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

MERCURY 179
2. HEALTH EFFECTS
1992; Nielsen and Andersen 1991a, 1991b; Soria et al. 1992; Suzuki et al. 1992). Hair mercury levels,
determined using segmental hair analysis, can be used to monitor exposure to mercury and may leave a
historical record of exposure or uptake (Phelps et al. 1980; Suzuki et al. 1992). The concentration of
mercury in the hair is considered proportional to the concentration of mercury in the blood. Correlations
can be drawn to determine blood concentrations of mercury relative to its concentration in the hair (see the
discussion of the methylmercury MRL in Section 2.5). Mercury concentrations in maternal hair were
significantly correlated with cord blood levels of mercury in pregnant women who had frequently ingested
whale meat throughout pregnancy (Grandjean et al. 1992). The frequent ingestion of whale meat dinners
during pregnancy and, to a lesser degree, the frequent consumption of fish, as well as increased parity or
age, were associated with high mercury concentrations in cord blood and hair. The incorporation of
mercury into hair is irreversible; the loss of hair mercury occurs as the result of hair loss (Nielsen and
Andersen 1991b).
As with metallic mercury, methylmercury can readily traverse the placental barrier. In humans with no
known exposure to mercury, blood mercury levels increased with advancing gestation such that the mean
blood mercury level on admission for delivery (1.15 ppb) was significantly higher than that of the first
prenatal visit (0.79 ppb) (Kuntz et al. 1982). Cord blood levels were similar to maternal blood values in
labor and postpartum. Concentrations of methylmercury in the fetal blood are slightly higher than in the
maternal blood (Inouye and Kajiwara 1988; Kuhnert et al. 1981). Following an oral dose of methylmercuric
chloride during gestation, accumulation of mercury was much greater in the fetal kidneys than in
the maternal kidneys of guinea pigs (Inouye and Kajiwara 1988). Mercury levels in the liver were slightly
higher in the fetus compared to the dam when exposed to organic mercury at late gestation but were
similar at early gestation. Distribution of mercury in the maternal and fetal brains was uneven, with the
highest concentrations in the neopallium, diencephalon, and mesencephalon and the lowest in the
rhombencephalon. Exposure at later gestational weeks resulted in higher concentrations for both maternal
and fetal brains (Inouye and Kajiwara 1988).
Methylmercury may also be secreted in mother's milk (Bakir et al. 1973). Following intravenous dosing of
methylmercuric chloride (1.6 mg Hg/kg) to pregnant mice on one of days 9–17 of pregnancy, methyl­
mercury was readily transferred to the fetuses from the mothers more predominantly at the later gestational
stage (Inouye and Kajiwara 1990). The placental transfer of methylmercury was more efficient compared
to the lactational transfer in rats exposed to methylmercury in the diet during 11 weeks prior to mating,
during gestation, and during lactation (Sundberg and Oskarsson 1992). A higher concentration of mercury