New Research Underscores Health Warnings on Mercury in Fish

U.S. mercury levels in precipitation have decreased slightly in the past few years. Still, new research findings of high mercury levels of fish in lakes, streams, and reservoirs show that mercury continues to pose a health concern for pregnant women and children who eat fish in their daily diets.

The Fish-Methyl Mercury Connection

Rain and snow flush mercury out of the atmosphere and into waterways. There, natural processes convert mercury to the highly toxic methyl mercury, starting a food-chain reaction. Small organisms absorb methyl mercury from water and lake sediment. Small fish consume the organisms, and in turn, are consumed by larger fish. With each step up the food chain, methyl mercury further accumulates, so that the largest predator fish often have high levels of methyl mercury. These concentrations in large fish can be more than a million-fold higher than in the surrounding water, according to the U.S. Environmental Protection Agency (USEPA). Fish with the most concentrated levels of methyl mercury include predator fish from oceans and lakes: sharks, swordfish, king mackerel, largemouth bass, walleye, and northern pike.

New research has shown the problem is widespread. From 1998 to 2005, researchers at the United States Geological Survey tested methyl mercury contamination in 34 varieties of predator fish (1,000 fish total) in nearly 300 rivers and streams across the United States. Mercury concentrations at 27 percent of sites exceeded the USEPA guidelines for safe fish consumption (0.3 parts per million).

A USEPA study in late 2009 found that mercury concentrations exceeded recommended levels at 48 percent of lakes and reservoirs nationwide; mercury concentrations in fish were found in nearly all 50 states.

Health Effects

For most people, fish consumption does not cause a health concern. However, special populations are at risk for detrimental health effects, including damage to the heart, kidneys, and the central nervous system. Most at risk are pregnant women, women who might become pregnant, nursing mothers, and children.

Exposure to methyl mercury has been shown to impact the developing nervous system of unborn and young children, resulting in various disabilities. Based on studies of women with high mercury levels, the USEPA estimated that more than 300,000 newborns each year in the U.S. may have an increased risk of learning disabilities associated with methyl mercury exposure.

Mercury can accumulate in people, as well as in fish. When methyl mercury is consumed, it takes about 70 days for half of the mercury ingested to be removed from the body, according to the Illinois Department of Public Health.

In efforts to address this problem, every state in the U.S. except for Wyoming and Alaska has issued advisories warning at-risk groups to limit fish consumption, especially avoiding the large fish varieties. Special mercury advisories are issued for lakes, streams, or reservoirs when higher concentrations of methyl mercury are found in fish compared with fish from similar water sources.

A Global Issue

Mercury is emitted into the atmosphere and surface waters from natural sources, including volcanoes and wildfires. It also occurs naturally in some soils. Yet most of the attention on mercury in the environment has focused on the man-made sources: coal combustion, medical waste incineration, and chlorine production, among others.

Only 17 percent of mercury deposited in the U.S. originates there, according to the USEPA. As much as 83 percent comes from international sources. USEPA research indicates that U.S. mercury sources influence concentrations in the environment much more in the eastern U.S., and global sources are more significant contributors in the West.

The Mercury Deposition Network (MDN) at NADP, which traces mercury in wet deposition (through rain and snow), has indicated that the highest deposition of mercury from the atmosphere occurs along the Gulf of Mexico and in Florida.

Across the nation, acidic waters draining from forests and wetlands typically produce fish with high levels of mercury concentrations due to their particular water chemistry, and land cover characteristics promote the transport and accumulation of mercury.

A Healthy Balance

Nutritionists have long known that fish is a part of a healthy diet. Fish contain lean protein, some essential nutrients, and omega-3 fatty acids, which are linked to heart health. In recent years, federal agencies have weighed the health costs of consuming neurotoxins against the nutritional benefits of fish. The U.S. Food and Drug Administration (FDA) has provided at-risk populations three recommendations for eating fish. By following these recommendations, women and young children will receive the benefits of eating fish while reducing their exposure to the harmful effects of mercury.

  1. Do not eat shark, swordfish, king mackerel, or tilefish because they contain high levels of mercury.
  2. Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury (shrimp, canned light tuna, salmon, pollock, and catfish).
  3. Check local advisories about the safety of fish caught by family and friends in local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish from local waters, but don't consume any other fish during that week.

The Mercury Deposition Network (MDN) compiles data on mercury concentrations in precipitation from 94 sites across the U.S. and six sites in Canada. Data are available on the MDN Web site:

Also see our Mercury Presentations

  • For Elementary School Students: Mercury in Fish [PPT] [PDF]
  • For High School Students: Contaminated Fish: The Mercury Connection [PPT] [PDF]