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An environmental study of mercury contamination in dental offices
An en viro n m e n ta l h ealth survey of 19 d en tal offices w ith 284 d en tal w o rke rs w as co n d u cte d to d e te rm in e sources o f u n c o n tro lle d ex p o s u re to m ercury vapor. An ad d itio n a l o bjective w as to assess th e d e g re e of risk o f th e various d en tal gro up s, by o cc u p a tio n , in th e h an dling o f m ercury. D a ta o b ta in ed fro m analysis o f co llected w ip e, air, and u rin e sam p les w e re used to ev alu ate th e risk o f a b so rp tio n o f m ercury vapo r. D e ntal assistants w h o p re p a re a m a lg a m fo r fillin g to o th cavities sh o w th e g rea tes t risk o f ex p o s u re to m ercury vapo r. N o ev id e n c e of a c u te e x p o s u re to m ercury v a p o r w a s fo u n d fo r an y o f th e o th e r g ro u p s studied. R e c o m m e n d a tio n s a b o u t th e h an d lin g o f m ercury an d th e p rep aratio n o f a m a lg a m a re m ade. F u rth e r stu d ies a re w arran ted to d e te rm in e e ffe cts on th e health of th ose d en tal p erson n el w h o se u rinary m e rc u ry co n ce n tra tio n s in d icate a b so rp tio n of a b n o rm a l am o u n ts of m ercury va p o r.
An environmental study of mercury contamination in dental offices M e ie r S c h n e id e r, M S, Los A n g e l e s B B H H B H B
The U S dental profession uses more than 200,000 lb of mercury metal a year in dental amalgams to restore decayed teeth.1 The knowledge of the toxicity of this metal to humans dates to ancient Greece. Although reports in the literature indicate that inorganic metallic mercury can be absorbed by inhalation of its vapor or through the unbroken skin, insufficient data exist to establish the latter route as a significant hazard for those who handle this metal. The most common form of occupa tional exposure of humans to mercury vapor is chronic exposure.210 D ental office personnel may be exposed to mercury vapor when they prepare and handle dental amalgam or are in the immediate vicinity of such preparation and handling operations; spilled mercury or improperly stored mercurycontaining wastes also are sources of exposure. Concern about environmental pollution has fo cused attention on the use of mercury in den tistry. Questions regarding the potential hazards to dental personnel and patients from mercury in dental offices continue to be raised, although several studies have been made of this subject. The literature on hazards from the use of mer cury in dental offices consists of the popular type of articles as well as those of a scientific nature. The conclusions of the authors range from casual dismissal of the hazard of mercury to dental pa tients and office employees to dire predictions of damage to health.11’22 A recent publication10 reports a fatality in volving a dental assistant with a 20-year history 1092 ■ JADA, Vol. 89, November 1974
of exposure to dental amalgams containing about 40% mercury. The dental assistant died from acute renal failure attributed to mercury intox ication. The patient apparently had .no indication of chronic mercury poisoning until she became suddenly ill with vomiting, passing of dark urine, pain in the right lumbar region of the abdomen, and edema of the face and legs. The manner in which death occurred is considered somewhat uncommon for those exposed to mercury va pors. Unfortunately, the description of this case omitted environmental data pertaining to the handling of mercury by this individual. The relatively voluminous data published about mercury in dental offices generally con clude that no danger of systemic poisoning ex ists for patients who have their teeth restored with amalgam. However, individuals in medical and dental practices who handle mercury can suffer either acute or chronic mercury intoxica tion because of inhalation of mercury vapor. The extent of exposure to mercury vapor depends on several factors. These include the quantity of metal handled, ventilation, and the manner in which the mercury is handled.23 A thorough review of the literature on haz ards of mercury in dental offices revealed sev eral field studies conducted in the United States and some foreign countries. F or the most part these studies provided little environmental in formation relating to details of mercury handling in dental offices. It was believed that a more searching study, with greater attention to the manner in which m ercury and the resultant amai-
gam are handled by dentists and their assistants, would provide information for the control of mercury vapor exposures in dental offices. One such study was conducted in Alberta, Canada, during the summer of 1968.24 Inasmuch as clim atic conditions and practices in the m anner of preparing amalgam can vary considerably from country to country, as well as from one part of a country to another, it was decided to repeat the Alberta study in Southern California.
