JADA Continuing Education
Potential health and environmental issues of mercury-contaminated amalgamators
HOWARD W. ROBERTS, D.M.D.,
DANIEL LEONARD, D.D.S. and
JOHN OSBORNE, D.D.S., M.S.D.
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ABSTRACT
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Background. Dental amalgamators may become contaminated internally with metallic mercury. This contamination may result from mercury leakage from capsules during trituration or from the long-term accrual from microscopic exterior contaminants that result from the industrial assembly process. The potential health risk to dental personnel from this contamination is unknown.
Methods. The authors assessed used amalgamators from the federal service inventory for the amounts of mercury vapor levels, as well as the visual presence of mercury contamination. They evaluated these amalgamators for potential mercury vapor health risk, using established National Institute for Occupational Safety and Health methods and American Conference of Governmental Industrial Hygienists standards.
Results. Ten of the 11 amalgamators assessed had measurable mercury vapor levels. Four amalgamators were found to have internal static mercury vapor levels above Occupational Safety and Health Administration ceiling limit thresholds. During a simulated worst-case clinical use protocol, the authors found that no amalgamators produced mercury vapor in the breathing space of dental personnel that exceeded established time-weighted federal mercury vapor limits.
Conclusions. Amalgamators may be contaminated internally with metallic mercury. Although the authors detected mercury vapor from these units during aggressive, simulated clinical use, dilution factors combined with room air exchange were found to keep health risks below established federal safety thresholds.
Clinical Implications. Dental personnel should be aware that amalgamators may be contaminated with mercury and produce minute amounts of mercury vapor. These contaminated amalgamators may require disposal as environmentally hazardous waste.
Dental amalgam remains the most-used restorative material despite of its lack of esthetics.1 Several factors contribute to its success: durability, sealing the restoration-tooth interface, ease of manipulation and finishing, and a low technique sensitivity to operator variability.2 Dental amalgam, however, has received widespread negative publicity in the world press. The mercury issue began dominating the research on dental amalgam 10 years ago.3
Although the findings of this study suggest that the health hazards of mercury-contaminated amalgamators are minimal, concerns regarding the proper disposal of these devices remain.
The research literature, involving many retrospective studies, has shown the safety of dental amalgam.1,4–9 In fact, The World Health Organization/Federation Dentaire International issued a consensus statement in 1995 that dental amalgam has not been shown to have an adverse health effect.10 This same report, however, did emphasize that mercury should be an environmental concern both within the dental office and when disposing of amalgam waste. The impact of dental amalgam disposal on the environment has been a source of interest.11
During a U.S. Air Force Dental Investigation Service, or DIS, Problem Resolution and Assistance Program action, it was reported to and confirmed by DIS that metallic mercury was being released from precapsulated amalgam products manufactured by Sybron Dental Specialties.12 Although Tytin (Sybron Dental Specialties) was the most-implicated product, metallic mercury leakage during trituration was confirmed in other products using the same capsule design.12
During other amalgam product evaluations, microscopic metallic mercury and alloy powder have been found to contaminate the capsule exterior of almost every amalgam product.12–14 This microscopic contamination most likely is the result of the industrial processes during capsule assembly in which mercury spillage has not been cleaned. This microscopic exterior contamination, although not as great of a concern as the visible mercury leakage from internal parts of the triturated capsules, may have long-term implications. During trituration the microscopic alloy and mercury contaminants can be dislodged from the capsule and deposited in the amalgamator. Over the lifetime of the amalgamator, the accretion of these unwanted contaminants could result in a substantial amount of mercury’s accumulating inside the amalgamator.
DIS surveyed numerous federal medical equipment repair personnel and received information that mercury contamination of amalgamators is found routinely during equipment maintenance. As mercury contamination in amalgamators has not been reported universally in the literature, we conducted this study to confirm the existence of mercury contamination within amalgamators and to determine if any health implications to dental personnel exist.
