Dent Mater 8:176-180, May, 1992

Quantitation of total mercury vapor released during dental procedures J. H. Engle, J. L. Ferracane, J. Wichmann and T. Okabe ~

Department of Dental Materials Science, Oregon Health Sciences University, Portland, OR, USA 1Department of Dental Materials, Baylor College of Dentistry, Dallas, TX, USA

Abstract. An in vitro method is described in which measurements were made of the total amount of mercury vapor released from three types of amalgam during routine dental procedures. It was found that the greatest amount of mercury was released during dry polishing of one amalgam (44 p.g). Removal of amalgam from a Class ! cavity under water spray and high volume evacuation also generated large amounts of mercury as expected (15-20 ~g). However, under the more clinically relevant conditions of extending evacuation for one minute to remove residual amalgam and mercury after cutting, this value was reduced by approximately 90%. The total amount of mercury generated during placement (6-8 t~g), wet polishing (2-4 ~g) and trituration (1-2 ~g) were also measured. The study showed that dental procedures associated with amalgam do potentially expose the patient and operator to mercury vapor. However, the total amount of mercury released during any procedure was far below the total exposure level calculated from the daily threshold limits established by regulatory agencies for occupational exposure.

INTRODUCTION Previous toxicological studies, as reviewed by the Environmental Protection Agency (EPA), have led to the classification of mercury (Hg) as an environmental hazard (USEPA, 1984). The studies encompassed all environmental areas, both within and outside of the dental community, and prompted the EPA to establish guidelines for the reduction of exposure to mercury, as well as to establish environmental limits. Globally, the World Health Organization (WHO) has also established recommendations for maximum allowable exposure to mercury in the environment over time (WHO, 1980). Within the dental community there have been investigations into the exposure of dental personnel to mercury (Brown et al., 1984; Langanet al., 1987; Moller-Madsenet al., 1988; Nalewayetal., 1985; Newman, 1979; Nilsson and Nilsson, 1986), as well as into mercury contamination ofthe dental environment (Kantor and Woodcock, 1981; Wilson and Wilson, 1985). Results of these investigations led to the development of guidelines from the profession aimed at reducing mercury exposure in the dental operatory (Pohl et al., 1988; Koski et al., 1981). Early studies were responsible for the current mercury hygiene practices, including the widespread use of pre-packaged capsules for triturating silver amalgam filling materials, as well as the establishment of guidelines for proper mercury care and hygiene in the office environment (Cooley and

176 Engleet alJMercury release during dentalprocedures

Lubow, 1985; Cooley et al., 1985). It is important to note that these studies have all been sampling studies, and have not made measurements of total mercury availability for exposure to the patient and dental personnel. It is generally believed that both the patient and the dental personnel are subjected to the highest levels of mercury vapor during certain dental procedures, when liquid or airborne mercury may be present. Therefore, studies which attempt to quantitate one's total possible exposure to mercury during these procedures have great significance. The purpose of the present investigation was to provide a quantitative evaluation of the total Hg vapor released into the patient's and operator's environment during a simulation of the dental operative procedures of silver amalgam trituration, restoration placement, subsequent polishing and eventual removal. Evaluation of the contribution ofeach component was made to further the understanding of the role of each phase in the maximum exposure of patients and dental personnel to elemental Hg vapor.

MATERIALSAND METHODS The selected dental procedures of amalgam alloy trituration, placement, polishing and subsequent removal were carried out in a modified controlled environment chamber (Demerculator, Futurecraft Corp., City of Industry, CA, USA) with the approximate dimensions of 30 cm x 30 cm x 30 cm (Fig. 1). The chamber had a volume of approximately 27,000 mL, and was sealed with the exception of air intake and exhaust ports. Procedures were performed through the use of a sealed glove arrangement. Fresh air was blown through the chamber at a constant 850 mL/min. In order to more closely approximate clinical conditions, the temperature was controlled at 24°C for trituration studies and at 28°C for the condensation, polishing and removal procedures by placing a light bulb in the chamber. All measurements of mercury levels were taken with a Jerome 511 Gold Film Mercury Vapor Analyzer (Arizona Instrument Corp., Jerome, AZ, USA). The analyzer was attached to a two-way valve which was connected to the exhaust port of the chamber. The instrument was calibrated before each series of measurements. Setting the sampling time for 10 s at the instrument flow rate of 850 mL/min yielded a sampling volume of 142 mL. When not actively sampling for mercury vapor, the airflow was ported through an acti-

