A test used to gauge mercury exposure from dental fillings greatly overestimates the amount of the toxic metal they may release, causing unnecessary concerns about health risks, new research suggests.
The findings, by University of Michigan researchers, cast new doubts on the notion that mercury exposure from dental amalgam fillings poses a significant danger — an issue that the investigators acknowledged is highly controversial.
In a new report in the journal Environmental Science & Technology, the UM scientists said there is no question that dental amalgam fillings slowly release mercury vapor into the mouth. But the amount of mercury released is typically small and is not likely to pose a major health risk.
They noted public health studies often assume that mercury in urine can be used to estimate exposure to mercury vapor from amalgam fillings and that mercury in hair indicates exposure to mercury from diet. But the UM study, which 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.
"These results challenge the common assumption that mercury in urine is entirely derived from inhaled mercury vapor," said lead researcher Laura Sherman, a postdoctoral research fellow in the Department of Earth and Environmental Sciences.
Added co-researcher Joel D. Blum, a UM biogeochemist: "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."
Mercury is a naturally occurring element, but is emitted into the atmosphere from coal-fired power plants, small-scale gold-mining operations, metals and cement production, incineration, and caustic soda production. A highly toxic organic form of mercury can build up in fish and humans. It can cause damage to the central nervous system, heart, and immune system.
Inorganic mercury can also cause central nervous system and kidney damage. Exposure to inorganic mercury can occur through the inhalation of elemental mercury vapor by industrial workers and dentists who install mercury amalgam fillings.
Because the mercury found in urine is almost entirely inorganic, total mercury concentrations in urine are commonly used as an indicator of exposure from dental amalgams. But the UM study suggests that urine contains a mix of inorganic mercury from dental amalgams and mercury from fish.
The study was funded, in part, by the National Institute for Occupational Safety and Health.
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