Dental amalgam can release minute amounts of elemental mercury, a heavy metal whose toxicity at high intake levels (such as in industrial exposures) is well-established. Under the aegis of the PHS Committee to Coordinate Environmental Health and Related Programs, the Subcommittee on Risk Assessment performed a comprehensive review of existing information on the uptake of mercury from dental amalgam. (For details, consult the Evaluation of Risks Associated with Mercury Vapor From Dental Amalgam in Appendix III.) The Subcommittee concentrated on the toxicity of mercury, looking in particular for evidence of biological effects from the low doses to which patients might be exposed from amalgam.
It is clear from the Subcommittee's review that a fraction of the mercury in amalgam is absorbed by the body; people with amalgam have higher concentrations of mercury in various tissues (including blood, urine, kidney, and brain) than those without amalgam. Also, a small proportion of individuals may manifest allergic reactions to these restorations.
Mercury is absorbed from many sources, including food and ambient air. Thus, it is not known whether the vast majority of people with amalgam experience any clinical effect from this small additional body burden of mercury—and this is the key question which must be answered in order to resolve the issue of whether amalgam poses a public health risk.
Part of the reason for the dearth of information on whether there are health effects from the mercury in amalgam is that the few human studies that have investigated this issue have been too small or flawed in design to detect an effect. To add to the difficulty, if there were long-term effects from the mercury in amalgam, it is likely that they would be subtle in nature—slight neurological or behavioral changes, for example—and thus would be very difficult to detect and assess in human populations. An alternative approach would be to extrapolate from the effects known to occur after high doses of mercury (such as those received from poorly controlled occupational exposure) in order to predict whether biological effects might occur after low doses (such as those received from dental amalgam). But the nature of the dose-response relationship for mercury toxicity is not well enough understood to permit this.
In the absence of adequate human studies, the Subcommittee on Risk Assessment could not conclude with certainty whether or not the mercury in amalgam might pose a public health risk; on the one hand, there is no evidence at present that the health of people with amalgam is compromised in any way. Likewise, there is no evidence that removing amalgam has a beneficial effect on health, despite anecdotal reports of "improvement" after amalgam removal in patients with certain chronic illnesses. (It should also be noted that the removal process itself may expose the patient to additional mercury, and that alternative dental restorative materials could have long-term toxicity problems of their own.) On the other hand, given that the evaluation of potential health effects from dental restorative materials, including dental amalgam, will be an ongoing process, the possibility that these materials could pose health risks cannot be ruled out.
It is important to view mercury exposure from dental amalgam in the context of other sources of mercury. Table 11 of the report by the Subcommittee on Risk Assessment (in Appendix III), puts the issue into perspective by comparing several sources of mercury exposure.