Fig. 1. Photograph of the chamber, showing the location of the air pump (A), flowmeter (B), air inlet port (C), air outlet port (D), 3-way valve (E, only used as 2-way), gold film mercury analyzer (F), activated charcoal filter (G), sealable port used to introduce handpiece and suction (H), specimen (I), water drip device (J), and the light bulb used to maintain constant temperature (K).

vated charcoal filter connected to the two-way valve, which allowed for a rapid switching of the chamber exhaust between the film analyzer and the filter. After each sampling cycle was followed to conclusion, the chamber was opened, cleaned of mercury residue with a cloth dampened with a sulfide solution (HgX, Acton Associates, Pittston, PA, USA), and tested to determine a background level of mercury vapor prior to the next sampling. During the early experiments, mercury vapor concentration was analyzed at several sites within the chamber to verify that mercury was not being sequestered in certain areas due to variations in the air flow. Since mercury is heavier than air and has a tendency to adsorb onto surfaces, it is possible that the values calculated for total mercury release were slightly underestimated. The sampling technique allowed for reporting of Hg vapor release in graphic form as Hg per sampling volume (ng/142 mL) vs. time (min). The area under the curve generated for each trial was then integrated with a planimeter, and the total Hg vapor released was calculated by correcting for the instrument flow rate (850 mL/min) according to equation (3) from the previous work of Berglund et al. (1988). Three alloy systems were selected to provide a range of compositions. These alloys included: a low copper spherical - Spheraloy (Kerr, Romulus, MI, USA), containing approximately 3-4% Cu; a high copper spherical - Tytin (Kerr), containing approximately 12% Cu; and a higher copper spherical - Sybralloy (Kerr), containing approximately 30% Cu. Trituration was performed for Tytin with a Caulk Vari-Mix II-M amalgamator (L.D. Caulk Co., Milford, DE, USA) placed inside the test chamber. Mercury vapor release was investigated for two types of capsules: 1) Pre-encapsulated Tytin (n=5) as furnished by the manufacturer, and 2) a screw-closed (Kerr) capsule (n=7) containing 600 mg of Tytin alloy and 443 mg of rig dispensed with a Kerr Universal Alloy Proportioner. The mercury and alloy from several amalgam capsules were emptied, pooled and then accurately weighed and redistributed to their individual capsules to insure the uniformity of each mix. The amalgamator was emptied of any residual mercury and cleaned with a sulfide solution before being placed into the chamber. The background mercury vapor level

