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

MERCURY 188
2. HEALTH EFFECTS
workers occupationally exposed to mercury: a metallic form, a mercuric-cysteine complex that is reducible,
and a large complex in which the mercury can only be released by organic destruction (Henderson et al.
1974).
Data are limited on elimination of metallic and inorganic mercury in animals. Initial excretion of mercury
is predominantly in the fecal matter following inhalation of metallic mercury vapor, but as mercury
concentrations increase in the kidneys, urinary excretion increases (Rothstein and Hayes 1964). After
inhalation, approximately 10–20% of the total excreted metallic mercury is by exhalation (Rothstein and
Hayes 1964). Mercury is excreted in the urine of mice exposed orally to mercuric sulfide
(.8–200 mg Hg/kg) (Yeoh et al. 1986, 1989). The amount of mercury in the urine of the treated group was
4.5–15-fold greater than the control levels. Urinary rates of mercury excretion were 1.6–2.2 ng/hour.
Neonatal rats (1, 8, and 15 days old) eliminated mercury slower than older rats (22 and 29 days old) given
mercuric chloride subcutaneously (Daston et al. 1986).
Inorganic mercury is also excreted in breast milk (Yoshida et al. 1992). Newborn guinea pigs were exposed
to inorganic mercury in breast milk from mothers exposed to mercury vapor (6–9 mg/m 3 ) for 120 minutes
after parturition (Yoshida et al. 1992). Mercury concentrations in breast milk were slightly lower than
plasma mercury concentrations of the maternal guinea pigs over the observation period. Ratios of milk to
plasma were 0.24–0.44 on day 3, 0.45–0.46 on day 5, and 0.46–0.66 on day 10. The decrease in the
mercury concentration in breast milk with time was slower than that in maternal plasma. The distribution of
mercury to organs in the suckling neonates indicated that they were exposed to the inorganic rather than to
elemental mercury.
Sundberg et al. (1998) studied the elimination of radiolabeled inorganic mercury in lactating and
nonlactating mice exposed to mercuric chloride via a single intravenous injection at 0.5 mg Hg/kg body
weight. A three-compartment pharmacokinetic model was used to fit the data. The study was designed to
provide additional information on the speciation of mercury in breast milk and the differences between
methylmercury and inorganic mercury migration into milk. Unlike placenta, where methylmercury moves
more easily across the placental border than inorganic mercury, inorganic mercury is more readily
eliminated in milk than methylmercury. For inorganic mercury, no significant differences were observed
between lactating and nonlactating mice for plasma clearance (43.3 and 44.4 mL/hour/kg, respectively) and
volume of
distribution (4,950 and 3,780 mL/kg). The terminal half-lives of inorganic mercury in plasma were
297 hours for lactating, and 162 hours for nonlactating mice. The milk-to-plasma concentration ratio