Mercury in Seafood Causes Neurological Problems in Humans

The Scare:
Mercury is a toxic metal that is naturally present in the environment and can also be released by human activity such as emissions from coal-burning power plants. Fifty-five percent of global mercury emissions are from natural sources such as underwater volcanoes, and another 42% are from man-made sources outside the United States.(76) When in water, mercury is converted into methylmercury, a potent neurotoxin, which then enters the food chain. Therefore, methylmercury is present in small quantities in fish. Larger fish accumulate more methylmercury than do smaller fish, partly because they are higher up in the food chain.

The Environmental Protection Agency (EPA) sets a “reference dose” for human blood levels of mercury (see “The bottom line” section for more information), and recommends amounts and types of seafood that women who are or may become pregnant and young children should eat in order to limit their exposure to mercury. Media coverage in 2004 of a few mercury-related reports and recommendations set off new alarm about the effects of mercury found in seafood.

Where Did the Scare Come From?
Several events in the beginning of 2004 provided new fuel for scary reports about mercury in fish. In January, the EPA released an estimate that as many as 630,000 babies were born yearly in the United States with blood levels of mercury greater than the EPA’s reference dose. A study published in February 2004 in the Journal of Pediatrics indicated that higher than usual mercury levels in the blood of mothers in the Faroe Islands had caused neurological problems in their children.(77) In March, the Food and Drug Administration (FDA) and EPA combined their previously separate guidelines regarding mercury in seafood, and for the first time also included specific information about canned tuna and tuna steak.(78) Additionally, due to opposition to President Bush’s policies regarding mercury emissions, some election-related political advertisements capitalized on the scare regarding mercury in seafood.

The Media Coverage:
While the EPA’s reference dose for mercury does not indicate a threshold for harm by mercury, many media reports from well-respected sources indicated that the 630,000 babies born yearly with mercury above the reference dose were born with “unsafe”(79) or “dangerous”(80) levels of mercury in their blood. Some articles about the study of children from the Faroe Islands did not mention why the levels of mercury consumption there may be exceptionally high.(81) One report took this issue a step further and inaccurately linked the unsafe levels of mercury in the Faroe Islands study to the statistic about levels of mercury higher than the EPA reference dose in American babies. On CBS’s The Early Show, Jane Houlihan, spokesperson for the Environmental Working Group, stated that, “This new study shows a baby’s brain can be permanently damaged from the mercury pollution in the fish women eat during pregnancy. This is a big deal because one in six babies born in the U.S. is exposed to mercury at unsafe levels in the womb from a mother’s consumption of canned tuna and other fish high in mercury.”(82) An additional flurry of alarmist media coverage surfaced in March in response to the EPA and FDA’s new guidelines on fish consumption. For example, when mentioning the new EPA/FDA guidelines for fish, an article in U.S. News & World Report warned, “your favorite fish may come with a side of toxic mercury.”(83)

Several political advertisements in newspapers and on television also used scare tactics about the hazards of mercury in fish in order to advance political agendas.(84) For example, one television advertisement (by MoveOn.org and the Environmental Working Group Action Fund) aimed at criticizing President Bush’s policies regarding mercury emissions showed a poison symbol morphing into a happy face on a child’s lunchbox. The advertisement’s narrator announced that, “our children will go on eating mercury in their tuna, risking brain damage.”

The Bottom Line:
As with many scares about trace levels of substances, the scare involving mercury in seafood stems from a lack of understanding about the fact that it is the dose that makes the poison. It is true that mercury is toxic, and reports of its tragic effects in humans were reported in the 1950s near Minamata Bay, Japan. There, residents had extremely high exposure to seafood from waters where tons of mercury had been released. However, numerous studies of less extreme circumstances, including a nine-year-long study of children with high prenatal exposure to mercury, have not found evidence of low levels of mercury causing harm in humans.(85) Some studies of children in the Faroe Islands, including the aforementioned one published in February 2004, have found neurological effects associated with prenatal exposure to mercury.(86) However, the results may not be applicable to other groups because the island’s residents eat large quantities of very high-mercury whale meat.(87)

One source for confusion over the safety of mercury is the concept of the Reference Dose (RfD) for mercury in blood, which is a level established by the EPA. Based on studies of toxicity of methylmercury for fetuses, the EPA determined a benchmark dose (BMD), the level of in utero exposure that is associated with an increase in prevalence of children’s abnormal scores on cognitive function tests. The lower 95% confidence limit of the BMD, known as the benchmark dose lower limit (BMDL), was 58 parts of mercury per billion of blood (ppb). The EPA then built in a safety factor of 10 in order to determine the reference dose (set at 5.8 ppb), a regulatory target level for mercury in blood. The EPA’s estimate that 630,000 babies were born annually in the United States with blood levels greater than the reference dose did not indicate, as some media reports claimed, that those babies were at risk for damage.

The recommended mercury exposure levels set by other scientific bodies (including the Joint Food and Agriculture Organization of the United Nations and World Health Organization Expert Committee on Food Additives) are multiple times higher than that set by the EPA.(88) Even with a very strict EPA reference dose, a recent large nationally representative study found that only 5.66% of women of childbearing age had blood levels of mercury higher than the reference dose.(89)

It is important to note that due to the nutritional benefits of eating fish, there is a cost to trying to “be safe” by avoiding fish. The importance of fish to the human diet is even recognized in the EPA and FDA’s guidelines about mercury in seafood, which indicate that “women and young children in particular should include fish or shellfish in their diets due to the many nutritional benefits.”(90,91) Therefore, media-produced scares about relatively low levels of mercury in fish are counterproductive to efforts to improve the health of women and young children.

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