A high-level debate about the health risks of mercury appeared to be settled on Tuesday when a National Academy of Sciences' panel endorsed strict safety levels set by the Environmental Protection Agency (EPA). But the debate continues beneath the surface. Some scientists hotly contest the panel's conclusions, which may also pose problems for other federal agencies.
Mercury gets into the air and water mainly from coal-burning power plants. People can be exposed to a neurotoxic form called methylmercury by eating fish that have absorbed the toxin through their gills or eaten smaller animals. Five years ago, a controversy erupted when EPA proposed tightening the safe level for mercury exposure to 0.1 micrograms per kilogram of body weight per day. That level was up to five times more stringent than the amount set by other federal agencies, such as the Food and Drug Administration.
Critics argued that low levels of mercury are harmless, citing a major 1998 study that found no damage to neurological development in 700 children born to fish-eating mothers in the Seychelles Islands in the Indian Ocean. But EPA scientists said a Danish study of children in the Faroe Islands in the North Atlantic, which did find neurological harm at low levels, justified its position. The results were less clear-cut, however: The mercury-tainted whale meat that the Faroe islanders ate also contained polychlorinated biphenyls (PCBs) and other pollutants known to affect neurodevelopment. When scientists couldn't agree on which study was more reliable, Congress requested the academy report.
To the surprise of some, the panel placed more faith in the Faroe Islands study. At the panel's request, the Danish investigators excluded the data for children who were also exposed to PCBs; the remaining subjects still showed neurological effects from mercury, says retired pathologist Robert Goyer from Chapel Hill, North Carolina, who chaired the committee. "We're not really clear why the Seychelles Islands is different, but we feel very confident in the results," says Goyer. A recently published New Zealand study, he adds, supports that position.
But some scientists don't buy that conclusion. "We're very disappointed," says University of Rochester neurologist Gary Myers, a member of the Seychelles study team. Myers and others--including a scientist at the Department of Health and Human Services who spoke with Science--argue that the Danish researchers should have measured other pollutants in the Faroe islander's diets, not just certain PCBs; and they say the New Zealand study, which involved about 200 children, was flawed in part because it was too small. Another critic, Alaska state epidemiologist John Middaugh, complains that some panelists were "among the most extreme proponents" of stricter levels.
The critics say they don't oppose EPA's plans to clamp down on industrial mercury emissions. But they worry that people will stop eating fish, which has many beneficial health effects--a tradeoff agencies such as the Food and Drug Administration must now weigh in deciding whether to adjust their safety levels for mercury. "For a presumed risk that has not been proved, it would be a mistake," Myers says.