vas determined, and then a pre-weighed capsule placed inside ,hrough the sealable inlet port and triturated for the specified :ime (Vari-Mix M-2 for 8 s). The procedure used for the screwclosed capsule was similar; however, the precise amounts of mercury and alloy were introduced after the capsule was placed into the chamber, and the chamber resealed. During the entire experiment for the screw capsule, the mercury dispenser remained sealed in a plastic bag through which only the spout protruded to allow the mercury to be dispensed into the capsule. Following trituration, the capsules were immediately removed from the amalgamator and placed in a capped polyethylene bottle to eliminate any further release of mercury vapor into the test chamber. The first measurement of mercury vapor was taken within 30 s of the end oftrituration. Subsequent measurements were taken at intervals of 2.5 rain for the first 10 min of sampling, then every 5 min for the next 30 min, and approximately every 10-20 min for the remaining time until the levels returned to background. This design allowed for a determination oftotal mercury vapor release with greater accuracy than has previously been reported (Cooley et al., 1985; Newman, 1979; Pines and Schulman, 1989). Placement of the amalgam restoration was performed using a standardized technique within the chamber. Prefabricated lower first permanent molar resin teeth with standardized occlusal cavity preparations (Viade Products, Inc., Camarillo, CA, USA) were used. The capsule was triturated outside and then introduced through the inlet port of the chamber. It was subsequently opened (t--0) and its contents emptied into a dappen dish. The capsule was then removed and the port resealed. Background levels were recorded before introducing the capsule to the chamber. Proportioning of the alloys was done by weight to insure a uniform consistency of each mix: Tytin (n=7) -- 443 mg Hg / 600 mg alloy (42.5% Hg) Sybraloy (n=6) -- 481 mg Hg / 600 mg alloy (45.5% Hg) Spheraloy (n--8) -- 500 mg Hg / 600 mg alloy (44.5% Hg) The slight differences in final mercury percentage for each amalgam were necessary to produce clinically acceptable mixes. The operator used standard operative instrumentation with hand condensation of small incremental additions of the alloy. The sequence took place above a plastic petri dish filled with water in order to trap any scrap amalgam. The cavity was overpacked prior to the start of carving. Each preparation received approximately 0.33 g of amalgam. The time required for condensation (3 min) and carving (2 min) were monitored to insure reproducibility. As soon as condensation was accomplished, all residual amalgam was collected and placed into the plastic dish and sealed with a lid. The plastic tooth was placed atop the lid in the bottom center of the chamber. Following placement, burnishing, and carving of the restoration, the tooth was left in the chamber for evaluation of mercury vapor over the next 300 min. The first reading took place within 5 min from opening the capsule, and usually coincided with the initiation of carving. Measurements were made every 5 rain for the first 30 min, then at 10-20 min intervals until the levels returned to background. Polishing of the restoration took place at a subsequent "appointment". The plastic teeth with the carved restorations were stored two weeks between placement and polishing in artificial saliva (Tani and Zucci, 1967) at 37°C. A standardized, timed polishing technique was utilized in the chamber following the storage. Since air exhausted from the low speed

Dental Materials~May 1992 177

air turbine handpiece made accurate vapor analysis impossible, a Dremel tool was used and operated at the same speed as a low-speed handpiece. The speed was determined by a strobe. The following polishing sequence was undertaken using rotary abrasive points to marginate the restoration: green stone for one minute, white stone for one minute, and finally brown and green points and cups (Shofu, Menlo Park, CA, USA) for one minute each to achieve final polish. Polishing in both a dry and moist environment was studied in an attempt to duplicate possible clinical conditions of treatment with and without water spray. The moist environment was created by having the sample polished under a constant drip of water (one drop per second) provided by a modified I.V. irrigation device during polishing and the entire measuring period. Additionally, the use of high volume evacuation was investigated in the dry polishing technique to determine if it could eliminate mercury vapor from the patient's immediate environment. The first measurements were made at 2.5 and 5 rain after the procedure was initiated. Sampling was performed every 5 min during the first 30, every 10 min during the next hour, then every 30 rain until levels returned to background (150 min total). The polished restorations were stored in artificial saliva at 37°C for four months before being removed from the teeth. Removal was accomplished in the controlled environment using a new #245, pear-shaped carbide bur operated in a high speed handpiece (Adec, Newberg, OR, USA) using both water spray and high volume evacuation. The evacuator from a dental unit was placed into the chamber through a sealable port in the rear and operated within one inch of the specimen during the 45 s required to remove the Tytin and Sybralloy amalgams. Upon completion, the evacuator was withdrawn and the rear exhaust port resealed. Vapor measurement readings were continued for up to 5 h until previous background levels were obtained. Mercury levels from Spheraloy removal were monitored with two different procedures. The first was as described above. The second procedure was similar, except that there was an additional 1 rain of high volume evacuation at the end of the removal period. Free amalgam slurry and chips within the chamber were removed as much as possible during this process. Differences in the release of mercury during the different procedures for each amalgam, as well as between the amalgams for each procedure, were determined by ANOVA and Scheff4 test for making multiple comparisons between means (p < 0.05).

Mercury From Dental Procedures [Hg], ng/142 ml. 120 100 8O 60 4O 20 0~ 0

100

200

300

400

Time in minutes Trituration

--+-- Placement

Polish (dry)

~

+

Polish (wet)

Removal

Fig. 2. Typical plots of Mercury ([Hg] - ng/142 mL of air sampled over 10 s) monitored in the chamber during the various dental procedures. The data represent an average of the results from the experiments with Tytin. All measuring periods are not shown for each procedure in order to simplify the figure. The time t=0 represents that at which trituration and placement were initiated, and when polishing and removal were just completed.