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METHODS AND MATERIALS
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We obtained 11 used dental amalgamators (Varimix II, Dentsply) from Dunn Dental Clinic, Lackland Air Force Base, Texas. These amalgamators had been in service for 10 years and were being shipped to the Department of Defense’s Defense Reutilization Marketing Office because they were at the end of their projected life cycles. Review of Dunn Dental Clinic’s logistics records revealed that Tytin had been the predominant amalgam product triturated in these units. Other amalgam alloy products prepared in these units included Dispersalloy (Dentsply, L.D. Caulk), Valiant (Ivoclar North America) and Valiant PhD (Ivoclar North America).
We placed each amalgamator in a clean, well-ventilated work area that previously had been evaluated for and was found to be free of existing mercury levels. We took five internal mercury vapor readings for each amalgamator with a calibrated Jerome 431 Mercury Vapor Analyzer (Arizona Instrument Corp.). The analyzer tip was placed approximately two inches inside the mechanical compartment, with sample readings obtained from each corner and from the center of each amalgamator. When we observed positive readings, we allowed the analyzer to return to a zero reading before recording the next reading. A mean mercury vapor reading and its standard deviation were recorded for each amalgamator. Then, we operated each amalgamator for approximately two minutes to effect heating of its motor and obtained five mercury vapor readings as described previously with a mean vapor reading calculated for each amalgamator. After we took the readings, we disassembled each corresponding amalgamator and visually examined it. Photographs were taken to document qualitative presence of any contamination.
We conducted tests at the DIS dental operatory to determine mercury vapor levels for dental personnel. We assessed the four amalgamators that were visibly contaminated with mercury and demonstrated the highest static internal mercury vapor readings. A scenario to replicate a schedule busier than usual for typical clinical amalgamator use was devised. The testing method for mercury level was in accordance with National Institute for Occupational Safety and Health, or NIOSH, and Occupational Safety and Health Administration, or OSHA, methods. One researcher (H.W.R.) wore a charcoal mercury sorbent tube attached to an air sampling unit (Li’l Pump, Omega Specialty Instrument Co.) as a detection device. The detector was located at chest level and was attached to the researcher’s right breast pocket.
The researcher, functioning as a dental technician, operated the contaminated amalgamator (without an amalgam capsule in it) for 15 seconds four times. This was accomplished every 30 minutes during a three-hour period. Another sampling unit was positioned approximately two inches from the rear of the contaminated amalgamator during simulated use. At the end of the three-hour evaluation period, the mercury detection tubes were submitted for evaluation. Each of the four contaminated amalgamators was evaluated three times, for a total of 12 evaluations. Samples were analyzed by the Industrial Hygiene Section (IERA/RSH) at Brooks Air Force Base, Texas, using NIOSH method 6009 with mean time-weighted mercury vapor exposure calculated and compared to the American College of Government Industrial Hygienists, or ACGIH, standard of 0.025 milligrams per cubic meter for an eight-hour time-weighted limit.
In addition, testing took place in the DIS dental materials testing laboratory to evaluate medical equipment technician exposure to mercury vapor levels. We undertook this aspect of the evaluation because periodic maintenance of dental equipment is performed in federal dental service clinics. We used a scenario that duplicated a medical equipment technician’s performing maintenance service (for example, calibration) on the same four mercury-contaminated amalgamators that were used to determine mercury vapor levels for dental personnel. One researcher (H.W.R.) wore the mercury detection apparatus as described previously. An additional sampling device was placed approximately two inches behind the amalgamator to identify mercury vapor levels near the contaminated unit. The researcher removed the cover of each amalgamator and calibrated the unit according to the manufacturer’s and U.S. Air Force medical equipment guidelines. At the end of the maintenance, he replaced the cover and recorded the total time for maintenance for each unit. The Carulite mercury detection tubes were submitted for evaluation. The time-weighted mercury vapor exposure was determined by IERA/RSH as described previously for the entire maintenance session and the value was compared to the established ACGIH limits.
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RESULTS
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Internal amalgamator static mercury vapor levels.