Statistical treatment and study methods The association between personnel groups in dental offices and their exposures to mercury vapor was assessed. Urinary mercury levels were used to determine exposure to mercury vapor. The P values were based on the chisquare test with one degree of freedom and the application of the Yates’s correction for contin uity. The study participants worked in 19 dental offices. F or the study 101 dentists, 88 dental as sistants, 13 dental hygienists, 20 dental labor atory technicians, 9 dental X-ray technicians, 6 maintenance men, and 47 receptionist-clerical personnel participated. The survey included small offices as well as large clinics. M ost dentists practiced general dentistry, and a few restricted their practice to pedodontics. The pedodontists use significantly larger quanti ties of amalgam than most practitioners of gen eral dentistry. D ata were collected on contamination of sur faces, by taking wipe samples, and the atmos phere, by use of a direct-reading instrument (Mercury Sniffer*), as well as by scrubbing of the dental office air through an absorbing solu tion that was subsequently analyzed in the labor atory for mercury in vapor or particulate form or both. D ata for excreted mercury were obtained by analysis of collected urine samples.25,26 These data were analyzed to determine environ mental factors primarily responsible for absorp tion of mercury by dental office personnel. ■ Urine samples: The literature27,28 dealing with excretion of mercury in urine indicates no consistent correlation between excretion and duration of exposure for individuals. However, on a group basis, a good correlation between mercury exposure levels- and levels of urinary excretion apparently exists. M oreover, wide fluctuations are found in hourly, as well as daily,
excretion of mercury in the urine of exposed per sons. One study28 indicated a rhythmic pattern to the excretion of mercury. This pattern seems more related to some physiologic mechanism rather than to an immediate environmental ex posure. A considerable time lag exists between increased exposure and a corresponding in crease in urinary mercury excretion, as noted by Molyneux28 in 1966 and in a letter of March 1971. During the planning phase of this study, a de cision was made to use urinary mercury values to identify groups of individuals in dental offices who exhibit urinary mercury levels in excess of those for the average nonoccupationally ex posed population. F o r the most part, all urine voidings during a normal workday were collect ed from personnel in the dental offices surveyed. A few spot-voidings were collected in instances in which it was impractical to obtain a full day’s voidings. All participants in this study were involved with some aspect of dental practice and were subject to some degree of exposure to mercury vapor. The participants were classified into groups according to functions performed with respect to mercury handling in the dental prac tice. Urinary mercury concentrations were separ ated into two classes. Those concentrations equal to or less than 30/ug/liter were placed in the normal category. Those exceeding the upper limit of this arbitrarily chosen value were con sidered abnormal in that they were indicative of increased mercury absorption over previously established “ normal” limits for nonoccupation ally exposed individuals. The literature on uri nary mercury excretion of nonoccupationally exposed individuals indicates an upper limit of about 10/xg/liter of urine.3,29 The arbitrarily chosen upper limit, used here for the average “ normal” category, is higher than the generally accepted value because personnel employed in dental offices cannot be considered nonoccupa tionally exposed, regardless of their job func tion. M ercury urine values are not considered abso lute indicators of exposure to mercury vapor. However, they can be useful on a group basis for the evaluation of human exposure to mer cury, provided that the individuals in the groups surveyed exhibit normal kidney function. ■ Am bient air concentrations: During this study, air in the dental offices was analyzed dir ectly as well as indirectly for mercury vapor. Schneider: MERCURY CONTAMINATION ■ 1093
T a b le 1 ■ S u r fa c e c o n ta m in a tio n b y m e rc u ry . A v e ra g e d s u rfa c e c o n ta m in a tio n ( f i g H g/100 c m 2 s u rfa c e )
D e nta l o ffic e
1
24 557 2 171 13 93 6 247 41 37 2 8 2 21 43 8
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
T ab le 2 ■ Urine mercury levels for functional groups in the dental offices. U rin e m e rc u ry le vels ( m 3 H g /lite r) G rou p D e n tis ts H a n d lin g m e rc u ry N o t h a n d lin g m e rc u ry D e nta l a s s is ta n ts H a n d lin g m e rc u ry N o t h a n d lin g m e rc u ry O th e r p e rs o n n e l N o n o p e ra to ry
No.
