developmental neurotoxicity as the health effect of greatest concern following high-level episodic exposures. Individuals poisoned by MeHg through consumption of contaminated fish in Japan exhibited paresthesia, ataxia, sensory disturbances, tremors, impairment of hearing, and difficulty walking (Harada 1995). In Iraq, exposure was due to the consumption of home-made bread that was made with grain treated with MeHg as a fungicide. In that outbreak, the most common symptom in adults was paresthesia; the most severely affected individuals exhibited ataxia, blurred vision, slurred speech, hearing difficulties, blindness, deafness, and death (Marsh et al. 1987). In both Iraq and Japan, the effects in offspring who were exposed to MeHg in utero were more serious, and in some cases seen at lower doses, than in adults. Both exposure episodes have been studied to determine the doses and the effects resulting from exposure to MeHg. Although the doses that produced those effects in the Japanese and Iraqi populations were undoubtedly quite high, precise dose-response relationships have not been established, and the exposure scenarios are not comparable to the low-dose chronic exposure that the general population in North America might experience.
In an attempt to establish dose-response relationships, three large prospective epidemiological studies have evaluated subtle end points of neurotoxicity. One study was conducted in the Republic of the Seychelles, a nation of islands located in the Indian Ocean off the coast of East Africa (Davidson et al. 1995, 1998). Another major study was conducted in the Faroe Islands (part of Denmark), which are located in the North Sea between Scotland and Iceland (Grandjean et al. 1997, 1998, 1999). The other major study was conducted in New Zealand (Kjellström et al. 1986, 1989). The populations of the Seychelles, Faroe Islands, and New Zealand were chosen for study, because their dietary dependence on fish and marine mammals provides an ongoing source of exposure to MeHg. Prenatal MeHg exposures in those populations were within the range of at least some U.S. population exposures. All three studies evaluated large numbers of subjects.
The 66-month study of 711 children in the Seychelles islands assessed the effects of prenatal MeHg in tests of global intelligence and developmental milestones. No adverse effects were seen that could be attributed to MeHg. Maternal hair samples collected at birth contained Hg concentrations that ranged from 0.5 to 27 ppm (mean, 6.8 ppm). Meanwhile,