Mercury

Mercury is a metal that is present in the environment from natural and man-made sources (e.g., coal-burning or other industrial pollution). It is converted primarily by microorganisms to a more toxic form, methylmercury, which is bioaccumulated in the aquatic food chain, reaching it highest levels in large, longer living predatory fish. Among humans, the sole source of exposure to methylmercury is the consumption of fish and sea mammals.

Methylmercury is neurotoxic and accumulates in the brain and central nervous system. It inhibits the division and migration of neuronal cells and disrupts the cytoarchitecture of the developing brain. Although a mother may show no signs of neurotoxi-city, the developing fetus may be damaged following exposure to methylmercury. The concentration of methylmercury in fetal brain has been shown to be 5-7 times higher than that in maternal blood, and it has been estimated that the fetus is 5-10 times more sensitive to methylmercury exposure than an adult, although the reason for this is unknown.

Disasters in Minamata, Japan, in the 1950s and in Iraq in 1971-1972 demonstrated that acute prenatal exposure may result in severe mental retardation, cerebral palsy, blindness, and deafness. However, whether exposure to lower chronic doses, which may occur if pregnant women consume large amounts of fish, can also lead to adverse neurodevelopmental consequences is less certain. Large, long-term prospective epidemio-logical studies of high fish-eating populations have not found a consistent pattern of association between exposure and neuropsychological outcomes. Although subtle neuropsychological changes were reported in a study of children in the Faroe Islands study, where exposure was mainly from whale consumption, a similar study in the Seychelles found no adverse effects from fish consumption alone.

The Joint FAO/WHO Committee on Food Additives revised its safety guideline for weekly intake of methylmercury, known as the provisional tolerable weekly intake (PTWI), to 1.6 mg/kg body weight per week. The UK government's independent expert Committee on Toxicity of Chemicals in Food, Consumer Products, and the Environment (COT) has applied a lower PTWI limit of 0.7 mg/kg body weight per week to women who are pregnant or those intending to become pregnant and to mothers who are breast feeding.

Any public health recommendations to pregnant women regarding fish consumption must recognize the important role that it plays as part of a healthy, balanced diet. Most fish contain trace amounts of methylmercury, but high concentrations of the metal have only been found in large, predatory fish, such as shark, marlin, and swordfish (Table 5). If a pregnant or breast feeding mother were to consume one portion of these predatory fish, she would exceed the lower PTWI set by COT and the EPA by 400%. Therefore, as a precaution, pregnant women, breast feeding mothers, and those who intend to become pregnant within the next 12 months are advised to avoid consumption of these types of fish (in the United States, this also includes king mackerel and tilefish). Some samples of tuna have also been found to have higher levels than other species. In the United Kingdom, pregnant women (and those who may become pregnant) are advised to restrict their weekly intake to two 140-g portions of fresh tuna or four 140-g portions of canned tuna.

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