Critics said the studies were not designed to detect problems that might manifest themselves in adulthood, as has been observed with some other metals such as lead.

"If I were a parent or a dentist looking at these trials, I would have to conclude that we should stick with amalgam," said epidemiologist Sonja M. McKinlay of the New England Research Institutes, who led one of the trials.

The strength of the evidence is "particularly impressive," added Dr. Lawrence Tabak, director of the National Institute of Dental and Craniofacial Research, which sponsored both trials. "The studies evaluated mercury exposure in two large, geographically distinct groups of children and reached similar conclusions about the safety of amalgam."

Subtle Effects?

But critics said the studies were not designed to detect problems that might manifest themselves in adulthood, as has been observed with some other metals such as lead.

"The question of subtle effects remains open," wrote Dr. Herbert Needleman of the University of Pittsburgh School of Medicine in an editorial in the same journal.

Mercury amalgam is a silver-colored powder of silver, copper, zinc and other metals held together by mercury. It has been used to fill cavities for more than 150 years and an estimated 70 million fillings are installed each year in the United States.

Mercury amalgam fillings are the most commonly used filling because they are cheaper, easier to use and more durable. Resin composites are typically used for cosmetic reasons in front teeth, but are not as durable as amalgam and their long-term health effects are unknown.

Gold fillings, which do not contain mercury, are also used, but they are much more expensive and more difficult to install.

Traces of Mercury Vapor

Mercury is toxic in sizable doses such as those ingested when eating substantial quantities of mercury-contaminated fish, for example, and critics have said that it must therefore produce damaging effects at lower doses as well.

A series of epidemiological studies over the years, however, have shown no link between dental fillings and neurological or other problems.

Two decades ago, the development of new, more sensitive instruments revealed that minute traces of mercury vapor escape from the fillings, sparking renewed concern about the potential health effects.

The studies also found no changes to the kidney, which has been shown to be sensitive to damage from higher levels of mercury.