Metal fillings in your mouth are harmless, UW study finds

A pioneering UW study confirms that metal dental fillings are safe, but critics still aren't satisfied.

More than 100 million people in the U.S. have toxic metal permanently implanted in their teeth—and if you're over age 30, there's a good chance you're one of them. Dental amalgam, which most people know as their metal-colored "silver fillings," is approximately 50 percent mercury.

Yet according to Dentistry Associate Dean Tim DeRouen, people don’t need to worry about their fillings; the mercury in your mouth won’t harm you. DeRouen is the lead author of a study published in the journal of the American Medical Association last year, which compared 253 children who had amalgam fillings with 254 children who had mercury-free fillings. It found that amalgam had no harmful effects.

Not everyone is persuaded by the study’s results. Some organizations that want to end the use of mercury in dentistry have criticized the study’s design and maintain that amalgam is dangerous. But DeRouen says the science is clear. “I think this pretty much settles the question as far as the average response of most kids,” he says. “The average kid who gets an amalgam filling is not at any higher risk of neurodevelopmental problems or neurobehavioral problems than kids who don’t.” Since children are thought to be more sensitive to the effects of mercury due to their still-developing brains and bodies, the implication is that adults can breathe easier about their fillings too.

To make dental amalgam, elemental mercury—a liquid at room temperature—is mixed with an alloy powder containing silver and small amounts of other metals such as copper, tin and zinc. When this mixture is placed in a damaged tooth, it hardens quickly into a tough, long-lasting shield against decay. For much of the last 150 years, amalgam has been the most commonly used material for repairing cavities in teeth—and also the subject of periodic debate.

The results of a study by Oral Medicine and Epidemiology Professor Michael Martin (left) and Dentistry Associate Dean Timothy DeRouen drew fire from opponents of dental fillings made with mercury. Photo by Kathy Sauber.

The current round of controversy was sparked in the late 1970s and 1980s, when highly sensitive technologies revealed that amalgam fillings—long believed to be chemically inert once they had hardened in the mouth—actually release small amounts of mercury vapor whenever a person chews or brushes their teeth. Videos of this process are dramatic, showing a sinister-looking smoke rising from the surface of molars. Some of this mercury vapor is absorbed by the lungs; studies have shown a direct relationship between the number of amalgam fillings and the level of mercury in a person’s blood, urine and tissues.

At high doses, mercury can cause a slew of neurological effects such as tremors, anxiety, personality changes, and loss of muscle control, memory or coordination. It also damages the kidneys, digestive system and other organs throughout the body. However, the amount of mercury vapor released by fillings is extremely small, about 5 to 9 micrograms per day, depending on the number of fillings you have and what you’re chewing. Anti-amalgam groups say mercury is so toxic that even these miniscule amounts—a microgram is a millionth of a gram—are dangerous. They blame dental mercury for autism, multiple sclerosis, Alzheimer’s disease, immunological problems and a host of other ills.

Yet studies have failed to confirm a link between amalgam fillings and such diseases, and a variety of public health bodies—including the U.S. Food and Drug Administration, the U.S. Public Health Service, the Centers for Disease Control and Prevention, the National Institutes of Health and the World Health Organization—have all declared amalgam safe. Then again, studies of long-term, low-level mercury exposure are relatively scarce. Despite 150 years of widespread use, dental amalgam has never been the subject of a randomized controlled clinical trial—the gold standard, scientifically speaking, for deciding questions of treatment safety.

Never, that is, until now.

We were not doing anything experimental. We were offering standard-of-care, good dental treatment to all of these kids.

Timothy DeRouen, associate dean, School of Dentistry

“We decided to do this study—we knew it was controversial—because all that was out there was basically heated opinion,” says DeRouen. Funded by the National Institutes of Health, DeRouen collaborated with a group of researchers at the UW and the University of Lisbon on the trial, which began in 1996. The UW team included Oral Medicine and Epidemiology Professor Michael Martin; Biostatistics and Dentistry Professor Brian Leroux; Environmental Health Research Professor James Woods, a toxicologist specializing in mercury; and Brenda Townes and Gail Rosenbaum, experts on neurobehavioral testing.

DeRouen and his colleagues were in the unusual position of conducting a clinical trial to test the safety of a treatment that has long been part of standard dental care. Most clinical trials, by contrast, are aimed at determining the safety or effectiveness of new treatments. That situation posed some challenges in designing the study—and the solutions have provoked further controversy.

