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Seafood Choices: Balancing Benefits and Risks
Mercury is a heavy metal that is present in the environment as a result of both human activities (referred to as anthropogenic sources) and natural processes. The primary anthropogenic source is the combustion of fossil carbon fuels, particulary from coal-fired utility boilers; other sources include municipal, medical, and hazardous waste incineration (NRC, 2000). The natural sources include volcanic emissions and the weathering of rock containing mercury ore. Mercury can be deposited locally or travel long distances in the atmosphere and contaminate sites far from its point of release. Further, the complex biogeochemistry of mercury fate and transport creates uncertainty in efforts to apportion the relative contributions of these processes to global mercury pollution. The US Environmental Protection Agency (US EPA) estimated that 50 to 75 percent of the total yearly input of mercury into the environment is anthropogenic (US EPA, 1997), while the United Nations Environment Programme (UNEP) suggests that this source accounts for more than half of the inputs (UNEP, 2002).
Mercury exists in the environment in several different forms, including metallic, inorganic, and organic, and interconversion between forms can occur. The form of mercury of greatest concern with regard to seafood consumption is methylmercury (MeHg). Methylmercury results when mercury in other forms is deposited in water bodies and biotransformed through the process of methylation by microorganisms. It bioaccumulates up the aquatic trophic food chain as smaller organisms are consumed by larger organisms. Because methylmercury is persistent, this bioaccumulation process results in large long-lived predatory species, such as certain sharks, swordfish, and tuna, or freshwater species such as bass, walleye, and pickerel having the highest concentrations (Kraepiel et al., 2003). Methylmercury levels can also be high in marine mammals such as whales, and in animals that feed on marine life, such as polar bears and sea birds. Consumption of aquatic life is the major route of human exposure to methylmercury. The seafood choices a consumer makes, and the frequency with which different species are consumed, are thus important determinants of methylmercury intake. Because of the global dispersion of methylmercury and migration of species, the extent of regional variation in body burdens among different aquatic animals is less striking than the regional variations in certain other water contaminants, such as PCBs or dioxin-like compounds (DLCs). This implies that the location in which an aquatic animal was caught might provide relatively little information about its methylmercury content.
Methylmercury is not lipophilic (lipid soluble) and is thus present in the largest concentrations in the muscle tissue of aquatic animals rather than in fat or oils. Approximately 95 percent of ingested methylmercury is absorbed across the gastrointestinal tract into the blood. The half-life