H U M A N F A C T 0 R S , 1975, 17(4), 346-355
Metallic Mercury Contamination of the Dental Operatory RICHARD G . DOMEY, Department of Bioengitieeririg, The Uiiiversity of Texas Health Science Center, Saiz Atitoriio, Texas
TlreA?iiericaiiCorifereiice of GovcniiiieiifalItiditstrialHygieiiisfsi 971 standard tltresliold limit value (TLV) of 0.55 ~iigHglin’of air was fottiid t o be exceeded frcqtietrtly iii a saiiiple a f 60 deiital operatories iri Sail Antoriio, Texas. Sigtiificarit differences it1 contaniinafiotz ariioiig laboratories rLwe foitizd, anioiig sites rvitliiii operatories, and aiiioiig tinies o f day, correlated with kilograiiis of tiiercctry used, average rimtrber of awalgarm iiiserted per day. age of operatories, Iieiglit of carpeting pile, cleaititig methods, freqtiericy of cleaning, aitd general lack of roiltitie nioiiitoriiig atid ttse of bioassays. Recot?tniendatio,isfor coiitrol of metallic iiierctiry coritariiiiiatioii iii derital operatories are offered.
INTRODUCTION
conclusive data, differences in assessment arise. Overall, the literature appears to Metallic mercury will kill all types of body demonstrate the reality of this hazard. Howcells, may be classified as chronic (hydrar- ever, since bioassays and monitoring of the gyrism)oracute,andcanresult indeathofthe working environment of the dentist and his organism. The physical symptoms of chronic personnel are not routine, the scope and demetallic mercury poisoning are anemia, gree of this hazard are suggested almost exleukopenia, the significant reduction of leuko- clusively by intermittent publication of highly cytes in the blood, anorexia, renal toxicity, variable research findings and community permanent liver and renal damage, diarrhea, surveys. Almost all such data are published in salivation, headaches, tremors, photophobia, papers and journals of interest to persons and restlessness. Psychological symptoms are primarily associated with dentistry and hence anxiety and depression. Acute metallic mer- are, in fact, limited to a relatively small group cury poisoning is characterized by a metallic of professionals only marginally familiar with taste, abdominal and other gastrointestinal the human factors discipline. Increasing the pains, vomiting, bloody diarrhea, dermatitis, range of reporting and the size of the techand central nervous system ulceration. Cer- nically sophisticated readership may also intain behavioral signs are dizziness, clumsi- crease the probability of improving solutions ness, ataxia, and slurred speech. to the problem, for example, by way of deThere is growing evidence that the dental veloping simplified and convenient bioassay operatory work space is one among several methods and inexpensive and easy-to-operate sources of metallic mercury contamination instruments useful for monitoring metallic (Rupp and Paffenbarger, 1971). However, Hef- mercury contamination in the environment. ferren (1974) stated that dental operatorycon- This topic embraces many other elements tarnination was “minor”. In the absence of closely connected with the interests of human
346
August, 1975-347
RICHARD G. DOMEY
factors specialists. Among these interests are: (1) the design of the dental workspace, (2)
handling of toxic materials, (3) dental instrumentation, (4) teaching and maintaining control, (5) industrial hygiene, (6) occupational safety, (7) individual and team performance, as well as (8) work methods and procedures.
Characteristics of Mercury
~
Mercury is a relatively volatile substance since it. has an equilibrium concentration of about 2 mgHg/m3 of air at 25°C and its vapor pressure rises rapidly with increases in temperature; there is about an eight-fold increase as temperature rises from 20°C to 50°C.a range which brackets room-comfort temperatures. Temperatures exceeding 50°C are common in hcaterairducts. a common conduit for dispersal of aerosols of all kinds including metallic mercury. Volatility coupled with conversion to small droplets in aerosol form, conjoined with effective dispersal mechanisms to which is added physical contact, provide the conditions for the development of contamination hazards in the dental environment.
