Whether dental work containing mercury can raise blood levels of the metal is controversial, but a new study finds that this indeed may be the case especially in people who have eight fillings or more.

Dental amalgam has been the go-to dental filling material for more than 150 years, because it's affordable and durable. However, about half of the compound contains mercury, a heavy metal known to be toxic at high levels, causing brain, heart, kidney, lung and immune system damage.

Using data from nearly 15,000 people, University of Georgia researchers found that those people with more than eight fillings had about 150 percent more mercury in their blood than those with none.

In addition, the researchers analyzed exposure by specific types of mercury and found a significant increase in methyl mercury, the most toxic form of mercury, related to dental fillings, the study finds. New research suggests that methyl mercury may cause damage even at low levels, they say.

The average American has three dental fillings, while 25 percent of the population has 11 or more fillings, the researchers say.

Mercury exposure from dental fillings is not a new concern, but previous studies were inconsistent and limited. This study was the first study to use a nationally representative sample and also control for other causes of mercury, including smoking and seafood consumption

"As toxicologists, we know that mercury is poison, but it all depends on the dose. So, if you have one dental filling, maybe it's OK. But if you have more than eight dental filings, the potential risk for adverse effect is higher," says Xiaozhong "John" Yu, the study’s co-author. People with numerous dental fillings who are also exposed to mercury from other sources, such as seafood or work environments, are most at risk, he adds.

According to its website, the U.S. Food and Drug Administration considers dental amalgam fillings safe for adults, but says, "pregnant women and parents with children under six who are concerned about the absence of clinical data as to long-term health outcomes should talk to their dentist,” the researchers note.

They are hoping the study, which appears online in Ecotoxicology and Environmental Safety, will encourage dentists to discuss the choice of dental fillings with their patients.