"Methylmercury" is a shorthand for "monomethylmercury", and is more correctly "monomethylmercury(II) cation". It is composed of a methyl group (CH3-) bonded to a mercury ion; its chemical formula is CH3Hg+ (sometimes written as MeHg+). As a positively charged ion it readily combines with anions such as chloride (Cl−), hydroxide (OH−) and nitrate (NO3−). It also has very high affinity for sulfur-containing anions, particularly the thiol (-SH) groups on the amino acid cysteine and hence in proteins containing cysteine, forming a covalent bond. More than one cysteine moiety may coordinate with methylmercury, and methylmercury may migrate to other metal-binding sites in proteins.
Sources of methylmercury
Methylmercury is formed from inorganic mercury by the action of microbes that live in aquatic systems including lakes, rivers, wetlands, sediments, soils and the open ocean. 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 tonnes) of elemental mercury to the atmosphere each year, out of total anthropogenic mercury emissions in the United States of 158 tons (144 tonnes)/year. Natural sources of mercury to the atmosphere include volcanoes, forest fires, volatization from the ocean  and weathering of mercury-bearing rocks.
In the past, methylmercury was produced directly and indirectly as part of several industrial processes such as the manufacture of acetaldehyde. Currently there are few anthropogenic sources of methylmercury pollution in the United States other than as an indirect consequence of the burning of wastes containing inorganic mercury and from the burning of fossil fuels, particularly coal. Flooding of soils associated with reservoir creation (e.g. for hydroelectric power generation) has been linked to increased methylmercury concentrations in reservoir water and fish.
Methylmercury production in inland and marine ecosystems has been primarily attributed to anaerobic bacteria in the sediment. However, peaks in methylmercury in ocean water column and strong associations between methylmercury, nutrients and organic matter remineralization suggest water column production of methylmercury during carbon remineralization. Direct measurements of methylmercury production using stable mercury isotopes were made in oxic waters, but the microbes involved are still unknown.
Acute 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 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.
Because methylmercury is formed in aquatic systems and because it is not readily eliminated from organisms it is biomagnified in aquatic food chains from bacteria, to plankton, through macroinvertebrates, to herbivorous fish and to piscivorous (fish-eating) fish. At each step in the food chain, the concentration of methylmercury in the organism increases. The concentration of methylmercury in the top level aquatic predators can reach a level a million times higher than the level in the water. This is because methylmercury has a half-life of about 72 days in aquatic organisms resulting in its bioaccumulation within these food chains. Organisms, including humans, fish-eating birds, and fish-eating mammals such as otters and whales that consume fish from the top of the aquatic food chain receive the methylmercury that has accumulated through this process. Fish and other aquatic species are the only significant source of human methylmercury exposure.
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. 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 acidic also tend to have higher levels of methylmercury.
Human health effects
Ingested methylmercury is readily and completely absorbed by the gastrointestinal tract. It is mostly found complexed with free cysteine and with proteins and peptides containing that amino acid. The methylmercuric-cysteinyl complex is recognized by amino acid transporting proteins in the body as methionine, another essential amino acid. Because of this mimicry, it is transported freely throughout the body including across the blood–brain barrier and across the placenta, where it is absorbed by the developing fetus. Also for this reason as well as its strong binding to proteins, methylmercury is not readily eliminated. Methylmercury has a half-life in human blood of about 50 days.
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. Some evidence also suggests that methylmercury can cause autoimmune effects in sensitive individuals. Despite some concerns about the relationship between methylmercury exposure and autism, there are few data that support such a link. 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 
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 pollution and subsequent mass poisoning 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 neurological symptoms including paresthesias, loss of physical coordination, difficulty in speech, 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; 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.
Effects on fish and wildlife
In recent years, there has been increasing recognition that methylmercury affects fish and wildlife health, both in acutely polluted ecosystems and ecosystems with modest methylmercury levels. Two reviews document numerous studies of diminished reproductive success of both fish, fish-eating birds, and mammals due to methylmercury contamination in aquatic ecosystems.