Mecharrisrirs of Coirtarrrirratioir More than 2.72 million kg of mercury are used annually in the United States, of which 95,281 kg, or 496, are used in the dental profession (Rupp and Paffenbarger, 1971). On the average, each dentist uses 1.13 kg of mercury per year, mostly for amalgam fillings which contain about 50% mercury and 25% silver. Of this weight a certain amount of free mercury enters the working environment and is dispersed as a consequence of (1) the conventional design of the dental suite, especially the operatory, (2) procedures, (3) methods of using mercury, and (4) the increasing sophistication of dental technology and instrumentation. The result is inadvertent physical contact through direct handling and exposure to mer-
cury aerosols and vapors. More specifically, loss occurs through spillage from handling, leakage from capsules in the trituration (pulverizing) process, and droppage from the mulling procedure (Rupp and Paffenbarger, 1971); collection, storage, and reclamation of valuable scrap (Barber and Reisbick, 1973); absorption by carpeting; handling, without duccaution,of mechanical amalgamators and ultrasonic condensers (Chandler, Rupp, and Paffenbarger. 1971; von Nossek and Seidel, 1968); a n d dispersion i n t o aerosols from high-speed rotary-cutting instruments (Meyer, 1962). Air-conditioncr/hcater units provide a means of increasing the probability of vaporization and distribution of vapors (Gronka, Bobkoskie, Tomchick, Bach, and Rakow, 1970; Joselow, et al., 1968; Shepherd, e f al., 1941). In addition, conventional methods of cleaning apparently do not always effectively remove residual mercury (Joselow, et al., 1968).
Iiidttstrial Starrdards In one study of industrial workers who breathed 0.1 0 mgHg/m3 of air for eight hours a day (Sax, 1968), it was reported that blood mercury was successfully removed by the kidneys but vapor concentrations above that level resulted in residual traces in some workers. This provided some justification for the original 0.10 mgHg/m3 of air threshold limit value (TLV) recommended by The American Conference of Governmental Industrial Hygienists (1971), and also adopted by the Department of Employmcnt in the United Kingdom (Anonymous, 1972). Atmospheric contamination does not account for physical ‘contact through direct handling, clearly an additional source of contamination. It follows that the industrial study by Sax could not be considered as providing sufficient validation of the original recommended criterion. Reduction of theO.10 mgHglm3ofairTLV toO.05
348-August, 1975
rngHdm3 has been recommended and presumably is a far safer standard of reference for studies of metallic mercury contamination of the dentist's suite. Patiertt atid Worker Exposlire to Metallic MerCllry
Exposure of patients to metallic mercury is usually infrequent and brief, and, except for rare cases of sensitization to mercury, the hazard to the consumer is said to be minimal (Rupp and Paffenbarger, 197 1). However, the professional dentist and his personnel are continuously exposed for prolonged periods of time to greater o r lesser amounts of free mercury, which, when absorbed, is not readily excreted. The much publicized death through mercury poisoning of a dental assistant (Cook and Yates, 1969) was preceded and followed by several studies and summaries of research into this health hazard. The dangers which arise if precautions are not taken when handling mercury and in the trituration (grinding and mixing) of amalgams have been pointed out by Giese (1948). Grossman and Dannenberg (1949), Docking (1962), and Knapp (1963). Hygienically significant over-exposure to mercury in dental offices has been reported by Frykholm (1957), Noe (1959). McCord (1961). Preussner, Klocking, and Bast (1963), Nixon and Smith (1965), Hoover and Goldwater (1966), Joselow, et al. (1 968), Gronka, et al. (1970), and Frykholrn (1970). In their study, Joselow, et al. (1968) showed that in a sample of 50 dental offices, one office in seven was reported as having mercury vapor levels which exceeded the TLV. Gronka, et al. (1970) reported t h a t of workers i n 59 d e n t a l surgeries, six dentists and four assistants,were exposed to excessive levels of mercury contamination. Even more important were physical studies of dental workers by Hertst, er al. (1963),Nixonand Smith(1965),andFrykholm (1970), all of whom documented the presence
HUMAN FACTORS
of mercury in hair, fingernails, and urine of dental workers. More specifically, Herbst, et al. (1963) said that the urine mercury of 8 dentists averaged 6.3 pglliter, and that of 13 dental assistants, who averaged 17 years of employment in dentistry, averaged 14.3 pglliter, which was more than double that of dentists' urine mercury excretion. However, the urine test may be an underestimate of the levels of contamination since urine mercury decreases after damage to the kidneys resulting from mercurial poisoning has occurred. It was noted that room airconcentrationsof mercury vapor for each group was less than the 0.10 mgHdm3 of air TLV standard accepted at that time. In a carefully conducted study, Buchwald ( I 972) pointed out: Perhaps the most significant finding of the survey
