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.