Martin explains: “We felt you needed at least 500 children for statistical power”—that is, in order to be able to pick up small, subtle effects of amalgam. ”All of those children had to have untreated dental decay. They all had to have no prior exposure to mercury from dental sources …. And you had to be able to follow them,” because harmful effects might not show up right away.

In the U.S., fluoridated water and toothpaste and the wipe availability of dental care have greatly reduced the rates of tooth decay—many elementary-school children in the U.S. don’t have any cavities at all.

The situation was different, however, in Portugal—access to routine dental care was mostly limited to the well-to-do. The School of Dentistry has close ties with the University of Lisbon, and Martin had previously collaborated with their researchers on a clinical trial involving students in the Casa Pia school system.

While children who received amalgam fillings were exposed to more mercury, this didn't have any neurological consequences that the researchers could find.

The school system was founded over 200 years ago to educate Lisbon’s orphan and homeless children, and has since grown into a widely respected educational institution. Today about 20 percent of Casa Pia students are wards of the state, but the school also draws students from families around the city. These families represent a wide spectrum of socioeconomic backgrounds—yet few of them have access to good preventive dental care. The Casa Pia students on average had at least six times as many cavities as U.S. children of the same age.

But the decision to conduct the trial among Casa Pia students has also sparked controversy. Charlie Brown, a lawyer with Consumers for Dental Choice, which opposes the use of dental amalgam, calls the study “morally inexcusable and scientifically indefensible.” In a complaint filed with the federal Office of Human Research Protection last July, Brown’s group charged that students and their families weren’t properly informed of the mercury content and dangers of amalgam fillings, and that the researchers took advantage of a vulnerable population.

DeRouen responds that the consent forms used in the study were approved by multiple ethics panels, and emphasizes, “We were not doing anything experimental. We were offering standard-of-care, good dental treatment to all of these kids. We didn’t do anything that’s not done hundreds of thousands of times every day in dental offices around the world.”

Altogether, the study included 507 8- to 10-year-olds, half of whom were randomly assigned to have cavities in their back teeth filled with dental amalgam, and half of whom had cavities in their back teeth filled with composite, a more recently developed material made out of plastic resin mixed with quartz or glass filler. At the end of seven years, the researchers found that, as expected, children who got amalgam fillings had more mercury in their urine—a widely used marker of mercury exposure—than children who got composite fillings.

However, even in the amalgam group, the average urinary mercury level remained within the normal or “background” range, under 4 micrograms of mercury per liter of urine. By contrast, neurological and kidney damage has been consistently shown at urinary mercury levels above 50 micrograms per liter.

And while children who received amalgam fillings were exposed to more mercury, this didn’t have any neurological consequences that the researchers could find. Each year, all the children underwent a battery of simple, standard tests of memory, concentration and motor skills. The results showed no differences between the two groups.

DeRouen’s team also measured how fast the children’s nerve fibers were able to fire, and again the results of the amalgam group were statistically indistinguishable from those of the composite group. Finally, they compared the 20 percent of the amalgam group that had the most fillings (and therefore the greatest exposure to mercury) with the composite group, but no differences emerged at this level, either.

The preponderance of evidence to date is that a very little bit of mercury is just not a big deal.

Michael Martin, oral medicine and epidemiology professor

Another clinical trial, published in the same issue of the Journal of the American Medical Association, found similar results. Children in Boston and Farmington, Maine, were followed for five years. Those who received amalgam fillings had more mercury in their urine, but no differences in IQ or other neurological measures when compared to children who received only composite fillings.

“This adds to a growing body of opinion that dental amalgam is a safe and effective material. I am absolutely convinced that it is not a health issue,” University of Connecticut Professor J. Robert Kelly, an American Dental Association spokesman, told the Chicago Tribune.

But this opinion is not universally held. David Kennedy, a dentist and past president of the International Academy of Oral Medicine and Toxicology, which opposes the use of amalgam, sees evidence of harm in the study. Among children in the amalgam group, urinary mercury levels peaked two years after they received most of their fillings, then gradually declined. Kennedy says this indicates that the children’s kidneys were damaged by mercury and could no longer excrete the poison as efficiently.

DeRouen and his colleagues measured the urinary content of several chemicals that indicate kidney function in their study. Toxicologist Woods says that the results are still being analyzed in detail, and will probably be published sometime this year. However, the researchers did not see any very high values of these chemicals that would have indicated clear kidney damage. In the New England study, the amalgam and composite groups showed no differences in the levels of urinary albumin, the presence of which indicates structural damage to the kidneys.