was the general unawareness of the dentists, especially their assistants, that mercury could be harmful . . . . If the results of the survey are representative of dental practices in the remainder of Canada, then it is possible that some750 dentists (of 5,000) and 1,250 assistants in Canada alone are absorbing more mercury than they should. OBJECTIVES AS part of a research training program selected dental students undertook the task of measuring metallic mercury contamination in dental operatories in the local community of metropolitan San Antonio, Texas. The topic is of current interest partly because it has been and is intermittently studied and reported in a wide variety of literature and partly because nothing was known in this regard about the dental facilities in this city. In Texas there is no record of dentists or paradental assistants having taken advantage of the annual American Dental Association Health Screening Program. The Division of Allied Health Manpower, Public Health Service, showed considerable interest in this project and supported it with a Public Health Apprentice Training Grant. To be investigated were the mercury vapor
August, 1975-349
RICHARD G. DOMEY
r
levels as a function of (1) age of the operatory, (2) time of day, (3) height of observations above the floor, (4) site of observations, (5) type of air-conditionerheater systems, (6) frequency of replacement of air-conditioner/heater systems “ d u s t ” filters, (7) type of floor covering, (8) amount of mercury used in the practice with the number of amalgam fillings removed as a separate measure of the amount used, (9) amount of scrap accumulated for reclamation (50% mercury,25%siIver),and(lO)the typeofdentistry practiced, that is, sit-down us. stand-up. METHODOLOGY Sa tiiple
Ziistniiizeiztatioii
The Bachrach Mercury Sniffer (BMS), Model MV-2, which replaced the original BMS, Model MV-2, both loaned by the Texas State Department of Public Health, was used to measure vapor levels. The principle of operation of this type of mercury sniffer depends upon atomic absorption of ultraviolet light wavelength specific for mercury (253.7 nanometers). Meter graduations permit direct readings in increments of 0.01 mgHg/m3 air with interpolations to the nearest 0.001 mgHg/m3 air. Beyond this range, up to 1.0 mgHg/m3 air minor scale divisions correspond to 0.1 mgHg/m3 air increments. Each night the power supply of the sniffer was recharged. The apparatus was calibrated before and after the study and the close correspondence of the obtained values lends credence to the validity of the observations made during the study (Adrian, 1973). All operatories were air-conditioned, and variations in temperat u r e were considered to be randomized throughout. To avoid systematic biases due to timedependent changes in temperature, humidity, barometric pressure, and other factors which could influence mercury vapor concentrations, observations were made during treatment hours (morning, noon, and afternoon). Data were obtained only in those operatories used by the dentist in each dental suite.
The population studied was the 257 general dental practitioners and pedodontists listed in the Yellow Pages of the September 1972 metropolitan area San Antonio Telephone Directory. Numbers were assigned to each name according to a table of random numbers. Because of death, or retirement, or refusal to participate, 76 names were drawn before the sample complement of 50 cooperative dentists was obtained. Names of participants and office locations were held in confidence. As a result of publicity given this project, additional dentists volunteered to participate in the study. Although their requests were honored as a service to the profession, the obtained data were not included in the main study except from two respondents whose names coincidentally appeared on the ran- Observa tioiis Observations were independently duplidom sample list. The random sample eventually was reduced cated by two investigators at two different to 30 dentists and 60 operatories, the reasons levels (floor and operator heights) in each of being conflicts with vacation schedules, can- seven different sites within each operatory cellations, retirement, and initial mechanical measured (each of the four corners of the failure of the mercury vapor sensing equip- ‘ room, at the chair, the unit, and the work ment. Since order of operatories tested was cabinet) fora total of 28 readings, duringeach randomized, there was no special reason to of the three daily time periods, yieldinga total assume that the remaining operatories were of 84 observations for each operatory, for a less random except by way of the smaller grand total of 5040 readings overall. The number of observations was slightly different sample.
