Should I be concerned about mercury in fish and what fish are safe to eat?

All health experts agree that fish is an excellent source of lean protein accompanied by heart-healthy omega-3 fats, and many health organizations, such as the American Heart Association, recommend eating fish two or three times each week.

On the other hand, the FDA has issued a caution for pregnant or breastfeeding women and women of child-bearing age to avoid consumption of swordfish, king mackerel, shark and tilefish, and to limit consumption of fresh or frozen tuna steaks due to the high levels of mercury that have been found in these fish.

Human pollution is the problem, not natural occurrence of mercury

Although mercury is a naturally occurring heavy metal, both in the environment and in the body, its natural occurrence is miniscule and insignificant. Mercury has only become a problem in our environment and in our health because human activities, like the manufacturing of mercury batteries, thermometers, and mercury-containing fluorescent lights, have liberated mercury from ground ores and thrown it into the category of hazardous waste. (Vermont is the first state in the nation to take action in this area, and beginning on November 30th of this year, all lamps sold in Vermont containing mercury will have to be labeled to that effect).

Mercury from discarded mercury-containing products has leeched its way from landfills and hazardous waste sites into our soil and water, and all of us now harbor bits of mercury from this process. For example, the Centers for Disease control in Atlanta, Georgia report that 8% of all women of childbearing age in the U.S. now have mercury levels in their bodies that exceed governmental guidelines.

Symptoms of toxic levels of mercury in your system:

  • Slurred speech
  • Shakiness
  • Numbness in hands and feet
  • Crushing fatigue
  • Depression
  • Lack of concentration
  • Hair loss
  • Impaired memory
  • Disorientation
  • Irritability
  • Headaches
  • Joint pain
  • Metallic taste in mouth

Mercury and fish toxicity

As the FDA has correctly pointed out in its position papers issued on mercury, this heavy metal is indeed found almost everywhere in our environment. At the World’s Healthiest Foods, we agree completely with the following description of mercury and fish toxicity published by the FDA in consumer education materials:

“Approximately 2,700 to 6,000 tons of mercury are released annually into the atmosphere naturally by degassing from the Earth's crust and oceans. Another 2,000 to 3,000 tons are released annually into the atmosphere by human activities, primarily from burning household and industrial wastes, and especially from fossil fuels such as coal.

Mercury vapor is easily transported in the atmosphere, deposited on land and water, and then, in part, released again to the atmosphere. Trace amounts of mercury are soluble in bodies of water, where bacteria can cause chemical changes that transform mercury to methyl mercury, a more toxic form.

How fish get involved

Once mercury makes its way into lakes and streams, it comes into contact with a wide variety of organisms. Bacteria in water can metabolize mercury, and can convert it into several of its many different forms.

Fish absorb methyl mercury from water as it passes over their gills and as they feed on aquatic organisms. Larger predator fish are exposed to higher levels of methyl mercury from their prey.

Methyl mercury binds tightly to the proteins in fish tissue, including muscle. It is a form of mercury that can make its way up into the muscle proteins of fish (as well as humans). When we eat a piece of fish, we are eating mostly muscle, and if the fish has lived for a long time in mercury-polluted water, its muscle will have stored significant amounts of methylmercury.

Cooking does not appreciably reduce the methyl mercury content of the fish.

Nearly all fish contain trace amounts of methyl mercury, some more than others. In areas where there is industrial mercury pollution, the levels in the fish can be quite elevated.”

How much mercury is safe?

The safety of eating mercury-contaminated fish is controversial because “safe” really depends on who is trying to stay safe and what they are trying to stay safe from. A very unhealthy person, perhaps in the hospital from weakness and poor nourishment, can withstand very little toxic exposure, including mercury-contaminated fish. An extremely healthy person, full of vitality, and optimally nourished, with good nutrient supplies and a strong ability to get rid of toxins, can definitely eat mercury-contaminated fish and remain healthy. How much could such a person eat? Here the answer hinges on what the person is trying to stay safe from. An athlete facing endurance training might not want to deplete his or her nutrient supplies at all, and might not want to ask his or her body to engage in any unnecessary detoxification of mercury. In this case, the choice might be to avoid any mercury-contaminated fish. A well-nourished, healthy person just wanting to stay generally healthy, i.e., stay safe from premature aging or premature onset of chronic disease, might choose to eat canned tuna once or twice a week and do just fine. But let’s look a little more closely at the numbers.

