If You’ve Ever Had a Cavity, Read This

The next time you have a cavity, you may want to weigh your filling options carefully. New research from the University of Georgia suggests that dental fillings made of amalgam—a mixture of mercury, silver, and other metals—can contribute to elevated mercury levels in the body.

In the study, people with more than eight fillings had blood mercury levels more than twice as high as people with no fillings. The authors say these findings raise questions about safety for people who already have high mercury levels, like those who eat a lot of seafood. The American Dental Association, however, maintains that amalgam fillings are safe.

Dental amalgam has been the go-to dental filling material for more than 150 years. It’s affordable and durable, but it does contain mercury—a heavy metal that’s been shown to cause brain, heart, kidney, lung and immune system damage at high levels. Research suggests that one form, methylmercury, may cause damage even at low levels.

Concerns about mercury in dental fillings is not new, but previous studies have been inconsistent and limited, says co-author Xiaozhong “John” Yu, PhD, assistant professor of environmental health science.

The U.S. Food and Drug Administration acknowledges that amalgam can release small amounts of mercury vapor. It considers amalgam fillings safe for adults, but says that pregnant women and parents with children under 6 “who are concerned about the absence of clinical data as to long-term health outcomes” should talk to their dentists about other options.

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In search of more definitive answers, Yu and his colleagues looked at data from nearly 15,000 people during two different study periods—2003-2004 and 2011-2012. In the earlier group, they found that people with more than eight fillings had blood mercury levels 2.4 times the levels of those who had none.

A similar association was also seen in the later group, although the increase in mercury levels was slightly less—probably because mercury-free alternatives to amalgam have become more common, the authors wrote.

In both groups, the average mercury levels for people with more than eight fillings were below the safety thresholds established by the EPA and the World Health Organization.

But Yu says there is still cause for concern. “That is just the average,” he told Health. “A small percentage of those people did exceed those threshold levels. If you have other exposures, like if you eat fish every day, those amounts can add up in the body.”

He also points out that the study didn’t just show an increase in total mercury levels, but that it also showed an increase in methylmercury—the metal’s most toxic form.

The average American has three dental fillings, the authors wrote in the journal Ecotoxicology and Environmental Safety, while 25% of the population has 11 or more.

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The study is the first to demonstrate a relationship between dental fillings and mercury exposure in a nationally representative population. It’s also the first to control for age, education, ethnicity, race, gender, smoking, and seafood consumption, which is a known contributor to mercury levels in the body.

In response to the new research, the American Dental Association released a statement Friday reaffirming its position that “dental amalgam is a safe, durable, and effective cavity-filling option.”

No conclusions about safety should be drawn from the study, says the statement. The ADA also says that the findings may be “prone to over-interpretation” because it included dental fillings made of both amalgam and composite materials—but Yu points out that this would only lead to an under-estimation of the association between amalgam fillings and mercury levels, not an over-estimation. (An expert from the ADA was not available for comment.)

“The ADA says ‘Talk to your dentist if you’re concerned,’ but the truth is that most dentists don’t know there is a risk,” says Yu. “They only know what the ADA tells them—that amalgam is safe.”

Yu says that he doesn’t want to raise alarm with his findings, and that more research is needed before conclusions can be drawn about how much—and how many—fillings might actually affect health.

But he does say that patients should be aware of the potential risks, and should consider their own unique situations when faced with decisions about dental fillings.

For example, people of Asian descent tend to have higher levels of mercury in their blood, he says—likely because they eat more fish—and may want to be extra cautious. The same goes for people who are exposed to mercury at work, and for vulnerable groups like pregnant women and young children.

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“As toxicologists, we know that mercury is poison, but it all depends on the dose,” he said in a press release. “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.”

In most cases, patients do have another option: mercury-free composite resin fillings. These fillings are white instead of silver, and made of acrylic. They do include another hazardous chemical, bisphenol A (BPA)—but Yu’s study found no association between number of fillings and increased BPA levels in the blood, suggesting that they do not release the chemical the way that amalgam releases mercury.

“If a patient came to me and said that she eats fish every day, I think I will recommend she use the amalgam-free material because her levels are already really high,” he says.

Composite fillings are more expensive, and may not be covered to the same extent under insurance. They may also not last as long as amalgam, according to the FDA, and can be harder to place.

But patients should know that they have choices, says Yu, which is something that many dentists don’t ever bring up. “Patient education and doctor education on this topic is very important,” he says.

This article originally appeared on Health.com