EPA's methylmercury guidelines will protect most Americans, but some may be at risk

provided by National Academy of Sciences


hile the US Environmental Protection Agency's guideline for protecting the public from a toxic form of mercury is justifiable, based on 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 Academies' National Research Council. Congress requested that the Research Council provide independent, scientific advice in establishing appropriate exposure limits for methylmercury.

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.

"Although we believe 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 US 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 of adverse health effects. However, the committee estimated that each year about 60,000 children may be born in the United States with neurological problems that could lead to poor school performance because of exposure to methylmercury in utero.


Had your dose today?


EPA's 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. EPA's reference dose is used to guide risk-management decisions and regulatory policies ranging from fish-consumption advisories to air-emission permits. According to EPA, the typical American consumer eats less than a third of an ounce of fish per day, and would be exposed to considerably less than its current guideline.

To draw its conclusions, the committee evaluated the range of data on which risk assessments conducted by EPA and other regulatory agencies are based. It also reviewed new findings that have emerged since the development of EPA's 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 EPA's reference dose is scientifically justifiable for protecting the health of the vast majority of Americans.

When the agency first developed its guideline five years ago, EPA judged data from a 1971 Iraqi poisoning incident to be the most relevant. To provide EPA with more appropriate data in formulating its reference dose, the committee analyzed population studies in the Faroe Islands, Seychelles Islands, and New Zealand. It concluded that the Faroe Islands analysis should be used by EPA as the critical study for deriving the reference dose, the report says.


It gets on your nerves


Neurodevelopmental problems are the most appropriate basis for setting an exposure limit, the committee found. Strong scientific evidence exists from human and animal studies to link certain levels of methylmercury exposure and neurological problems, including poor performance on tests that measure attention and motor function. However, 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 low-dose exposure to methylmercury 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. In addition to methylmercury, research on exposure to other forms of mercury, including mercury from dental fillings, is needed to see if they affect the human body's response to methylmercury.


Environmental sources

Mercury exists naturally in the environment and finds its way into the air through both natural processes and human activities. Power plants that burn fossil fuels, particularly coal, generate the greatest amount of 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: EPA and the Food and Drug Administration (FDA). FDA is charged with regulating commercially sold fish and seafood. EPA monitors concentrations in the environment and regulates industrial releases of mercury to surface water and air.

The study was sponsored by the US Environmental Protection Agency. The National Research Council is the principal operating arm of the National Academy of Sciences and the National Academy of Engineering. It is a private, nonprofit institution that provides independent advice on science and technology issues under a congressional charter.

Copies of Toxicological Effects of Methylmercury will be available at the end of August from the National Academy Press, (202) 334-3313 or (800) 624-6242. The cost of the report is $54.00 (prepaid) plus shipping charges of $4.50 for the first copy and $.95 for each additional copy