The Food and Drug Administration is reviewing the scientific evidence behind its pronouncement last year that dental fillings containing mercury do not harm patients, the Los Angeles Times reports.

The review this week by an FDA advisory panel is focusing on amalgam, a silver-colored material considered the cheapest and longest-lasting dental filling, but one that also releases a mercury vapor. High levels of mercury vapor are associated with brain and kidney disorders. In July, 2009, however, the FDA classified amalgam as a “moderate risk item” that released levels of mercury “not high enough to cause harm in patients.”

Nancy Stade, deputy director for policy in the FDA’s medical device center, said the review was prompted by four petitions received from consumer advocates questioning the rigor of the agency’s scientific evidence.

The American Dental Association opposes restrictions on amalgam, which has been used in hundreds of millions of people over the past 150 years, saying that decisions about use of the material should be made by patients and their dentists. But activists who support a ban on amalgam fillings say that harm from the material has been overlooked because mercury can build up gradually in the body over the years.

The issue is potentially far-reaching because if the FDA determines that amalgam fillings pose a risk, millions of people would be faced with the decision of whether to have their fillings removed. The use of amalgam fillings has fallen in recent years because of concerns about mercury and a consumer preference for composite resin fillings that match the color of teeth.

The advisory committee will not vote on specific recommendations. However, it will discuss technical questions about how exposure to mercury is measured, along with whether safe levels of exposure have been set correctly and whether studies on the effects of mercury on humans are reliable.