350-August. 1975
HUMAN FACTORS
for the five operatories in which cabinets occupied corners. RESULTS All operatories tested showed the presence of mercury vapor although not all 84 observations for each laboratory showed positive results. Thirty-eight operatories had no concentrations which reached the TLV. The distribution of operatories by the frequency of mercury vapor readings equal to o r greater than the TLV is shown in Table 1. Of a total of 5030 observations, in the 60 operatories surveyed, 436 were equal to or greater than the TLV a n d were distributed among 22 operatories. Of particular interest were the five most contaminated operatories, each of which showed 36 or more observations equal to o r greater than the standard. Of these five operatories, one had 48 observations at or above the standard, one had 57, one had 64, one had 83, and for one operatory all 84 observations were equal to or greater than the TLV. Table 2 shows the time distribution of the 436 high value observations. In the morning 182 high levels were recorded in 13 operatories, 84 were found in 13 operatories at noon, and f 70 were observed in I8 operatories in the afternoon. From the 84 observations made in each of
the 60 operatories, a mean mercury vapor level was computed. The frequency distribution of the operatories by mercury vapor levels is shown in Table 3. From this table, the inverse relationship of these two variables is apparent. Operatory, site, time, and height observations were evaluated through a four-way analysis of variance (Sokal and Rohlf, 1969). All distributions were skewed toward the higher mercury vapor levels as shown by Fisher’s test and by Kolmogorov-Smirnov D,,, test (Siegel, 1956) for normality of distribution. However, it is noted, for purposes of interpreting the analysis of variance, that samples were large, departures from normality were consistently in the same direction, and all but one of the 15 F-ratios were highly significant. Thus, by and large the results were considered reasonable and tenable (Young TABLE 2 Time Distributions for 436 Hg Vapor Concentration Observations Eaual To or Greater Than the TLV Time
AM (9-10O’clock) Noon (12-1o’clock) PM (3-4O’clock)
Number 5 TLV Observations Operatories
182 a4 170
13 13 18
TABLE 3 TABLE 1 DistributionofOperatories by Numbersofobservations Equal to or in Excess of the TLV for hIercury Vapor Frequency of Observations TLV
Number of Operatories
0 1-5 6-1 0 11-15 1620 21-25 2630 31-35 3683
38 12 3 0 0 1 0
>84
1 4 1
.
Distribution of Dental Operatorics By Mean hlercury Vapor Levels Mercury Levels (mg Hglm3 air) All levels
.001-.010 .011-.020 .021-.030 ,031-.040 .041-.050 .051-.060 .061-.070
.071-.080 .081-.090
.091-.loo
Operatories Number Percent
60 21 14 8 10
1 1 0 4 1 0
100 35 23 13 17 2 2 0 7 2 0
August, 1975-351
RICHARD G . DOMEY
TABLE 4 Four-way Analysis of Variance of Mercury Vapor Levels Source of Variation
Operatory (0)
Site (S) Time (7) Height (H)
oxs
OxT SxT OXH SXH TxH OxSxT OXSXH OxTxH SxTXH OxSxTxff Pooled Error
Total
ss 1.1871805 0.0133689 0.0133249 0.0053747 0.0640437 0.2922557 0.0009552 0.0258690 0.0056075 0.0005663 0.0392169 0.0580553 0.0195536 0.0003446 0.0348473 0.0571338 1.8176918
df
59 3 2 1 177 118 6 59 3 2 354 177 118 6 354 1440 2879
MS
0.0201217 0.0044563 0.0066625 0.0053747 0.0003618 0.0024767 0.0001592 0.0004385 0.0018692 0.0002831 0.0001108 0.0003280 0.0001657 0.0000574 0.0000984 0.0000397 0.0006314
F
P
597.147
Metallic Mercury Contamination of the Dental Operatory RICHARD G . DOMEY, Department of Bioengitieeririg, The Uiiiversity of Texas Health Science Center, Saiz Atitoriio, Texas
TlreA?iiericaiiCorifereiice of GovcniiiieiifalItiditstrialHygieiiisfsi 971 standard tltresliold limit value (TLV) of 0.55 ~iigHglin’of air was fottiid t o be exceeded frcqtietrtly iii a saiiiple a f 60 deiital operatories iri Sail Antoriio, Texas. Sigtiificarit differences it1 contaniinafiotz ariioiig laboratories rLwe foitizd, anioiig sites rvitliiii operatories, and aiiioiig tinies o f day, correlated with kilograiiis of tiiercctry used, average rimtrber of awalgarm iiiserted per day. age of operatories, Iieiglit of carpeting pile, cleaititig methods, freqtiericy of cleaning, aitd general lack of roiltitie nioiiitoriiig atid ttse of bioassays. Recot?tniendatio,isfor coiitrol of metallic iiierctiry coritariiiiiatioii iii derital operatories are offered.
