Does methylmercury have a role in causing developmental disabilities in children?

Methylmercury (MeHg) is a potent neurotoxin that in high exposures can caus e mental retardation, cerebral palsy, and seizures. The developing brain ap pears particularly sensitive to MeHg. Exposure levels in pregnant experimen tal animals that do not result in detectable signs or symptoms in the mothe r can adversely affect the offspring's development. Studies of human poison ings suggest this may also occur in humans. Human exposure to MeHg is prima rily dietary through the consumption of fish: MeHg is present in all fresh and saltwater fish. Populations that depend on fish as a major source of di etary protein may achieve MeHg exposure levels hypothesized to adversely af fect brain development increasing mercury levels in the environment have he ightened concerns about dietary exposure and a possible role for MeHg in de velopmental disabilities. Follow-up studies of an outbreak of MeHg poisonin g in Iraq revealed a dose-response relationship for prenatal MeHg exposure. That relationship suggested that prenatal exposure as low as 10 ppm (measu red in maternal hair growing during pregnancy) could adversely affect fetal brain development. However, using the same end points as were used in the iraq study, no associations have been reported in fish-eating populations. Using a more extensive range of developmental end points, some studies of p opulations consuming seafood have reported associations with prenatal MeHg exposure, whereas others have found none. This paper reviews the data prese ntly available associating MeHg exposure with development and poses some of the unanswered questions in this field.