ROCHESTER, N.Y. -- Even though the world's fish contain slight amounts of mercury, eating lots of fish carries no detectable health risk from low levels of the substance, even for very young children and pregnant women, concludes the most comprehensive study of the subject yet.
The findings come from a nine-year University of Rochester study conducted in the Republic of the Seychelles, an island nation in the Indian Ocean where most people eat nearly a dozen fish meals each week and whose mercury levels are about 10 times higher than most U.S. citizens. Indeed, no harmful effects were seen in children at levels up to 20 times the average U.S. level. The work is published in the August 26 issue of the Journal of the American Medical Association.
"We look at the Seychelles people as a sentinel population," says pediatric neurologist Gary Myers, who examined the children. "If somebody who eats fish twice a day does not show effects from mercury exposure, it's unlikely that somebody who eats fish twice a week will be affected. And the fish they eat in the Seychelles contains the same amount of mercury as fish sold at supermarkets and eaten in the United States."
Philip Davidson, an expert on developmental disabilities who designed a battery of the most sophisticated tests available to examine the children, said, "What we found in the Seychelles is applicable to every woman, every man, and every child around the world who eats ocean fish."
In the United States the green light applies only to fish bought and sold commercially, at grocery stores, supermarkets, fish shops, and in restaurants. Those fish are already regulated based on their mercury levels, and current regulations are sufficient to safeguard frequent fish eaters against mercury exposure, say the investigators. Consumers still should follow advisories about eating fish caught in lakes and rivers, since there are hundreds of polluted waterways whose fish are dangerous to eat in abundance, often because of other pollutants such as PCBs.
The Seychelles study began in 1989 when Rochester researchers, with decades of expertise studying mercury exposure, chose the Republic of the Seychelles with about 65,000 people as an ideal site to study the effects of mercury exposure. Myers enrolled 779 newborn children in the study, about half the births on the islands that year. From the children's mothers, Myers and the team took samples of hair, which lock in a record of mercury exposure of the child during gestation.
A neurologist, a childhood development expert, and nurses then studied the children at 6, 19, 29 and 66 months of age, visiting their homes, talking to their parents, and performing nearly three dozen sensitive developmental and neurological tests designed to detect subtle effects of mercury exposure. The analysis included noting when the children learned to walk and talk, measurements of reflexes, word recognition, and social behavior, and neuropsychological tests developed to evaluate children at these ages. At each interval, the results of the longitudinal study have been consistent -- no ill effects from a high-fish diet. The JAMA paper details the 66-month evaluation, which included 711 of the original children.
Mercury is a deadly neurotoxin that at high levels kills nerve cells, causing blurry vision, lack of coordination, slurred speech, and even death. Children exposed to high levels of the compound pre-natally can suffer slowed development, blindness, cerebral palsy, and other birth defects. according to the researchers.
While high amounts of mercury are obviously toxic, scientists for years have debated the health effects of lower levels. Late last year, the federal Environmental Protection Agency (EPA) proposed slashing the amount of mercury that is acceptable for people to ingest from 30 micrograms per day, the level recommended both by the World Health Organization and the federal Agency for Toxic Substances and Disease Registry, to just 6 micrograms per day. If the Food and Drug Administration (FDA) follows this guideline, it will need to slash the current level of mercury allowable in ocean fish that are sold in the United States below the current level of 1 part per million (ppm).
That action would remove from the market a significant proportion of the fish now available, especially large predatory fish like swordfish, shark, and red snapper, and could even affect tuna. The team fears that it might also convince consumers who associate mercury with health dangers to limit their intake of fish, a remarkably healthy form of nutrition. Under the proposed rules, scientists estimate that the average person would be able to eat only a few ounces of fish per week before bumping up against the new limit.
"Eating lots of ocean fish isn't much of a hazard compared to missing out on the benefits from not eating fish," says Thomas Clarkson, professor of environmental medicine and an internationally recognized authority on mercury. Clarkson is principal investigator of the study, which is being funded by the National Institutes of Health, the Food and Drug Administration, and the Republic of the Seychelles.
"A slew of scientific reports have shown that eating fish helps protect against cardiovascular disease and enhances brain development before and after birth. Fish is a rich source of low-fat protein and is full of fatty acids known to lower cholesterol. Overstating the almost negligible risk of mercury could adversely affect millions of people who face the risk of heart disease," says Clarkson. He adds that FDA's current guideline already helps people avoid excessive mercury exposure, which would be a danger primarily for someone eating frequent meals of fish like swordfish and shark.
Fish are the primary source of exposure to mercury around the world. Scientists estimate that about half the mercury in the Earth and its atmosphere originates from natural sources such as volcanoes. Man-made sources include coal-fired power plants, smoke from cigarettes, and incinerators that burn items like fluorescent bulbs, batteries, and mercury thermometers. Mercury vapor enters the atmosphere and falls in rainwater to the Earth. Then, in a poorly understood process in the oceans and other bodies of water, microbes play a key role, transforming the mercury into a substance known as methyl mercury, which works its way up the food chain and accumulates primarily in virtually all fish around the globe.
While the study focused on healthy fish from ocean waters, its implications spill over into the freshwater arena too. Mercury is one of many pollutants that limit consumption of fish from lakes and rivers across the nation, and individual states rely on federal guidelines when developing recommendations on how many fish can be eaten per week or month. If federal agencies lower the level of mercury they say is acceptable in the diet, that would likely force states to recommend that residents eat fewer fish from local waters.
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