Mercury is part of the silver dental fillings that have long been used to treat cavities; in this form, mercury is believed to be safe.

However, when dentists use drills to remove these fillings, the tiny mercury particles that end up in dental wastewater are exposed to sulfate-reducing bacteria that convert the particles into methyl mercury.

The new findings, published in the journal Environmental Science & Technology, raise the concern that dental offices may be an important source of methyl mercury in the public water supply.

"We found the highest levels of methyl mercury ever reported in any environmental water sample," researcher Dr. Karl J. Rockne, of the University of Illinois at Chicago, said in a statement.

He and his colleagues estimate that up to 11 pounds of methyl mercury from dental wastewater may enter the US public water supply each year. The amount sounds small, but they note that minute amounts of this form of mercury can be toxic.

The findings are, however, based on tests from only two dental practices -- one "single-chair" office and one 12-chair clinic.

More research is still needed to confirm the results, Rockne said.

In the US, public drinking water supplies are monitored for mercury, and if a system's levels are consistently above a certain threshold, steps must be taken to reduce them to acceptable levels.

Mercury that gets into the water can also be consumed by fish and work its way into the food supply.

Dental offices are far from the most significant source of environmental mercury pollution in the US Coal-fired power plants emit about 50 tons of mercury into the air each year.

However, the problem of mercury in dental wastewater can be fixed. Devices called amalgam separators can help remove mercury particles from wastewater, and are a "good first step," Rockne said.

But additional measures may be necessary, he added.