FIGURE 1-1 Cycling of Hg in aquatic system. CH3Hg+, methylmercury ion; CH3HgCH3, dimethylmercury; Hg(ll), mercuric mercury; Hg0, elemental mercury; H2S, hydrogen sulfide; HgS, cinnabar. Source: Adapted from EPA 1997b.

episodes. The fetus is considered much more sensitive than the adult. Prenatal exposures interfere with the growth and migration of neurons and have the potential to cause irreversible damage to the developing central nervous system (EPA 1997a). Infants exposed in utero to MeHg during the Minamata and Iraqi episodes were born with severe disabilities, such as mental retardation, seizure disorders, cerebral palsy, blindness, and deafness. At much lower doses that result from chronic maternal fish consumption, infants might appear normal during the first few months of life but might later display deficits in subtle neurological end points (e.g., IQ deficits, abnormal muscle tone, decrements in motor function, attention, and visuospatial performance).

Exposures that occur during childhood and adulthood can also cause damage to the central nervous system, as evidenced by human poison-

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