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

MERCURY 259
2. HEALTH EFFECTS
where methylmercury is present in combination with other known or suspected neurodevelopmental toxicants,
such as lead or polychlorinated biphenyls (PCBs), and in which exposure is primarily episodic in nature, the
health assessor might consider using a value below the chronic oral MRL for methylmercury as a starting
point for determination of further site investigation. (A more complete description of the uses of MRLs and
other HGVs can be found in Chou et al. 1998 and Risher and De Rosa 1997.)
Background and general population exposures relevant to the oral MRL for methylmercury
Mercury hair levels have been monitored in a variety of populations and generally range from 1 to 4 ppm,
depending upon the level of fish consumption. Table 2-10 summarizes the mean (or median) values and the
maximum value from a number of these studies.
Diet. Based on the FDA total diet study of 1982–1984 (Gunderson 1988), FDA estimated that the average
intake for total mercury (both inorganic and organic) is 50–100 ng/kg/day. Based on the more recent
1989–1990 FDA total diet study, the estimated intake of total mercury is 27–60 ng/kg/day (Cramer 1994).
An estimated 86% of the mercury in the total diet study is derived from fish (Tollefson and Cordle 1986). A
separate estimate of the average intake of methylmercury alone, based on a survey of fish eaters and average
levels of methylmercury in fish, places the average intake of methylmercury at 36 ng/kg/day, with a 99%
upper bound at 243 ng/kg/day (Clarkson 1990).
Potential protective effect of selenium in fish. Selenium is known to bioconcentrate in fish, and selenium has
been observed to correlate with mercury levels in the blood of fish consumed (Grandjean et al. 1992).
Furthermore, there is evidence suggesting that consumption of methylmercury from fish, in conjunction with
other beneficial constituents in fish (e.g., omega-3 fatty acids) may not result in the same toxicity dose-
response relationship observed with methylmercury exposure from consumption of contaminated grain (as in
the Iraqi population) (Davidson et al. 1998).
Regarding the bioavailability of methylmercury in fish, the available data indicate that methylmercury
uptake is not affected by its presence in fish. Experimental studies on the metabolism of methylmercury
in humans following the ingestion of contaminated fish (using methylmercury bound to fish muscle protein)
have shown that absorption is almost complete (95% absorbed) (Miettinen 1973). Animal studies also
support this absorption value. Data on cats given fish homogenates indicate absorptions of $90% of
methylmercury, whether added to the homogenate, accumulated by fish in vivo, or from methylmercury
proteinate (Berglund et al. 1971). Using blood and tissue levels as evidence of absorption, Charbonneau
et al. (1976) concluded that there was no difference in the biological availability of methylmercury