Exposure to mercury, even in small amounts, is a great danger to humans and wildlife.When mercury enters the body it acts as a neurotoxin, which means it harms our brain and nervous system. Mercury exposure is especially dangerous to pregnant women and young children, but all adults are at risk for serious medical problems. Most mercury pollution is produced by coal-fired power plants and other industrial processes. The most common way we are exposed to mercury is by eating contaminated fish.(1)
Fish and shellfish concentrate mercury in their bodies, often in the form of methylmercury, a highly toxic organic compound of mercury. Fish products have been shown to contain varying amounts of heavy metals, particularly mercury and fat-soluble pollutants from water pollution. Species of fish that are long-lived and high on the food chain, such as marlin, tuna, shark,swordfish, king mackerel, tilefish (Gulf of Mexico), and northern pike, contain higher concentrations of mercury than others.(2)
Mercury is known to bioaccumulate in humans, so bioaccumulation in seafood carries over into human populations, where it can result in mercury poisoning. Mercury is dangerous to both natural ecosystems and humans because it is a metal known to be highly toxic, especially due to its ability to damage the central nervous system. In human-controlled ecosystems of fish, usually done for market production of wanted seafood species, mercury clearly rises through the food chain via fish consuming small plankton, as well as through non-food sources such as underwater sediment. This mercury grows in concentration within the bodies of fish and can be measured in the tissues of selected species.(3)
Do you need to avoid the mercury found in fish?
For most people, the level of mercury absorbed by eating fish and shellfish is not a health concern. Overall, fish and shellfish are healthy foods. They contain high-quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids, a type of essential fatty acid. A balanced diet that includes fish and shellfish can contribute to heart health and children’s growth and development.
Nearly all fish and shellfish contain traces of mercury. But some contain high levels. Eating large amounts of these fish and shellfish can result in high levels of mercury in the human body. In a fetus or young child, this can damage the brain and nerves (nervous system).(4)
Because of the mercury found in fish, the U.S. Food and Drug Administration (FDA) and U.S. Environmental Protection Agency (EPA) advise the following people to avoid eating fish high in mercury and to eat limited amounts of fish and shellfish that are lower in mercury:(5)
- Women who may become pregnant
- Pregnant women
- Nursing mothers
- Young children
Environmental Protection Agency (EPA). The EPA recommendation is based on body weight and is, therefore, dependent on a person’s size. The guidance below is based on a 6oz serving of cooked fish for a 130lb/60kg woman. People with small children who want to use the list as a guide should reduce portion sizes.(6)
Levels of contamination
The danger level from consuming fish depends on species and size. Size is the best predictor of increased levels of accumulated mercury. Sharks, such as the mako shark, have very high levels of mercury. A study on New Jersey coastal fish indicated that one third of the sampled fish had levels of mercury above 0.5 parts per million, a level that could pose a human health concern for consumers who regularly eat this fish. Another study of marketplace fish caught in waters surrounding Southern Italy showed that, undoubtedly, greater fish weight leads to additional mercury found in fish body tissues. Moreover, the concentration, measured in milligrams of mercury per kilogram of fish, steadily increases with the size of the fish. Anglerfish off the coast of Italy were found with concentrations as high as 2.2 milligrams of mercury per kilogram, higher than the recommended limit of 1 milligram of mercury per kilogram. Annually, Italy catches approximately a third of its fish from the Adriatic Sea, where these anglerfish were found.(7)
Protecting yourself — and the fish: Certain fish, even some that are low in mercury, make poor choices for other reasons, most often because they have been fished so extensively that their numbers are perilously low. These fish are marked with an asterisk:(8)
Enjoy these fish:
Anchovies, Butterfish, Catfish, Clam, Crab (Domestic), Crawfish/Crayfish, Croaker (Atlantic), Haddock (Atlantic), Hake, Herring, Mackerel (N. Atlantic, Chub), Mullet, Oyster, Perch (Ocean), Plaice, Pollock, Salmon (Canned), Salmon(Fresh), Sardine, Scallop, Shad (American), Shrimp, Sole(Pacific), Squid (Calamari), Trout (Freshwater), Whitefish
Eat six servings or less per month:
Bass (Striped, Black), Carp, Cod (Alaskan), Croaker (White Pacific), Halibut (Atlantic), Halibut (Pacific), Jacksmelt (Silverside), Lobster, Mahi Mahi, Monkfish, Perch (Freshwater), Sablefish, Skate, Snapper, Tuna (Canned chunk light), Tuna (Skipjack), Weakfish (Sea Trout)
Eat three servings or less per month:
Bluefish, Grouper, Mackerel (Spanish, Gulf), Sea Bass (Chilean), Tuna (Canned Albacore), Tuna (Yellowfin)
Mackerel (King), Marlin, Orange Roughy, Shark, Swordfish, Tilefish, Tuna (Bigeye,Ahi)
* Fish in Trouble! These fish are perilously low in numbers or are caught using environmentally destructive methods. To learn more, see the Monterey Bay Aquariumand the The Safina Center (formerly Blue Ocean Institute), both of which provide guides to fish to enjoy or avoid on the basis of environmental factors.
