Federal health officials are again examining what’s known and what’s still to be learned about the safety of a mercury mixture that’s stirred controversy since dentists began using it to fill cavities in the 1800s. In Maine, the news has revived interest in a hotly debated issue.

A joint panel of outside experts voted last week to reject a draft report from the federal Food and Drug Administration that concluded that “silver” dental fillings used by millions of people are safe. The panel did not go so far as to declare the mercury-laden amalgam a danger, only that more study is needed because of the risk it poses to some groups.

“For the general population, amalgams are safe. There is evidence of that,” said Dr. Karl Kieburtz, a University of Rochester professor and chairman of one of the two panels brought together by the Food and Drug Administration.

An FDA survey of 34 recent research studies, including one that recently ended in Maine, did not counter what the agency has said for years: The fillings don’t harm patients, except in rare cases where they have allergic reactions.

But more research is needed on the effect of dental mercury on children, the fetuses of pregnant women with fillings and others whose bodies may absorb, distribute, process and eliminate mercury differently, Kieburtz and other panelists said.

“There are too many things we don’t know, too many things that were excluded,” said Michael Aschner, a professor of pediatrics and pharmacology at Vanderbilt University and a panel consultant.

Dr. Ralph Sacco of Columbia University said consumers shouldn’t panic and that there was no need to have their amalgam fillings removed.

Still, consumer activists who have pushed for a ban welcomed the panel’s rejection of the FDA report, saying it would spark debate and foster public awareness of the issue. They maintain the toxic metal shouldn’t be a part of dentistry at all.

“If we don’t want it in our fish, we don’t want it in our thermometers, what is it doing in our heads?” asked Sara Moore-Hines, 57, a Pennsylvania counselor.

In Maine, mercury-free dentistry advocate Pamela Anderson said this week that the ruling will reopen debate in the state over abolishing the material outright.

Anderson and her husband, Thomas Anderson, who practices dentistry in Houlton, have been staunch supporters of legislative measures to inform consumers about the potential hazards of mercury amalgam fillings and regulate the use and disposal of mercury in dentists’ offices. “We’ll be back before the Legislature this year,” she promised.

Pediatric dentist and child health advocate Jonathan Shenkin of Bangor said the panel’s call for more research won’t change the way he practices.

Like most dentists, Shenkin uses more of the cosmetically pleasing white composite material for fillings, but in some cases mercury fillings are best, he said. Especially in children with behavioral problems who can’t or won’t sit still or cooperate in the dentists’ chair, the silver material is easier and faster to place and lasts longer once it hardens, Shenkin said.

“But there should definitely be more research into the safety of all filling materials, particularly as they affect young children and pregnant women,” he said.

Frances Miliano, executive director of the Maine Dental Association, which has resisted efforts to regulate the use of the material, said the organization concurs with the independent panels in calling for further research.

“We believe research will support the fact that there is no detrimental health effect with the use of amalgam fillings,” she said.

Amalgam fillings by weight are about 50 percent mercury, joined with silver, copper and tin. Tens of millions of Americans receive mercury fillings each year. Amalgam use has begun to taper off, though, with many doctors switching to resin composite fillings that blend better with the natural coloring of teeth.

With amalgam fillings, mercury vapor is released when patients chew and when they brush their teeth.

Significant levels of mercury exposure can cause permanent damage to the brain and kidneys. Fetuses and children are especially sensitive.

The FDA will take the recommendations and comments and evaluate its next steps, Dr. Norris Alderson, the FDA’s associate commissioner for science, told the panelists at the close of the two-day meeting.

The FDA said patients with amalgam fillings are exposed to mercury at levels well below those known to be harmful.

Still, Dr. Michael Fleming, a Durham, N.C., dentist and a consumer representative on the panel, asked the agency to consider restricting the use of amalgam in children younger than 6 and in pregnant women.

Bangor Daily News reporter Meg Haskell contributed to this story.