Mercury isotope fractionation provides new tool to trace the source of human exposure

Scientists agree that dental amalgam fillings slowly release mercury vapor into the mouth. About 80 percent of inhaled mercury vapor is absorbed into the bloodstream in the lungs and transported to the kidneys, where it is excreted in urine. In the absence of occupational exposure and because the mercury found in urine is almost entirely inorganic, total mercury concentrations in urine are commonly used as an indicator, or biomarker, for exposure to inorganic mercury from dental amalgams.

Results from a new study:
Researcher from the University of Michigan, who measured mercury isotopes in the hair and urine from 12 Michigan dentists, found that their urine contained a mix of mercury from two sources: the consumption of fish containing organic mercury and inorganic mercury vapor from the dentists' own amalgam fillings.

Mercury has seven stable (nonradioactive) isotopes. The scientists used the mercury isotopic distribution to distinguish between the two sources of exposure. Using this method to trace the metabolic pathway, the researchers could trace that part of methylmercury from fish that underwent a type of chemical breakdown in the body called demethylation. The demethylated mercury from fish contributes significantly to the amount of inorganic mercury in the urine.

The method relies on the effect called mass-independent fractionation that occurs during such metabolic reactions, in which different mercury isotopes react to form new compounds at slightly different rates. The specific isotopic patterns resulting from the mass-independcent fractionation was used as the chemical fingerprints that enabled them to distinguish between exposure to methylmercury from fish and mercury vapor from dental amalgam fillings.

"These results challenge the common assumption that mercury in urine is entirely derived from inhaled mercury vapor," said Laura Sherman, a postdoctoral research fellow in the Department of Earth and Environmental Sciences and lead author of a paper in the journal Environmental Science & Technology. "These data suggest that in populations that eat fish but lack occupational exposure to mercury vapor, mercury concentrations in urine may overestimate exposure to mercury vapor from dental amalgams. This is an important consideration for studies seeking to determine the health risks of mercury vapor inhalation from dental amalgams," said biogeochemist Joel D. Blum, a co-author of the paper and a professor in the Department of Earth and Environmental Sciences. In addition to Sherman and Blum, co-authors of the Environmental Science & Technology paper are Alfred Franzblau and Niladri Basu of the U-M School of Public Health.