Karl Rockne of the University of Illinois at Chicago and colleagues at the University of Illinois Urbana-Champaign said mercury is a large component of dental fillings, but it is not believed to pose immediate health risks in that form. However, when exposed to sulfate-reducing bacteria, mercury undergoes a chemical change and becomes methylated -- a potent, ingestible neurotoxin.

The study, published in Environmental Science and Technology, estimated that 2 to 5 kilograms, or up to 11 pounds, of methyl mercury could be entering the public U.S. water supply annually from dental waste water. Methyl mercury is highly toxic in minute amounts.

Rockne gathered waste water samples generated from a single-chair dentist's office and a 12-chair dental clinic.

The researchers found fine, slow-settling particles of mercury get into the waste water mostly after dentists using high-speed drills to remove old amalgam fillings.

"We have to take more steps to prevent the problem from occurring in the first place," Rockne said in a statement. "We're dealing with a pipe, a control point. As an engineer, I see this as a problem we can definitely do something about."