The element mercury has been shown to damage the human nervous system. Yet since 1856, mercury amalgam is "the" filling dentists have used to repair our teeth. Now a study in the journal Chemical Research in Toxicology indicates that mercury fillings actually lose their toxic potential over time, Canadian scientists used x-ray spectroscopy to compare new mercury amalgams to those about 20 years old. After two decades, 95 percent of the elemental mercury on the surface of a filling had gone through a chemical reaction. It became beta mercury sulfide, a black-colored solid. Which explains why fillings dull with age. And the compound isn't absorbed by the body, which means it can't affect the body the way elemental mercury can. Of course, this study does not settle the question of how potentially harmful a new mercury filling is.