Mercury and Seafood

By Ellen Kuwana
Neuroscience for Kids Staff Writer
July 23, 2004

Mercury is a naturally occurring heavy metal. It is also a result of industrial pollution: coal-fired power plants in the US spew 48 tons of mercury into the atmosphere each year. The mercury from the air settles into the soil and rivers, where it is converted by bacteria into methylmercury. It is this form of mercury that is particularly dangerous to animals, including humans.

In March 2004, the US Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) issued a joint consumer advisory about mercury in seafood. Researchers measured methylmercury levels in the tissues of various fish and shellfish. Fish at the top of the food chain tend to live longer. The combination of eating other fish and living a long life leads to a higher mercury build-up in their tissue. Four types of fish (swordfish, king mackerel, shark and tilefish) were shown to have mercury levels high enough to cause damage to a developing baby's brain. Therefore, pregnant and nursing women should not eat these types of fish.

The Association of Reproductive Health Professionals recommends women of childbearing age and children should limit seafood intake to one serving (6 ounces) per week of the following fish: fresh or canned albacore tuna, grouper, marlin, bluefish and orange roughy. These recommendations are based on mercury levels and the level of contaminants such as PCBs, which also impair development and functioning of the nervous system. The government advises that people (pregnant women included) limit intake of seafood to 12 ounces (340 grams, or 2 average meals) of low-mercury seafood per week. Low-mercury seafoods include shrimp, salmon, pollock, catfish, monkfish, bass, trout, cod, haddock, tilapia and canned light tuna. (Albacore "white" tuna has higher mercury levels than canned light tuna.) It is recommended that people eat a variety of seafood, not just one type.

Although all seafood has trace levels of mercury, there are many benefits to eating seafood. For example, the omega-3 fatty acids that are found in fish help prevent heart disease and improve symptoms of psychiatric disorders such as depression, schizophrenia, bipolar disorder and possibly attention deficit hyperactivity disorder. British researchers have shown that children who are given capsules of omega-3 fatty acids improve in their schoolwork.

Mercury is known to damage the nervous system. A mother who is exposed to mercury passes it on to her unborn child; similarly, mercury can be passed to a baby from the mother's breast milk. Problems with memory, attention and language have been documented in children who were exposed to "moderate levels" of mercury while in the womb. Mercury exposure can lead to cognitive impairment, blindness, seizures, kidney damage, as well as general problems with the nervous system and digestive system. Young children are more sensitive to the effects of mercury than adults. Therefore, pregnant women (or women considering getting pregnant), children and nursing mothers should be especially careful about how much and what type of seafood they eat. Many people will be happy to know that fish sticks and most fast food fish sandwiches are made with fish that are low in mercury.

Some folk remedies and rituals use mercury in the form of metallic droplets, which are typically stored in a sealed pouch. Sold as "azogue" in stores called "botanicas," the mercury is used in rites of Esperitismo (from Puerto Rico), Santeria (Cuban-based) and voodoo. The mercury is sprinkled into bathwater, perfume, devotional candles, or placed in the home or car. Short- and long-term use is hazardous, and damage can be done before any symptoms appear.

For more information about seafood and mercury, see resources and references below, or call 1-888-SAFEFOOD (the US FDA).

Did You Know?

  • Packaged whale meat in Japan was found to contain dangerously high levels of mercury. The Japanese government set the limit at 0.4 parts per million (0.4 ppm); whale meat samples tested at 160-200 times that limit. (Source: June 15, 2003, Journal of Environmental Science and Technology; written up in Scientific American, May 20, 2003)

  • As many as 60,000 newborns a year in the US are at risk for neurodevelopmental damage as a result of dietary mercury exposure. (Source: The National Research Council)

  • As many as 4.9 million women of childbearing age in the US (8%) have unsafe mercury levels in their blood. (Source: Centers for Disease Control and Prevention)

  • Fish oil supplements have been found to be high in pollutants such as PCBs and mercury, both of which impair development and functioning of the nervous system. (Source: Reuters Health, June 24, 2004)

References and Further Information:

  1. Environmental Protection Agency home page on mercury
  2. Mercury Health Hazards from the National Institutes of Health. Includes a schematic of how mercury gets into wildlife.
  3. How mercury affects your health from the EPA
  4. Fish Advisories from the EPA, in English and Spanish
  5. Use of mercury in ritualistic healing, folk medicine from the Agency for Toxic Substances and Disease Registry
  6. Effects of mercury on the nervous system from Neuroscience for Kids

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