Methylmercury in fish

Clarkson, T.W. 1990. Human health risks from methylmercury in fish. Environ, Toxicol. Chem. 9 957-961. 

Mercury is mined predominantly as HgS in cinnabar ore and is then converted commercially to a variety of chemical forms. Key industrial and commercial applications of mercury are found in the electrolytic production of chlorine and caustic soda the manufacture of electrical equipment, thermometers, and other instruments fluorescent lamps dental amalgam and artisanal gold production. Use in pharmaceuticals and in biocides has declined substantially in recent years, but occasional use in antiseptics and folk medicines is still encountered. Thimerosal, an organomercurial preservative that is metabolized in part to ethylmercury, has been removed from almost all the vaccines in which it was formerly present. Environmental exposure to mercury from the burning of fossil fuels, or the bioaccumulation of methylmercury in fish, remains a concern in some regions of the world. Low-level exposure to mercury released from dental amalgam fillings occurs, but systemic toxicity from this source has not been established. 

On the basis of chemical profile, Wood (38) predicted that arsenic, selenium, and tellurium will be methylated in the environment, and lead, cadmium, and zinc will not. Elemental concentration in the aquatic food chain has been reported for As (39), Hg (40), Cd (41), Pb (42), and Cu (43). The biological half-life of methylmercury in fish, for example, is one to two years (44). Pillay et al. (40) implicated heavy coal burning in the mercurial contamination of plankton and fish populations of Lake Erie. Other metals, notably cadmium, have been shown to be incorporated into the grazing grasses surrounding a coal burning source (27). Trace element contamination, therefore, can enter the food chain at various points. Disposal of solid wastes in the form of ash and slag is yet another environmental consideration (45). 

Sherlock, J., J. Hislop, D. Newton, G. Topping, and K. Whittle. 1984. Elevation of mercury in human blood from controlled chronic ingestion of methylmercury in fish. Hum. Toxicol. 3(2) 117-131. 

Dioxins and polychlorinated biphenyls (PCBs) in fatty foods, methylmercury in fish, tetrabutyltin in molluscs or fumonisins in com products are some examples of chemical contaminants. Frequently problems concerning chemical contaminants in food reach the headlines of major newspapers and the reader may be left with a scare, sometimes with good reason, often not. In any event, those responsible for the sale of foods must be alert. They bear the responsibility for the safety of the foods they offer for sale, and they also know that any such alert may instantly influence the sale, whether the problem raised is more or less serious. 

Fischer, R., Rapsomanikis, S. and Andreae, M.O. (1993) Determination of methylmercury in fish samples using GC/AA and sodium tetraethylborate derivatization. Anal. Chem., 65, 763-766. 

Westoo, G. Determination of methylmercury compounds in foodstuffs. Determination of methylmercury in fish, egg, meat and liver. Acta Chem. Scand. 21, 1790 (1967). 

R. Rai, W. Maher, F. Kirkowa, Measurement of inorganic and methylmercury in fish tissues by enzymatic hydrolysis and HPLC-ICP-MS, J. Anal. Atom. Spectrom., 12 (2002), 1560-1563. 

H. E. L. Palmieri, L. V. Leonel, Determination of methylmercury in fish tissue by gas chromatography with microwave-induced plasma atomic emission spectrometry after derivatization with sodium tetraphenylborate, Fresenius J. Anal. Chem., 366 (2000), 466-469. 

X. P. Yan, Y. Li, Y. Jiang, Selective measurement of ultratrace methylmercury in fish by flow injection on-line microcolumn displacement sorption preconcentration and separation coupled with electrothermal atomic absorption spectrometry, Anal. Chem., 75 (2003), 2251-2255. 

L. Yang, Z. Mester, R. E. Sturgeon, Determination of methylmercury in fish tissues by isotope dilution SPME-GC-ICP-MS, J. Anal. At. Spectrom., 18 (2003), 431-436. 

Currently, hair is used routinely to monitor exposure to methylmercury in fish-eating native populations of northern Canada. The objective is to adjust consumption of fish, an important, yet contaminated nutritional source, so as not to exceed concentrations of 30pgg of hair in the general adult population and 15 pg in fertile women. 

The concerns aroused by methylmercury in fish, arising from the susceptibility of the developing brain to this neurotoxicant, led to the design and execution of two large prospective studies. One was located in the Seychelle islands, which lie in the Indian Ocean. The other was located in the Faroe Islands, which lie in the North Sea. Both communities consume large quantities of seafood. In the Seychelles, it is almost exclusively in the form of fish. In the Faroes, virtually all the methylmercury comes from the consumption of pilot whales, which are also contaminated with polychlorinated biphenyls (PCBs). Both studies assayed maternal exposures to methylmercury. In the Seychelles, maternal hair was used as the index it reflects the history of blood levels. The Faroes study relied primarily on cord blood. 

Diet. Based on the FDA total diet study of 1982-1984 (Gunderson 1988), FDA estimated that the average intake for total mercury (both inorganic and organic) is 50-100 ng/kg/day. Based on the more recent 1989-1990 FDA total diet study, the estimated intake of total mercury is 27-60 ng/kg/day (Cramer 1994). An estimated 86% of the mercury in the total diet study is derived from fish (Tollefson and Cordle 1986). A separate estimate of the average intake of methylmercury alone, based on a survey of fish eaters and average levels of methylmercury in fish, places the average intake of methylmercury at 36 ng/kg/day, with a 99% upper bound at 243 ng/kg/day (Clarkson 1990). 

Regarding the bioavailability of methylmercury in fish, the available data indicate that methylmercury uptake is not affected by its presence in fish. Experimental studies on the metabolism of methylmercury in humans following the ingestion of contaminated fish (using methylmercury bound to fish muscle protein) have shown that absorption is almost complete (95% absorbed) (Miettinen 1973). Animal studies also support this absorption value. Data on cats given fish homogenates indicate absorptions of 90% of methylmercury, whether added to the homogenate, accumulated by fish in vivo, or from methyl-... 

There are differences in the outcomes of these epidemiology studies on low level chronic exposures to methylmercury in foods. Davidson et al. (1998) report no adverse developmental effects associated with prenatal and postnatal exposure to methylmercury in fish in a Seychelles Island cohort of children at age 66 months (n=708). The exposure levels are reflected in maternal hair levels of 6.8 ppm for the prenatal exposure (SD=4.5, n=711) and children s hair levels of 6.5 ppm (SD=3.3, n=708) for both the prenatal and subsequent postnatal exposure. The age-appropriate main outcome measures included (1) the... 

Malm O, Branches FJ, Akagi H, et al. 1995. Mercury and methylmercury in fish and human hair from the Tapajos River Basin, Brazil. Sci Total Environ 175(2) 141-150. 

Tollefson L, Cordle F. 1986. Methylmercury in fish A review of residue levels, fish consumption and regulatory action in the United States. Environ Health Perspect 68 203-8. 

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