R a nge
> 3 0 (% )
M ean
75 25
2-110 2-60
26.3 20.0
24.7 19.3
20.1 17.4
74 33
2-300 2-92
40.6 12.1
37.6 16.8
44.5 19.6
95
2-224
11.1
18.3
29.3
±
SE
T able 3 ■ Comparison of urinary mercury levels of dental office personnel. H Q Hg/liter
G ro u p
<30
>30
132
56
188 j I
N o n o p e ra to ry
85
10
95 !
D e n tis ts H a n d lin g m e rcu ry
56
20
I 761►
20
5
44
30
N o t h a n d lin g m e rc u ry
29
4
D e n tis ts h a n d lin g m e rc u ry
56
20
76 iI
D e n ta l a s s is ta n ts h a n d lin g m e rcu ry
44
30
74!
A ll d e n tis ts
76
25
D e nta l a s s is ta n ts h a n d lin g m e rc u ry
44
30
O p e ra to ry
T o ta l
P
< 0 .0 1
N o t h a n d lin g m e rc u ry D e n ta l a s s is ta n ts H a n d lin g m e ru ry
N ot s ig n ific a n t
25 !
74 I» J
< 0 .0 1
33
-
101
N o t s ig n ific a n t
}i N o t s ig n ific a n t
74 !
Since most offices surveyed had ambient air con centrations less than the currently accepted threshold limit value30 (TLV) of 0.05 mg Hg/m3 of air, it was decided that ambient air mercury concentrations would not be suitable for estab lishing levels of mercury contamination in these offices. (This value of 0.05 mg Hg/m3 of air is an eight-hour day, 40-hour workweek concentration that most persons can be exposed to without ad verse effect.) The average ambient air concen tration obtained for 19 dental offices was 0.03 mg Hg/m3, with a range of 0.00 to 0.16 mg Hg/m3. ■ Contamination o f surfaces: Mercury drop lets oxidize rapidly in air, with the result that the metal is coated with oxide or sulfide. This oxi1094 ■ JADA, Vol. 89, November 1974
dation coating, as well as grease and dust, sup presses the vaporization of the underlying metal if the droplet is undisturbed. After various sur faces such as bench tops, carpets, flooring tile, and operatory units were rubbed, a mercury va por meter was used to detect mercury vapor a few inches above the disturbed surface. The rub bing action not only broke the coating, expos ing bare metal, but also raised the surface tem perature slightly because of frictional heating. The latter served to increase the vaporization of the mercury contaminating the surface. Wipe samples were taken from surfaces in the dental offices to obtain an indication of the degree of mercury contamination.
Results
Table 1 contains a list of those offices surveyed where wipe samples were obtained. About six wipes were obtained for each office, and the in dividual samples were averaged for each office surveyed. Urine mercury levels for the functional groups surveyed are summarized in Table 2. A rank or dering of functional groups according to risk of absorbing mercury reveals that dental assistants who handle amalgam have the greatest risk and that nonoperatory personnel have the least risk; this is based on the percentage of individuals in each functional group who exhibited urinary mercury levels greater than 30/ug/liter. Table 3 shows a comparison of urinary mer cury levels by functional groups. A significant difference in urinary mercury levels was found between dental operatory personnel and dental nonoperatory personnel, and between dental assistants who handle mercury amalgams and dental assistants who do not. N o significant dif ferences were found for dentists who handle mercury compared with dentists who do not, or between dentists who handle mercury and dental assistants who also handle amalgams.
