Amalgam dental fillings are safe for most kids, two new studies show.

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 appear in the April 19 issue of 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."

Debate Far From Over

Do the two trials prove once and for all that amalgam fillings are safe? No, says psychiatrist Herbert L. Needleman, MD, of the University of Pittsburgh. Needleman was among the first doctors to realize that U.S. children were suffering lead poisoning at exposure levels once widely -- and wrongly -- believed to be safe.

Now he warns that we just don't know enough about mercury to say it's safe to put into kids' mouths.

In an editorial accompanying the McKinlay and DeRouen reports, Needleman agrees that the studies show no obvious problems in children who get amalgam fillings. But he argues that the studies can't account for rare side effects. Because so many children get amalgam fillings, even a side effect that struck one in 100 children would affect hundreds of thousands of kids.

"These are good studies, but they are not enough to assert that amalgam in fillings is safe," Needleman tells WebMD. "I know people will parade this as showing there is no harmful effect of fillings. But I have heard this argument made with other toxins, too."

Needleman remembers being taught that lead was toxic only at blood levels six times higher than those now believed to be safe. And, he says, there's troubling data that even this much lead might be harmful. Mercury in dental fillings gives him a troubling sense of déjà vu.

"As better studies are done, harmful effects will become visible at doses of mercury now considered safe. I think that will be inevitable," Needleman says. "The question of whether amalgam fillings are safe is still open. But if my children needed fillings, I would use an alternative."

Amalgam vs. Resin Composite

The resin composite tested in the McKinlay and DeRouen studies did not contain mercury. But it contained several other chemicals -- and nobody knows whether these chemicals are safe in the long term.

"We know even less about that than about the safety of amalgam," DeRouen says. "Yes, there is a small percentage of kids that might react negatively to amalgam -- but there is no data on that. What we do have data on, is that in these kids we studied, we did not see any safety differences between amalgam and resin-composite fillings. While there could be rare events linked to amalgam, there is no hint of it here."

McKinlay says if there is a danger from amalgam fillings, it should be most apparent in growing children.

"We look at the group that would be most vulnerable to toxic effects of mercury from amalgam -- and we are not seeing it," she says. "Therefore the chance it is safe, in children going forward, is probably a good bet. … There is always going to be the rare exception to any rule. But we now have real information on the safety of amalgam from two very well-designed trials. We had none before this. This is the only evidence available."

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Bellinger, D.C. The Journal of the American Medical Association, April 19, 2006; vol 295: pp 1775-1783. DeRouen, T.A. The Journal of the American Medical Association,April 19, 2006; vol 295: pp 1784-1792. Sonja M. McKinlay, PhD, president, New England Research Institute Inc., Watertown, Mass. Timothy DeRouen, PhD, professor of biostatistics, School of Public Health; professor of dental public health sciences and executive associate dean for research and academic affairs, School of Dentistry, University of Washington, Seattle. Herbert L. Needleman, MD, professor of child psychiatry and pediatrics, University of Pittsburgh.