What is Methylmercury?
Mercury is a mineral that exists naturally in the environment. In addition, thousands of tons are released into the air each year through pollution and waste. Bacteria and natural processes can transform mercury into the organic mercury compound methylmercury (MeHg), which is a poisonous substance.
Unfortunately, this toxin is in the fish we eat. Methylmercury can accumulate in streams and oceans. It also accumulates in the food chain, as each fish absorbs all the mercury of the smaller fish or organisms it has eaten. That is why the oldest and largest fish, such as shark or swordfish, have the highest levels. Methylmercury levels are higher in people who regularly eat fish.
Who Is At Risk?
Anyone can be harmed by high levels of mercury. The Centers for Disease Control and Prevention (CDC) reports that people most sensitive to mercury are pregnant and nursing women, children under the age of six (especially younger than three), people with impaired kidney function, and those with very sensitive immune responses to metals.
Methylmercury easily crosses the placenta and accumulates in the blood and tissues of the developing fetus. It can be passed to newborns through breast milk, and a baby’s growing brain and nervous system are even more sensitive to this toxin than an adult’s. Children remain particularly vulnerable for at least several years because, compared to adults, they eat more food relative to their body size.
According to a 2005 Environmental Protection Agency (EPA) study, women living in US coastal communities – and presumably eating more fish than inland residents do – had higher average blood levels of methylmercury. Women living on the Atlantic coast had the highest average levels, followed by women on the Pacific and then women on the Gulf coasts. Many had methylmercury levels that the EPA considers unsafe for adults.
Which fish are harmful? There is limited information about methylmercury in fish because there is no national or statewide system in place to monitor amounts. Most states, Native American tribes, and U.S. territories issue advisories that warn people when they are aware of methylmercury contamination. The advisories indicate what types, size, and amounts of fish are of concern. Pollution can result in high mercury levels in fish. Otherwise, methylmercury levels for many fish are relatively low, ranging from less than 0.01 part per million (ppm) to 0.5 ppm, but 1 ppm is considered too high by the Food and Drug Administration (FDA).
A few fish are so high in methylmercury that they should be totally avoided by pregnant or nursing women, young children, and other at-risk populations. (Please see chart below.) In March 2004, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) issued a joint Consumer Advisory warning about methylmercury in fish. The advisory continues a previous warning against four particular species of fish and for the first time includes a specific warning about the consumption of tuna.
The advisory recommends that women who might become pregnant, who are pregnant or nursing, and young children:
- Avoid eating shark, swordfish, king mackerel, and tilefish (also known as golden bass or golden snapper);
- Limit consumption of all other types of fish to 12 ounces (2 average meals) per week;
- Limit consumption of canned albacore (“white”) tuna or fresh tuna to no more than 6 ounces per week;
- Limit the fish eaten by young children to even smaller portions per week (no specific advice is given);
- Check local advisories about the safety of fish caught in local lakes, rivers, and coastal areas. If no advice is available, eat no more than 6 ounces per week of locally caught fish, and do not consume any other fish during that week;
- If more than the recommended amount of fish is eaten in one week, eat less in the following weeks.
The advice given by the EPA and other experts is more cautious than more recently released recommendations by some other groups, several of which have been criticized for accepting funding from the fish industry. Calculating safe levels of methylmercury based on individual body weight, the EPA determined that 6 oz of canned albacore tuna or fresh tuna per week could expose a pregnant or nursing woman to too much mercury, especially if she also ate other fish.
Tuna: A Cause for Concern
Levels of methylmercury in tuna are lower than in the four fish on the “do not eat” list but higher than many other fish. Since Americans eat so much tuna, the potential for unsafe mercury exposure could be greater. There are two types of canned tuna generally found in the U.S. market – albacore white and “light.” Fresh tuna and canned albacore white tuna are 3 times as high in methylmercury as canned light tuna, which comes from smaller fish. The amount of mercury in canned tuna varies from can to can, with canned albacore white tuna sometimes exceeding the danger zone of 1 part per million.
While tuna is high in omega 3 acids, low in saturated fat and high in protein and vitamin E, and can therefore be a part of a healthy diet, we recommend that women who may become pregnant, pregnant women, and nursing mothers avoid fresh tuna, eat no more than one 6 oz. can of albacore (white) tuna per month, and eat no more than one 6 oz. can of light tuna per week. (For most women, that would be two or three tuna salad sandwiches, depending on what the tuna salad contains.) If a woman in these high-risk categories does eat tuna steak or tuna sushi, we recommend that she reduce her consumption of canned tuna over the next weeks to reduce her mercury intake. It is possible that eating more could result in neurological damage to children. Studies within the past five years have shown that children whose mothers were exposed to higher levels of mercury from eating large amounts of fish while pregnant were more likely to have delays or problems with fine motor coordination, language, and memory.
