Methylmercury Science

Methylmercury Science

July 17, 2008

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During the past few years, environmental activists, public health officials, and the media have become increasingly concerned about consumers’ exposure to mercury, primarily methylmercury in fish. In response to exaggerated risk estimates, many consumers have been advised to reduce their fish consumption. However, the most reliable scientific analyses continue to show that eating at least two servings of fish each week produces health benefits that substantially outweigh any hypothesized risk— even for young children and pregnant mothers. 

The advisory warnings that spawned this unhealthy trend were not created on a scientific or nutritional basis. Rather, they were created for political reasons. Environmental activists have buoyed the fish scare in an attempt to increase regulation of mercury emissions from electric power plants. The theory is that methylmercury in fish is unhealthy for pregnant women and children and that mercury emissions must therefore be significantly reduced. 

A substantial body of evidence indicates, however, that (a) the amount of mercury in the American diet is so low that it has little or no health effect on even at-risk populations, and (b) even sizable reductions in mercury emissions would have no appreciable effect on American exposure to mercury. Furthermore, the cost of complying with new power plant emissions regulations is estimated to have a large human impact. 

Very large doses of mercury are known to have substantial adverse health effects, including impacts on neurodevelopment in both adults and children. Effects on developing fetuses are of special concern, because mercury in the diet of pregnant women can affect the hearing, intelligence, and other cognitive functions of those children. However, all that we currently know about the health effects of methylmercury exposure is derived from (a) the study of mass poisonings in Iraq and Japan, (b) epidemiological studies conducted with populations that are different from Americans in important ways, and (c) experimental studies on lab animals. Each of these sources suffers from shortcomings, but the existing science suggests that methylmercury exposure at the very small current dietary levels does not pose a genuine health or developmental risk. 

The dangerous effects of methylmercury exposure were first highlighted when, from the 1930s to the 1960s, people living around Minamata Bay, Japan, ate fish heavily contaminated with mercury wastes discharged from a plastics factory. Hundreds died, and thousands more were left with varying degrees of neurological damage. The precise level of mercury exposure was never accurately calculated, but it is generally believed to be far higher—perhaps several hundreds of times higher—than current U.S. dietary exposure from fish. A similarly tragic case, resulting from mercury-contaminated grain in Iraq, occurred in the 1970s—again, however, with mercury exposure thousands of times greater than seen today with normal dietary fish consumption.3 Because the exact dosage is important in determining whether exposure to a substance will be harmful, these mass poisoning scenarios have little or no relevance for determining whether fish consumption, under normal circumstances, poses any legitimate health threat. 

Researchers have instead turned to epidemiological studies of various fish-eating populations to determine whether typical dietary mercury exposure poses a real risk. Several different study populations have been examined, but the two largest and most extensive studies have considered populations in the Faroe Islands and the Seychelles Islands. Researchers conducting the Faroe Islands study claim to have found a link between small dosages of mercury and negative health impacts, whereas authors of the Seychelles study are convinced there is no such link.