revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY
2. HEALTH EFFECTS
acquiring all eight pellets. Generally, the results indicate that prenatal exposure to elemental mercury
causes alterations to both spontaneous and learned behaviors, suggesting some deficit in the adaptive
functions of the rats. Co-exposure to methylmercury, which by itself did not alter these functions at the
dose given in this study, served to aggravate the changes significantly. Brain mercury concentrations in
offspring were 1 ng/g w/w in the controls, 4 ng/g in the methylmercury group, 5 ng/g in the elemental Hg
group, and 12 ng/g in the methylmercury + elemental Hg group (Fredriksson et al. 1996).
Adult female rats were exposed to metallic mercury vapor at 2.5 mg/m 3 for 3 weeks prior to fertilization and
during Gd 7–20 (Baranski and Szymczyk 1973). A decrease in the number of living fetuses was observed
in these dams compared to unexposed controls, and all pups born to the exposed dams died by the sixth day
after birth. However, no difference in the occurrence of developmental abnormalities was observed
between exposed and control groups. The cause of death of the pups in the mercury-exposed group was
unknown, although an unspecified percentage of the deaths was attributed by the authors to a failure of
lactation in the dams. Death of pups was also observed in another experiment in which dams were only
exposed to the same dose level prior to fertilization, supporting the conclusion that high mortality in the
first experiment was due, at least in part, to the poor health of the mothers. Without further information,
this study must be considered inconclusive regarding developmental effects.
Newland et al. (1996) studied the offspring of pregnant squirrel monkeys exposed to 0.5 or 1 mg/m 3 of
mercury vapor for 4 or 7 hours per day, 5 days per week during the last two-thirds or more of the gestation
period. One female and 2 male offspring came from mothers exposed to 0.5 mg/m3 mercury vapor during
gestation weeks 5–19, 5–21, or 6–22 for a total of 247–510 hours, resulting in total doses of
1,304–2,900 µg (20–38 µg/day); and 3 male offspring came from mothers exposed to 1 mg/m3 mercury
vapor during gestation weeks 7–21, 3–18, or 8–21 for a total of 283–402 hours, resulting in total doses of
2,901–4,305 µg (42–62 µg/day). Five male offspring born about the same time as the exposed monkeys
served as controls. Lever pressing was maintained under a Concurrent Random-Interval 30 schedule of
reinforcement. Time allocation on each lever was examined during behavioral transitions and in a steady
state. Median maternal blood levels ranged from 0.025 to 0.09 µg/g in animals exposed to 0.5 mg/m 3 and
from 0.12 to 0.18 µg/g in animals exposed to 1 mg/m3 . No differences in birth weight, weight gain, or
body weight at time of behavioral testing were observed between exposed and control offspring. No
difference in sensitivity to reinforcer ratios was identified in the steady state, but there was much more
variability in the steady-state performance of exposed monkeys, as indicated by the standard deviation of
the regression, than in controls. Logistic regression was used to examine the transition to new schedule
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