Discussion
Every dental office surveyed during this study was contaminated with mercury. The extent of contamination was governed by many variables including the number of amalgam restorations prepared, the number of operations in the office, the method of mixing the amalgam, the care in the handling of mercury and dental amalgam, the
type of floor covering in the dental office, house keeping procedures, and the type of ventilation. Because finely divided mercury can form a covering of either oxide, sulfide, dust, or grease as a result of contact with air, the presence of such longstanding contamination may not pose a health hazard to personnel in the dental office unless this residual contamination is disturbed mechanically or by heating. If ambient temper atures are relatively high, the vapor pressure of the mercury can increase to such an extent that vapor can penetrate and escape through the coating of the mercury droplet. In this situation hazardous concentrations of mercury may result in closed spaces with insufficient ventilation to dilute the mercury vapor to safe levels. Thus, the lack of association between elevated urine mer cury levels and surface contamination levels in the offices is not surprising. Of all the offices surveyed, only one showed an association between urine mercury levels of personnel and surface contamination. This of fice had a heavy patient load composed of union members associated with a prepaid dental plan. The building housing the various operatories was completely air-conditioned; about 80% of the air was recirculated. Puddles of mercury were noted on a ceramic tile floor, and dental office personnel were observed stepping on the liquid mercury and tracking it throughout the building. Shoe soles of a few dental assistants were checked with the Mercury Sniffer for evi dence of contamination. In all instances the me ter went off scale. This office was so badly con taminated that the air in the waiting room mea sured 0.07 mg Hg/m3, which is in excess of the current TLV of 0.05 mg Hg/m3. The major cause of the mercury contamination in this office was the misuse of amalgamators. The amalgamators used in this office were designed for the prepar ation of only single batches of amalgam, that is, one measure of alloy plus one measure of mer cury. However, the dentists instructed the den tal assistants to prepare triple-size batches of amalgam. Since this amalgamator is unable to accommodate such large amounts of alloy and mercury, excess mercury was discharged from the machine during the initial amalgamation pro cedure before the final mulling procedure. This excess mercury spilled on to bench tops and rolled off the bench to the tile floor. Dental office personnel whose functions in volve the full-time use of the dental operatory where amalgam is used are more likely to be ex posed to mercury vapor than those whose pri
mary duties are conducted outside of the dental operatory. Therefore, elevated mercury levels in urine of dental operatory personnel, compared with the more normal mercury levels in urine of nonoperatory personnel, are to be expected. During amalgam preparation, the mercury and alloy mixture is subjected to severe vibratory motion to permit maximum contact between the liquid mercury and metal alloy. This intense vi bration occurs while the amalgam ingredients are enclosed in a small plastic capsule. Frictional heating occurs during this operation and is large ly responsible for increasing the vapor pressure of the mercury above that associated with nor mal ambient temperatures extant in the dental operatory. Thus, when the dental assistant opens the triturating capsule and drops the plastic amalgam onto a squeeze cloth or into a glass cup, before handing it to the dentist, she encounters a relatively high, but short (in the order of sec onds) exposure to mercury vapor. This phenomenon was noted by sampling of the breathing zone of the dental assistant during all phases of amalgam preparation, including the act of transferring amalgam from the amalgam ator area to the dental unit. Air sampling with the Mercury Sniffer during amalgam prepara tion and handling revealed values ranging from about twice the current TLV for mercury vapor in the workroom (0.05 mg Hg/m3) to readings that were so high that they went off the scale of the meter. The highest readings on the Mercury Sniffer were noted during the time the triturating capsule was opened and just before the transfer of the amalgam to the dental unit. Moreover, dental assistants receive greater mercury vapor exposures because they work in the immediate vicinity of the amalgamator, whereas dentists usually do not. Generally, the greatest contam ination of mercury occurs within a radius of 1 foot of the bench area where the amalgamation of the alloy and mercury is conducted. Mercury vapor is released as a result of spills of mercury metal and improper storage of waste amalgam and recovered mercury metal. The constant activity in this area of the dental operatory dis turbs mercury globules and very finely divided mercury on various surfaces; this exposes fresh metal surfaces that give off vapor. Many dentists prefer a more splashy amalgam, which necessi tates the use of an excess of mercury during amal gamation, rather than the recommended 1:1 ratio of alloy and mercury. Much of this excess mer cury must be expressed by hand squeezing in a squeeze cloth. The excess mercury usually is Schneider: MERCURY CONTAMINATION ■ 1095
deposited in some type of container such as aja r, flat dish or glass, paper, or plastic cup. Most storage containers for excess mercury and waste amalgam were not secured with lids. During the squeezing operation, the dental assistant receives an additional vapor exposure. The squeezing is normally done in the immediate vicinity of the amalgamator. With the exception of the report of the fatal ity ,10 the literature on mercurialism lacks these data as they pertain to individuals in dentistry. O f all the personnel involved in the handling of dental amalgams in the dental operatory, the risk of absorption of the metal by the body is greatest for the dental assistant. The lack of data relating to mercurialism among dental assistants may be explained in that many dental assistants are women in their early 20s who tend to leave the profession after a few years as a result of mar riage, and because the vapor exposures are inter mittent and are of relatively short duration. N o data are available regarding the effects of these intermittent exposures to mercury vapor on human health. However, since there may be danger to the fetus, pregnant women should not be exposed to mercury vapor for any prolonged period.
