Brain Food

What women should know about mercury contamination of fish

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On January 12, 2001, government health officials issued new advisories warning women to limit fish consumption during pregnancy to avoid exposing their unborn children to unsafe levels of methylmercury. Methylmercury can cross the placenta and cause learning deficits and developmental delays in children who are exposed even to relatively low levels in the womb. The principal exposure route for the fetus is fish consumption by the mother.

The Food and Drug Administration (FDA), which regulates commercially sold fish, recommends that pregnant and nursing women and young children not eat any shark, swordfish, tilefish, or king mackerel, but then recommends 12 ounces per week of any other fish. The Environmental Protection Agency (EPA), which makes recommendations to states about safe mercury levels in sport fish, allows up to 8 ounces of any fish per week for pregnant women with no prohibitions on consumption of any individual fish caught recreationally.

These restrictions are steps in the right direction, but they need to be tightened significantly to adequately protect women and their unborn children from the toxic effects of methylmercury.

The nutritional benefits of fish complicate the task faced by health officials when protecting the public from methylmercury. Protein, omega-3 fatty acids, Vitamin D, and other nutrients make fish an exceptionally good food for pregnant mothers and their developing babies. At the same time, there is no doubt that methylmercury is toxic to the fetal brain and nervous system, and that many beneficial fish species are contaminated. EPA’s safe exposure estimate for methylmercury has dropped twice in the past 16 years, as new science has identified adverse effects in children exposed in the womb at lower and lower doses. Emerging evidence indicates that the safe dose may drop even lower in the future (NAS 2000). Just how long a fetus can tolerate a dose of methylmercury above a “safe level” with no observable adverse effects is a matter of ongoing debate.

Compounding this uncertainty is the lack of effective education and outreach to pregnant women about methylmercury risks and the near total absence of information for pregnant women on the levels of mercury in the fish they buy. New data from the Centers for Disease Control and Prevention (CDC) show that about 10 percent of all women of childbearing age have blood methylmercury levels above the dose that may put their fetus at risk for adverse neurological effects (CDC 2001). If these women were to increase their consumption of certain fish species in hopes of benefiting their babies during pregnancy, they could expose their fetuses to potentially hazardous levels of methylmercury.

FDA’s protections fall short

FDA’s methylmercury safeguards are designed to protect an averagesized woman eating an average fish contaminated with an average amount of methylmercury that decays in her body at an average rate. These assumptions rarely apply to the risks faced by any individual. Instead, risks are unevenly distributed throughout the population, with a small but significant number of pregnancies exposed to far higher and potentially unsafe levels of methylmercury than the average fetus. The 10 percent most-heavily exposed American women already have blood methylmercury levels that would increase health risks to their fetuses if they became pregnant (CDC 2001). FDA’s health advisory, based on average exposures, does little to protect these children.

The Environmental Working Group assessed fetal exposure to methylmercury taking into account a host of real world differences in individual exposure, including a mother’s body weight and blood volume, varying methylmercury absorption and distribution rates, and variable rates of methylmercury decay in different pregnant women (Stern 1997, CDC 2001, NAS 2000). These biological differences were matched up with a unique database of fish contamination that contains 56,000 records of methylmercury test results from seven different government sources. Fish consumption, fish contamination levels, and biological variables were matched thousands of times to create a distribution of blood methylmercury levels in women similar to that occurring in the general population.

This distribution was compared to the benchmark dose of methylmercury recommended by the Committee on the Toxicological Effects of Methylmercury of the National Academy of Sciences (NAS 2000).


EWG’s analysis shows that:

  • FDA’s recommendation of 76 6-ounce fish meals during pregnancy could actually be detrimental to the health of unborn children. Fish are an important part of a healthy diet and women should be encouraged to eat fish with low methylmercury levels during pregnancy. But if American women ate a varied diet of FDA’s recommended 12 ounces of fish a week (and none of the four prohibited fish) they would expose more than one-fourth of all babies born each year (1 million infants) to a potentially harmful dose of methylmercury for at least one month during pregnancy. About 20,000 of these children would be exposed to a dose of methylmercury that increases the risk of adverse neurological effects for the entire pregnancy.

The EPA and state fish advisories for sport fish

EPA provides guidance on safe methylmercury exposure levels to state officials who in turn issue consumption advisories for sport fish caught by recreational anglers. State authorities typically post fish advisories for individual water bodies where fish are contaminated with methylmercury at a level that they deem unsafe for women of childbearing age.