In an editorial in the Journal of the American Medical Association accompanying the publication of the two studies, University of Pittsburgh Medicine Professor Herbert Needleman cautioned that the results of the two studies are not definitive. While he didn’t take issue with the clinical-trial approach, he did note that mercury fillings might still be harmful to a small number of children.

“What we’re saying is that we don’t see evidence that it’s harmful; we can’t rule out rare occurrences of things that we didn’t have the power in the study to detect,” acknowledges DeRouen. There might be effects of amalgam that the study didn’t test for, effects might not become evident until later in life, or a few children might be genetically programmed to absorb and sequester a larger fraction of mercury in their bodies.

Martin and Woods are now working on a follow-up study. The researchers are collecting DNA from the Portuguese children—now young adults—who participated in the earlier study. They plan to analyze these samples of genetic material, then compare the neurological and kidney function test results of children who have the susceptibility variations with chose of children who don’t.

The use of dental amalgam has been on the decline in the U.S. since the late 1970s, partly because people are getting fewer and smaller cavities and partly because of the development of composite fillings. In the U.S., composite recently overtook amalgam as the most commonly used material for filling cavities, although whether the shift to amalgam is being driven by aesthetic concerns—composite is tooth-colored and therefore more natural looking than amalgam—or by public perception that amalgam is unsafe isn’t clear.

If amalgam might have harmful effects in even a small number of people, why not stop using it and rely on composites instead? Scientists who support the continued use of amalgam say that the two materials simply aren’t interchangeable. For example, DeRouen and his colleagues found that the children in the composite group in their study needed 50 percent more replacement fillings than children in the amalgam group after five years.

That not only has medical consequences—“Every time you have to cut a tooth to take out a filling, you run the risk of harming or killing the tooth,” Martin explains—but economic ones as well. A composite filling is about twice as expensive as an amalgam filling. Banning amalgam as an option for filling cavities could decrease access to dental care, particularly for people who don’t have dental insurance and people in less-wealthy countries.

Moreover, there have been no long-term trials of the safety of composite fillings, “and we know that some of the components of composites are carcinogens and endocrine disruptors,” Martin says. His point is not that we should be afraid of the composite fillings in our teeth, only that there are unknowns about chis option, too.

If, as their study suggests, amalgam fillings are safe for most people, it wouldn’t be the only instance of a dangerous substance being used, in small doses, for medical benefit. Radiation, for example, is deadly at high doses, but most people are willing to accept the small risk that comes with an X-ray.

“It may turn out that mercury is a bad thing, even at very low levels, and we just didn’t do the right study, didn’t look at the right thing,” says Martin. “But the preponderance of evidence to date is that a very little bit of mercury is just not a big deal.”

What risk are the study’s authors choosing for themselves? Asked what kinds of fillings he has, DeRouen shrugs—at his age, probably a mixture of amalgam and composite, plus some crowns made of who-can-remember-what. Martin is more definite about putting his, well, mouth where his mouth is.

“I’m getting a filling done next week,” he says. “I’m going to be asking for mercury amalgam, because I know it’s going to last me pretty much forever.”

Heavy metal

Even those of us with perfect teeth and no fillings have mercury in our bodies. Where does it come from?

Scientists disagree about which source is responsible for most of the mercury we carry, but we encounter the metal in two principal forms: elemental mercury, mostly in the air, and organic mercury, primarily in the seafood we eat.

Some of the mercury in the air gets there naturally, released from the earth’s crust by volcanic eruptions and other geologic processes. The burning of fossil fuels, especially coal, is also an important source of atmospheric mercury. Coal-fired power plants in the U.S. release about 50 tons of mercury each year.

Just like the mercury vapor released by amalgam fillings, elemental mercury in the air is absorbed by the lungs when we breathe it in. When mercury enters streams, lakes and oceans—from the air or through release of industrial waste—bacteria transform it into organic mercury, typically a form that scientists call methyl mercury. Organisms living in these waters absorb this mercury, and as bigger fish eat little fish methyl mercury becomes more and more concentrated.

When we humans act as top predators in the aquatic food chain—that is, when we catch and eat the biggest fish—all that accumulated mercury gets stored in our own bodies. Methyl­mercury is particularly toxic because it binds easily to proteins and to amino acids, the building blocks of proteins. When bound this way, methyl mercury is transported throughout the body—including across the blood-brain barrier and, in pregnant women, across the placenta.

The highest amounts of mercury are found in large, long-lived, predatory fish such as swordfish, shark, king mackerel, albacore tuna and tilefish. The U.S. Environmental Protection Agency recommends limiting intake of these fish—especially for women who are nursing, pregnant or may become pregnant—and young children.