during amalgam removal
John W. Reinhardt, D.D.S., M.S.,* Kai Chiu Chan, D.D.S., MS.,** and Thomas M. Schulein, D.D.S.*+* The University of Iowa, College of Dentistry, Iowa City, Iowa
ercury vapor exposure in the dental office is significant to the health of dentists and auxiliaries, and hazards of chronic overexposure to mercury are numerous. Symptoms usually involve the central nervous system, although the kidneys are also known to be affected. The U.S. government has established a guideline for mercury exposure in the workplace that is called the Threshold Limit Value (TLV). This safety level, set at 0.05 mg of mercury per cubic meter of air (0.05 mg Hg/m3), is considered the maximum limit to which a worker should be exposed during a 40-hour workweek. It is interesting to note that the TLV for elemental mercury in the Soviet Union is 0.01 mg/m3 (five times less than the U.S. standard).’ Unfortunately, random surveys of mercury levels in dental offices have found offices that exceed the TLV of the United States. One report noted that 10% to 25% of oflices examined in several states during the late 1960s exceeded the TLV.2 A survey of Kentucky dental offices reported in 1979 that 15% of those offices exceeded the TLV at breathing level: while another study done in 1980 found that of 56 offices surveyed in Iowa, none exceeded the TLV at breathing level4 In an effort to overcome the danger of mercury exposure in dental offices, the American Dental ASSOciation has prepared a set of guidelines for mercury hygiene in dentistry.5 These recommendations for the use of mercury and amalgam are based on research findings and are designed to lower the ambient mercury levels in dental offices. In addition, various products are now being marketed to monitor mercury vapor as well as contain and remove mercury from the oflice.6 One source that can raise ambient mercury vapor levels in the dental offtce is the high-speed removal of amalgam restorations.7 The inhalation of vapors SO produced has been shown to cause high concentra-
*Assistant Professor, Department of Operative Dentistry. **Professor, Department of Operative Dentistry. ***Clinical Instructor, Department of Operative Dentistry.
tions of mercury in various organs of laboratory animals.* It has also been shown that a dental patient may demonstrate a measurable increase in exhaled mercury levels from the removal of one amalgam restoration.’ This study determines the amount of mercury vapor present in the air at various distances from the site of amalgam removal and determines the effects of both dry and wet cutting on the levels of mercury vapor released.
A total of 120 amalgam specimens were used in this study. Cylindrical cavity preparations 2 mm in diameter and 3 mm in depth were cut into 0.25 inch plexiglass plates with a drill press. Each cavity was then restored by hand condensation using high-copper amalgam of low mercury content (43%) (Tytin, S. S. White Dental Mfg. Co., Philadelphia, Pa.). The samples had an average weight of approximately 125 mg each. The samples were stored at 37” F and 100% relative humidity for 7 days prior to the beginning of the mercury vapor experiment. The amalgam specimens were removed by one operator with a high-speed dental handpiece operated at approximately 225,000 rpm and a No. 56 straight fissure bur. Each sample was removed in a 30-second time period. Half (60) the samples were removed without using water coolant, and the other half were removed with the handpiece water spray at a flow rate of 25 ml/min. Neither high- nor low-velocity suction was used during this experiment. Maximum mercury vapor concentrations were measured with a model MV-2 Mercury Vapor Sniffer (Bachrach Instrument Co, Pittsburgh, Pa.) at 6,12, 18, 24, 36, and 48 inches from the site of removal. Ten samples were removed at each distance and under both conditions (wet and dry). The measuring instrument was a portable ultraviolet photometer that had measuring ranges of 0 to 0.2 and 0 to 1 mg/m3. It had a sensitivity of 0.1 mg/m3 and a repeatability of f 5%. The experiment was conducted in a dental operatory
Table I. Mercury levels measured during amalgam removal (overall means) Condition Dry Wet t = 3.769 p < .Ol
measuring 8 X 9 feet and enclosed by walls on three sides. Only a few samples were measured on any given day to ensure that the background levels of ambient mercury vapor would remain at zero. Background levels were determined prior to each sampling, and the experiment was discontinued when a measurable increase occurred.
RESULTS Mean mercury levels recorded during the amalgam removal procedure are shown in Fig. 1. Higher vapor levels were obtained with dry cutting than with wet cutting at each distance. Levels for dry cutting remained similar at all distances measured within 24 inches. Samples that were cut wet showed a general decrease in vapor levels as distance increased, although levels at 6 and 12 inches were similar and levels at 24 and 36 inches were equal. Sample means were compared statistically using a Student t-test. Table I shows that those amalgams removed dry caused significantly higher levels of mercury vapor than those removed wet cjl < .Ol).
