Silver amalgam fillings, which have plugged American cavities for more than 150 years, have lost their luster over the last couple of decades thanks to the rise of more attractive tooth-colored fillings and concerns about the environmental and health impact of their chief ingredient: mercury. Although use of amalgam fillings has dropped 30 percent in the last decade, according to the American Dental Association, these fillings are still sitting in hundreds of millions of mouths, and dentists continue to fill some 50 million teeth with amalgam each year - especially in children. Mercury, a known neurotoxin, makes up 50 percent (in weight) of amalgam fillings, which also contain silver, copper and tin. For some dentists, toxicologists and advocates, the fact that mercury has been shown to hurt the neurological system, kidneys and other organs is reason enough to keep it out of people's mouths. When you plant a neurotoxin two inches from the brain, can you say no one is ever harmed from that?" said Charlie Brown, director of Consumers for Dental Choice. His group advocates that dentists be required to disclose the mercury content of amalgam fillings to patients. The buildup of mercury from vapors 24 hours a day, over a lifetime, is the greatest concern, said Boyd Haley, retired professor emeritus at the University of Kentucky. Eighty percent of mercury vapor stays in body tissue for days, months, even years, because the body doesn't have a good system for excreting it, he said.