For 150 years, dentists have filled cavities with a silver mixture called amalgam. By weight, it's about half mercury. Nearly everyone assumed that the mercury became inert once the amalgam hardened. But it's now clear that tiny amounts of mercury vapor leach out of the fillings. Some of it gets into the bloodstream.Is it harmful? Mercury is highly toxic at low concentrations. However, it's never been proven that mercury from dental fillings is harmful. There's no proof it isn't, either.That's why the U.S. National Institutes of Health commissioned two separate clinical trials. Each trial enrolled children with cavities. Half the kids got mercury-containing amalgam fillings. Half got fillings made from a newer resin-based composite material. For seven years in one study, and for five years in the other, the kids underwent extensive neuropsychological tests, IQ tests, and tests of nerve function.'Consistent Findings'The results of both studies appear in the The Journal of the American Medical Association. Sonja McKinlay, PhD, president of New England Research Institute Inc., led a five-year study in 534 U.S. children who got fillings at ages 6-10. Timothy A. DeRouen, PhD, led a seven-year study of 507 Portuguese children who got fillings at ages 8-10."With consistent findings in two trials in very different groups of children, this is very strong evidence that amalgam is safe," McKinlay tells WebMD. "If I was a parent or a dentist worried whether or not to use amalgam in a child's mouth, this trial says it's OK and it's safe. Both trials say, resoundingly, it is safe."DeRouen says amalgam is safe for the vast majority of children. He does not rule out the possibility that some children might be especially sensitive to the harmful effects of mercury from amalgam fillings. But he did not see such children in his study."I would not have a problem with my children getting amalgam fillings," DeRouen tells WebMD. "I would hesitate to make an absolute statement about the safety of amalgam. The study was not designed to detect rare effects that children might have from low levels of mercury in amalgam. � But on a battery of neurological tests, over seven years, we don't see any differences between children who get amalgam and children who get resin composite."