Results of the tests on each amalgamator are summarized in Table 1
. The numbers in boldface indicate mercury vapor readings in excess of the current OSHA one-time ceiling limit of 0.1 mg/m3.15 One amalgamator (unit 43926) registered a high static mercury vapor reading of 0.126 mg/m3, which exceeds the OSHA permissible exposure limit, or PEL, before its motor was operated. We could not operate this unit because it had electrical defects, so we were not able to take warm readings. Three amalgamators displayed elevated vapor levels that exceeded the OSHA PEL after their motors were operated: unit 43925 (0.327 mg/m3), unit 43972 (0.126 mg/m3) and unit 43924 (0.354 mg/m3).
If mercury contamination exists within amalgamators, elevated mercury vapor levels inside the confined and restricted airflow chamber of the internal amalgamator compartment can be expected. The amount of mercury vapor that may leak from this confined space can be diluted by several magnitudes to negligible levels by room air. We tested this hypothesis by assessing the time-weighted mercury vapor levels in dental and medical equipment personnel’s breathing space.
Mercury vapor levels for dental personnel.
The time-weighted average, or TWA, mean mercury vapor levels we found during the evaluation for dental personnel are reported in Table 2. In addition, the mean TWA mercury vapor level for the operator was 0.0007903 mg/m3 (± 0.0004855 mg/m3, standard deviation, or SD) and the mean TWA mercury vapor level two inches from the contaminated amalgamator was 0.0002901 mg/m3 ± 0.0000936 mg/m3 SD.
Medical equipment maintenance personnel mercury vapor levels.
The TWA mercury vapor results during the simulated medical equipment maintenance are summarized in Table 3. The TWA mercury vapor level for the maintenance session for the four contaminated amalgamators was 0.001062 mg/m3. It took approximately one hour to complete the required maintenance for the four amalgamators. The TWA mercury vapor level two inches behind each amalgamator was 0.000292 mg/m3 during the maintenance session for the four amalgamators.
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DISCUSSION
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Amalgamator findings.
We evaluated 11 dental amalgamators, each with a 10-year history of use, for mercury vapor. All but one registered static mercury vapor values. The mean mercury vapor level found in the internal compartment of these amalgamators was 0.044 mg/m3 ± .04 SD cold and 0.104 mg/m3 ± .13 SD after a two-minute run. These static mercury vapor readings were determined using a mercury vapor analyzer, which uses a gold film sensor for the detection and quantification of mercury vapor and is a method more stable and selective for mercury vapor detection than older ultraviolet mercury analyzers.16 The analyzer samples approximately 190 cubic centimeters of air over 13 seconds, which should provide a representative sample of the interior environment of the amalgamator. Although no amalgam product requires two minutes of trituration, we chose this length of time to obtain a worst-case scenario of mercury vapor readings when the amalgamator was warm and simulating heavy periods of clinical use.
Internal visual examination of the 11 amalgamators revealed generalized mercury contamination, especially immediately below the capsule mixing area and on the electric drive motor casing (Figure 1). When we disassembled the amalgamator motors, we found mercury contamination within the electrical motor wiring (Figure 2). There did not appear to be a chemical interaction between the copper wiring and the mercury, as the wiring in these units was encased in clear insulating plastic.
Mercury vapor exposure.
A person’s exposure to mercury vapor should not exceed OSHA’s PEL for mercury vapor of 0.1 mg/m3 of air.15 When evaluating possible health hazards from chronic mercury exposure, federal and civilian agencies use TWA mercury vapor exposure measurements to assess health safety thresholds.15 In addition to the OSHA mercury vapor PEL ceiling limit, NIOSH has established a recommended exposure limit for mercury vapor of 0.05 mg/m3 TWA for up to a 10-hour workday and a 40-hour workweek.15 ACGIH has assigned mercury vapor a threshold limit value of 0.025 mg/m3 TWA for a normal eight-hour workday and a 40-hour workweek.15 In view of these different mercury exposure limits, it would seem prudent to follow the more stringent ACGIH guideline.
Dental personnel exposure.
