Mercury Rising

Important information on fish and mercury.


For more information on fish and mercury, check out the following websites:National Geographic's TheGreenGuide offers a comprehensive list of the best seafood choices—those that are not only low in mercury but are also not over-fished or farmed destructively—as well as those to avoid. Visit • To calculate your weekly intake of mercury, visit • For a quick link to the FDA's consumer advisory about mercury in fish, visit

More on Mercury If you're pregnant, chances are you've been assaulted with news that eating too much seafood during these all-important nine months could put your baby at risk for developmental and other problems. In fact, if you're like many women, the warnings may have even alarmed you to the point where you're actually eating too little fish—which, some experts warn, is potentially more detrimental to a growing fetus than eating too much.

The issue all boils down to a concern about methylmercury in seafood. But how does this environmental toxin get into fish in the first place? And how much of a danger is it? Here's a look at this perplexing topic:

How does mercury contaminate seafood? Methylmercury is a byproduct of mercury, which can occur in nature but is more commonly released into the air through industrial pollution. When mercury falls from the air into oceans, streams and rivers, bacteria in the water transform the compound into methylmercury, a known toxin. As fish live and feed in these waters, they absorb the methylmercury; as bigger fish feed on smaller fish, they absorb increasing amounts of the compound.

As a result of this process, all seafood contains some amount of methylmercury. "The higher up the food chain you go, the more methylmercury you get," explains Gary J. Myers, M.D., a professor of neurology, pediatrics and environmental health at the University of Rochester Medical Center in Rochester, N.Y. "It's a simple matter of bio-accumulation: big fish eating little fish." Almost all natural exposures to methylmercury are from exposure to seafood, Myers adds; most true mercury poisonings are due to environmental contamination (See question below for more information.)

Where did the concern about methylmercury come from? The so-called Minamata Bay Disaster was the genesis. The event took place in Japan in the 1950s and 60s, during which time a local corporation dumped wastewater into Minamata Bay, causing heavy pollution and illness among thousands of people who ate fish from those waters. Some of these people were pregnant women whose fetuses were also affected by the poisoning. "Infants born to mothers contaminated by mercury in Japan's Minamata Bay in 1956 had profound neurological disabilities, including deafness, blindness, mental retardation and cerebral palsy," reports the FDA.

"This was the first time people realized a fetus could be damaged by its mother consuming contaminated fish," Myers explains. "This led to the concept of prenatal exposure leading to neurological damage: There was a realization that individuals who did not actually consume fish, but were in utero, could be neurologically damaged."

What are the risks to a fetus of a woman eating too much seafood while pregnant? "This is difficult to determine because the only times seafood consumption has ever led to clinical manifestations were due to environmental poisonings," Myers says. "The first was at Minamata, where other contaminants in addition to methylmercury were involved. Also, Minamata involved a very large exposure over a very long time: 10 years."

Myers points out that if a woman living near Minamata Bay had been eating contaminated fish for an extended period and then became pregnant, she would have dramatically higher levels of mercury in her body than a woman eating seafood from an uncontaminated environment. What's more, women living in that time and place typically ate much more seafood than today's typical Western woman.

"The second outbreak was in Niigata, Japan, which involved a similar environmental poisoning about 10 years later," Myers adds. "Yet, at Niigata, the mother of the only child actually diagnosed with Congenital Minamata Disease (which includes such effects as severe mental impairment and motor dysfunction), had a hair level of 293 parts per million of mercury." (This is in comparison to the average U.S. woman's level of less than 1 part per million, Myers says.)

Researcher Emily Oken, M.D., a primary care physician at the Fish Center for Women's Health at Brigham and Women's Hospital and an assistant professor at Harvard Medical School, both in Boston, says there is a potential detrimental effect on the IQ of babies exposed to mercury in utero from the mother's diet. But, she adds, the effects are very, very small. "For every 1 part per million of mercury in maternal hair (mercury is typically measured in the body by analyzing its presence in the hair), studies have shown a 0.18-point reduction in IQ," she explains. "To put that in perspective, in the U.S., the average level of mercury in a woman's hair is one-half part per million or less."

At what level of contamination does methylmercury become a concern? A third event—in Iraq in 1972—led Myers and his fellow researchers to investigate what level of mercury contamination can cause problems for developing fetuses. Though this event involved people eating seed grain that had been treated with methylmercury and didn't involve the intake of seafood, Myers and his research team found that women who were pregnant when they ate the seed grain might have subtle neurological consequences for their fetus—including delayed motor development—with exposures as low as 10 parts per million. But again, keep in mind that most women in the U.S. have far less than 1 part per million in their bodies at any given time.

"There clearly are detrimental effects at very high levels of methylmercury exposure, but these are from accidents such as Minamata and Iraq, which involved exposures exponentially higher than what you might get by eating normal seafood," says Dariush Mozaffarian, M.D., DrPH, an assistant professor at Harvard Medical School and a cardiologist at Brigham and Women's Hospital. "In Minamata, the mercury levels were between 20 and 600 parts per million—orders of magnitude higher than what you might get from eating seafood from an uncontaminated environment. At that high level, there's an effect. But at much lower levels, there are very subtle differences in the neurodevelopment of fetuses."

What is a safe intake of methylmercury for pregnant women? "Different agencies have different guidelines, but the most conservative is from the U.S. Environmental Protection Agency," says Myers. "While complicated, their guidelines were calculated by taking the lowest amount anyone thinks might possibly damage a fetus and dividing that number by 10. This translates to a tissue concentration of about 1 part per million."

To meet this guideline, the EPA suggests that pregnant women follow these guidelines:

• Eat no more than 12 ounces of lower-mercury seafood (such as shrimp, canned light tuna, salmon, pollock and catfish) per week.

• Do not eat shark, swordfish, king mackerel and tilefish. These varieties tend to have the highest amounts of mercury.

• Limit albacore tuna (which is higher in mercury than canned light tuna) to no more than 6 ounces per week (one average meal) but still don't eat more than 12 ounces of any seafood per week.

• Before eating any non-commercial seafood (such as that caught by family or friends), check with your local branch of the EPA or health department to make sure the water it was taken from is safe. If you can't obtain this information, eat up to 6 ounces per week of this fish, but don't eat any other seafood that week. So, what's the bottom line? Think the way a toxicologist would: It's not just the toxin that counts but the dose. And keep the situation in perspective: Eating an entire whale (if you could) might expose your baby to enough mercury to pose a risk, but a poached-salmon entrée is unlikely to cause harm. "The whole issue is very interesting because mercury is a poison—there's no doubt about that," Myers says. "There's no question that if you get enough of it, it causes problems. The issue is whether or not very low exposures—the type you get by eating seafood—actually cause problems in developing fetuses."