March 21, 2013
Urine, Hair Samples Overestimate Mercury Exposure From Dental Fillings
April Flowers for redOrbit.com - Your Universe Online
A new study from the University of Michigan reveals that a common test currently used to determine mercury exposure from dental amalgam fillings may significantly overestimate the amount of toxic metal released from fillings.
Although scientists agree that mercury vapor is released into the mouth by dental amalgam fillings, the amount released remains controversial, as does the question of the health risk posed by this release.
Previous public health studies have assumed that the mostly inorganic mercury in urine can be used to estimate exposure to mercury vapor from amalgam fillings, while the mostly organic mercury in hair is used to estimate exposure to organic mercury from a person's diet.
The new study, published in a recent issue of Environmental Science and Technology, measured mercury isotopes in the hair and urine of 12 Michigan dentists. The researchers found that the urine contained a mixture of mercury from two sources: organic mercury from the dietary consumption of fish, and inorganic mercury vapor from 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 (EES).
"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," U-M biogeochemist Joel D. Blum, professor in the Department of Earth and Environmental Sciences, said in a statement.
The study findings demonstrate that mercury isotopes can be used to more accurately test for exposure to the metal and the related health risks, than traditional measurements of mercury concentrations in hair and urine samples. Isotopes provide a unique chemical tracer that the scientists can use to "fingerprint" both organic and inorganic mercury sources.
Mercury is an element that occurs naturally, but more than 2,000 tons are emitted into the atmosphere annually from man-made sources such as coal-fired power plants, small-scale gold-mining operations, metals and cement production, incineration and caustic soda production.
These mercury emissions are deposited onto land and into water. Micro-organisms then convert some of it to methylmercury, which is a highly toxic organic form that builds up in fish and the animals that eat them, including humans. There are harmful side effects to this build up, including damage to the central nervous system, heart and immune system. Fetuses and young children are especially vulnerable with their developing brains.
Inorganic mercury, of the sort found in dental amalgam fillings, can also cause central nervous system and kidney damage. Inhalation of elemental mercury vapor accounts for the majority of exposure to inorganic mercury, with industrial workers, gold miners and dentists being at highest risk. The risk to dentists has been decreasing as they have increasingly switched to resin-based composite fillings and restorations in recent years.
A vast majority of inhaled mercury, approximately 80 percent, is absorbed into the bloodstream in the lungs and transported to the kidneys. It is then excreted in urine. The mercury in urine is almost entirely inorganic, leading scientists to use it as an indicator, or biomarker, for exposure to mercury from dental fillings.
The U-M study suggests that the mercury found in urine consists of a mixture of inorganic mercury from dental amalgams and methylmercury from fish that has undergone a chemical breakdown in the body called demethylation. The amount of inorganic mercury in the urine contains a significant amount of inorganic demethylated mercury from fish.
The research team used a natural phenomenon called isotopic fractionation to distinguish between the two types of mercury. All the atoms of a particular element, in this case mercury, have the same number of protons in the nucleus. There can be various forms of any given element, called isotopes, each with a different number of neutrons in the nucleus.
There are seven stable, or nonradioactive, isotopes of mercury. Different mercury isotopes react to form new compounds at slightly different rates during isotopic fractionation. Specifically, the U-M research team used a type of isotopic fractionation called mass-independent fractionation to obtain the chemical fingerprints. These chemical fingerprints enabled them to distinguish between exposure to methylmercury from fish and mercury vapor from dental amalgam fillings.