Division of Environmental Quality

Mercury in the Environment and Fish

Effects of Mercury on the Environment

Mercury released outdoors is also a problem. Natural sources of mercury releases to the environment include volcanic eruptions and forest fires. In addition to natural sources, there have been decades of mercury releases from the burning of coal, the production of paper and chlorine, waste incineration, and other industrial processes. Because it is an element, mercury does not break down into less harmful substances. It will circulate in the atmosphere until it falls to earth attached to rain or dust particles. Eventually it makes its way to lakes and streams where bacteria transform it into methylmercury, a much more toxic form. In an aquatic environment, mercury reaches its highest concentrations in the larger predatory fish - the type most often sought by anglers.

Illustration of the Mercury Cycle in the Environment.
Graphic: Dynamic cycling of mercury, or Hg, in the environment. United States Geological Survey.

Mercury in Fish

The majority of mercury in the environment is in a gaseous form, shown here as elemental mercury or Hg(0). In the atmosphere, elemental mercury is converted to a more water-soluble form, ionic mercury or Hg(II), which is returned to the earth's surface in precipitation. Bacteria can convert these forms of mercury to methylmercury (MeHg), a much more toxic form that can concentrate quickly up the food chain. The amount of mercury in the air, plants, animals, land and water is constantly cycling between each of these components.

Organic mercury compounds are formed when elemental mercury combines with carbon. When mercury is released to the environment some of it gets converted by microscopic organisms into methylmercury, a highly toxic organic mercury compound. Methylmercury is highly soluble, mobile and bioaccumulative in the food chain. Bioaccumulation is the process where organisms take up a contaminant faster than their bodies can eliminate it. Methylmercury can move quickly from water to sediment, aquatic plants, invertebrates and fish. It concentrates, or biomagnifies, as it moves up through the food chain. Biomagnification is the incremental increase in a contaminant's concentration at each level of the food chain. In an aquatic environment, mercury reaches its highest concentrations in the larger predatory fish - the type most often sought by anglers.

In Missouri, mercury concentrations in some fish exceed what is considered to be safe levels for human consumption. The Missouri Department of Health and Senior Services, or DHSS, advises women who are pregnant or who may become pregnant, nursing mothers and children 12 years of age and younger not to eat any largemouth bass more than 12 inches in length from anywhere in Missouri. For more information, see the DHSS Fish Advisory.  See also the DNR publication Mercury in Missouri Streams and Lakes, Fact Sheet for further information on mercury in the aquatic environment and fish consumption advisories.

In addition to Missouri's advisories, the U.S. Food and Drug Administration and Environmental Protection Agency have issued a saltwater fish consumption advisory for women of childbearing age who are pregnant or may become pregnant, nursing mothers and young children. The FDA recommends these individuals should not eat shark, swordfish, king mackerel and tilefish, and should limit consumption of shrimp, canned light tuna, salmon, pollock and catfish. Albacore tuna has higher levels of mercury than other tuna and so greater limitations on consumption of albacore tuna are also recommended. For further information, see EPA's brochure What You Need to Know about Mercury in Fish and Shellfish.

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