In public policy
Methylmercury contamination in fish, along with fish consumption advisories, has the potential to disrupt people's eating habits, fishing traditions, and the livelihoods of people involved in the capture, distribution, and preparation of fish as a foodstuff for humans. Furthermore, proposed limits on mercury emissions have the potential to add costly pollution controls on coal-fired utility boilers. Therefore, the methylmercury issue has attracted the attention of many levels of government (see selected external links), environmental groups, consumer advocates, science groups, food-industry-funded groups that question the science, and significant interest from electric utilities.
- Dimethylmercury, mercury with a second methyl group
- Ethylmercury, a related cation
- Mercury poisoning
- Mercury regulation in the United States
- Minamata disease
- Canadian Reference Material of methylmercury
- Halliday, Tim; Davey, Basiro (2007). Water and health in an overcrowded world. Oxford: Oxford University Press. pp. 79, 80, 95. ISBN 9780199237302.
- Govindaswamy, N.; Moy, J.; Millar, M.; Koch, S. A. (1992). "A distorted mercury [Hg(SR)4]2− complex with alkanethiolate ligands: the fictile coordination sphere of monomeric [Hg(SR)x] complexes". Inorg. Chem. 26 (26): 5343–5344. doi:10.1021/ic00052a001.
- Erni, I.; Geier, G. (1979). "Kinetics of Extremely Fast Ligand Exchange Reactions with Methylmercury(II)-complexes of 1-Methylpyridine-2-thione and 1-Methyl-quinaldine-4-thione: Rate-Equilibria Correlations". Helv. Chim. Acta 62 (4): 1007–1015. doi:10.1002/hlca.19790620411.
- Ullrich, Susanne; Tanton, Trevor; Abdrashitova, Svetlana (2001). "Mercury in the Aquatic Environment: A Review of Factors Affecting Methylation". Critical Reviews in Environmental Science and Technology 31 (3): 241–293. doi:10.1080/20016491089226.
- 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.
- "Mercury in the Environment". U.S. Geological Survey. Retrieved 2013-09-20.
- 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.
- Schartup, Amina T.; Balcom, Prentiss H.; Soerensen, Anne L.; Gosnell, Kathleen J.; Calder, Ryan S. D.; Mason, Robert P.; Sunderland, Elsie M. (2015-09-22). "Freshwater discharges drive high levels of methylmercury in Arctic marine biota". Proceedings of the National Academy of Sciences 112 (38): 11789–11794. doi:10.1073/pnas.1505541112. ISSN 0027-8424. PMC 4586882. PMID 26351688.
- St.Louis, Vincent L.; Rudd, John W. M.; Kelly, Carol A.; Bodaly, R. A. (Drew); Paterson, Michael J.; Beaty, Kenneth G.; Hesslein, Raymond H.; Heyes, Andrew; Majewski, Andrew R. (2004-03-01). "The Rise and Fall of Mercury Methylation in an Experimental Reservoir". Environmental Science & Technology 38 (5): 1348–1358. doi:10.1021/es034424f. ISSN 0013-936X.
- Compeau, G. C.; Bartha, R. (1985-08-01). "Sulfate-Reducing Bacteria: Principal Methylators of Mercury in Anoxic Estuarine Sediment". Applied and Environmental Microbiology 50 (2): 498–502. ISSN 0099-2240. PMC 238649. PMID 16346866.
- Mason, R. P.; Fitzgerald, W. F. (1990-10-04). "Alkylmercury species in the equatorial Pacific". Nature 347 (6292): 457–459. doi:10.1038/347457a0.
- Sunderland, Elsie M.; Krabbenhoft, David P.; Moreau, John W.; Strode, Sarah A.; Landing, William M. (2009-06-01). "Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models". Global Biogeochemical Cycles 23 (2): GB2010. doi:10.1029/2008GB003425. ISSN 1944-9224.
