ANN ARBOR, Mich., USA: The findings of a recently published study suggest that among populations that consume fish, tests of urine mercury concentrations may significantly overestimate mercury exposure from dental amalgam. Through a new test, researchers from the U.S. have found that mercury levels in the urine of individuals with amalgam fillings are primarily derived from diet.

Although mercury occurs naturally in the environment, more than 2,000 tons are emitted into the atmosphere from human-generated sources every year. This mercury is deposited onto land and into water, where microorganisms convert it to methylmercury, a highly toxic organic form that builds up in fish and the animals that eat them.

Conventionally, human exposure to methylmercury and elemental mercury vapor from dental amalgam is often estimated using total mercury concentrations in hair and urine. In the current study, however, the researchers measured mercury isotopes, variants of the element, in the hair and urine of 12 dentists from Michigan. They found that their urine contained a mix of demethylated fish-derived mercury and amalgam-derived inorganic mercury. They estimated that more than 70 percent of the element in the urine of individuals with up to ten amalgam fillings is derived from ingestion of fish.

The study demonstrated that mercury isotopes can be used to assess mercury concentrations in hair and urine samples more accurately than conventional measurement methods. In particular, the researchers were able to identify both organic mercury from fish and inorganic vapor from dental amalgam independently.

"These findings challenge the common assumption that mercury in urine is entirely derived from inhaled mercury vapor," said Laura Sherman, lead author and postdoctoral fellow from the University of Michigan's Department of Earth and Environmental Sciences.

Despite the increasing use of resin-based composites, dentists who place amalgam fillings are still at particular risk of exposure to inorganic mercury that primarily occurs through the inhalation of mercury vapor. The researchers estimated that about 80 percent of the inhaled vapor is absorbed into the bloodstream. Known effects of mercury exposure on humans include damage to the central nervous system, heart and immune system, as well as to brain development in fetuses and small children.

The study was published online on March 6 in the Environmental Science and Technology journal.