ria. A.O. Summers (University of Georgia, personal commun., Dec. 1999) estimated that 9 µg of MeHg can be formed per day in the gut of humans. That estimate is based on the bacterial species reported to occur in the human gut and assumes that there are 454 g of feces in the lower bowel of an adult human. However, not all the MeHg that is synthesized would be absorbed. Some of the methylation would occur in the colon, where absorption is less. In addition, intestinal flora can demethylate MeHg to inorganic Hg, which is poorly absorbed by the GI tract (Nakamura et al. 1977; Rowland et al. 1980).

The major source of exposure to elemental Hg in the general U.S. population is due to Hg vapor released from dental amalgams (Goering et al. 1992; Halbach 1994; Lorscheider et al. 1995). Approximately 300 metric tons of Hg are used annually by dentists for amalgams (Arenholt-Bindslev and Larsen 1996). Most amalgams used in the United States contain approximately 50% Hg (IPCS 1991; Aposhian et al. 1992a; Lorscheider et al. 1995). In a study of college students who have dental amalgams, two-thirds of the Hg excreted in the urine appeared to be derived from the Hg vapor released from their amalgams (Aposhian et al. 1992a). Evidence shows that Hg vapor from dental amalgams enters tissues, including the brain, where it is oxidized to inorganic Hg. Pregnant sheep given amalgam fillings labeled with radioactive Hg accumulated radioactivity in maternal and fetal tissues within a few days (Vimy et al. 1990). Significant positive correlations between the number of amalgams in the mouth and the mercury content of human tissues, including the brain, are also seen (Drasch et al. 1994). The mean concentration of total Hg in whole blood (in the absence of consumption of fish with high concentrations of MeHg) is probably of the order of 5-10 µg/L (IPCS 1991; Mahaffey and Mergler 1998). This concentration is most likely due to exposure to Hg vapors from amalgams, because retention of inorganic Hg is very low compared with retention of organic and elemental Hg. Furthermore, exposure to MeHg from non-fish sources is also very low (IPCS 1991).

Occupational exposure to elemental Hg has occurred because of accidents in chloralkali plants (Bluhm et al. 1992). However, there are other potential occupational exposures to elemental Hg. In addition, some Caribbean religions use elemental Hg in religious ceremonies (Wendroff 1995). Children have been known to play with elemental Hg because of its fascinating physical properties (i.e., liquid silver), possibly