- Lehnherr, Igor; Louis, Vincent L. St.; Hintelmann, Holger; Kirk, Jane L. "Methylation of inorganic mercury in polar marine waters". Nature Geoscience 4 (5): 298–302. doi:10.1038/ngeo1134.
- reviewed in Wiener, J.G., Krabbenhoft, D.P., Heinz, G.H., and Scheuhammer, A.M., 2003, "Ecotoxicology of mercury," Chapter 16 in Hoffman, D.J., B.A. Rattner, G.A. Burton, Jr., and J. Cairns, Jr., eds., Handbook of Ecotoxicology, 2nd edition.: Boca Raton, Florida, CRC Press, p. 409-463.
- Burros, Marian (2008-01-23). "High Mercury Levels Are Found in Tuna Sushi". The New York Times.
- 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.
- Kerper, L.E.; Ballatori, N.; Clarkson, T.W. (May 1992). "Methylmercury transport across the blood–brain barrier by an amino acid carrier". Am. J. Physiol. 262: R761–5. PMID 1590471.
- Carrier, G; Bouchard, M; Brunet, RC; Caza, M (2001). "A Toxicokinetic Model for Predicting the Tissue Distribution and Elimination of Organic and Inorganic Mercury Following Exposure to Methyl Mercury in Animals and Humans. II. Application and Validation of the Model in Humans". Toxicology and Applied Pharmacology 171 (1): 50–60. doi:10.1006/taap.2000.9113. PMID 11181111.
- Rice, DC; Schoeny, R; Mahaffey, K (2003). "Methods and rationale for derivation of a reference dose for methylmercury by the U.S. EPA". Risk analysis : an official publication of the Society for Risk Analysis 23 (1): 107–15. doi:10.1111/1539-6924.00294. PMID 12635727.
- Salonen, J. T.; Seppänen, K.; Nyyssönen, K.; Korpela, H.; Kauhanen, J.; Kantola, M.; Tuomilehto, J.; Esterbauer, H.; Tatzber, F.; Salonen, R. (1995). "Intake of Mercury from Fish, Lipid Peroxidation, and the Risk of Myocardial Infarction and Coronary, Cardiovascular, and Any Death in Eastern Finnish Men". Circulation 91 (3): 645–55. doi:10.1161/01.CIR.91.3.645. PMID 7828289.
- Guallar E, Sanz-Gallardo I, van't Veer P, et al., 2002, Mercury, Fish Oils, and the Risk of Myocardial Infarction, New England Journal of Medicine, vol. 347, p. 1747-1754.
- 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.
- Hultman, P; Hansson-Georgiadis, H (1999). "Methyl mercury-induced autoimmunity in mice". Toxicology and applied pharmacology 154 (3): 203–11. doi:10.1006/taap.1998.8576. PMID 9931279.
- Guallar, E; Sanz-Gallardo, MI; Van't Veer, P; Bode, P; Aro, A; Gómez-Aracena, J; Kark, JD; Riemersma, RA; Martín-Moreno, JM; Kok, FJ; Heavy Metals Myocardial Infarction Study Group (2002). "Mercury, fish oils, and the risk of myocardial infarction". The New England Journal of Medicine 347 (22): 1747–54. doi:10.1056/NEJMoa020157. PMID 12456850.
- Choi, AL; Cordier, S; Weihe, P; Grandjean, P (2008). "Negative confounding in the evaluation of toxicity: The case of methylmercury in fish and seafood". Critical Reviews in Toxicology 38 (10): 877–93. doi:10.1080/10408440802273164. PMC 2597522. PMID 19012089. Review. Erratum in: "Erratum". Critical Reviews in Toxicology 39: 95. 2009. doi:10.1080/10408440802661707.
