Dental caries are practically standard issue during childhood, with more than 70 million amalgam fillings placed in caries each year. A dental amalgam is a mixture of metals, including silver, copper, and zinc, that is held together with mercury. Mercury makes up 40%-50% of this mixture. Exposure to high-doses of mercury has been linked to neurologic effects and kidney damage. However, there has been little concern about toxicity from dental amalgam during its 150 years of use. Until recently, exposure was believed to occur only briefly, while the filling was being applied. More sophisticated testing has found that exposure is not brief; the amalgam continually releases small amounts of mercury vapors that are absorbed by the body.

Two separate groups of researchers set out to determine if mercury-containing amalgam is safe. Although conducted in different countries and focusing on slightly different factors, both studies reported no greater health risks among children treated with dental amalgam. The two studies are published in the April 19, 2006 Journal of the American Medical Association .

About the Study

Researchers from Children’s Hospital Boston conducted the New England Children’s Amalgam Trial (NECAT), which enrolled 534 children ages 6-10 from Massachusetts and Maine. The Portuguese trial, run out of the University of Washington, Seattle, enlisted 507 children ages 8-10 from Lisbon, Portugal. In both studies, children were randomly assigned to receive one of two types of fillings for all their cavities: amalgam or a mercury-free resin composite. The NECAT trial monitored children for five years, and compared changes in IQ, memory, visuomotor ability, and kidney function between the amalgam and composite groups. In the seven-year Portuguese study, researchers compared neurobehavioral functions, such as memory, concentration, visuomotor ability, and nerve conduction speed, between the two treatment groups.

In both studies, the children receiving amalgam had higher levels of mercury in their urine than the children receiving composite, indicating greater exposure to mercury. However, this greater exposure did not translate into adverse health effects. Neither trial reported significant differences between the amalgam and composite groups in IQ, kidney function, or any neurobehavioral functions.

Having similar designs, these two studies have similar limitations as well. For example, although no adverse effects were seen after five and seven years, it’s possible that toxic effects may not appear until later in life. In addition, certain subgroups of people, such as those with a genetic predisposition to mercury toxicity, may be at greater risk from the low-level exposure associated with amalgam. The NECAT trial was designed to detect a decrease in IQ of three points or more; any smaller decreases would not have been detected (IQ generally ranges from 30-150).

How Does This Affect You?

Children who receive dental amalgam have no greater risk for depressed IQ, kidney damage, or neurologic dysfunction than children whose cavities are filled with a non-mercury composite for at least five years. The strength of this finding is bolstered by the fact that two independently run studies in two different countries reported the same results. Armed with these findings, dentists can continue to fill cavities with amalgam, and parents can comfortably bring their children for these procedures, without cause for concern.

Although both studies reported a rise in urinary mercury levels in the amalgam groups, there is little cause for concern. The elevated levels stayed within the safe range of 0-4µg/L, and well below the 50 µg/L level that is associated with mercury toxicity.

Given mercury’s know toxic effects, why not play it absolutely safe and only use mercury-free composites for dental caries? In the Portuguese trial, the need for further treatment was 50% greater in the composite group after five years. This finding is consistent with previous reports that non-mercury composites are not as durable as amalgam. In addition, composites cost more, require a more sensitive technique, and have not been thoroughly assessed for safety.