Methylmercury and fish consumption: weighing the risks.

In this issue (page 1439) Dr. Charles Dumont and colleagues report a substantial fall in mercury levels (as measured by the concentration of methylmercury in hair samples) from 1988 to 1993 among Crees living in the James Bay area of Quebec. Fish in hydroelectric reservoirs had been found to be contaminated with mercury and, in 1988, 14.2% of the population of the area had levels of 15.0 mg/kg or greater. By 1993/94, this proportion had fallen to only 2.7%. Dumont and colleagues conclude that this reduction was achieved by a number of educational activities, chief among which was an effort to encourage the Cree to avoid contaminated species of fish while maintaining their traditional way of life and continuing to eat traditional food. Concern over the contamination of fish by mercury first arose in Canada with a report in 1971 of elevated levels in fish in Lake St. Claire. At about the same time, a report by a Swedish expert group evaluated the health risks posed by methylmercury in fish. They concluded that the threshold for toxic effects in adults corresponded to a hair level of 50 mg/kg. To take into account the presumed greater sensitivity of the developing brain, a safety factor of 10 was applied to the adult threshold by the World Health Organization to cover exposure of the general population. Thus, a level of 5 mg/kg in hair was taken as the upper “tolerable” limit. This limit has been consistently supported by subsequent epidemiologic studies, including those involving the James Bay Cree and studies now in progress in the Seychelles. Dumont and colleagues conclude on the basis of mercury levels in hair that the health risks posed by mercury contamination in the region are low. Even before the intervention, only 5% of adults had hair mercury levels exceeding 30 mg/kg. Few indeed must have exceeded the adult threshold of 50 mg/kg. They did find, however, that mercury levels increased with the age of the study participants. In addition, levels found in husbands and wives had a strong positive correlation with one another but not with those found in other members of the family. This may reflect a difference in lifestyle between older and younger Crees. The question the authors raise as to whether “seasonal life-long exposure . . . may cause effects at doses lower than those observed with shorter-term exposure” has haunted public health authorities for many decades, since current estimates of health risks for adults are based on data from short-term exposures of a few months to a year or so. Although age-related increases in the neurologic signs of mercury poisoning have been reported among the James Bay Cree, no association with mercury levels could be detected. Other studies involving adults with lifetime exposures have failed to reveal health risks any greater than those resulting from short-term exposures. With respect to prenatal risks, no women of childbearing age had levels exceeding 30 mg/kg in 1988, and 1.1% had levels higher than 15 mg/kg in a tested population of 1285 (which also appears to have been the total or “target” population). At these levels, the actual risks are probably small. Effects on child development have not been demonstrated in studies in Canada or elsewhere involving fish-eating populations among whom hair levels were found to be below 15 mg/kg. In Dumont and colleagues’ study, most of the 1.1% (i.e., a total of about 14 in all) of women of childbearing age stated to have hair mercury levels in the Editorial