Millions of Americans have cavity fillings made of amalgam, a blend of about 50 percent mercury, a neurotoxin, plus tin, silver and other metals. (Fillings called "silver" are actually amalgam.) Although they've been widely used for more than 150 years, some people say amalgam fillings can emit mercury, causing damage to the brain, kidneys or nervous system.

Several studies published in medical journals have linked amalgam fillings to increased levels of bodily mercury. But not everyone is convinced: Matthew Messina, a spokesman for the American Dental Association and a practicing dentist in Ohio, says that "dental amalgam is a stable, solid compound."

A recently settled lawsuit, filed against the Food and Drug Administration by a group led by an organization called Moms Against Mercury, will require the FDA to complete its unfinished process of reviewing and possibly reclassifying amalgam fillings by next July. Now a Class I device, amalgam might be reclassified to Class II, which means that special controls would be issued governing its use. Those could be anything from simply requiring that patients be notified of the potential risks of mercury before receiving fillings to restricting amalgam's use in vulnerable populations such as pregnant women and small children.

The FDA's Web site recently changed its statement on amalgam. The site previously said that government agencies "have found no scientific studies that demonstrate dental amalgam harms children or adults." It now says that mercury in fillings "may have neurotoxic effects on the nervous systems of developing children and fetuses." It also states that "Pregnant women and persons who may have a health condition that makes them more sensitive to mercury exposure, including individuals with existing high levels of mercury bioburden, should not avoid seeking dental care, but should discuss options with their health practitioner."