TABLE: TOTAL MERCURYRELEASE(~g) FOR THE THREEAMALGAMS

DURING THE FOUR PROCEDURES TYTIN

SPHERALOY

TRITURATION (pre-cap)

0.86+0.09 (n=5)

PLACEMENT

7.24+1.36

SYBRALQY

. . . . . . 8.95+1.50

(n=7)

(n=8)

8.25+1.50 (n=6)

POLISHING (wet)

2.39+0.90 (n=7)

2.47+0.88 (n=7)

4.28+1.25 (n=6)

REMOVAL

20.8+2.80

15.7+6.2

16.2+7.70

(n=5)

(n=5)

(n=6)

TRITURATION TYTIN 2.5

TOTAL Hg (Micrograms)

RESULTS As demonstrated in previous studies, mercury vapor was released during all of the procedures, but fell thereafter until background levels were reached within several hours (Fig. 2) It should be noted that these plots do not demonstrate that mercury was continuously released for this length of time. Mercury was allowed to '%uild-up" in the chamber during the procedures, as evidenced by the rise to a peak in Fig. 2. The time needed to return to baseline was a function of the flow rate into and out of the chamber. This was set at 850 mL/min to match the rate of sampling of the analyzer. Therefore, the most important information was the total amount of mercury released in micrograms, which was obtained from the integration of the curves (Table). Trituration. Results of the mercury release during trituration of Tytin (Fig. 3) show that the average mercury

178

EngleetalJMercuryreleaseduringdentalprocedures

2 1.5 1 0.5 0 SCREW-CAP m

PRE-CAP MEAN

~

+ 1 S.D.

SCREW CAPSULE VS PRE-ENCAPSULATED

Rg.3. Histogram of the mean (lal~lled) total mercury released during the tritumtion of Tytin comparing the screw-cap to the pre-encapsulated form. The cap above the mean represents one standard deviation unit.

release from the pre-encapsulated alloy (n=5) was significantly less than for the screw-closed capsules (n=7) (Student t-test; p < 0.05). Placement. A relatively small amount of mercury vapor was released into the environment during the placement of the amalgam fillings (Table), but this amount was significantly greater than that generated during trituration for Tytin (Scheff~ crtical difference = 2.74). There were no significant differences between the values obtained for the three alloys (Scheff~ critical difference = 2.15). Polishing. Wet polishing of the Tytin and Spheraloy restorations released significantly less mercury vapor than did placement (Table). However, the amount of mercury generated during placement and polishingofSybraloy was not different (Scheff~ critical difference = 7.18). Polishing in the wet environment produced more mercury from Sybralloy than from Tytin or Spheraloy (Scheff~ critical difference = 1.50). Polishing Tytin dry (Fig. 4) generated nearly twenty times more mercury than wet polishing (Student t-test; p < 0.05). However, the use of high-volume evacuation during the dry polishing procedure reduced the total mercury vapor release to the same level as wet polishing for Tytin. Removal. With the exception ofdry polishing, the greatest amount of mercury vapor was released for each amalgam during the removal procedure (Table). However, the more clinically relevant case in which 60 s of additional evacuation was used (Fig. 5) produced a 90% reduction in mercury vapor release for Spheraloy (Student t-test; p < 0.05).

DISCUSSION This study confirmed that different dental procedures generate different amounts ofmercuryvapor. The greatest amounts of mercury were generated during dry polishing and dry removal of the amalgam without extended evacuation. In addition, mercury release during placement was greater than for wet polishing or trituration (assuming that trituration of all alloys produce an amount that is similar to that generated from Tytin). However, it should also be noted that during certain procedures, such as polishing and removal, it was not possible to remove all of the amalgam fragments generated. This would not be typical ofthe clinical situation where rinsing

