Unfortunately, this toxin is in the fish we eat. Methylmercury can enter the water or soil, accumulating 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 have the highest methylmercury levels. Saltwater fish that live a long time, such as shark and swordfish tend to have the highest levels. Methylmercury levels are higher in people who regularly eat fish.

Who is at risk?

At greatest risk is the developing fetus, when a pregnant mother eats a lot of fish. Methylmercury easily crosses the placenta, and accumulates in an unborn baby’s blood. A baby’s developing brain and nervous system are very sensitive to this toxin, and remains vulnerable after birth. Methylmercury can also be passed to newborns through breast milk. Young children who eat fish may also be at risk because, compared to adults, they eat more food relative to their body size.

The Center For Disease Control and Prevention (CDC) defines populations sensitive to mercury exposure as pregnant and nursing women, children under the age of six (especially up to the age of three), people with impaired kidney function, and those with very sensitive immune responses to metals.

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 most fish are relatively low, ranging from less than .01 part per million (ppm) to .5 ppm.

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 2001, the Food and Drug Administration (FDA) issued a Consumer Advisory warning about methylmercury in fish. The FDA indicated that fish are safe to eat when they contain less than 1 part per million (1 ppm) of methylmercury.

The FDA found four species of fish that have methylmercury levels that exceed this limit: Shark, Swordfish, King Mackerel, Tilefish (also known as Golden Bass or Golden Snapper). The FDA indicated that these fish should be avoided by pregnant women, nursing mothers, and children. The advisory recommends that pregnant women limit their consumption of all other types of fish to 12 oz per week.

Stricter guidelines were issued by the Environmental Protection Agency (EPA). The EPA calculates safe levels of methylmercury consumption is the amount that is safe to consume in your diet based on individual bodyweight. Freshwater fish from contaminated waters have been shown to have particularly high levels of methylmercury. In response, the EPA has also issued a national advisory recommending that women who are pregnant or may become pregnant, nursing mothers, and young children limit their consumption of freshwater fish caught by friends and family to one meal per week (6 oz. cooked for adults, 2 oz. cooked for children). So, if you want to be cautious, and use the EPA standards instead of the FDA’s, you should be even more careful about the fish you eat.

Tuna: A cause for concern

Many consumer advocates are concerned that neither fresh nor canned tuna was included in the recent FDA advisory. Levels of methylmercury in tuna are lower than the fish included in the advisory, but since Americans eat so much tuna the risk could be greater. Fortunately, canned tuna usually has lower levels of mercury than fresh tuna, because it is typically canned using smaller fish.

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 pregnant women, nursing mothers, and young children eat no more than one 6 oz. can of tuna per week. It is possible that more could result in neurological damages to babies and young children.

The risks

The effects of methylmercury toxicity include paraesthesia (a pricking, tingling or creeping sensation on the skin), depression, and blurred vision. Research also suggests neurobehavioral effects on fetuses and developing infants, including 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.

What if I'm a man or older woman?

Unless there is an advisory about fish in your area, the FDA and EPA have not expressed concern about methylmercury exposure for men or women who are beyond reproductive age. There are no guidelines, but it makes sense to consider moderation if you like to eat the fish that are higher in mercury.

See chart below for a summary of mercury risk posed by specific types of fish.

References:
www.fda.gov/bbs/topics/ANSWERS/2003/ANS01209.html
www.epa.gov/ost/criteria/methylmercury/
www.epa.gov/waterscience/fishadvice/factsheet.html