high consumption of fish, the committee noted. Research on exposure to other forms of mercury, including mercury from dental fillings, is needed to see if that exposure affects the response of the human body to methylmercury.

Mercury exists naturally in the environment and finds its way into the air through natural processes and human activities. Power plants that burn fossil fuels, particularly coal, generate the greatest mercury emissions. Once mercury is deposited in lakes, rivers, and oceans, it is converted to methylmercury by aquatic organisms. Humans are exposed to the chemical when they eat fish.

In the United States, responsibility for regulating mercury is shared by two federal agencies: U.S. EPA and the Food and Drug Administration (FDA). FDA is charged with regulating fish and seafood commercially U.S. EPA monitors concentrations in the environment and regulates industrial releases of mercury to surface water and air.

Copies of Toxicological Effects of Methylmercury are available from the National Academy Press at [phone] or [phone]. The cost of the report is $54 (prepaid) plus shipping charges of $4.50 for the first copy and $0.95 for each additional copy.