Recommendations These precautions are recommended for the safe handling of mercury by dental personnel. ■ Education: All personnel involved in the handling of mercury and amalgam should be made aware of the toxic nature of these sub stances and should be instructed in safe handling practices. ■ Storage: All mercury should be kept in tight ly sealed containers of glass, plastic, or glazed ceramic when not being used. W aste mercury and amalgam should be stored in similar contain ers that have tight-fitting covers. If a tightly cov ered container is not available for stored waste amalgam, this material should be covered with about 14 inch of water. All mercury and waste amalgam should be stored in a cool place. All stored mercury and waste amalgam should be labeled. The label should indicate the contents of the container, as well as its hazardous nature. ■ Disposal: W aste
mercury
1096 ■ JADA, Vol. 89, November 1974
and
amalgam
should be accumulated in tightly sealed contain ers. These materials should not be flushed down the sink, dropped on the floor, thrown into waste baskets, or allowed to accumulate in open trays, beakers, or on bench tops. Operatory units should be equipped with suit able traps to trap waste amalgam expectorated into sinks by patients. The dentist should use a rubber dam when filling a tooth with amalgam or when removing amalgam restorations. Waste amalgam accumulated on the rubber dam can be suctioned off by the dental assistant. All articles such as paper tissues and squeeze cloths that have become contaminated with amalgam should be placed in plastic bags that are kept sealed until ready for disposal. ■ Operatory: All amalgam preparation should be conducted in a ventilated enclosure. The en closure should be under slight negative pressure with respect to the ambient air, and air discharged from the enclosure should be passed through iodized activated charcoal to remove mercury vapor. The enclosure should be constructed of smooth metal or plastic, with no blind comers where mercury can collect. This unit should be equipped with a removable tray to catch any spilled mercury or amalgam. The unit should be cleaned each day of all free mercury and amal gam. The interior surfaces and removable tray should be washed with a water solution contain ing a mercury decontaminant. Mercury filters should be checked with a mercury meter at least once each six months to ensure that no break through of mercury vapor has occurred. During removal of amalgam restorations, den tists should wear a tight-fitting oral-nasal dust mask capable of filtering out particulates as small as 0.5/im in diameter to prevent inhalation of finely divided amalgam particulates. The use of a drill equipped with a cooling water spray pre vents frictional heating of the amalgam and sub sequent release of mercury vapor during remov al of the restoration from the patient’s mouth. All surfaces where mercury and amalgam are used should be free of cracks or comers where mercury can be trapped. Stainless steel or For mica with concave comers and edges are suitable for surface coverings. Floors should be onepiece, with a vinyl coating; comers and abutting walls should be covered. Carpets should not be used in operatories because it is almost impos sible to decontaminate them after a mercury spill. Furthermore, vacuuming of mercury-con taminated carpets can create a temporary severe
inhalation hazard because the mercury, passing through the heated exhaust of the vacuum clean er, is readily vaporized. Operatory chairs should be covered with a smooth vinyl upholstery material. Cloth cover ings should be avoided. Excess amalgam should not be permitted to fall from the patient’s clothing because it can contaminate operatory floors, dental chairs, cor ridors, and waiting-room chairs and floors. Waste amalgam and mercury can be accum ulated for eventual disposal to a metal reclaim er. If the latter procedure is not feasible, the mer cury should be placed in a container that can be sealed. The waste container should be labeled as to the nature of its contents, and then disposed of through a waste chemical collection service for ultimate burial in a legal dump designated for hazardous chemicals. M ercury contamination can be eliminated during handling of the metal if encapsulated pre proportioned materials are used when the amal gam is being prepared. M anufacturers’ recommendations with re spect to maximum quantities of materials to be used in their amalgamators should be followed. Mulling cups that contain old amalgam should never be héated to remove the amalgam unless this procedure is carried out in a chemical fume hood equipped with mechanical exhaust ventila tion to carry the fumes outside of the building. All fumes vented to the outside should be passed through an iodized charcoal