Nearly all fish contain trace amounts of methylmercury. How does this element get into our fish supply? Mercury occurs both naturally and from man-made sources. Some of it can be traced to coal-burning power plants. Smokestacks release toxic mercury emissions which rain down into rivers, lakes, and oceans. Bacteria convert the mercury to a form that's easily absorbed by insects and other small organisms. Mercury moves up the food chain as small fish eat the small organisms and big fish eat the smaller fish. The highest concentrations accumulate in large predators such as shark, swordfish and tuna...some of America's favorite fish.
Pregnant women, nursing mothers, and very young children are cautioned against excessive consumption of these fish. Read the FDA consumer advisory for pregnant women about the risks of mercury in fish. The FDA also offers a chart on mercury levels in seafood species.
Until the 1950's, the problems that can occur with excessive mercury intake were not well-known. However, at that time, an epidemic hit fishermen and their families in villages on Japan's Minamata Bay. People whose diet was primarily seafood showed signs of brain damage; some were even fatally stricken with disease and seizures. The investigation linked the health problems to methylmercury poisoning from a local chemical plant that was discharging organic mercury into the bay. The villagers were getting sick from eating the fish that had absorbed the mercury. (Learn more about The Poisoning of Minamata.)
In 1969, the FDA first set an action level for total mercury in fish; 0.5ppm (part-per-million) was considered the maximum safe limit. (Action levels represent the limit at or above which FDA will take legal action to remove a product from the market.) In 1979, the action level was raised to 1ppm. In 1984, there was another major change. The FDA stopped measuring on a basis of total mercury and instead started checking levels in terms of methylmercury only. In 1998, the FDA stopped widely testing for mercury in fish.
Around the world, there is concern about mercury contamination through fish, but specific recommendations vary. For example, Health Canada advises consumers to limit their consumption of swordfish, shark or fresh and frozen tuna to one meal per week; for young children and women of child-bearing age, the recommended limit is one meal per month. Health Canada's guideline is 0.5ppm total mercury content — more stringent than in the U.S. Britain's Food Standards Agency is advising pregnant and breastfeeding women and women who intend to become pregnant to limit their consumption of tuna to no more than two medium-size cans or one fresh tuna steak per week.
Even within the United States, women are hearing different advice from different sources, especially where tuna is concerned. The EPA's methylmercury guideline is a recommended limit on mercury consumption based on bodyweight, also known as a "reference dose." EPA's methylmercury reference dose is .1 micrograms/kg body weight per day. In July 2000, the National Academy of Sciences found the EPA's reference dose as "scientifically justifiable" for protecting most Americans.
So exactly how much mercury a 45 lb. child would ingest by eating one 6 ounce can of tuna per week, and how does that compare to the EPA's reference dose? Take a look at the following calculations:
Step 1 - DETERMINE EPA's RECOMMENDED LEVEL FOR A 45 LB CHILD
Multiply child's body weight by EPA's reference dose.
Convert 45 pounds to kilograms = 20.45 kilograms
20.45 kilograms x .1 micrograms per kilogram per day
EPA RECOMMENDED LEVEL = 2.05 micrograms per day = 14.35 micrograms per week.
Step 2 - HOW MUCH MERCURY IS IN 6 OUNCES OF CHUNK WHITE TUNA?
Multiply amount of fish by average mercury level for chunk white albacore.
Convert 6 ounces to grams = 170 grams 170 grams X .31 ppm (or micrograms per gram)**
MERCURY INGESTED = 52.7 micrograms per gram
Step 3 – COMPARE MERCURY INGESTED WITH EPA'S RECOMMENDED LEVEL
Divide 52.7 micrograms by 14.35 micrograms = 3.7
BY EATING 6 OUNCES OF CHUNK WHITE TUNA A WEEK, THE CHILD IS INGESTING ALMOST FOUR TIMES EPA'S RECOMMENDED DOSE.
NOW. Science & Health. Mercury in Fish | PBS