INTRODUCTION
conclusive data, differences in assessment arise. Overall, the literature appears to Metallic mercury will kill all types of body demonstrate the reality of this hazard. Howcells, may be classified as chronic (hydrar- ever, since bioassays and monitoring of the gyrism)oracute,andcanresult indeathofthe working environment of the dentist and his organism. The physical symptoms of chronic personnel are not routine, the scope and demetallic mercury poisoning are anemia, gree of this hazard are suggested almost exleukopenia, the significant reduction of leuko- clusively by intermittent publication of highly cytes in the blood, anorexia, renal toxicity, variable research findings and community permanent liver and renal damage, diarrhea, surveys. Almost all such data are published in salivation, headaches, tremors, photophobia, papers and journals of interest to persons and restlessness. Psychological symptoms are primarily associated with dentistry and hence anxiety and depression. Acute metallic mer- are, in fact, limited to a relatively small group cury poisoning is characterized by a metallic of professionals only marginally familiar with taste, abdominal and other gastrointestinal the human factors discipline. Increasing the pains, vomiting, bloody diarrhea, dermatitis, range of reporting and the size of the techand central nervous system ulceration. Cer- nically sophisticated readership may also intain behavioral signs are dizziness, clumsi- crease the probability of improving solutions ness, ataxia, and slurred speech. to the problem, for example, by way of deThere is growing evidence that the dental veloping simplified and convenient bioassay operatory work space is one among several methods and inexpensive and easy-to-operate sources of metallic mercury contamination instruments useful for monitoring metallic (Rupp and Paffenbarger, 1971). However, Hef- mercury contamination in the environment. ferren (1974) stated that dental operatorycon- This topic embraces many other elements tarnination was “minor”. In the absence of closely connected with the interests of human
346
August, 1975-347
RICHARD G. DOMEY
factors specialists. Among these interests are: (1) the design of the dental workspace, (2)
handling of toxic materials, (3) dental instrumentation, (4) teaching and maintaining control, (5) industrial hygiene, (6) occupational safety, (7) individual and team performance, as well as (8) work methods and procedures.
Characteristics of Mercury
~
Mercury is a relatively volatile substance since it. has an equilibrium concentration of about 2 mgHg/m3 of air at 25°C and its vapor pressure rises rapidly with increases in temperature; there is about an eight-fold increase as temperature rises from 20°C to 50°C.a range which brackets room-comfort temperatures. Temperatures exceeding 50°C are common in hcaterairducts. a common conduit for dispersal of aerosols of all kinds including metallic mercury. Volatility coupled with conversion to small droplets in aerosol form, conjoined with effective dispersal mechanisms to which is added physical contact, provide the conditions for the development of contamination hazards in the dental environment.
