Mercury

Mercury is a toxic metal that can cause neurological damage, especially during early development. Mercury is transported globally in the atmosphere and seafood consumption is an important source of exposure.

The health effects of mercury depend on the dose, timing, and route of exposure, as well as the form of mercury involved. Mercury exists in three chemical forms: elemental or metallic mercury, inorganic mercury salts, and organic mercury. Inhalation of elemental mercury can harm the brain and kidneys, but inorganic mercury compounds have relatively limited ability to enter the brain, according to the U.S. Agency for Toxic Substances and Disease Registry (ATSDR). Unlike other forms of mercury, the most common type of organic mercury, methylmercury (HgCH3 or MeHg), is readily absorbed into the body through ingestion.

Incidents resulting in high methylmercury exposures have shown that neurotoxicity is the most prominent outcome and prenatal and childhood exposure can be particularly hazardous. Neurological damage observed in adults and especially children in Japan in the 1950s was linked to consumption of fish contaminated by industrial dumping of mercury into Minamata Bay. Mercury-contaminated grain in Iraq in the early 1970s had similar effects. Epidemiologic studies in the Faroe Islands and the Seychelles have informed risk assessment.

Exposure to methylmercury typically occurs through seafood, but the original mercury source can be far removed from the fish. ATSDR's Toxicological Profile for Mercury says that between 33 and 66 percent of mercury releases are estimated to come from anthropogenic sources and “approximately 80 percent of the mercury released from human activities is elemental mercury released to the air, primarily from fossil fuel combustion, mining, and smelting, and from solid waste incineration.” This mercury can travel long distances globally and ultimately deposit in soil or bodies of water in a distant region.

Aquatic microorganisms convert elemental mercury into methylmercury, which bioaccumulates in the tissues of fish and biomagnifies so higher levels of the food chain have higher concentrations of mercury. Thus, large predatory fish can have high methylmercury levels.

Although reducing mercury emissions might have a greater impact, interventions often focus on consumer education. The U.S. Environmental Protection Agency (EPA) and Food and Drug Administration (FDA) issued a Joint Advisory in 2004 recommending that women who are or may become pregnant, nursing mothers, and young children avoid eating shark, swordfish, king [Page 1117]mackerel, and tilefish—the species typically containing the most mercury—and eat up to 12 ounces per week of a variety of fish including no more than 6 ounces of albacore tuna. Advocacy groups have urged governments to take stronger action.

The EPA, FDA, and the Joint Food and Agriculture (FAO)/World Health Organization (WHO) Expert Committee on Food Additives (JECFA) point out that the risks of consuming seafood contaminated by methylmercury or other toxins should be weighed against seafood's nutritional benefits, particularly in the context of varying regional diets. Seafood contains nutrients including omega-3 fatty acids, which have been associated with cardiovascular benefits. Certain fish, such as wild salmon, typically have low mercury but high omega-3 fatty acid levels.

In addition to diet, occupational exposure to mercury in gold mining and other industries is a hazard in certain regions. People can be exposed to high levels of elemental mercury from amalgam dental fillings, but the potential health effects of these fillings have been debated.

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