No harm to health from dental amalgam that contains mercury, studyshows

Two studies—one in Europe, one in the United States—have independently reached the conclusion that children whose cavities are filled with dental amalgam aren't at risk of related adverse health effects. The news comes from scientists who reported their findings, in the April 19 issue of the Journal of the American Medical Association, on the first randomized clinical trials to evaluate the safety of placing amalgam fillings that contain mercury in the teeth of children. The research was supported by the National Institute of Dental and Craniofacial Research (NIDCR), part of the National Institutes of Health.

The findings of the studies included no detectable loss of intelligence, memory, coordination, concentration, nerve conduction, or kidney function during the five to seven years the children were followed. Researchers looked for measurable signs of damage to the brain and kidneys because earlier studies in adults indicated these organs might be especially sensitive to mercury.

The authors noted that children in both studies who received amalgam, informally known as "silver fillings," had a slightly elevated level of mercury in their urine. But after several years of analysis, they determined that mercury levels remained low and did not correlate with symptoms of mercury poisoning.

Dentists have used silver-colored amalgam to fill cavities for more than 150 years. The material is made from an alloy powder of silver, copper, zinc, and other metals held together by liquid mercury. The mercury comprises about half the total weight of a filling. For decades, it was believed that a person's direct exposure to the mercury in amalgam was brief-occurring only while the dentist packed the filling into the tooth. But with the arrival of more sensitive laboratory tools in the late 1970s and into the 1980s, scientists showed that dental amalgam continuously releases a mercury vapor into the mouth, which is inhaled and absorbed by the body.

That discovery raised concerns about the possible toxicity of chronic low-level exposure to mercury from dental amalgam. The toxicity of mercury at higher levels, such as from industrial exposure, is well established. Possible symptoms of mercury poisoning include irritability, memory loss, tremors, poor physical coordination, insomnia, kidney failure, and anorexia.

The two studies are the New England study, undertaken in urban Boston and rural Farmington, Maine, and the Portuguese study, conducted in Lisbon. Each study enrolled children who had existing untreated decay in permanent posterior, or back, teeth but no previously placed dental amalgam fillings. Each child was randomly assigned to receive either amalgam or composite resin (tooth-colored) fillings while participating in the research studies. All were evaluated for several years to determine if health changes occurred, with an emphasis on IQ changes in the New England study and memory, concentration, coordination, and nerve conduction measures in the Portuguese study.

In the Portuguese study, which enrolled children ages 8 through 10, Dr. Tim DeRouen, principal investigator and a scientist at the University of Washington in Seattle, and colleagues found no differences over seven years between 253 participants who received amalgam fillings and 254 volunteers who were treated with composites. This conclusion was reached following annual standardized tests of memory, attention, physical coordination, and velocity of nerve conduction. The scientists likewise did not detect a pattern of decline in the test scores of individual children who received amalgam fillings.

Dr. DeRouen noted that children who received amalgam fillings had a slightly elevated level of mercury in their urine-measuring, on average, 1.5 μg/L of urine for the first two years and leveling off to 1.0 μg/L or less thereafter. However, these numbers fall within the "background" level (0-4 μg/L) that is typical for an average person not exposed to industrial or other known sources of mercury.

The scientists noted that children in both groups were in great need of dental care. Among those in the amalgam group, children had, on average, 10.1 tooth surfaces treated upon entry into the study. By Year 7 of the study, they had received, on average, a total of 18.7 surface restorations. Each tooth has either four or five defined geographic surface areas, totaling 128 surfaces in a human's 32 permanent teeth.

In the New England study, which enrolled children ages 6 through 10, the scientists also found, after five years, no significant differences in the well-being of the 267 participants who randomly received amalgam and their 267 counterparts who received composite fillings.

"We took great pains to design our study in a way that our tests would be sensitive enough to detect as little as a three-point drop in IQ," said Dr. Sonja McKinlay, principal investigator of the American study and a scientist at New England Research Institutes in Watertown, Mass. "What we found over the course of the study is that the amalgam fillings had no adverse effects on the IQ of these children or on a range of other neuropsychological measures and kidney function."