Mecharrisrirs of Coirtarrrirratioir More than 2.72 million kg of mercury are used annually in the United States, of which 95,281 kg, or 496, are used in the dental profession (Rupp and Paffenbarger, 1971). On the average, each dentist uses 1.13 kg of mercury per year, mostly for amalgam fillings which contain about 50% mercury and 25% silver. Of this weight a certain amount of free mercury enters the working environment and is dispersed as a consequence of (1) the conventional design of the dental suite, especially the operatory, (2) procedures, (3) methods of using mercury, and (4) the increasing sophistication of dental technology and instrumentation. The result is inadvertent physical contact through direct handling and exposure to mer-
cury aerosols and vapors. More specifically, loss occurs through spillage from handling, leakage from capsules in the trituration (pulverizing) process, and droppage from the mulling procedure (Rupp and Paffenbarger, 1971); collection, storage, and reclamation of valuable scrap (Barber and Reisbick, 1973); absorption by carpeting; handling, without duccaution,of mechanical amalgamators and ultrasonic condensers (Chandler, Rupp, and Paffenbarger. 1971; von Nossek and Seidel, 1968); a n d dispersion i n t o aerosols from high-speed rotary-cutting instruments (Meyer, 1962). Air-conditioncr/hcater units provide a means of increasing the probability of vaporization and distribution of vapors (Gronka, Bobkoskie, Tomchick, Bach, and Rakow, 1970; Joselow, et al., 1968; Shepherd, e f al., 1941). In addition, conventional methods of cleaning apparently do not always effectively remove residual mercury (Joselow, et al., 1968).
Iiidttstrial Starrdards In one study of industrial workers who breathed 0.1 0 mgHg/m3 of air for eight hours a day (Sax, 1968), it was reported that blood mercury was successfully removed by the kidneys but vapor concentrations above that level resulted in residual traces in some workers. This provided some justification for the original 0.10 mgHg/m3 of air threshold limit value (TLV) recommended by The American Conference of Governmental Industrial Hygienists (1971), and also adopted by the Department of Employmcnt in the United Kingdom (Anonymous, 1972). Atmospheric contamination does not account for physical ‘contact through direct handling, clearly an additional source of contamination. It follows that the industrial study by Sax could not be considered as providing sufficient validation of the original recommended criterion. Reduction of theO.10 mgHglm3ofairTLV toO.05
348-August, 1975
rngHdm3 has been recommended and presumably is a far safer standard of reference for studies of metallic mercury contamination of the dentist's suite. Patiertt atid Worker Exposlire to Metallic MerCllry
Exposure of patients to metallic mercury is usually infrequent and brief, and, except for rare cases of sensitization to mercury, the hazard to the consumer is said to be minimal (Rupp and Paffenbarger, 197 1). However, the professional dentist and his personnel are continuously exposed for prolonged periods of time to greater o r lesser amounts of free mercury, which, when absorbed, is not readily excreted. The much publicized death through mercury poisoning of a dental assistant (Cook and Yates, 1969) was preceded and followed by several studies and summaries of research into this health hazard. The dangers which arise if precautions are not taken when handling mercury and in the trituration (grinding and mixing) of amalgams have been pointed out by Giese (1948). Grossman and Dannenberg (1949), Docking (1962), and Knapp (1963). Hygienically significant over-exposure to mercury in dental offices has been reported by Frykholm (1957), Noe (1959). McCord (1961). Preussner, Klocking, and Bast (1963), Nixon and Smith (1965), Hoover and Goldwater (1966), Joselow, et al. (1 968), Gronka, et al. (1970), and Frykholrn (1970). In their study, Joselow, et al. (1968) showed that in a sample of 50 dental offices, one office in seven was reported as having mercury vapor levels which exceeded the TLV. Gronka, et al. (1970) reported t h a t of workers i n 59 d e n t a l surgeries, six dentists and four assistants,were exposed to excessive levels of mercury contamination. Even more important were physical studies of dental workers by Hertst, er al. (1963),Nixonand Smith(1965),andFrykholm (1970), all of whom documented the presence
HUMAN FACTORS
of mercury in hair, fingernails, and urine of dental workers. More specifically, Herbst, et al. (1963) said that the urine mercury of 8 dentists averaged 6.3 pglliter, and that of 13 dental assistants, who averaged 17 years of employment in dentistry, averaged 14.3 pglliter, which was more than double that of dentists' urine mercury excretion. However, the urine test may be an underestimate of the levels of contamination since urine mercury decreases after damage to the kidneys resulting from mercurial poisoning has occurred. It was noted that room airconcentrationsof mercury vapor for each group was less than the 0.10 mgHdm3 of air TLV standard accepted at that time. In a carefully conducted study, Buchwald ( I 972) pointed out: Perhaps the most significant finding of the survey
