Methylmercury’s toxic toll

More than 60,000 children are born each year in the United States with neurodevelopmental impairments caused by exposure in the womb to methylmercury compounds, according to new estimates by an expert panel convened by the National Academy of Sciences. The panel released its 290-page review of methylmercury’s toxicity earlier this month.

Many industrial processes, especially fossil-fuel burning, spew inorganic mercury into the air. Once it rains down, the pollutant usually undergoes a chemical transformation—methylation—into a much more toxic form. Fish and other aquatic life readily pick up and store the resulting methylmercury (MeHg) in their fat (SN: 3/9/91, p. 152).

At highest risk, therefore, are children whose mothers ate large amounts of tainted fish and other seafood during pregnancy. Because fish provide a host of healthy nutrients, the panel argues that the long-term goal should be reducing MeHg concentrations in U.S. fish, not cutting fish from the diet.

Although the effects of MeHg exposure are subtle, the panel argues, they could “result in an increase in the number of children who have to struggle to keep up in school and who might require remedial classes or special education.”

Philippe A. Grandjean of Boston University School of Public Health found such subtle problems in language, attention, and memory among more than 900 Faroe Islands 7-year-olds, whose mothers had eaten MeHg-tainted seafood during pregnancy. Most cognitive problems were linked to MeHg exposures “considered safe,” he reported last month in Bar Harbor, Maine, at a meeting on endocrine disruptors in the marine environment.

The National Academy of Sciences panel also cited studies linking MeHg exposures to high blood pressure and abnormal heart rates. The studies sometimes found such effects from exposures even lower than those causing neurodevelopmental harm.

The report concludes, however, that toxicity has emerged mainly within a special, fairly small population—people who eat large amounts of local fish. For everyone else, it finds EPA’s current MeHg guidelines “justifiable.”

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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