Mercury within dental fillings is not by itself harmful, but when exposed to sulfate- reducing bacteria, the element undergoes a chemical change that turns it into a potent, ingestible neurotoxin. Researchers at the University of Illinois at Chicago and at Urbana-Champaign have found that mercury particles entering drain water from dental offices is an ample source of methyl mercury, a compound long associated with coal-fired electric power plants.

"We found the highest levels of methyl mercury ever reported in any environmental water sample," said Karl Rockne, associate professor of environmental engineering at the University of Illinois in Chicago. Rockne, working with James Drummond, professor of restorative dentistry, observed fine, slow-settling mercury in waste water samples from both a single-chair dentist's office and a 12-chair dental clinic, mostly after dentists used high-speed drills to remove old amalgams.

Asked whether the culprit bacteria were living in the mouths of dental patients, Rockne responded, "We don't have the answer." Based on their sample studies, the researchers estimate that 2 to 5 kilograms, or up to 11 pounds, of methyl mercury could enter the public water supply in the U.S. each year from dental waste water - significant considering that even traces of the compound are highly toxic.

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