- Strain, JJ; Davidson, PW; Bonham, MP; Duffy, EM; Stokes-Riner, A; Thurston, SW; Wallace, JM; Robson, PJ; Shamlaye, CF; Georger, LA; Sloane-Reeves, J; Cernichiari, E; Canfield, RL; Cox, C; Huang, LS; Janciuras, J; Myers, GJ; Clarkson, TW (2008). "Associations of maternal long-chain polyunsaturated fatty acids, methyl mercury, and infant development in the Seychelles Child Development Nutrition Study". Neurotoxicology 29 (5): 776–82. doi:10.1016/j.neuro.2008.06.002. PMC 2574624. PMID 18590765.
- Khan, MA; Wang, F (2009). "Mercury-selenium compounds and their toxicological significance: Toward a molecular understanding of the mercury-selenium antagonism". Environmental toxicology and chemistry / SETAC 28 (8): 1567–77. doi:10.1897/08-375.1. PMID 19374471. Review.
- Heath, JC; Banna, KM; Reed, MN; Pesek, EF; Cole, N; Li, J; Newland, MC (2010). "Dietary selenium protects against selected signs of aging and methylmercury exposure". Neurotoxicology 31 (2): 169–79. doi:10.1016/j.neuro.2010.01.003. PMC 2853007. PMID 20079371.
- Myers, G. J.; Davidson, P. W.; Weiss, B. (2004). "Methyl mercury exposure and poisoning at Niigata, Japan" (PDF). SMDJ Seychelles Medical and Dental Journal 7 (Special Issue): 132–133. Archived from the original (PDF) on May 5, 2006. Retrieved January 12, 2006.[dead link]
- For example: Hightower, JM; Moore, D (2003). "Mercury levels in high-end consumers of fish". Environmental Health Perspectives 111 (4): 604–8. doi:10.1289/ehp.5837. PMC 1241452. PMID 12676623.
- Information on characteristic levels of methylmercury by species can be found at http://www.cfsan.fda.gov/~frf/sea-mehg.html
- A wallet-card guide for consumers can be found at http://www.nrdc.org/health/effects/mercury/protect.asp
- Scheuhammer, Anton M.; Meyer, Michael W.; Sandheinrich, Mark B.; Murray, Michael W. (2007). "Effects of Environmental Methylmercury on the Health of Wild Birds, Mammals, and Fish". AMBIO: A Journal of the Human Environment 36: 12–19. doi:10.1579/0044-7447(2007)36[12:EOEMOT]2.0.CO;2. ISSN 0044-7447.
- Wheatley, B; Wheatley, M (2000). "Methylmercury and the health of indigenous peoples: a risk management challenge for physical and social sciences and for public health policy". The Science of the Total Environment 259 (1–3): 23–9. doi:10.1016/S0048-9697(00)00546-5. PMID 11032132.
- for example, National Wildlife Federation and Environmental Working Group
- EPA Ignored Science When Regulating Power Plant Mercury Emissions
- for example,  and 
- ATSDR - ToxFAQs: Mercury
- ATSDR - Public Health Statement: Mercury
- ATSDR - ALERT! Patterns of Metallic Mercury Exposure, 6/26/97
- ATSDR - MMG: Mercury
- ATSDR - Toxicological Profile: Mercury
- National Pollutant Inventory - Mercury and compounds Fact Sheet
- Methylmercury-in-fish exposure calculator provided by GotMercury.Org, which uses FDA mercury data with the EPA's calculated safe exposure levels.
- Methylmercury Contamination in Fish and Shellfish
- nytimes.com, Tuna Fish Stories: The Candidates Spin the Sushi
- U.S. Environmental Protection Agency's mercury site
- U.S. Geological Survey's mercury site
- Environment Canada's mercury site
- Health Canada's mercury site
- International Conference on Mercury as a Global Pollutant 2006-Madison, WI USA 2009-Guizhou, China 2011-Halifax, NS Canada