To evaluate the possible health hazards of dental personnel’s operating such contaminated equipment, we established a simulated worst-case scenario. The amalgamators we chose for this aspect of the evaluation were the four amalgamators with the highest identified internal mercury vapor readings. During the simulated dental treatment conditions of this evaluation, we followed a schedule busier than that normally experienced at a USAF dental treatment facility. In this scenario, a dental patient would be seen every 30 minutes, with four three-spill amalgam capsules prepared for each patient. Also, the mixing time we used for each amalgam capsule was longer than manufacturer’s recommendations for any disposable amalgam capsule. Under this busy, worst-case set-up, we found that no established TWA mercury vapor exposure limits were exceeded in the personnel’s breathing space or in the immediate proximity of the contaminated amalgamators. Although Tables 2
and 3 show that mercury vapor levels were detected, most were one-tenth to one-hundredth of the most stringent ACGIH-established TWA thresholds. Mean time-weighted mercury levels detected in this study are compared to OSHA’s, NIOSH’s and ACGIH’s standards in Figure 3.
Medical equipment technician exposure.
During the simulated medical maintenance portion of this evaluation, we followed established amalgamator medical equipment maintenance procedures. The researcher removed the exterior cover of the contaminated dental amalgamator, performed an electrical safety inspection and evaluated the unit’s trituration speed. Maintenance of a contaminated amalgamator has the potential for causing greater mercury vapor exposure. The technician may be exposed to more mercury vapor because the unit’s motor is operated with the cover removed during maintenance and calibration. Even so, under the conditions of this evaluation, we found that no TWA mercury vapor thresholds were exceeded. Review of USAF medical maintenance requirements for dental amalgamators revealed that routine maintenance is required only once a year. Hence, the overall chance of adverse mercury exposure for medical maintenance personnel is remote.
Environmental considerations.
Although the findings of this study suggest that the health hazards of mercury-contaminated amalgamators are minimal, concerns regarding the proper disposal of these devices remain. Discharge of mercury into the environment from any source has come under increased scrutiny. Mercury is classified as a persistent bioaccumulative toxin, or PBT, and is listed among the top 12 hazardous substances listed on U.S. Environmental Protection Agency’s, or EPA, priority PBT list.17 In 1997, the EPA emphasized in a 2,000-page report to Congress the need for closer scrutiny and regulation of mercury emissions.18 In 1998, the American Hospital Association and the EPA signed a landmark agreement to advance pollution prevention efforts in our nation’s hospitals. This agreement calls for the elimination of mercury-containing waste from hospital waste streams by 2005.19
Owing to the amounts of metallic mercury present in the amalgamators we used in this study, these units could be classified as hazardous waste and would require proper decontamination before disposal. Review of the literature by both DIS and outside environmental agencies revealed that no peer-reviewed protocol for the effective decontamination of amalgamators exists (K. Boyle, director, PRO-ACT, USAF Center for Environmental Excellence, personal communication, April 23, 1999). An effort to develop effective means of amalgamator mercury decontamination to allow proper disposal is the subject of a current evaluation by DIS within federal agencies. This ongoing research will be the subject of a future report to the dental profession.
Mercury considerations in today’s dental practice.
