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Dental Amalgam: Update on Safety Concerns Ada Council on Scientific Affairs

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Dental Amalgam: Update on Safety Concerns Ada Council on Scientific Affairs ASSOCIATION REPORT DENTAL AMALGAM: UPDATE ON SAFETY CONCERNS ADA COUNCIL ON SCIENTIFIC AFFAIRS ABSTRACT Dental amalgam is an alloy composed of a mixture of approxi- mately equal parts of elemental liquid mercury and an alloy pow- This report of the Council on der.1 The first use of amalgam was recorded in the Chinese litera- Scientific Affairs reviews and ture in the year 659,2 and for the last 150 years, amalgam has been discusses recent studies the most popular and effective restorative material used in den- tistry. The popularity of amalgam arises from its excellent long- concerning the safety of dental term performance, ease of use and low cost.1,3 Before the 1970s, amalgam, with an emphasis on amalgam accounted for more than 75 percent of all restorations.4 In studies that have been 1979, the total number of amalgam restorations placed by dentists in the United States was estimated at 157 million.3,5 During the published since the 1993 review past 20 years, however, the use of amalgam in the United States of dental amalgam by the U.S. has been declining, largely due to the decreasing incidence of dental Public Health Service Committee caries, more frequent use of crowns and the availability of tooth-col- ored alternative restorative materials for certain applications.3 In to Coordinate Environmental 1991, the total number of amalgam restorations placed was esti- Health and Related Programs. mated at approximately 96 million, which accounted for about 50 5 The Council concludes that, percent of all restorations. Despite the long history and popularity of dental amalgam as a based on currently available restorative material, there have been periodic concerns regarding scientific information, amalgam the potential adverse health effects arising from exposure to mer- 6-10 continues to be a safe and cury in amalgam. As early as 1850, some U.S. dentists claimed that removing amalgam fillings could bring miraculous cures in pa- effective restorative material. tients with chronic disease.11 Even today, some dentists remove amalgam restorations from patients as a result of claims that amal- gam restorations result in serious adverse health effects.12,13 Concerns in the public sector also were demonstrated in a 1991 sur- vey conducted by the American Dental Association, which revealed that nearly half of the 1,000 American adults surveyed believed that health problems could develop as a result of dental amal- gam.14,15 The safety of dental amalgam has been the subject of a number of previous publications, expert panel meetings and national and international conferences.3,16,17,18 During 1991 and 1992, the National Institutes of Health and the U.S. Public Health Service, or PHS, separately convened panels of experts to review the current state of knowledge on amalgam safety. The expert panels were unable to 494 JADA, Vol. 129, April 1998 ASSOCIATION REPORT identify, in the general popula- values ranging from 2 to 15 other hand, has adopted the tion, any human health detri- µg/day.22,23 Estimates of inhaled lower limit of 25 µg/m3 as the ments arising from the place- elemental mercury from air TLV for occupational mercury ment of dental amalgam range from 40 to 120 exposure.28 restorations, and all concluded nanograms per day.16,20 In 1983, a study by Fawer that amalgam was a safe and Controversy still exists as to and colleagues29 reported that effective restorative material.3,19 whether mercury from amalgam industrial workers who had oc- This article reviews more re- is a significant contributor to cupational mercury exposure at cent studies on the safety of the total body mercury burden. a time-weighted average of 26 dental amalgams, with an em- The toxicological effects of µg/m3 in the workplace for an phasis on those that have been various forms of mercury have average of 15.3 years showed a published since the 1993 report been well-documented and in- significant increase in tremor by the PHS Committee to Coor- vestigated, mainly in popula- when compared with a control dinate Environmental Health tions with excessive occupation- group. Concerns about this and Related Programs.3 For ref- al or environmental study have been expressed by erence, a brief summary on mer- exposures.9,20,21,24,25 Besides aller- Mackert and Berglund,30 who cury toxicity and current safety gic reactions, symptoms associ- re-evaluated the hand tremors guidelines also is provided. ated with mercury toxicity in- in this group of 26 occupational- clude tremor, ataxia, personality ly exposed industrial workers. MERCURY TOXICITY AND SAFETY GUIDELINES Concerns with the study design Controversy still noted that the hand-tremor test Chemically, mercury exists in apparently was not blinded, and three major forms: elemental exists as to whether the medical and previous expo- (valence 0), inorganic (valence mercury from amal- sure histories of the workers +1 and +2) and organic (alkyl gam is a significant were not