Fish, Women and Mercury

©Johan Boshoff

Scientists are warning that the danger of mercury poisoning may be greater than previously thought, especially for children and babies developing in the mother's womb. People who eat a lot of fish are the most at risk, from exposure to a compound known as methyl mercury. Mercury emitted into the atmosphere by industrial processes enters water bodies, where bacteria convert it into methyl mercury.

This compound then passes into the food chain, where it accumulates in the bodies of fish, especially those that live the longest. According to the American Food and Drug Administration (FDA) the concentrations of methyl mercury in large fish can be over a million times higher than in the surrounding water. Just as it accumulates in fish, methyl mercury also accumulates in humans, and can pass through a mother's blood stream into a developing baby.

An adult woman can have no signs of mercury poisoning, but her baby can be born disabled, as methyl mercury can enter its developing brain and wreak havoc. The FDA reports that foetuses are the most at risk in the human population. As the effects are cumulative, women should consider their intake of fish over their reproductive lifetime, and especially during the time that they are pregnant or breastfeeding.

Although fish and shellfish are important to a healthy diet because of their fatty acid content, certain fish have higher mercury concentrations, and the FDA warns that these should not be eaten. They include shark, swordfish, king mackerel or tilefish. Other species that are low in mercury include shrimp, canned light tuna, salmon, pollock and catfish. Albacore or white tuna has more mercury in it than canned tuna. Mercury vapour coming out of liquid or solid mercury is also considered to be harmful to people, but the severity of the risk has not been quantified by scientists.

People are exposed to mercury vapour when anything containing elemental mercury breaks open. This includes thermometers, barometers, thermostats and some electrical switches. Dental fillings also contain mercury which releases vapour, but the effect of these has not been conclusively studied. Some medicines also contain mercury salts, including fungicides, antiseptics and disinfectants. A mercury compound called thimerosal is also found in vaccines and has been accused of causing autism.

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