was the general unawareness of the dentists, especially their assistants, that mercury could be harmful . . . . If the results of the survey are representative of dental practices in the remainder of Canada, then it is possible that some750 dentists (of 5,000) and 1,250 assistants in Canada alone are absorbing more mercury than they should. OBJECTIVES AS part of a research training program selected dental students undertook the task of measuring metallic mercury contamination in dental operatories in the local community of metropolitan San Antonio, Texas. The topic is of current interest partly because it has been and is intermittently studied and reported in a wide variety of literature and partly because nothing was known in this regard about the dental facilities in this city. In Texas there is no record of dentists or paradental assistants having taken advantage of the annual American Dental Association Health Screening Program. The Division of Allied Health Manpower, Public Health Service, showed considerable interest in this project and supported it with a Public Health Apprentice Training Grant. To be investigated were the mercury vapor
August, 1975-349
RICHARD G. DOMEY
r
levels as a function of (1) age of the operatory, (2) time of day, (3) height of observations above the floor, (4) site of observations, (5) type of air-conditionerheater systems, (6) frequency of replacement of air-conditioner/heater systems “ d u s t ” filters, (7) type of floor covering, (8) amount of mercury used in the practice with the number of amalgam fillings removed as a separate measure of the amount used, (9) amount of scrap accumulated for reclamation (50% mercury,25%siIver),and(lO)the typeofdentistry practiced, that is, sit-down us. stand-up. METHODOLOGY Sa tiiple
Ziistniiizeiztatioii
The Bachrach Mercury Sniffer (BMS), Model MV-2, which replaced the original BMS, Model MV-2, both loaned by the Texas State Department of Public Health, was used to measure vapor levels. The principle of operation of this type of mercury sniffer depends upon atomic absorption of ultraviolet light wavelength specific for mercury (253.7 nanometers). Meter graduations permit direct readings in increments of 0.01 mgHg/m3 air with interpolations to the nearest 0.001 mgHg/m3 air. Beyond this range, up to 1.0 mgHg/m3 air minor scale divisions correspond to 0.1 mgHg/m3 air increments. Each night the power supply of the sniffer was recharged. The apparatus was calibrated before and after the study and the close correspondence of the obtained values lends credence to the validity of the observations made during the study (Adrian, 1973). All operatories were air-conditioned, and variations in temperat u r e were considered to be randomized throughout. To avoid systematic biases due to timedependent changes in temperature, humidity, barometric pressure, and other factors which could influence mercury vapor concentrations, observations were made during treatment hours (morning, noon, and afternoon). Data were obtained only in those operatories used by the dentist in each dental suite.
The population studied was the 257 general dental practitioners and pedodontists listed in the Yellow Pages of the September 1972 metropolitan area San Antonio Telephone Directory. Numbers were assigned to each name according to a table of random numbers. Because of death, or retirement, or refusal to participate, 76 names were drawn before the sample complement of 50 cooperative dentists was obtained. Names of participants and office locations were held in confidence. As a result of publicity given this project, additional dentists volunteered to participate in the study. Although their requests were honored as a service to the profession, the obtained data were not included in the main study except from two respondents whose names coincidentally appeared on the ran- Observa tioiis Observations were independently duplidom sample list. The random sample eventually was reduced cated by two investigators at two different to 30 dentists and 60 operatories, the reasons levels (floor and operator heights) in each of being conflicts with vacation schedules, can- seven different sites within each operatory cellations, retirement, and initial mechanical measured (each of the four corners of the failure of the mercury vapor sensing equip- ‘ room, at the chair, the unit, and the work ment. Since order of operatories tested was cabinet) fora total of 28 readings, duringeach randomized, there was no special reason to of the three daily time periods, yieldinga total assume that the remaining operatories were of 84 observations for each operatory, for a less random except by way of the smaller grand total of 5040 readings overall. The number of observations was slightly different sample.
