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Methylmercury is an organometallic cation with the formula . It is the simplest organomercury compound. Methylmercury is extremely toxic, and its derivatives are the major source of organic mercury for humans. It is a bioaccumulative environmental toxicant with a 50-day half-life. (2025). 9780199237302, Oxford University Press. ISBN 9780199237302
Methylmercury (derived biologically from dimethylmercury) is the causative agent of the infamous Minamata disease.
As a positively charged ion, it readily combines with such as chloride (), hydroxide () and nitrate (). It has particular affinity for sulfur-containing anions, particularly (). Thiols are generated when the amino acid cysteine and the peptide glutathione form strong complexes with methylmercury:
There are various sources of inorganic mercury that may indirectly contribute to the production of methylmercury from microbes in the environment. Natural sources of mercury released to the atmosphere include , wildfire, volatilization from the ocean and weathering of Cinnabar rocks.Tewalt, S. J.; Bragg, L. J.; Finkelman, R. B., 2005, Mercury in U.S. coal -- Abundance, distribution, and modes of occurrence, U.S. Geological Survey Fact Sheet 095-01. Access-date=January 12, 2006. Anthropogenic sources of mercury include the burning of wastes containing inorganic mercury and from the burning of , particularly coal. Although inorganic mercury is only a trace constituent of such fuels, their large scale combustion in utility and commercial/industrial boilers in the United States alone results in release of some 80.2 tons (73 Tonne) of elemental mercury to the atmosphere each year, out of total anthropogenic mercury emissions in the United States of 158 tons (144 metric tons)/year.U. S. Environmental Protection Agency, 1997, "Mercury study report to congress, Volume II: An inventory of anthropogenic mercury emissions in the United States" , table ES-3, sum of Utility boilers and Commercial/industrial boilers. Report: EPA-452/R-97-004.
In the past, methylmercury was produced directly and indirectly as part of several industrial processes such as the manufacture of acetaldehyde. However, currently there are few direct anthropogenic sources of methylmercury pollution in the United States.
Whole-lake ecosystem experiments at IISD-ELA in Ontario, Canada, showed that mercury falling directly on a lake had the fastest impacts on aquatic ecosystems as opposed to mercury falling on the surrounding land. This inorganic mercury is converted to methylmercury by bacteria. Different Isotope analysis of mercury were added to lakes, , and uplands, simulating rain, and then mercury concentrations in fish were analyzed to find their source. The mercury applied to lakes was found in young-of-the-year yellow perch within two months, whereas the mercury applied to wetlands and uplands had a slower but longer influx.
Acute methylmercury poisoning can occur either directly from the release of methylmercury into the environment or indirectly from the release of inorganic mercury that is subsequently methylated in the environment. For example, methylmercury poisoning occurred at Grassy Narrows in Ontario, Canada (see Ontario Minamata disease), as a result of mercury released from the mercury-cell Chloralkali process, which uses liquid mercury as an electrode in a process that entails electrolytic decomposition of brine, followed by mercury methylation in the aquatic environment. An acute methylmercury poisoning tragedy occurred also in Minamata, Japan, following release of methylmercury into Minamata Bay and its tributaries (see Minamata disease). In the Ontario case, inorganic mercury discharged into the environment was methylated in the environment; whereas, in Minamata, Japan, there was direct industrial discharge of methylmercury.
The concentration of mercury in any given fish depends on the species of fish, the age and size of the fish and the type of water body in which it is found. In general, fish-eating fish such as shark, swordfish, marlin, larger species of tuna, walleye, largemouth bass, and northern pike, have higher levels of methylmercury than herbivorous fish or smaller fish such as tilapia and herring. Mercury Levels in Commercial Fish and Shellfish Accessed March 25, 2009. What You Need to Know about Mercury in Fish and Shellfish Accessed March 25, 2009. Within a given species of fish, older and larger fish have higher levels of methylmercury than smaller fish. Fish that develop in water bodies that are more also tend to have higher levels of methylmercury.
