Methylmercury

Abstract Methylmercury (MeHg) is a heavy metal that is dispersed into the environment as the result of both natural and anthropogenic processes. The primary pathway of human exposure is the consumption of seafood from contaminated waters. Because of biomagnification and bioaccumulation as MeHg ascends the aquatic food chain, MeHg levels are highest in the muscles of long‐lived predatory fish. MeHg is actively transported to the brain and across the placenta. The devastating effects of high‐dose MeHg exposures were revealed by poisoning episodes in Japan and Iraq. These episodes resulted in the recognition that fetal exposure is particularly deleterious. They also provided motivation for the conduct of many epidemiological studies seeking to determine whether adverse effects result from chronic low‐dose MeHg exposure as the result of frequent consumption of seafood. For risk assessment purposes, the most important of these studies, some of which are ongoing, were conducted in New Zealand, the Faroe Islands, and the Seychelles Islands. The findings have been mixed, and different governmental agencies have made different choices in selecting the data on which to base MeHg risk assessments. The benchmark dose modeling approach used by the US Environmental Protection Agency to establish a MeHg reference dose (RfD) is described. The sensitivity of the RfD to the underlying assumptions is illustrated. All MeHg risk assessments are complicated by the fact that the consumption of seafood also provides beneficial nutrients whose health effects can be antagonistic to those of MeHg.