Dental Amalgam: Update on Safety Concerns

identify, in the general population, any human health detriments arising from the placement of dental amalgam restorations, and all concluded that amalgam was a safe and effective restorative material. This article reviews more recent studies on the safety of dental amalgams, with an emphasis on those that have been published since the 1993 report by the PHS Committee to Coordinate Environmental Health and Related Programs. For reference, a brief summary on mercury toxicity and current safety guidelines also is provided. MERCURY TOXICITY AND SAFETY GUIDELINES Chemically, mercury exists in three major forms: elemental (valence 0), inorganic (valence +1 and +2) and organic (alkyl and aryl). These three forms are different in their physical and chemical properties, their rates of absorption and excretion, and their distribution patterns in tissues. The chemical form of mercury, therefore, determines its toxicological profile. Elemental mercury is the most volatile of the three, and mercury vapor in air is the predominant form of elemental mercury. Sources of mercury in drinking water and food are generally inorganic and organic mercury compounds, with organic compounds being particularly associated with seafood. Total daily exposure to methylmercury (a prototype of organomercury), primarily stemming from the ingestion of food (> 98 percent), is estimated at 5.8 micrograms by the Environmental Protection Agency, or EPA, and 2.3 μg by Clarkson and colleagues. Other studies have reported JADA, Vol. 129, April 1998 495 ASSOCIATION REPORT values ranging from 2 to 15 μg/day. Estimates of inhaled elemental mercury from air range from 40 to 120 nanograms per day. Controversy still exists as to whether mercury from amalgam is a significant contributor to the total body mercury burden. The toxicological effects of various forms of mercury have been well-documented and investigated, mainly in populations with excessive occupational or environmental exposures. Besides allergic reactions, symptoms associated with mercury toxicity include tremor, ataxia, personality change, loss of memory, insomnia, anxiety, fatigue, depression, headaches, irritability, slowed nerve conduction, weight loss, appetite loss, gastrointestinal problems, psychological distress and gingivitis. Consequently, various guidelines to prevent excessive occupational exposure to mercury have been developed. Both the National Institute for Occupational Safety and Health, or NIOSH, and the Occupational Safety and Health Administration have adopted a threshold limit value, or TLV, of 50 μg mercury vapor per cubic meter of the breathing zone air for eight hours per day, 40 hours per week. The World Health Organization, or WHO, on the other hand, has adopted the lower limit of 25 μg/m3 as the TLV for occupational mercury exposure. In 1983, a study by Fawer and colleagues reported that industrial workers who had occupational mercury exposure at a time-weighted average of 26 μg/m3 in the workplace for an average of 15.3 years showed a significant increase in tremor when compared with a control group. Concerns about this study have been expressed by Mackert and Berglund, who re-evaluated the hand tremors in this group of 26 occupationally exposed industrial workers. Concerns with the study design noted that the hand-tremor test apparently was not blinded, and the medical and previous exposure histories of the workers were not known. In addition, the researchers make no mention of any corrections for other sources of mercury intake or elimination. The sample pool was small, and no dose-response relation was found. Assuming that confounding factors were similar between the exposed and control groups, it can be estimated that the mercury level in the air for the control subjects was between 8 and 10 μg mercury/m, which is exceptionally high. Furthermore, in a study by Nilsson and Nilsson, urinary mercury concentrations found in Swedish dentists, dental assistants and the rest of the staff were 2.5, 3.6 and 1.8 nanomole mercury/millimole creatinine, respectively. These concentrations were similar to those found in the supposedly nonoccupationally exposed control subjects in the Fawer and colleagues study, who exhibited an average of 3.4 nmol mercury/mmol creaControversy still exists as to whether