Several studies indicate that methylmercury is linked to subtle developmental deficits in children exposed in utero such as loss of IQ points, and decreased performance in tests of language skills, memory function and attention deficits. Methylmercury exposure in adults has also been linked to increased risk of cardiovascular disease including heart attack.Choi, A.L., Weihe, P., Budtz-Jørgensen, E., Jørgensen, P.J., Salonen, J.T., Tuomainen, T.-P., Murata, K., Nielsen, H.P., Petersen, M.S., Askham, J., and Grandjean, P., 2009, Methylmercury Exposure and Adverse Cardiovascular Effects in Faroese Whaling Men: Environmental Health Perspectives, v. 117, no. 3, p. 367–372. Some evidence also suggests that methylmercury can cause autoimmune effects in sensitive individuals. There is some evidence suggesting a possible connection between post-natal mercury exposure and autism; however, it is not clear whether methylmercury intake in particular is linked in a similar way. Although there is no doubt that methylmercury is toxic in several respects, including through exposure of the developing fetus, there is still some controversy as to the levels of methylmercury in the diet that can result in adverse effects. Recent evidence suggests that the developmental and cardiovascular toxicity of methylmercury may be mitigated by co-exposures to omega-3 fatty acids and perhaps selenium, both found in fish and elsewhere. Review. Erratum in: Review.
There have been several episodes in which large numbers of people were severely poisoned by food contaminated with high levels of methylmercury, notably the dumping of industrial waste that resulted in the Minamata disease in Minamata and Niigata, Japan and the situation in Iraq in the 1960s and 1970s in which wheat treated with methylmercury as a preservative and intended as seed grain was fed to animals and directly consumed by people (see Basra poison grain disaster). These episodes resulted in neurology including , loss of physical coordination, dysarthria, narrowing of the visual field, hearing impairment, blindness, and death. Children who had been exposed in utero through their mothers' ingestion were also affected with a range of symptoms including motor difficulties, sensory problems and intellectual disability.
At present, exposures of this magnitude are rarely seen and are confined to isolated incidents. Accordingly, concern over methylmercury pollution is currently focused on more subtle effects that may be linked to levels of exposure presently seen in populations with high to moderate levels of dietary fish consumption. These effects are not necessarily identifiable on an individual level or may not be uniquely recognizable as due to methylmercury. However, such effects may be detected by comparing populations with different levels of exposure. There are isolated reports of various clinical health effects in individuals who consume large amounts of fish;For example: however, the specific health effects and exposure patterns have not been verified with larger, controlled studies.
Many governmental agencies, the most notable ones being the United States Environmental Protection Agency (EPA), the United States Food and Drug Administration (FDA), Health Canada, and the European Union Health and Consumer Protection Directorate-General, as well as the World Health Organization (WHO) and the United Nations Food and Agriculture Organization (FAO), have issued guidance for fish consumers that is designed to limit methylmercury exposure from fish consumption. At present, most of this guidance is based on protection of the developing fetus; future guidance, however, may also address cardiovascular risk. In general, fish consumption advice attempts to convey the message that fish is a good source of nutrition and has significant health benefits, but that consumers, in particular pregnant women, women of child-bearing age, nursing mothers, and young children, should avoid fish with high levels of methylmercury, limit their intake of fish with moderate levels of methylmercury, and consume fish with low levels of methylmercury no more than twice a week.Information on characteristic levels of methylmercury by species can be found at A wallet-card guide for consumers can be found at http://www.nrdc.org/health/effects/mercury/protect.asp
About 30% of the distributed mercury depositional input is from current anthropogenic sources, and 70% is from natural sources. The natural sources category includes re-emission of mercury previously deposited from anthropogenic sources. According to one study, based on modeled concentrations, pre-Anthropocene tissue-bound levels in freshwater fish may not have differed markedly from current levels. However, based on a comprehensive set of global measurements, the ocean contains about 60,000 to 80,000 tons of mercury from pollution, and mercury levels in the upper ocean have tripled since the beginning of the industrial revolution. Higher mercury levels in shallower ocean waters could increase the amount of the toxicant accumulating in food fish, exposing people to a greater risk of mercury poisoning.Carl H. Lamborg, Chad R. Hammerschmidt, Katlin L. Bowman, Gretchen J. Swarr, Kathleen M. Munson, Daniel C. Ohnemus, Phoebe J. Lam, Lars-Eric Heimbürger, Micha J. A. Rijkenberg & Mak A. Saito (2014) A global ocean inventory of anthropogenic mercury based on water column measurements, Nature, 512, 65–68, doi:10.1038/nature13563
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