Who Should Be Concerned?

See this related seafood lobby fable:
Fable: No one has ever been harmed by mercury in commercially-caught fish

A strong consensus exists among scientists and public health officials that pregnant women, women of childbearing age more generally, and young children (i.e., their parents) should be concerned about exposure to mercury, and should choose low-mercury fish.

A child’s brain grows and develops rapidly up to the age of about six years. Children in that age range who like to eat fish should be given low-mercury varieties like shrimp, salmon, flounder, tilapia and pollock.

There is less agreement about other populations, including adult men of all ages, women beyond their child-bearing years, and children older than six years. However, perception that “no one but pregnant women needs to worry” may be fairly common, because the government’s advice is focused exclusively on this population.

Fable: No one has ever been harmed by mercury in commercially-caught fish.

Facts: This often-repeated public relations claim is sometimes qualified with the phrase “No one in the United States has ever been harmed…,” probably because the studies documenting the adverse effects of mercury in populations with high (commercially-caught) fish diets were done in other countries.

The claim is also sometimes stated, “There has never been a single documented case [whatever that means—emphasis added] of methylmercury poisoning in the United States from eating commercially-caught fish.”

This, too, is a distortion. Methylmercury poisoning does occur, in people who eat commercially-caught fish, in the United States. Such cases are rare; they almost certainly occur in less than one-tenth of one percent of the population, i.e., less than 1 person in 1,000. But they do occur. At least two case reports have been published in medical journals. Mercury Policy Project (MPP) published a report in 2008, Over The Limit, which reviewed 24 case histories of people who got methylmercury poisoning by repeatedly eating high-mercury fish.

Most of the cases analyzed in Over The Limit were health-conscious individuals who felt their high-fish diet was a healthy choice. They were generally not aware that the types of fish they chose to eat—tuna, swordfish, sea bass and halibut, among others—contained enough mercury to cause toxic symptoms, if eaten too often. Consumer education about mercury risks and the mercury content of different fish choices, aimed at people who eat above-average amounts of fish, can reduce this risk. The seafood lobby’s denial that the risk exists, however, has significantly muddied the waters.

In addition to their unusually high fish consumption and preference for high-mercury fish varieties, patients described in Over The Limit probably were also more sensitive than average to the toxic effects of methylmercury. It is a well-established fact that individuals vary widely in their sensitivity to toxic substances, and many people with high mercury exposure will not experience toxic symptoms. Unfortunately, there is no practical way to know in advance who is sensitive and who less so. It makes sense therefore for everyone who eats fish frequently to try to limit their mercury exposure.

See our listing of fish and shellfish by mercury levels.

In the U.S., only about 5 percent of the population eats fish twice a week or more. Those high-end consumers should choose most of their seafood meals from the two lower-mercury categories shown in the chart.