filter. ■ Spills: Spilled mercury should be cleaned up immediately by use of a suction device that can trap the collected metal. Other mechanical mer cury-gathering devices may be obtained from a chemical supply house. A vacuum cleaner should never be used to clean up mercury spills unless it is specially designed for that purpose. It must have a mercury filter on the discharge. If mercury has spilled into places that are in accessible to a clean-up device, vaporization of the mercury can be suppressed by covering of the metal with a thin coating of flowers of sulfur. A commercial, water-soluble mercury decon taminant can be used to wash down table tops, benches, and equipment that are contaminated. Dental instruments that are contaminated with mercury in the course of tooth restoration can be soaked in this decontaminant solution for an hour before the washing and sterilizing of these instruments. If the commercial decontaminant is not available, technical grade photographic
sodium thiosulfate can be used in a 10% by weight water solution. The occupational health section of the state or local health department should be consulted for detailed information on mercury decontaminants. These materials do not destroy mercury. They merely change its character to prevent the vapor from contaminating the air. ■ Personal hygiene: Mercury will adhere to the skin. This poses the danger of inhalation of the vapor because the body temperature is suffi ciently high to cause vaporization of some of the metal adhering to the skin. M oreover, those who smoke can transfer mercury from the skin to cigarettes, with subsequent vaporization and in halation of the metal. Many individuals bring their hands near their nose from time to time and thus any metal vaporizing from the hands can be inhaled. Some absorption of mercury can take place through the skin. The rate of absorption of metallic mercury through unbroken skin is low compared with that for organomercury com pounds. However, specific data are not avail able for this phenomenon. Personnel handling mercury or amalgam should scrub their hands thoroughly, using soap or detergent, as soon as it is practical after con tact with the metal or its alloy. Smoking should be prohibited in areas where mercury is being handled. Individuals who handle amalgam should not smoke until they have thoroughly scrubbed their hands. ■ Medical: Periodic urine testing should be done of individuals handling mercury and amal gam to determine the adequacy of preventive measures for safeguarding employees’ health. The testing intervals can be established on the basis of the degree of exposure. Advice on med ical monitoring of personnel handling mercury and amalgam, as well as guidelines about the ex posure of pregnant employees to mercury vapor, can be obtained from the occupational health section of the state or local health departments or from a recently published report.31 This doc ument recommends comprehensive medical examinations for all persons subject to exposure to inorganic mercury before employee place ment and annually thereafter. Medical records are available to the medical representatives of the employer, of the Secretary of Labor, to the Secretary of Health, Education and Welfare, and of the employee at his request. Such records should be retained by the employer for at least five years after the employee’s last occupational Schneider: MERCURY CONTAMINATION ■ 1097
exposure to inorganic mercury. Other recom mendations relate to the posting of warning signs and the use of personal protective equipment, work clothing, and respiratory protection for those exposed to mercury vapor.
Conclusions Environmental contamination of dental offices by mercury does not seem to pose an acute health hazard for personnel. The risk of exposure to mercury vapor, measured by urinary mercury concentrations, is greater for dental office oper atory personnel than for nonoperatory person nel. In large offices in which general dentistry is practiced, no significant difference in mercury absorption exists between dentists who handle mercury and dentists who do not. Dental assis tants who handle mercury and amalgam have a greater risk of mercury vapor absorption than dental assistants whose duties do not include the handling of amalgam. N o significant difference in mercury absorption is found between dentists and dental assistants who handle mercury. How ever, compared with all dental office functional groups, dental assistants who handle mercury have the greatest risk of absorption of mercury vapor. The risk of absorption of mercury vapor is least for nonoperatory office personnel. A detailed health study of dental personnel is warranted for those individuals exhibiting higher than normal urinary mercury concentrations for occupationally exposed persons.
