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

MERCURY 308
2. HEALTH EFFECTS
most recent NOAEL of 6.8 ppm for the Seychelles cohort at 66 months of age strongly supports the findings
at earlier ages, and that the benefits of eating fish outweigh the small risk of adverse effects from an
increased exposure to methylmercury for this exposure pathway.
The differences in these studies highlight the importance of interpreting epidemiology results and, indeed, all
study results on mercury toxicity within a fairly comprehensive context of the numerous factors that might
affect the toxicokinetics and the amount absorbed (e.g., form of mercury, route of exposure, age, diet of
population exposed, health status, other potential sources of exposure to mercury, dose duration, constancy
of dose amount over time, etc.)
A route of exposure unique to children is breast milk. Both organic and inorganic mercury can move into
breast milk from a nursing woman’s body, and children will readily absorb this 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 (an exposure source for older children). 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. That is because, unlike the
placental barrier, which is crossed more easily by methylmercury than by inorganic mercury, inorganic
mercury moves more easily into breast milk. Some researchers think that a carrier mediated process is
involved (Sundberg et al 1998).
For the Swedish population in the study, Oskarsson et al. (1996) reports 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 one-half was inorganic mercury. This exposure corresponds to
approximately one-half the tolerable daily intake of total mercury for adults recommended by the World
Health organization. The authors concluded that efforts should be made to decrease total mercury burden in
women of reproductive age Oskarsson et al. (1996).
The metabolism of mercury is relatively straightforward compared, for example, to pesticides or some
organic solvents. No information was identified to indicate that metabolic pathways are different for
children and adults, or that children have unique metabolites. Once absorbed, metallic and inorganic