Some states have done a better job than others in protecting their populations from methylmercury, but an analysis by U.S. PIRG and the State PIRGs shows that only Massachusetts has adopted health safeguards that protect all women and children.

The broader issue with recreational fish, however, is whether these advisories translate into conscious choices by pregnant mothers to avoid eating contaminated fish. There is a substantial body of evidence indicating that they do not (Golden et al 2001).


Fish provide important health benefits to the developing fetus, and pregnant women should be encouraged to eat fish with consistently low methylmercury levels. With too many species, however, these nutritional pluses are outweighed by the hazards of methylmercury.

Federal health authorities need to take much stronger steps to protect a far greater portion of the population. They must move beyond their antiquated safeguards designed to protect an average woman from an average amount of methylmercury in fish and take a realistic and protective stance against dietary exposure to methylmercury.

Fish Advisories


There are three ways that the FDA methylmercury health advisory must be improved:

1. The list of fish to avoid during pregnancy must be expanded. By advising against the consumption of just four types of fish, FDA allows heavy consumption of many fish that have unacceptably high methylmercury levels. To protect women and their babies from methylmercury, the FDA must add the following species to the list of seafood that should not be eaten by pregnant women, nursing women, and women considering pregnancy:

Tuna steaks
Sea bass
Oysters (Gulf of Mexico)
White croaker
Largemouth bass

While not every serving of any of these fish is contaminated with dangerous levels of methylmercury, the odds are greater than one in 1,000 that consumption of a single meal of these fish will expose the fetus to a potentially hazardous amount of methylmercury for longer than 30 days.

2. FDA’s recommendation that pregnant women eat 12 ounces a week of any fish (except the four that are not allowed) must be radically revised. Ten percent of American women enter pregnancy with elevated methylmercury levels, and current FDA safeguards, which are based on average exposures, do almost nothing to protect these high exposure pregnancies. If these women follow FDA’s advice of 12 ounces of any fish a week, they could easily expose their fetuses to a level of methylmercury that presents a real risk of adverse neurological effects. To protect women and children, FDA must restrict consumption of the following fish to no more than one meal per month, for all species combined:

Canned tuna
Mahi mahi
Blue mussels
Eastern oyster
Salmon from the Great Lakes
Blue crab from the Gulf of Mexico
Channel catfish (wild)
Lake whitefish

3.Women who want to eat fish during pregnancy must have information about which species are least contaminated with methylmercury. Pregnant women have a right to this information, and FDA has a duty to provide it. In addition to strengthening restrictions on fish consumption by pregnant women, FDA should promote the following fish as safe options for pregnant women:

Trout (farmed)
Catfish (farmed)
Shrimp * (see sidebar)
Fish Sticks
Flounder (summer)
Salmon (wild Pacific)
Blue crab (mid Atlantic)

Freshwater Sport Fish

It was not possible for EWG to assess the methylmercury risk from every recreational fish caught in every lake in every state in the country. A review of the available data, however, shows that several large predator sport fish are so universally contaminated that FDA should add them to the list of fish that women should completely avoid during pregnancy. After analyzing the results of more than 10,000 samples from 792 lakes and rivers nationwide, we recommend that FDA add the following species to thier health advisory: walleye, northern pike, and largemouth bass. While FDA has no authority to regulate methylmercury levels in freshwater fish, they do have a responsibility to provide critical health information to the public. It is important that women receive a consistent message from one source, and the FDA is the appropriate agency to deliver this message.

Improve monitoring of fish for methylmercury contamination

A major flaw in FDA’s system is the agency’s own lack of comprehensive data on methylmercury in fish. In January 2001, FDA recommended that pregnant women avoid consumption of king mackerel based on methylmercury levels from a study published in 1979. There are many other species where the data on methylmercury contamination are similarly outdated, but where the available information indicates a potential problem.

FDA must immediately expand its methylmercury sampling program to include a host of fish where the data indicate that pregnant women and their babies could receive a potentially unsafe exposure from a relatively small amount of fish. These include:

Sea bass
Atlantic cod
Pacific cod
Flounder, various species
Grouper, black
Grouper, red
Orange roughy
Sand perch
White perch
Red snapper
Dover sole
Lake trout

Improve public access to mercury contamination data

Consumers have a right to know about contamination of the food supply, and FDA must be responsive to this right. Currently they are not. EWG had great difficulty obtaining relatively simple information about fish contamination from the agency through the Freedom of Information Act. FDA currently posts the results of its Total Diet Study on the web, and there is no reason that all of the agency’s mercury contamination information could not be posted as well.

