Each year, dentists fill 70 million cavities in the U.S. with a silver amalgam that contains mercury.

Some people have wondered about the health effects of this kind of mercury exposure in children, because it mercury can affect brain and kidney function. But a new study in the Journal of the American Medical Association says parents can rest easy.

"There has been increasing concern about the use of mercury-based amalgam for filling teeth, even though we've been using exactly this compound for about 150 years," said Dr. Sonja McKinlay of the New England Research Institutes.

McKinlay and colleagues at the Forsyth Institute in Boston studied the effects.

"Dentists and parents I think can be very assured from the results of this study that they can continue to put silver amalgam in children's mouths to fill cavities," she said.

The five-year study involved more than 500 kids ages 6 to 10 who had cavities but no fillings when it started. Half of the children got white fillings with no mercury, and the other half got the traditional amalgam.

The study showed that those with the silver had higher mercury levels in their urine, but they were still at a low level that didn't seem to affect them.

"We found that on I.Q. and other aspects of brain function, as well as kidney function, the group that received the mercury-based amalgam was exactly the same as those that received the composite," McKinlay said.

Andrea Kontos, whose sons were in the study, was not surprised. Her children got the white fillings, but she would have been fine with the other version.

"I did know that amalgam fillings have mercury, and I didn't have a problem with that because I have a whole mouth full of it myself," she said.