A closer look at the numbers

The United States Food and Drug Administration (FDA) considers fish safe for adults and even young children if it contains less than 1 part per million (ppm) of methyl mercury. One part per million is the same as 1,000 parts per billion (ppb). Generally speaking, ppb amounts of toxins have never been shown to directly cause disease. People don’t usually get visible symptoms of toxicity from ppb amounts.

While blood tests have been the most commonly used method for detecting toxic levels of mercury in the system, it is now believed that hair tests provide a more reliable and accurate measure of mercury levels. This is because heavy metals in your blood tend to vary dramatically with meals while hair samples provide a long term record of an individual's average level of mercury. This record can go back several months depending on the length of hair. The EPA has determined that a safe level for mercury as measured in hair is one part per million.

The absence of visible symptoms, however, doesn’t mean that the ppb amounts are safe. Many changes go on inside the body, inside of cells and cell parts, when ppb levels of toxins, including mercury, are consumed. We just don’t see the effects right away. Based on unwanted cellular changes and other “invisible” problematic effects, other governments have set tighter safety standards for mercury levels in fish.

The Canadian Food Inspection Agency (CFIA) considers fish safe if it contains less than 0.5 ppm—half the level of mercury considered safe by the FDA. (0.5 ppm is the same as 500 ppb). Research in Japan has shown that toxic effects from mercury can be present when there are 200 ppb amounts of mercury in the blood, and the Japanese government is considering even stricter recommendations for this reason.

In 2002, mercury-safe seafood bills were introduced into the legislatures of 19 states, and mercury-safe seafood legislation is also being reviewed by the U.S. Congress. The U.S. Environmental Protection Agency (EPA) has found that no more than 1 microgram/kilogram of body weight for mercury is recommended. This amount is the same as 1 ppb.

In early 2003, the United Kingdom’s Food Standards Agency (FSA) lowered its recommended limits for women who are pregnant, intend to become pregnant or who are breastfeeding, from 3.3 micrograms/kilogram bodyweight per week to 0.7 micrograms/kilogram bodyweight per week. The 3.3 mcg/kg limit had been set by the Joint FAO/WHO Expert Committee on Food Additives (JEFCA), but after an extensive review of the possible risks, the new lower limit was set by the Committee on Toxicity.

While it’s very different to measure amounts of mercury in the body than in the fish, the point here is that many organizations are moving in the direction of stricter standards when it comes to mercury levels, and for this reason, we at the World’s Healthiest Foods like the Canadian standard of 500 ppb better than the U.S. FDA standard of 1,000 ppb (or 1 ppm).

Our concerns

In summary, we suspect that the average amount of mercury found in fresh and frozen tuna steaks may be higher than the FDA has stated, and that the average amount found in canned tuna may be higher or lower, and that this information is critical to determine and pass on to consumers.

With respect to the health impact of eating mercury-contaminated fish, we do not agree with the FDA statement that levels of mercury below 1ppm (1,000ppb) are "safe." At the World's Healthiest Foods, we start with the assumption that everyone should enjoy optimal health - not just "adequate health" where there is no overt disease. Optimal health means maximally supporting all of the body's cells and organ tissues. Optimal health means being able to detoxify mercury if it is accidentally ingested into the body, and minimizing exposure to mercury whenever possible. We have seen research studies that clearly show disruption of cell function following ingestion of mercury at levels lower than 1ppm (1,000ppb). This damage to cell function depends on the relative health of the person involved, the person’s ability to detoxify mercury, and their overall diet.

With that in mind, where do things stand in the fish marketplace? Here are the numbers:

Species Mercury Range (ppm) Mercury Average (ppm)
Domestic Samples
Catfish ND*-0.16 ND, 0.07
Cod ND-0.17 0.13
Crab ND-0.27 0.13
Flounder ND ND
Hake ND ND
Halibut 0.12-0.63 ND
King mackerel Not available 0.73
Mahi mahi* Not available 0.19
Pollock ND ND
Salmon (canned) ND ND
Salmon (fresh or frozen) ND ND
Scallops(fresh or frozen) ND ND
Shark 0.30-3.52 0.84
Swordfish 0.36-1.68 0.88
Tilapia* Not available ND
Tilefish* Not available 1.45
Tuna (canned) ND-0.34 0.20
Tuna (fresh or frozen ND-0.76 0.38
Import Samples
Pollock ND-0.78 0.16
Shark ND-0.70 0.36
Swordfish 0.80-1.61 0.86
pole-caught albacore tuna, canned in water Not available 0.04
Tuna (canned) ND-0.39 0.14
Tuna (fresh or frozen) ND-0.75 0.27

Values from U.S.D.A. Nutrient Database

* ND in the above table stands for “not detectable”.

