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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."