EARTH Thailand

An Overview of Epidemiological Evidence on the Effects of Methylmercury on Brain Development, and A Rationale for a Lower Definition of Tolerable Exposure

Zero Mercury Working Group, prepared by Edward Groth, December 2012

It has been well known since about 1960 that methylmercury damages the developing brain. As research has progressed and methods have improved over the years, new evidence has consistently shown that harmful effects occur at lower levels of exposure than was previously recognized.

National and international government agencies have defined “tolerable exposure” limits, which are levels of intake of methylmercury believed, based on evidence available when they were set, to describe “safe” exposure, i.e., a level of intake sufficiently far below any exposure known to be harmful that it is reasonably certain to pose only a negligible risk, even to sensitive individuals and populations.

This document summarizes epidemiological research on the effects of methylmercury on the developing brain, beginning with severe pollution incidents in Japan that first documented methylmercury’s effects, examining studies that found subtler but similar effects in island populations with high-fish diets, and reviewing recent studies that raise concerns that methylmercury has adverse effects even at exposures typical of ordinary fish consumption in most countries.

We then describe two current definitions of “tolerable exposure,” and assess them in light of recent evidence that shows that neither definition currently provides an acceptable margin of safety. Finally, we propose a new definition of “safe” exposure based on all the evidence now available. The health effects of fish consumption and research studying them are complicated by the fact that fish contains beneficial nutrients; fish consumption during pregnancy and by young children is essential for brain development. In research, beneficial effects of nutrients can hide the negative effects of mercury, and vice versa. In risk management, women and children should not avoid fish or eat less fish (which could harm their health); instead they need guidance to choose low-mercury fish and shellfish.

Our proposed new definition of safe exposure balances these two important public-health concerns:

Encouraging consumption of widely available, popular low-mercury seafood varieties, while also keeping methylmercury exposure within acceptably safe limits.

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