In this current study, we sought to qualitatively ascertain the metallic mercury contamination of amalgamators and to investigate potential health hazards to dental personnel operating such equipment. As previously described, there are several possible sources of mercury that can contaminate an amalgamator. Historically, mercury leakage during trituration of reusable capsules commonly has been observed,20 but recently some precapsulated, self-activating amalgam products also have been shown to release mercury.12 In subsequent investigations, we have found routine microscopic mercury contamination on the exterior of precapsulated products that most likely is from the industrial assembly process.13,14
The dental profession should maintain a keen interest in the safe use of mercury. Mercury vapor can be absorbed across the pulmonary epithelium into the bloodstream, where it is oxidized to produce ionic forms that facilitate its transfer into the central nervous system.21 As a result, chronic exposure to mercury vapor produces a form of toxicity that is dominated by neurological effects.22 Elemental mercury liquid does not pose as much of a health hazard risk as does mercury vapor. When exposed to air, elemental mercury is oxidized readily, forming a protective surface film of mercuric oxide.22,23 This film is transparent, flexible and strong. The oxide layer reduces, but does not eliminate, released mercury vapor, because increased temperature raises mercury vapor pressure and thus promotes the formation of higher amounts of mercury vapor.25 In addition to the small amounts we detected in this study, other sources of mercury vapor in the dental operatory include dental amalgam trituration,26 the handling and placement of freshly-mixed amalgam, the polishing of amalgam restorations, used amalgam capsules and the removal of old amalgam with a dental handpiece.24,25,27 The total amount of mercury released during any of these procedures has been evaluated and shown to be far below the total exposure level threshold limits established by regulatory agencies for occupational exposure.25,27
Although the mercury contamination within the amalgamators we studied resulted in vapor levels that exceeded the OSHA PEL in some instances, there appears to be minimal risk to dental and medical equipment maintenance personnel. Within the confined internal compartment of the amalgamator, mercury vapor can concentrate and reach the levels reported in this evaluation. As this study has shown, however, if any of this mercury vapor does escape from the internal confines of the amalgamator, the mercury vapor concentration is diluted rapidly by several magnitudes in the larger volume of room air. With adequate room air ventilation, this high dilution of mercury vapors should not pose a health risk.
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FUTURE DIRECTIONS
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The long-term solution to this identified concern is preventing the source of contamination. Sybron Dental Specialties has marketed a new capsule design that has improvements to prevent the mercury leakage that was observed with the older capsule.28 Also, current draft amendments to International Standards Organization specifications will require manufacturers to produce amalgam capsules that are free from observable microscopic external contamination of mercury and alloy powder.29 These improvements should reduce the incidence of mercury contamination in amalgamators.
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CONCLUSIONS
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We determined the levels of mercury contamination inside dental amalgamators. This contamination is thought to be a result of gross mercury leakage from amalgam capsules, as well as long-term accrual of dislodged microscopic mercury from capsule exteriors—a contaminant from the industrial assembly process.
We detected mercury vapor in the internal compartment of mercury-contaminated amalgamators. Some units had mercury vapor levels that exceeded the OSHA-established PEL.
Under a worst-case usage scenario, we determined that contaminated amalgamators produce mercury vapor in dental and medical maintenance personnel’s breathing spaces. It is important to note, however, that under the conditions of this study the measured vapor levels were small and did not breach any established TWA mercury vapor thresholds. Mercury vapor levels detected were several magnitudes below the most stringent ACGIH guidelines. Regardless, it is important that personnel remain vigilant to possible sources of mercury vapor in dentistry and the potential health effects that may result from chronic exposure to elevated levels.
Amalgamators that are contaminated with mercury may require disposal as environmentally hazardous waste. Current investigation is ongoing to develop decontamination protocols to allow for proper disposal of contaminated amalgamators.
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Dr. Roberts is the director, Technical Evaluations, USAF Dental Investigation Service, Detachment 1, USAFSAM, 310C B St., Building 1H, Great Lakes, Ill. 60088, e-mail "howard.roberts{at}ndri.med.navy.mil". Address reprint requests to Dr. Roberts.
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Dr. Osborne is a professor and the director of clinical research, Department of Restorative Dentistry, University of Colorado Health Sciences Center, Denver, Colo.
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FOOTNOTES
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Any opinions expressed in this article are of the authors and do not constitute the official opinions of the U.S. Air Force, U.S. Department of Defense or the U.S. government.
The authors would like to acknowledge the diligent assistance of SSgt. Joyce M. Reichert, Noncommissioned Officer in Charge, Field Evaulation Team 3, Air Force Institute of Environment, Safety and Ocupational Health Risk Analysis, Brooks Air Force Base, Texas, in the preparation and processing of the mercury vapor air samples during this evaluation.
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REFERENCES
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ABSTRACT
METHODS AND MATERIALS
