the committee did not attempt to establish an RfD for MeHg. Instead, the committee provides guidance to EPA on the data sets, exposure-assessment approaches, modeling techniques, and statistical analysis that should be considered in deriving an appropriate Hg RfD.

SOURCES OF Hg

In the environment, Hg comes from natural and anthropogenic sources. Mercuric sulfide, or Hg in cinnabar, is the natural form of Hg. The concentration of cinnabar varies greatly with the location of deposits. Hg can be released into the air through weathering of rock containing Hg ore or through human activities, principally incineration and burning of fossil fuels. Hg is a global pollutant, that once released to the air can travel long distances and impact distant sites. Water contamination can occur from run-off water, contaminated by either natural or anthropogenic sources, or from air deposition. Potential sources of general population exposure to Hg include inhalation of Hg vapors in ambient air, ingestion of drinking water and foodstuffs contaminated with Hg, and exposure to Hg from dental amalgams and medical treatments. Dietary intake is one of the most important sources of non-occupational exposure to Hg, fish and other seafood products being the dominant source of Hg in the diet. Most of the Hg consumed in fish or other seafood is the highly absorbable MeHg form. The substantial variation in human MeHg exposure is based on the differences in frequency and amount of fish consumed and Hg concentration in the fish. MeHg exposure is a major problem in some populations, especially subsistence fish eaters who consume large amounts of fish (EPA 1997a). Intake of elemental Hg from dental analgams is another major contributing source to the total Hg body burden in humans in the general population (IPCS 1990, 1991).

The World Health Organization (WHO) has estimated that anthropogenic sources, mainly the combustion of fossil fuels, contribute 25% of the overall (natural and anthropogenic) Hg emissions to the atmosphere (ATSDR 1999). EPA has estimated that those sources account for 50% to 75% of the total yearly input of Hg into the atmosphere (EPA 1997a). In the United States, the majority of Hg emissions are from



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