Reference Dose for Methylmercury (External Review Draft)

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In 1997, U.S. EPA issued the Mercury Study Report to Congress (MSRC). Among the assessments in the MSRC was a state-of-the-science evaluation of the health effects of methylmercury. There has been considerable discussion within the scientific community regarding the level of exposure to methylmercury that is likely to be without appreciable risk of adverse health effects. Congress directed EPA through the House Appropriations Report for FY99 to contract with the National Research Council to evaluate the body of data on the health effects of methylmercury, with particular emphasis on new data since the 1997 Mercury Study Report to Congress, and provide recommendations regarding issues relevant to the derivation of an appropriate RfD for methylmercury. EPA has thoroughly reviewed this document, and concurs with the NRC findings and recommendations. The NRC document is used as the basis for the current EPA derivation of the RfD for methylmercury.

Methylmercury is a highly toxic substance; there are a number of adverse health effects associated with methylmercury exposure. Most extensive are the data for neurotoxicity, particularly in developing organisms. Therefore the brain is considered to be the most sensitive target organ for which there are data suitable for derivation of an RfD. The NRC considered three epidemiological longitudinal developmental studies suitable for quantitative risk assessment: the Seychelles Islands, the Faroe Islands, and New Zealand. The Seychelles study has yielded no evidence of impairment related to methylmercury exposure, while both the other studies have found dose-related adverse effects on a number of neuropsychological endpoints. The Faroe Islands study, the larger of the latter two studies, has been extensively peer-reviewed and is used for the derivation of the RfD. The NRC's major finding is that the results of the Faroe Islands study provide scientifically credible basis on which to base EPA's RfD.

The RfD derived in this assessment is 0.1 ug/kg per day. This is the same as the RfD derived by EPA in 1995 based on an earlier study of a poisoning episode in Iraq, in which data on adverse neurological effects in infants was used as the point of departure for derivation of the RfD.