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

MERCURY 175
2. HEALTH EFFECTS
followed by the liver and lungs (Yoshida et al. 1992). In the brain and whole blood, mercury
concentrations were slightly elevated compared to nonexposed controls. Levels of mercury in the fetus
were approximately 4 times higher after exposure to metallic mercury vapor than after mercuric chloride
administration for mice and 10–40 times higher for rats (Clarkson et al. 1972). The transport of the
mercuric ions is limited at the placental barrier by the presence of high-affinity binding sites (Dencker et
al. 1983).
Inorganic Mercury. No studies were located regarding the distribution of inorganic mercury in humans or
animals following inhalation exposure to inorganic mercury compounds.
Organic Mercury. No studies were located regarding the distribution of organic mercury in humans or
animals following inhalation exposure to organic mercury compounds.
2.3.2.2 Oral Exposure
Metallic and Inorganic Mercury. Data on the distribution of ingested elemental mercury were not
located, and data on the ingestion of inorganic mercury are limited. The metallic mercury that is absorbed
from an oral exposure is expected to resemble many aspects of the distribution of mercuric salts because
metallic mercury is oxidized to mercuric ion in biological fluids, and the resulting distribution reflects that
of the mercuric ion. Unlike elemental mercury, however, the amount of divalent mercury that crosses the
blood-brain and placental barriers is much lower because of its lower lipid solubility (Clarkson 1989).
In some studies there is a combined exposure to both organic and inorganic mercury. Oskarsson et al.
(1996) assessed the total and inorganic mercury content in breast milk and blood in relation to fish
consumption and amalgam fillings. Total mercury concentrations were evaluated in breast milk, blood,
and hair samples collected 6 weeks after delivery from 30 lactating Swedish women. In breast milk, about
half of the total mercury was inorganic and half was methylmercury, whereas in blood only 26% was
inorganic and 74% was methylmercury. The results of a regression analysis for mercury in hair, blood,
and milk indicated that there was an efficient transfer of inorganic mercury from blood to breast milk and
that mercury from amalgam fillings was probably the main source of mercury in breast milk, while
methylmercury levels in blood did not appear to be efficiently transferred to breast milk. Exposure of the
infant to mercury in breast milk was calculated to range up to 0.3 µg/kg/day, of which approximately onehalf
was inorganic mercury. This exposure corresponds to approximately one-half the tolerable daily