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

MERCURY 467
5. POTENTIAL FOR HUMAN EXPOSURE
freshwater sportfish, and marine fish and shellfish, the methylmercury content of which were characterized
by national distribution statistics, as were the consumption rates for marine fish. The consumption rates for
freshwater fish were specific to the region. The fish portion size was linked to body mass by a variable
correlation. Each meal was assumed to be an independent sample, so that as metabolic equilibrium was
approached, each person’s body burden of methylmercury tended to approach the value corresponding to the
mean methylmercury intake for the population. Predictions of methylmercury levels in hair by this model
compared well with an observed distribution in 1,437 women. Two neurological end points were examined:
adult paresthesia as related to methylmercury body burden and congenital neurological effects as associated
with average concentrations of methylmercury in maternal hair during pregnancy. In the baseline exposure
scenario, the source of the mercury in fish was background atmospheric deposition. In the worst-case
scenario, local mercury deposition and concentrations in fish were roughly doubled to represent additional
deposition from the hypothetical power plant. For both scenarios, the 99th percentile of methylmercury
body burden was more than an order of magnitude below the lowest level at which increased transient
paresthesia in adults was experienced in an acute methylmercury poisoning incident in Iraq. The authors
concluded that neurological risks to adults from methylmercury resulting from atmospheric deposition are
negligible. Based on three epidemiological studies of congenital neurological risks, they found that fetal
effects appeared to be more critical, and that there is a smaller margin of safety for pregnant consumers of
freshwater sportfish. However, there is still a considerable margin of safety, and uncertainties in the
relationships between maternal hair mercury and actual fetal exposures may have overstated the fetal risk
(Lipfert et al. 1996).
Recreational and Subsistence Fishers. Methylmercury concentrations in sport fish can be at least
an order of magnitude higher than in commercial fish purchased in a supermarket (see Section 5.4.4).
Therefore, recreational and subsistence fishers, including some Native American peoples who consume
locally caught fish from mercury-contaminated waterbodies or consume long-lived predatory oceanic species
such as shark and swordfish, can be exposed to higher mercury concentrations than individuals who
consume similar amounts of commercially marketed fish from a variety of sources (Ebert et al. 1996; EPA
1995k). The exposure to mercury will also be higher among people who regularly eat fish and other seafood
products, compared to those who only occasionally or never eat fish or other seafood products. This
increased exposure has been demonstrated by blood mercury levels several times higher in people who
regularly eat fish, compared to those who occasionally or never eat fish (Buzina et al. 1989; Cappon and
Smith 1982; Oskarsson et al. 1996; Phelps et al. 1980; Svensson et al. 1995). In addition, the consumption
of certain species of fish (e.g., shark and swordfish) is likely to contribute disproportionately to the observed