known. In addition, and aryl). These three forms are the researchers make no men- different in their physical and contributor to the tion of any corrections for other chemical properties, their rates total body mercury sources of mercury intake or of absorption and excretion, and burden. elimination. The sample pool their distribution patterns in was small, and no dose-re- tissues. The chemical form of sponse relation was found. mercury, therefore, determines change, loss of memory, insom- Assuming that confounding fac- its toxicological profile. nia, anxiety, fatigue, depression, tors were similar between the Elemental mercury is the most headaches, irritability, slowed exposed and control groups, it volatile of the three, and mer- nerve conduction, weight loss, can be estimated that the mer- cury vapor in air is the predom- appetite loss, gastrointestinal cury level in the air for the con- inant form of elemental mer- problems, psychological distress trol subjects was between 8 and cury. Sources of mercury in and gingivitis.16,20 10 µg mercury/m3, which is ex- drinking water and food are Consequently, various guide- ceptionally high. generally inorganic and organic lines to prevent excessive occu- Furthermore, in a study by mercury compounds, with or- pational exposure to mercury Nilsson and Nilsson,31 urinary ganic compounds being particu- have been developed. Both the mercury concentrations found larly associated with National Institute for Occu- in Swedish dentists, dental as- seafood.16,20,21 Total daily expo- pational Safety and Health, or sistants and the rest of the staff sure to methylmercury (a proto- NIOSH, and the Occupational were 2.5, 3.6 and 1.8 nanomole type of organomercury), primar- Safety and Health Adminis- mercury/millimole creatinine, re- ily stemming from the ingestion tration have adopted a thresh- spectively. These concentrations of food (> 98 percent), is esti- old limit value, or TLV, of 50 µg were similar to those found in mated at 5.8 micrograms by the mercury vapor per cubic meter the supposedly nonoccupation- Environmental Protection of the breathing zone air for ally exposed control subjects in Agency, or EPA,20 and 2.3 µg by eight hours per day, 40 hours the Fawer and colleagues29 Clarkson and colleagues.16 per week.26,27 The World Health study, who exhibited an average Other studies have reported Organization, or WHO, on the of 3.4 nmol mercury/mmol crea- JADA, Vol. 129, April 1998 495 ASSOCIATION REPORT tinine. It, therefore, can be in- ries; even in poorly ventilated organomercury compounds by ferred that the Fawer and col- operatories, mercury vapor con- microorganisms in the mouth leagues study is unsuitable for centrations returned to levels and gastrointestinal tract. To determining an occupational ex- below NIOSH’s TLV within 20 examine the potential of such a posure level at which preclinical to 30 min. The study concluded risk, Chang and co-workers41 symptoms can be established. that mercury remained in vapor conducted a study in both den- Nevertheless, using Fawer form for only limited periods, tists and nondentists. Although and colleagues’ data as the low- presumably because of its den- the investigation found that est observed adverse effect level sity and affinity for surfaces, blood inorganic mercury levels and a safety factor of 100, the and that a single accidental were higher among dentists Agency for Toxic Substances mercury spill probably would with poor mercury hygiene and Disease Registry set the not be a significant source of practices, blood organomercury minimal risk level, or MRL, at mercury in a dental operatory. levels were statistically in- 0.3 µg/m3 for long-term human Exposure to mercury vapor significant between the two exposure to mercury in ambient during the placement of amal- groups. Thus, researchers con- air.24 (The MRL is defined as gam restorations also was found cluded that biotransformation the level of mercury vapor to be minimal when appropriate of inorganic mercury to organo- below which a person can con- hygiene procedures were fol- mercury did not occur in vivo. tinuously be exposed with no lowed.37,38 Although significant One human study42 found harmful health effects.) The concentrations of mercury may that female dental assistants EPA also uses 0.3 µg/m3 as the be generated during restorative with high occupational exposure inhalation reference concentra- procedures, approximately 90 to mercury were less fertile tion for elemental mercury in percent can be eliminated by than unexposed control sub- air.3,32 using high-volume evacuation.39 jects. Interestingly, however, A recently published study by subjects with low mercury expo- DENTAL PROFESSIONALS 40 AND EXPOSURE TO Langworth and colleagues sure were more fertile than un- MERCURY FROM found that the levels in the den- exposed control subjects. In a AMALGAM tal clinics averaged approxi- 1994 study, Sundby and Dahl43 It has long been recognized in mately 2 µg mercury/m3, and no found no differences in fertility dentistry that chronic exposure adverse health effects on the and pregnancy outcome be- to mercury vapor owing to inap- personnel could be seen.
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