** Farmed Salmon may contain PCB’s, chemicals with serious long-term health effects.
US government scientists tested fish in 291 streams around the country for mercury contamination. They found mercury in every fish tested, according to the study by the U.S. Department of the Interior. They found mercury even in fish of isolated rural waterways. Twenty-five percent of the fish tested had mercury levels above the safety levels determined by the U.S. Environmental Protection Agency for people who eat the fish regularly.(9)
Seafood consumption benefits
The American College of Obstetricians and Gynecologists note that, considering all the dangers and benefits, the overall result of eating fish in the United States is likely to improve personal health rather than damage it. The college argues that the omega-3 polyunsaturated fatty acids found in fish have a health benefit that outweighs the harm from mercury or polychlorinated biphenyls. Even so, the College also suggests limiting fish consumption for pregnant women. A risk-benefit study weighing the risks of mercury consumption against the benefits derived from fish in Alaska showed that the benefits outweigh the risks when consuming salmon for both cardiovascular health and infant neurological development, and that MeHg data for non-oily fish needs to be of high quality before relative risk can be reliably identified. The Seychelles Child Development Study traced more than seven hundred mother-child pairs for nine years, and found no neurological problems in the children resulting from both prenatal and postnatal methylmercury exposure. A study done with marketed fish in Oman concluded that, except in a few rare cases, the fish available for consumption had lower levels of mercury than limits defined by various health organizations. Clearly, these studies call into question whether or not normal everyday consumption of fish is dangerous in any way, and at very least justify the creation of place-based and culturally relevant consumption advisories. They do not take into account cases of severe mercury poisoning, such as that found in Minamata disease.(10)
According to the US Food and Drug Administration (FDA), the risk from mercury by eating fish and shellfish is not a health concern for most people. However, certain seafood might contain levels of mercury that may cause harm to an unborn baby (and especially its brain development and nervous system). In a young child, high levels of mercury can interfere with the development of the nervous system. The FDA provides three recommendations for young children, pregnant women, and women of child-bearing age:(11)
- Do not eat shark, swordfish, king mackerel, or tilefish (Gulf of Mexico) because they might contain high levels of mercury.
- Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish and shellfish that are low in mercury are: shrimp, canned light tuna, salmon, pollock, and catfish. Another commonly eaten fish, albacore or big eye (“white”) tuna depending on its origin might have more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, it is recommended that you should not eat more than up to 6 ounces (one average meal) of albacore tuna per week.
- Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but consume no other fish during that week.
Mercury is a metal found naturally in the environment. Human activities, such as farming, burning coal, and using mercury in manufacturing, increase the mercury cycling through the air, water, and soil. In water, mercury changes its form and becomes methylmercury. Fish absorb this mercury. When you eat fish containing mercury, you absorb the mercury, and at high levels it can be harmful. Mercury will leave the body over time in the urine, feces, and breast milk.(12)