TOTAL Hg

would be expected to eliminate these particles as a source of mercury. Though it is expected that the release of mercury from these particles would be minimal due to their moist state, there is a possibility that they contribute a small amount of mercury to the chamber. This contamination would be the basis for a slight overestimate in the total amounts of mercury release calculated for these procedures, and could account for some of the difference in results between this study and that performed by Pohlet al. (1988), in which the release ofmercury was evaluated during dental procedures in a similar environmental chamber. Thus, the data from this study represent somewhat of a "worst case" scenario in terms of potential mercury exposure during dental procedures. The release of rig vapor from amalgam is dependent upon several factors, including the presence of free mercury within the structure as well as the temperature of the material. In addition, manipulation of the material imparts mechanical energy which can be converted to thermal energy, thereby increasing the surface temperature and the vapor pressure of mercury, enhancing its release. Thus, any procedure which involves the manipulation of mercury, such as placement, or vigorous instrumentation of amalgam, such as polishing or removal, will cause an increase in mercury vaporization, as was shown in this study. Similar results have been demonstrated in previous studies in which amalgams were brushed or chewed upon (Gay et al., 1979; Patterson et al., 1985; Vimy and Lorscheider, 1985), instrumented with an ultrasonic scaler (Hohenwald et al., 1987; Westermann et al., 1986), or removed with a handpiece (Pohl et al., 1988; Richards and Warren, 1985). Differences shown between wet and dry polishing can be explained by the fact that the water coolant held much of the slurryin suspension, severely reducing the release ofvapor. In addition, water serves as a coolant for the surface of the amalgam, reducing the vapor pressure of mercury at that site. It has been documented that there is reduced Hgvapor release if the amalgam sample is covered with liquid (Reinhardt et al., 1983; Haikel et al., 1990). Therefore, because the samples polished wet are maintained under a water drip, further mercury release after the actual polishing is reduced and possibly eliminated while the vapor levels in the chamber

POLISHING

REMOVAL

WET VS DRY

With Extended Evacuation

20=

(Micrograms)

TOTAL Hg (Micrograms)

60 50 40

15 30 10 20

10

o

2.39

2.85

i

0 TYTIN WET

TYTIN DRY MEAN

[~

TYTIN DRY - EVAC

* 1 S.D.

SPHERALOY m

SPHERALOY (ex. MEAN

~

evac)

* 1 S.D.

Fig.4. Histogram of the mean (labelled) total memury released during the polishing

Fig. 5. Histogram of the mean (labelled) total mercury released during the removal

of TyUn amalgam restorations under three conditions: wet, dry and dry with high volume evacuation. The cap above the mean represents one standard deviation unit,

of Spheraloy amalgam restorations under two conditions: normal and with one additional minuteof high volume evacuation. The cap above the mean representsone standard deviation unit.

Dental Materials~May 1992 179

return to background. Differences among alloys were not significant, with the exception of an inexplicably greater release of mercury from Sybraloy during polishing. The results of this study do not suggest a possible reason for this difference. Finally, the effect of prolonged evacuation on the total mercury released to the patient's and operator's breathing zones was significant, as expected. Pohl et al. (1988) previously evaluated the effectiveness of a mirror-evacuator device in removing mercury from the breathing zone during dental procedures. Although the final destination of mercury vapor removed from the operatory air by high-volume evacuation raises additional questions, it certainly has a dramatic influence on limiting vapor exposure during procedures which may create a substantial dosage otherwise. To lessen concerns, it is probable that a disposable trap-device, possibly containing activated charcoal, could be employed within the evacuation lines to scavenge mercury vapor and eliminate or reduce it as a source of contamination to both the operatory and environment. In order to evaluate the significance of the total amounts of mercury demonstrated in this study, one must examine them in light of currently acceptable exposure levels. The TLV established by the American Conference of Governmental Industrial Hygienists (ACGIH) for exposure over an 8 h day, 40 h week for those people occupationally exposed to mercury extrapolates to a total of approximately 250-300 ~g/d (Berglund, 1990). The total possible exposure to Hg during the removal of a single amalgam represents approximately 5% of this acceptable level. Obviously, the performance of multiple procedures would increase the level of exposure.

