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.

As the safety debate rages on, how concerned should you be?

Mercury, a known neurotoxin, makes up 50 percent (in weight) of amalgam fillings, which also contain silver, copper and tin. The ADA has steadfastly stood behind the safety of amalgam, saying the amount of mercury vapor released by the fillings is too little to cause harm.

In its "final rule" on amalgam fillings, issued in 2009, the Food and Drug Administration deemed them safe for adults and children older than 6. Clinical studies have found no link between dental amalgam fillings and health problems, and the amount of mercury measured in the bodies of people with these fillings is well below levels associated with adverse health effects, the FDA said.

Regarding kids younger than 6, whose developing bodies may be more sensitive to the neurotoxic effects of mercury vapor, as well as pregnant women and their developing fetuses, "very limited to no clinical information is available regarding long-term health outcomes," the FDA said, but existing evidence suggests there’s little to worry about.

Still, in response to concerns about its ruling, the FDA convened a panel last December to re-examine the issue and expects to make a new announcement by the end of this year.

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. Four states and several cities mandate such disclosure, though federal law does not, and the ADA has no such recommendation.

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, where he headed the chemistry department. 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. Cadaver studies have shown that bodies with more amalgam fillings have more mercury in their brain tissue.

Given people’s exposure to other sources of mercury, and how it might interact with other heavy metals built up in the body, it’s impossible to say that there is any safe level, said Haley, whose studies have linked mercury to Alzheimer’s disease.

Some people who worry their amalgam fillings are a source of their health problems, from allergies to multiple sclerosis, seek to get the amalgam removed.

Michael Fleming is a Raleigh, N.C.-based "mercury-free" dentist who hasn’t used amalgam in his practice for 28 years. He never guarantees that removing amalgam will improve health and advises of potential drawbacks: tooth sensitivity, potential future need of a root canal if there’s nerve damage in the process, the increased release of mercury vapors during removal and the cost, which can range from a few hundred to a few thousand dollars. He typically doesn’t recommend it for very ill people or those with life-threatening medical conditions.

But some patients, anecdotally, have reported feeling better after having their fillings removed.

"We have had a couple of patients whose thyroid disease cleared after removal of mercury, sleep and concentration improved, as did energy levels, fatigue," said Fleming, an appointed consultant to the FDA’s Center for Devices and Radiological Health (his views don’t represent the FDA’s). "And in others, you don’t see any improvement."

Edmond Hewlett, vice chairman of restorative dentistry at UCLA School of Dentistry and consumer adviser for the ADA, strongly warns against removing sound fillings, as more trauma to the tooth can weaken the tooth structure. Furthermore, he said, it’s "unethical" and "cruel" to give people hope that their symptoms will be cured post-amalgam without any evidence that that’s true.

"Until there’s compelling evidence there’s the slightest risk of harm, then (amalgam fillings) are a good option," Hewlett said.

Why amalgam?

Durability, affordability and speed of installation are the top reasons dentists use amalgam, which costs patients about 20 percent less than composite resin or glass ionomer fillings. Arguments against banning amalgam often suggest higher costs would discourage poorer people from fixing tooth decay.

Improvements in composite resin, which eclipsed amalgam as the most popular filling some seven years ago, have made it as durable as amalgam, and it requires less removal of healthy tooth structure, said dentist Edmond Hewlett. But it’s difficult to use on teeth that have trouble staying dry during the procedure, such as the back molars. Glass ionomer fillings, which are beginning to be used more, may present a good alternative, Hewlett said.