350-August. 1975
HUMAN FACTORS
for the five operatories in which cabinets occupied corners. RESULTS All operatories tested showed the presence of mercury vapor although not all 84 observations for each laboratory showed positive results. Thirty-eight operatories had no concentrations which reached the TLV. The distribution of operatories by the frequency of mercury vapor readings equal to o r greater than the TLV is shown in Table 1. Of a total of 5030 observations, in the 60 operatories surveyed, 436 were equal to or greater than the TLV a n d were distributed among 22 operatories. Of particular interest were the five most contaminated operatories, each of which showed 36 or more observations equal to o r greater than the standard. Of these five operatories, one had 48 observations at or above the standard, one had 57, one had 64, one had 83, and for one operatory all 84 observations were equal to or greater than the TLV. Table 2 shows the time distribution of the 436 high value observations. In the morning 182 high levels were recorded in 13 operatories, 84 were found in 13 operatories at noon, and f 70 were observed in I8 operatories in the afternoon. From the 84 observations made in each of
the 60 operatories, a mean mercury vapor level was computed. The frequency distribution of the operatories by mercury vapor levels is shown in Table 3. From this table, the inverse relationship of these two variables is apparent. Operatory, site, time, and height observations were evaluated through a four-way analysis of variance (Sokal and Rohlf, 1969). All distributions were skewed toward the higher mercury vapor levels as shown by Fisher’s test and by Kolmogorov-Smirnov D,,, test (Siegel, 1956) for normality of distribution. However, it is noted, for purposes of interpreting the analysis of variance, that samples were large, departures from normality were consistently in the same direction, and all but one of the 15 F-ratios were highly significant. Thus, by and large the results were considered reasonable and tenable (Young TABLE 2 Time Distributions for 436 Hg Vapor Concentration Observations Eaual To or Greater Than the TLV Time
AM (9-10O’clock) Noon (12-1o’clock) PM (3-4O’clock)
Number 5 TLV Observations Operatories
182 a4 170
13 13 18
TABLE 3 TABLE 1 DistributionofOperatories by Numbersofobservations Equal to or in Excess of the TLV for hIercury Vapor Frequency of Observations TLV
Number of Operatories
0 1-5 6-1 0 11-15 1620 21-25 2630 31-35 3683
38 12 3 0 0 1 0
>84
1 4 1
.
Distribution of Dental Operatorics By Mean hlercury Vapor Levels Mercury Levels (mg Hglm3 air) All levels
.001-.010 .011-.020 .021-.030 ,031-.040 .041-.050 .051-.060 .061-.070
.071-.080 .081-.090
.091-.loo
Operatories Number Percent
60 21 14 8 10
1 1 0 4 1 0
100 35 23 13 17 2 2 0 7 2 0
August, 1975-351
RICHARD G . DOMEY
TABLE 4 Four-way Analysis of Variance of Mercury Vapor Levels Source of Variation
Operatory (0)
Site (S) Time (7) Height (H)
oxs
OxT SxT OXH SXH TxH OxSxT OXSXH OxTxH SxTXH OxSxTxff Pooled Error
Total
ss 1.1871805 0.0133689 0.0133249 0.0053747 0.0640437 0.2922557 0.0009552 0.0258690 0.0056075 0.0005663 0.0392169 0.0580553 0.0195536 0.0003446 0.0348473 0.0571338 1.8176918
df
59 3 2 1 177 118 6 59 3 2 354 177 118 6 354 1440 2879
MS
0.0201217 0.0044563 0.0066625 0.0053747 0.0003618 0.0024767 0.0001592 0.0004385 0.0018692 0.0002831 0.0001108 0.0003280 0.0001657 0.0000574 0.0000984 0.0000397 0.0006314
F
P
597.147
