especially in emissions from coal-fired power plants. Because concerns have been raised about Hg exposure levels in the United States, particularly among sensitive populations, questions have arisen among federal agencies over what is an acceptable level of exposure to MeHg.

Due to disagreement over the appropriate level of concern for MeHg exposure, the potentially widespread implications for human health, and the challenges associated with further regulating Hg emissions, Congress directed EPA in the House Appropriations Report for EPA 's Fiscal 1999 funding to contract with the National Research Council (NRC) to prepare recommendations on the appropriate value for a Hg exposure reference dose (RfD). In response, the NRC convened the Committee on the Toxicological Effects of Mercury, whose membership includes experts in toxicology, pharmacology, medicine, epidemiology, developmental psychology, neurophysiology, neuropsychology, public health, nutrition, statistics, exposure assessment, and risk assessment. The committee was charged with the following specific tasks:

  1. Evaluate the body of evidence that led to the EPA-derived MeHg RfD. Human epidemiological and animal toxicity data should be the basis of the evaluation. The evaluation should determine the appropriateness of the critical study, end point of toxicity, and uncertainty factors used by EPA in deriving the RfD for MeHg. Sensitive populations should be considered.

  2. Evaluate any new data (e.g., mechanistic data) that were not considered in EPA's 1997 Hg report that are relevant to EPA's MeHg RfD for protecting human health.

  3. Consider exposure pathways (especially from the consumption of MeHg in fish) in evaluating likely human exposures, especially exposures of sensitive subpopulations. The evaluation should focus on those elements of exposure relevant to the establishment of an appropriate RfD.

  4. Identify data gaps and make recommendations for future research.

Although the committee name, the Committee on the Toxicological Effects of Mercury, does not limit the scope of this report to MeHg, the committee focused on the health effects of this organic form of Hg because the toxicity due to this form is of greatest concern. In addition,

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