Improve risk assessments

FDA needs to move beyond its antiquated and biologically implausible risk assessment methods based on average people and average fish and adopt state-of-theart risk assessment techniques that provide a much more realistic picture of mercury exposure and risk as it is distributed throughout the population.

It is not sufficient to protect the population from average exposures when it is clear that many individuals have far greater than average exposures for extended periods of time.

Reduce Mercury Pollution at its Source

Mercury emissions from coalfired power plants, the largest manmade source of environmental mercury, are currently completely unregulated. Federal decisionmakers should require power plants to reduce their mercury pollution by 90% and ultimately move away from polluting sources of power.

Monitor human exposure and health

The U.S. lacks a comprehensive program to track disease and exposure to environmental contaminants like methylmercury. This study is only one of many demonstrating the need for a nationwide comprehensive environmental health tracking network. Such a network would be our country’s first step toward assessing impacts of a range of environmental hazards on public health. In addition, it would provide a wealth of information to health care providers and health officials working to protect health. Specific recommendations include:

  • The network should begin in all 50 states by tracking: asthma and chronic respiratory diseases, birth defects, developmental and neurological conditions like those linked to methylmercury exposure, and cancers. The tracking of human exposures to hazards would start with priorities including PCBs and dioxin; heavy metals such as mercury and lead, pesticides, and water and air contaminants.
  • An Early Warning System would alert communities to immediate health crises such as heavy metal and pesticide poisonings. Similar to the monitoring currently in place for an outbreak of an infectious disease, this alert would help local communities identify more quickly and act immediately on health crises from environmental exposures.
  • Pilot Programs would allow 20 different regional and state initiatives to investigate local environmental health priorities, provide flexibility for local officials, allow community groups to gather more information and serve as a model for potential inclusion in the nationwide network.
  • Federal, state and local rapid response capability would enable health officials to investigate clusters, outbreaks and emerging threats. To respond effectively and protect the public from illness, health officials must be well-equipped and trained, and the network must be supported through state and federal resources. Steps taken to improve states’ capacity should include placing an Environmental Health Investigator in every state and training health officials in environmental epidemiology.
  • Support of community interests and scientific research will be crucial to further health tracking efforts. Five Centers of Excellence should be federally funded for environmental health research and training, and partnering with communities. Communities and the public have the right to know information that could improve their health; the information made available through the tracking network should be accessible to the public. Input from local groups on the design and implementation of the tracking and monitoring of chronic disease and environmental hazards will be needed to ensure the success of the Nationwide Health Tracking Network.

A nationwide comprehensive environmental health tracking network would be our country’s first step toward assessing impacts of a range of environmental hazards on public health. In addition, it would provide a wealth of information to health care providers and health officials working to protect health.

View and Download the report here: Brain Food

EWG’s Fish List

What Women Should Know About Mercury In Fish

Download this list as a PDF.

Avoid If Pregnant

  • Shark
  • Swordfish
  • King mackerel
  • Tilefish
  • Tuna steaks
  • Canned tuna
  • Sea bass
  • Gulf Coast Oysters
  • Marlin
  • Halibut
  • Pike
  • Walleye
  • White croaker
  • Largemouth bass


Eat No More Than One Serving From This List Per Month

  • Mahi mahi
  • Blue mussel
  • Eastern oyster
  • Cod
  • Pollock
  • Great Lakes salmon
  • Gulf Coast blue crab
  • Channel catfish (wild)
  • Lake whitefish

Lowest In Mercury

  • Blue crab (mid-Atlantic)
  • Croaker
  • Fish Sticks
  • Flounder (summer)
  • Haddock Trout (farmed)
  • Salmon (wild Pacific)
  • Shrimp *

* Shrimp fishing and farming practices have raised serious environmental concerns.
** Farmed catfish have low mercury levels but may contain PCBs in amounts of concern for pregnant women.

Data From The 1970s Show High Concentrations (No Recent Data Available)

  • Porgy
  • Orange Roughy
  • Snapper
  • Lake Trout
  • Bluefish
  • Bonito
  • Rockfish
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