Mercury in Fish

Some pollutants build up in the bodies of fish and other animals, reaching high concentrations at upper levels of the food chain. For mercury, this “bioaccumulation” process occurs primarily in aquatic systems such as lakes or oceans. Mercury that is deposited from air currents or released to water bodies tends to become attached to particles and deposited into sediments. There, under certain conditions, bacteria convert metallic or elemental forms of mercury into methylmercury.¹ Methylmercury can be absorbed in particles or from the water by small creatures such as shrimp or other invertebrates, which then are consumed by predators including fish.² As each organism builds up methylmercury in its own tissues, and as smaller fish are eaten by larger fish, concentrations of methylmercury can accumulate, particularly in those of large fish that live a relatively long time.^3-6 Examples of other chemicals that bioaccumulate include dioxins, PCBs, and chlorinated pesticides such as DDT or chlordane. Some of these chemicals also may pose risks to children.

Fish are the most common source of exposure to methylmercury for most people in the United States^{7, 8} and in many countries around the world.⁹ As noted in Part 2 of this report, about 8 percent of women of child-bearing age have blood mercury concentrations greater than 5.8 parts per billion (equivalent to EPA’s reference dose) based on data from the Centers for Disease Control and Prevention. EPA has determined that children born to such women may be at some increased risk of potential adverse health effects.¹⁰ Chemicals accumulated by women may pass through the umbilical cord, contributing to prenatal exposure in children. Prenatal exposure to such levels of methylmercury may cause developmental and cognitive effects in children, even at doses that do not result in effects in women who are or may become pregnant.^11-13

Some proportion of mercury that ends up in fish originates as emissions to the air. Mercury released into the atmosphere can travel long distances on global air currents and be deposited in areas far from its original source.^{14, 15} The largest human-generated source of mercury emissions in the United States is the burning of coal, which is roughly one percent of mercury in the global pool. Other sources include the combustion of waste and industrial processes that use mercury.¹⁴

Information regarding warnings to the public about elevated concentrations of methylmercury in fish provides some indication of the likelihood of exposure to mercury from fish that people catch for their own use. Fish advisory information is not a surrogate for exposure to the general population, because most people eat only commercial fish that they purchase in stores or restaurants. However, there are subpopulations who do consume fish they have caught from waters covered by fish advisories, and fish advisory information is an indirect surrogate for exposure to these populations.

The scope of the warnings issued by states varies considerably. States typically advise people to reduce their consumption of contaminated fish by switching to less-contaminated species or to smaller fish that have not accumulated as much mercury. For methylmercury, which accumulates in muscle tissue, changes in cooking practices such as trimming fat or cooking over a grill do not reduce exposure. States often provide guidance about the maximum number of meals of fish that can be safely consumed. Some warnings apply to entire states, others are issued for individual lakes or streams. States also issue warnings for other contaminants besides mercury.

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1. J. R. D. Guimaraes, J. Ikingura and H. Akagi. 2000. Methyl mercury production and distribution in river water-sediment systems investigated through radiochemical techniques. Water, Air, and Soil Pollution 124 (1-2):113-124.
   
2. C. Y. Chen, R. S. Stemberger, B. Klaue, J. D. Blum, P. C. Pickhardt and C. L. Folt. 2000. Accumulation of heavy metals in food web components across a gradient of lakes. Limnology and Oceanography 45 (7):1525-1536.
   
3. R. P. Mason, J. R. Reinfelder and F. M. M. Morel. 1995. Bioaccumulation of mercury and methylmercury. Water, Air, and Soil Pollution 80:915-921.
   
4. R. Dietz, F. Riget, M. Cleemann, A. Aarkrog, P. Johansen and J. C. Hansen. 2000. Comparison of contaminants from different trophic levels and ecosystems. Science of the Total Environment 245 (1-3):221-231.
   
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8. G. J. Myers and P. W. Davidson. 2000. Does methylmercury have a role in causing developmental disabilities in children? Environmental Health Perspectives 108 (Suppl. 3):413-20.
   
9. H. Galal-Gorchev. 1993. Dietary intake, levels in food and estimated intake of lead, cadmium, and mercury. Food Additives and Contaminants 10 (1):115-28.
   
10. U.S. Environmental Protection Agency. 2001. Integrated Risk Information System (IRIS) Risk Information for Methylmercury (MeHg). Washington, DC: National Center for Environmental Assessment. http://www.epa.gov/iris/subst/0073.htm.
    
11. P. Grandjean, R. F. White, A. Nielsen, D. Cleary and E. C. de Oliveira Santos. 1999. Methylmercury neurotoxicity in Amazonian children downstream from gold mining. Environmental Health Perspectives 107 (7):587-91.
    
12. P. Grandjean, P. Weihe, R. F. White and F. Debes. 1998. Cognitive performance of children prenatally exposed to “safe” levels of methylmercury. Environmental Research 77 (2):165-72.
    
13. P. Grandjean, P. Weihe, R. F. White, F. Debes, S. Araki, K. Yokoyama, K. Murata, N. Sorensen, R. Dahl and P. J. Jorgensen. 1997. Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury. Neurotoxicology and Teratology 19 (6):417-28.
    
14. U.S. Environmental Protection Agency. 1996. Mercury Study Report to Congress, Volumes I to VII. Washington, DC: Office of Air Quality Planning and Standards. http://www.epa.gov/oar/mercury.html.
    
15. W. F. Fitzgerald, D. R. Engstrom, R. P. Mason and E. A. Nater. 1998. The case for atmospheric mercury contamination in remote areas. Environmental Science and Technology 32 (1):1-7.

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