DISCUSSION The most important finding in this study is the effect of using water spray coolant during high-speed amalgam removal. The significant increase in mercury levels of samples cut dry emphasizes the need for coolant spray. Similar effects have been seen in mercury levels measured from the exhaled breath of dental patients.” The use of rubber dam, water spray, and high-volume evacuation was shown to prevent significant elevation of mercury levels in the expired air of dental patients following amalgam removal. Testing of breath samples following amalgam removal without rubber dam, coolant, and high-volume evacuation showed a brief elevation of mercury levels in the breath of dental patients. The similarity of elevated levels of mercury measured from 6 to 24 inches during dry cutting is also important to the dental staff since the dentist and assistant are normally within this range during amal-
Fig. 1. Mercury vapor levels recorded at various distances from amalgam removal. Standard deviations are in parentheses. gam removal. In contrast, mercury vapor levels decline rather dramatically in that same range when coolant spray is used. The mercury levels 36 and 48 inches from samples cut dry were similar to those of the samples cut wet at 6 and 12 inches. Standard deviations of the measurements indicate a wide fluctuation in levels of mercury measured at any particular distance. This variation may be caused by the changing air currents within the dental operator-y. Ventilation in the operatory consisted of a central forced-air heating system and was without noticeable draft. Amalgam removal is one of several sources of mercury vapor contributing to elevated ambient levels in the dental operatory. As noted earlier, each sample of amalgam used in this study weighed approximately 125 mg. This sample size is about twice that of an ideal conservative occlusal amalgam restoration on a premolar. It is important to note that the average level of mercury vapor exceeded the TLV in all instances during the removal procedure except those cut wet and measured at 48 inches. The overall mean for wet removal was 0.11 mg/m’, which exceeds the TLV by
2.2 times. The overall mean for dry removal was 0.65 mg/m3, 13 times greater than the TLV. Considering the difference between these means as well as the size and number of amalgams removed daily in a typical practice, the importance of wet cutting becomes even more obvious. In addition to reducing mercury vapor, water coolant is useful in reducing the temperature of the tooth surface and lessening dehydration of the dentin. In view of these facts, dentists should use water coolant for removing amalgam restorations. Because the mercury levels within the common working range distance are elevated for both dry and wet cutting, a well-ventilated environment is necessary to prevent chronic inhalation of this toxic vapor. The use of high volume evacuation to remove mercury vapor from the immediate vicinity of the operator is recommended. In addition, it would be advantageous for dental personnel to wear a mercury-filtering mask during amalgam removal. CONCLUSION Using water spray coolant can reduce mercury vapor levels caused during amalgam removal and therefore lessen one source of ambient mercury vapor in the dental office. With wet cutting, mercury levels generally decreased as distance from amalgam cutting increased. Dry cutting caused similar levels of mercury vaporization when measured within 24 inches. A mercury-filtering mask (Mercury Vapor Respirator, Item No. 8707, 3M Co., St. Paul, Minn.) is commercially available and recommended for use during amalgam removal.
REFERENCES Elkins, H. B.: Maximum acceptable concentrations. A comparison in Russia and the United States. Arch Environ Health ‘2~45, 1961. 2. Council on Dental Materials and Devices: Mercury surveys in dental offices. J Am Dent Assoc 89:900, 1974. 3. Block, J. B., and Peters, H. G.: Kentucky dental mercury survey. J Ky Dent Assoc 31:23, 1979. 4. Reinhardt, J. W., and Chan, K. C.: Mercury survey of dental 1.
offices in Iowa City and Cedar Rapids. Iowa Dent J 66:38, 1980. Council on in mercury Eames, W. devices: An
Dental Materials and Devices: Recommendations hygiene. J Am Dent Assoc 92:1217, 1976. B., and Palmertree, C. 0.: Twelve dental mercury evaluation of methods of monitoring, containment,
and removal of mercury. Oper Dent 5:72, 1980. 7.
R. L., and Barkmeier, W. W.: Mercury vapor emitted
during ultraspeed cutting of amalgam. J Indiana Dent Assoc 57:28, 1978. Cutright, D. E., Miller, R. A., Battistone, G. C., and Millikan, L. J.: Systemicmercury levels causedby inhaling mist during
high-speed amalgam grinding. J Oral Med Z&100, 1973. Reinhardt, J. W., Boyer, D. B., Gay, D. D., Cox, R., Frank, C. W., and &are, C. W.: Mercury vapor expired after restorative treatment: Preliminary study. J Dent Res 56~2005, 1979. 10. Reinhardt, J. W.: Mercury in the Expired Breath of Dental Patients. Thesis, University of Iowa, 1979. 9.
Reprint requests to: DR. JOHN W. REINHARDT THE UNIVERSITY OF IOWA COLLEGE OF DENTISTRY IOWA CITY. IA 52242
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