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

MERCURY 174
2. HEALTH EFFECTS
levels of mercuric ions in the plasma are similar to levels of mercuric ions in the red blood cells. Binding of
mercury also occurs in tissues, and retention varies, with the brain retaining mercury the longest.
The influence of age on mercury distribution following exposure to metallic mercury was evaluated in
neonatal (12 hours old) and adult guinea pigs exposed to 8 or 10 mg Hg/m 3 vapor for 120 minutes (Yoshida
et al. 1989). The mercury concentrations were 28, 58, and 64% higher in the brain, lungs, and heart,
respectively, of the neonates compared to the mothers. However, the mercury level in the kidneys was
approximately 50% lower in the neonates. The lower uptake of mercury in the kidneys of neonates may be
due to the functional immaturity of the kidneys at parturition. The higher levels in other highly perfused
tissues suggest that mercury accumulation in organs is dependent on how easily metallic mercury can reach
the tissues from blood. Similar findings were reported by Jugo (1976) who found higher mercury
concentrations in the liver, blood, and brain, but lower concentrations in the kidneys of 2-week-old rats
compared to similar tissues in 21-week-old rats. These results also suggest that infants may accumulate
mercury more readily after acute exposure and, therefore, may be more likely to exhibit neurotoxicity from
mercury vapors.
The extent of mercury accumulation with aging was studied in mice maintained under normal care
conditions in a conventional rodent colony without exposure to known mercury sources other than
background concentrations normally found in food, water, and air (Massie et al. 1993). There was no
significant change in the total amount of mercury in the organs (lungs, heart, brain, and liver) from male
C57BL/6J mice ranging in age from 133 to 904 days. However, the ratios of mercury levels in the brain to
mercury levels in the liver and kidneys were found to increase significantly and dramatically with age. The
increase with aging in the brain-to-liver and brain-to-kidneys ratios suggests that mercury removal from the
brain may be less efficient in some older organisms.
Metallic mercury vapor easily penetrates the placental barrier and accumulates in fetal tissues. The high
lipophilicity of metallic mercury favors its penetration across the barrier. The uptake of mercury appears
to increase during the later gestation period in mice, as indicated by increased mercury accumulation in the
fetus after exposure to metallic mercury (Dencker et al. 1983). Guinea pig fetuses that were exposed to
6–13 mg/m 3 mercury vapor during late gestation had elevated mercury concentrations in the liver, while
the levels in other tissues were only slightly increased relative to controls (Yoshida et al. 1990). Newborn
guinea pigs that were nursed by their mothers, who had been and exposed to mercury vapor (6–9 mg/m 3 )
for 120 minutes immediately after parturition, had the highest mercury concentrations in the kidneys,