These deficits can also crop up in children who are exposed to mercury through their own diets. Young children are mainly exposed to methylmercury through canned tuna. Research from a study published in August 2012 showed that the mercury content in tuna varies widely. Children weighing less than 55 pounds (25kg) should not eat light tuna more than once per month. Children that weigh more than 55 pounds should have no more than 6 oz (2 servings) of light tuna per month –in other words, they should not eat light tuna more than twice per month (see our chart below).
The effects of methylmercury toxicity include paraesthesia (a pricking, tingling or creeping sensation on the skin), depression, and blurred vision. Research also suggests prenatal and infant exposure can affect attention span, language, visual-spatial skills, memory, and coordination. The National Academy of Sciences estimates that nearly 60,000 children each year are born at risk for neurological problems due to methylmercury exposure in the womb.
Unless there is an advisory about fish in your area, the FDA and EPA have not expressed concern about methylmercury exposure from fish consumption for men or for women who are beyond reproductive age. However, people with impaired kidney function and those with very sensitive immune responses to metals should avoid mercury. It makes sense to consider moderation for anyone who likes to eat the fish that are higher in mercury. See the charts below for a summary of the risks of specific types of fish, and how much is safe to eat.
For Children, Women Who are Pregnant and/or Nursing:
|Child <55.lb (25 kg)||Child > 55lb (25 kg)||Pregnant/Nursing|
|White tuna||No||No||2 servings/1 can per month- (6oz) at most|
|Light Tuna||1 serving (2-3oz)/month at most||2 servings per month (6oz) at most||2 servings/1 can per week- (6oz) at most|
Avoid (Highest Mercury Levels)
|Shark, King Mackerel, Swordfish, Tilefish, Marlin, and Orange Roughy|
|No more than 3 6-oz servings/month
(High Mercury Levels)
|Sea Bass (Chilean), Bluefish, Grouper
Mackeral (Spanish, Gulf), Tuna (canned, White Albacore), Tuna ( Yellowfin)
|No more than 6 6-oz servings/month
(Lower Mercury Levels)
|Bass (striped, black), Carp, Cod (Alaskan), Croaker (White Pacific), Halibut (Pacific and Atlantic), Jacksmelt (Silverside), Lobster, Mahi Mahi, Monkfish, Perch (freshwater)
Sablefish, Skate, Snapper, Sea Trout (weakfish)
|2 6-oz. servings/week
(Lowest Mercury Levels)
|Anchovies, Butterfish, Catfish, Clam
Crab (domestic), Crawfish/Crayfish
Croaker, Flounder, Haddock
Hake, Herring, Mullet, Oysters, Perch (ocean), Plaice Salmon (canned, fresh), Sardines, Scallops, Shad, Shrimp, Sole, Squid (Calamari), Tilapia, Trout (freshwater), Whitefish, Whiting
Women who may become pregnant, are pregnant, or are nursing should avoid fish that are high in mercury. Pregnant and nursing women should try to eat 12 oz. of fish per week, but use the table above to choose fish with lower mercury levels, or instead take pills (dietary supplements) to provide omega-3 fatty acids. If taking fish oil supplements, look for those which are “molecularly distilled” and third-party tested to be free of heavy metals. Young children should eat less than 6 oz (2 small servings), regardless of the type of fish.
Other Resources to Check Out
- Fish advisories can differ depending on the state. State fish advisories are more frequently updated than FDA guidelines (which were last updated in 2004), and different contaminants may be present, depending on the body of water. To read more about specific fish advisories for local bodies of water in your state, check out the EPA’s interactive map, click here.
- See whether your mercury intake is within the EPA’s safe level with this mercury calculator from the Turtle Island Restoration Networks, check it out here.
All articles are reviewed and approved by Dr. Diana Zuckerman and other senior staff.
- Hong, Y., Kim, Y., & Lee, K. (2012). Methylmercury exposure and health effects. Journal of Preventive Medicine and Public Health, 45(6), 353-363.
- Agency for Toxic Substances and Disease Registry (CDC). (October 14, 2015). Retrieved April 8, 2018 from: https://www.atsdr.cdc.gov/sites/toxzine/mercury_toxzine.html
- Food and Drug Administration (FDA). (2017). Advice About Eating Fish, From the Environmental Protection Agency and Food and Drug Administration; Revised Fish Advice; Availability. Washington, D.C.: Federal Register.
- Food and Drug Administration (FDA). (2016). Retrieved April 9, 2018 from: https://www.fda.gov/downloads/ScienceResearch/SpecialTopics/PeerReviewofScientificInformationandAssessments/UCM531130.pdf
- Environmental Protection Agency (EPA). (January 17, 2017). https://www.epa.gov/fish-tech/epa-fda-fish-advice-technical-information
- The National Academy of Sciences. (July 11, 2000). Retrieved April 9, 2018 from: http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=9899