combustion sources. Medical and municipal waste incinerators and coal-fired utility boilers account for greater than 80% of the Hg emitted from point sources (EPA 1997b; ATSDR 1999).

FATE AND TRANSPORT

Hg has three valence states (Hg0, Hg1+, Hg2+) and is found in the environment in the metallic form and in various inorganic and organic complexes. The natural global bio-geochemical cycling of Hg is characterized by degassing of the element from soils and surface waters, atmospheric transport, deposition of Hg back to land and surface water, sorption of the compound onto soil or sediment particles, and revolatilization from land and surface water (see Figure 1-1). This emission, deposition, and revolatilization creates difficulties in tracing the movement of Hg to its sources (ATSDR 1999). Once in the environment, interconversion between the different forms of Hg can occur. Particulate-bound Hg can be converted to insoluble Hg sulfide and precipitated or bioconverted into more volatile or soluble forms that re-enter the atmosphere or are bioaccumulated in aquatic and terrestrial food chains. Conversion of inorganic Hg to MeHg occurs primarily in microorganisms especially in aquatic systems. Once in its methylated form, Hg bioaccumulates up the food chain; the microorganisms are consumed by fish, and the smaller fish are consumed by larger fish. Such bioaccumulation can result in very high concentrations of MeHg in some fish, which are one of the main sources of human and piscivorus wildlife exposure to MeHg.

HEALTH EFFECTS

Human exposure to MeHg from contaminated fish and seafood can pose a variety of health risks. A spectrum of adverse health effects has been observed following MeHg exposure, with the severity depending largely on the magnitude of the dose. Fatalities and devastating neurological damage were observed in association with the extremely high exposures that occurred during the Minamata and Iraqi poisoning



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