What Do You Know About Mercury in Fish?

Mercury is a liquid metal used in thermometers, barometers, blood pressure devices, antiseptic compounds, and fungicides. With the rise of manufacturing in the Industrial Revolution, the production of mercury increased. Because liquid mercury can vaporize directly into the atmosphere, the more mercury that is manufactured, the more that evaporates.

Methylmercury is an organic form of mercury that accumulates in the food chain. It becomes increasingly concentrated in the animals at the top of the food chain and thus causes them the most harm. Methylmercury accounts for more than 95 percent of the mercury found in food fish.

Tuna are a large predatory fish at the top of the food chain, where methylmercury tends to concentrate. Plankton is tiny food for almost all small (and some large) marine animals. Smaller marine animals are food for bigger marine animals. As mercury moves up the chain, the larger fish and the longest-lived fish accumulate the mercury that has been absorbed by the smaller plants and animals.

Again, the higher up the food chain you eat, the greater the concentrations of mercury tend to be. Tuna are high on the food chain.

The fetus is the most sensitive to the toxic effect of mercury. Unfortunately, the damage done to a fetus by mercury appears to be irreversible even when the exposure to mercury is removed. Although mercury is always toxic, it becomes less destructive with age. An adult is able to tolerate higher exposure without apparent adverse effects when compared with a fetus, child, or adolescent.

State and tribal health programs regularly issue fish consumption advisories regarding how much fish a person can safely eat from certain lakes, rivers, and coastal waters without developing mercury toxicity.

About 8 percent of women of childbearing age were found to have levels of mercury above the suggested upper safe limit. Women contemplating pregnancy might want to consider giving up fish (particularly if they eat a lot of fish). Methylmercury is eliminated from the body slowly and has a half-life of approximately 70 days. That means if you stopped all exposure to mercury, in 70 days you would have only half the original level of mercury and in 140 days only one-fourth the level.

Mercury damages the brain and can cause a broad spectrum of neurological problems. One of the worst exposures in history was at Minamata Bay, Japan, where hundreds of pregnant women consumed fish contaminated with methylmercury. Their children were born with devastating neurological handicaps.

Since Jan. 1, 2001, the FDA has determined that shark, swordfish, king mackerel, and tile-fish have such high levels of mercury that pregnant women, women who are breastfeeding, and young children should not have even a single serving of these types of fish.

This is approximately half the amount allowed for marine fish. It is important to take into account the type of fish and where they were caught. Some areas are so polluted that fishing is prohibited. You should check with your state health department if you are unsure about the pollution status of the area where the fish were caught.