ACKNOWLEDGMENTS This investigation was supported in part by U.S.P.H.S. Research Grant DE07644 from the National Institute of Dental Research, Bethesda, MD 20829 USA. Received August 19, 1991/Accepted November 27, 1991 Address correspondance and reprint requests to:

J. H. Engle DepartmentofDentalMaterialsScience OregonHealthSciencesUniversity 611 S.W.CampusDrive PortlandOR97201USA

REFERENCES Berglund A (1990). Estimation by a 24-hour study of the daily dose of intra-oral mercury vapor inhaled after release from dental amalgam. J Dent Res 69:1646-1651. Berglund A, Pohl L, Olsson S, Bergman M (1988). Determination of the rate of release of intra-oral mercury vapor from amalgam. JDent Res 67:1235-1242. Brown JW, Hosein HR, Horwood JH (1984). Mercury leakage during amalgam trituration. Can DentAssoc J50:234236. Cooley RL, Lubow RM (1985). Mercury vapor emitted from disposable capsules placed in trash containers. Gen Dent 33:498-500. 180 Engle et al./Mercury release during dental procedures

Cooley RL, Stilley J, Lubow RM (1985). Mercury vapor produced during sterilization of amalgam-contaminated instruments. J Prosthet Dent 53:304-308. Gay DD, Cox RD, Reinhardt JW (1979). Chewing releases mercury from fillings. Lancet (Letter) 8123:985-986. Haikel Y, Gasser P, Salek P, Voegel JC (1990). Exposure to mercury vapor during setting, removing, and polishing amalgam restorations. J Biomed Mater Res 24:15511558. Hohenwald R, Westermann H, Dermann K (1987). The effects of ultrasonic scaling on amalgam fillings. Dtsch Zahnarztl Z 42:105-108. Kantor ML, Woodcock RC (1981 ). Mercury vapor exposure in the dental office - does carpeting make a difference? J A m Dent Assoc 103:402-407. Koski RE, Kanter J, Gough J (1981). Controlling mercury vapor release within the dental operatory. J CalifDent Assoc 9:33-39. Langan DC, Steffek AJ, Naleway CA (1987). Mercury hygiene practises. CDA Journal 24-29. Moller-Madsen B, Hansen JC, Kragstrup J (1988). Mercury concentrations in blood from Danish dentists. Scand J Dent Res 96:56-59. Naleway C, Sakaguchi R, Mitchell E, Muller T, Ayer WA, Hefferren JJ (1985). Urinary mercury levels in US dentists, 1975-1983: review of Health Assessment Program. J A m Dent Assoc 111:37-42. Newman SM (1979). Mercury leakage from preportioned capsules. J Tenn Dent Assoc 59:19-22. Nilsson B, Nilsson B (1986). Mercury in dental practice. I. The working environment of dental personnel and their exposure to mercury vapor. Swed Dent J 10:1-14. Patterson JE, Weissberg BG, Dennison PJ (1985). Mercury in human breath from dental amalgams. Bull Environ Contain Toxicol 34:459-468. Pines M, Schulman A (1989). Detection of mercury leakage from amalgam capsules. J Dent Res 68:209, Abstr. No. 221. Pohl, L, Berglund A, Bergman M, Olsson S (1988). A new instrument for mercury vapor evacuation during clinical work with dental amalgam. Swed Dent J 12:193-199. Reinhardt JW, Chan KC, Schulien TM (1983). Mercury vaporization during amalgam removal. J Prosthet Dent 50:62-64. Richards JM, Warren PJ (1985). Mercury vapor released during the removal of old amalgam restorations. BrDent J 159:231-232. Tani G, Zucci F (1967). Electrochemical valuation of the corrosion resistance of commercially used metals in dental prosthesis. Minerva Stomat 16:710-13. USEPA (1984). Mercury Health Effects Update, Health Issue Assessment, Final Report, Washington, DC: United States Environmental Protection Agency, EPA-600/8-84019F. Vimy MJ, Lorscheider FL (1985). Intra-oral air mercury released from dental amalgam. JDentRes 64:1069-1071. Westermann R, Hohenwald R, Dermann K (1986): Mercury evaporation from amalgam after ultrasonic calculus removal. JDent Res 65:550, Abstr. No. 106. World Health Organization (1980). Technical Report Series 647. Wilson SJ, Wilson HJ (1985). Mercury vapour levels in a dental hospital environment. Br Dent J 159:233-234.