In 2005 alone, dentists in the United States plugged 52 million holes in our teeth with a material commonly known as “silver”– a material that, despite the name, is mostly mercury.

There is no question that mercury, a heavy metal, is toxic in large doses. It can cause memory impairment; itching; burning; pain; tissue swelling; loss of hair, teeth and nails; kidney dysfunction; and psychiatric problems.

Silver fillings are known to dentists as amalgam, and are made up of a mixture of elemental mercury, which is liquid at room temperature, and a powdered alloy of silver, tin and copper. The mixture forms a hard, stable compound.

Scientists have known for decades that vigorous chewing and even drinking hot beverages releases small amounts of mercury vapor from amalgam fillings.

But can long-term exposure to these low levels of mercury lead to health problems? Like most questions surrounding mercury safety — think preservatives in vaccines and methylmercury in fish — it depends on whom you ask.

A slap in the face

In 2008, the FDA settled a lawsuit with anti-mercury advocates by posting a statement on its Web site saying that questions remain about whether mercury may pose a threat to pregnant women and children under the age of 6. Part of the settlement was the requirement that the FDA would classify the risk level of mercury amalgam.

After a year for review and public comment, the FDA ruled in July that amalgam fillings contain too little mercury to harm the many millions of people whose teeth have been repaired with them — including pregnant women and young children — and that the only risk from the fillings is to the small number of people who may be allergic to the material.