Practical Tip

Based on these numbers, what do we recommend?

Larger fish, since they are longer-lived, accumulate and therefore contain greater quantities of mercury than smaller fish.

Fish to Avoid

Our recommendation for individuals who are concerned about mercury levels in thier diet and pregnant women and nursing mothers is to exclude large fish like swordfish, king mackerel (ono), shark and tilefish. As the above chart shows, these large fish average more than 500 ppb mercury. Chilean sea bass, fresh tuna, halibut, grouper, bluefish, amberjack, cobia and redfish should also be added to this list.

A draft advisory from the Food and Drug Administration and the Environmental Protection Agency containing new proposed guidelines on fish consumption was circulated at a meeting of the FDA's Food Advisory Committee December 10, 2003, in Washington, D.C. Recent FDA testing has indicated that canned albacore, known as white tuna, contains almost three times as much mercury as canned "light" tuna. The proposed advisory, which is expected to be formally promulgated early in 2004, currently states that although mercury levels in tuna vary, tuna steaks and canned albacore generally contain higher levels of mercury than canned light tuna.

While albacore is a biological type of tuna, "light" tuna may be biologically classified as skipjack, bluefin, yellowfin, and tongol. All types except tongol may be labeled "chunk light" or "solid light", while tongol may only be labeled "chunk light". Our recommendation: when you choose to eat tuna, either purchase canned light tuna or if buying canned albacore, purchase it from a reputable supplier who has had the mercury levels tested by an independent lab. (February 1, 2004)

The latest UK Food Standards Agency (FSA) recommendations (July 2004) emphasize the health benefits of eating oily fish and provide guidelines for the maximum levels at which the health benefits clearly outweigh possible risks from not only mercury but another contaminant found in fish – dioxins and dioxin-like polychlorinated biphenyls (PCBs). According to the FSA, women of childbearing age (including pregnant and breast-feeding women) and girls can safely eat up to 2-3 portions of oily fish per week – if they have not previously eaten more than this amount. Men, boys and women past childbearing age can safely eat up to 4 portions of oily fish per week. The full FSA report can be accessed from the FSA web site.
We still recommend choosing which oily fish you consume wisely. Tuna, unless tested and found to be uncontaminated, and farmed salmon, which have been identified as having some of the highest levels of PCBs found anywhere in the world, should be avoided. (August 1, 2004)

Recommended for Occasional Meals

Flounder, mahimahi, red snapper and trout have been determined by the EPA to be safe to be eaten once a week for men and for women who are no longer childbearing. Children and women still in their reproductive years should limit both red snapper and mahimahi to once a month.

Most Highly Recommended Fish to Eat

The mercury content of other types of fish will vary with the species. Some varieties recommended by the Environmental Working Group and the U.S. Public Interest Group include wild Pacific salmon, flounder, haddock, shrimp, farm-raised trout and catfish; there was no mercury detected in these fish.

Freshwater Fish and Fish Caught by Family or Friends

Feshwater fish may also be contaminated with mercury, either from natural or industrial sources. For freshwater fish caught by family or friends, check with your state and local health departments for current advisories regarding mercury, or call toll-free 1-888-SAFEFOOD.

Fish with lower levels of contamination, as evidenced by solid research from the manufacturer or other organizations, would be exempt from these recommendations and could be enjoyed more often.

Mercury Tests

Blood tests:Lab tests measure methylmercury levels based on micrograms per liter of blood. The EPA safety threshold is 5.8 micrograms of methylmercury per liter or one part per million (1ppm). In an optimally healthy individual half of the mercury that is accumulated in the body is eliminated every 50 days. 99% of the mercury that we eat today will be gone within the year.

Hair tests: The EPA has determined that a safe level for mercury as measured in hair is one part per million (1ppm).


Washington State Department of Health, Office of Environmental Assessments, Fish Facts, Fish Advisory for Mercury,

Report: “The State of the Nation’s Ecosystems,” September 2002. The H. John Heinz III Center for Science, Economics and the Environment, 1001 Pennsylvania Ave, NW Suite 735 South Washington, DC 20004


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This page was updated on: 2004-11-18 21:28:57
© 2002 The George Mateljan Foundation