The views expressed in this paper are those of the author and are not necessarily the views of the California State Department of Health. This project was sponsored jointly by the department of health science, California State University, Northridge, and by the Occu pational Health Section, California State Department of Health. The author thanks Hector P. Blejer, MD, head, medical unit, Occupational Health Section, California State Department of Health, and Mrs. Ester Baginsky, statistician with the California State Department of Health. Mr. Schneider is an industrial hygiene engineer for the City of Los Angeles, personnel department, medical services division, 1401 W Sixth St, Los Angeles, 90017. ‘ Mercury Sniffer model MV-2, J/W Bacharach Instrument Co., Pittsburgh, 15238. 1. Mineral facts and problems, 1970 edition. Bureau of Mines Bulletin 650. US Department of the Interior, 1970. 2. Bidstrup, P. Toxicity of mercury and its compounds. Elsevier monograph 34. New York, Elsevier Publishing Co., 1964, p 1. 1098 ■ JADA, Vol. 89, November 1974
3. Goldwater, L.J. Occupational exposure to mercury. Harben lectures, 1964. J Roy Inst Public Health 27:279 Nov-Dee 1964. 4. Maths, H.B., and others. Maximum allowable concentra tions of mercury compounds. Report of an international commit tee. Arch Environ Health 19:891 Dec 1969. 5. Ellis, R.W., and Fang, S C. Elimination, tissue accumulation, and cellular incorporation of mercury in rats receiving an oral dose of 203 Hg-labeled phenylmercuric acetate and mercuric acetate. Toxicol Appl Pharmacol 11:104 July 1967. 6. Browning, E. Toxicity of industrial metals, ed 2. London, Butterworth, 1969, p 229. 7. Albahary, C. Maladies medicamenteuses. Paris, Masson et Cie, 1953. 8. Hazards of mercury. Special report to the Secretary’s Pesti cide Advisory Committee. DHEW, Nov 1970. Environ Res 4:1 March 1971. 9. Ambrose, E.R. Pressure amalgamation. J Can Dent Assoc 28:571 Sept 1962. 10. Cook, T.A., and Yates, P.O. Fatal mercury intoxication in a dental surgery assistant. Br Dent J 127:553 Dec 16, 1969. 11. Lintz, W. Prevention and cure of occupational diseases of the dentist. JADA 22:2071 Dec 1935. 12. Souder, W., and Sweeney, W.T. Is mercury poisonous in dental amalgam restorations? Dent Cosmos 73:1145 Dec 1931. 13. Hoover, A.W., and Goldwater, L.J. Absorption and excretion of mercury in man. X. Dental amalgams as a source of urinary mercury. Arch Environ Health 12:506 April 1966. 14. McCord, C.P. Mercury poisoning in dentists. Ind Med Surg 30:554 Dec 1961. 15. Grossman, L.I., and Dannenberg, J.R. Amount of mercury vapor in the air of dental offices and laboratories. J Dent Res 28: 435 Oct 1949. 16. Preussner, S.,; Kloecking, H.P.; and Bast, G. [Chronic slow mercury poisoning in dental practice.] Arch Toxik 20:12, 1963. 17. Nixon, G.S., and Smith, H. Hazard of mercury poisoning in the dental surgery. J Oral Ther 1 :512 March 1965. 18. Joselow, M.M., and others. Absorption and excretion of mercury in man. XV. Occupational exposure among dentists. Arch Environ Health 17:39 July 1968. 19. Dalhamn, T. Mercury exposure among dentists and dental nurses. Nord Hyg Tidskr 34:32 No. 1-2, 1953. 20. Gronka, P.A., and others. Mercury vapor exposures in den tal offices. JADA 8 1 :923 Oct 1970. 21. Questions and answers. Siedlecki, J.T. Long-term expo sure of dentist to mercury. JAMA 215:647 Jan 25, 1971. 22. Frykholm, K.O. On mercury from dental amalgam. Its toxic and allergic effects and some comments on occupational hy giene. Acta Odontol Scand 15:71 Suppl 22, 1957. 23. Noe, R.E. Mercury as a potential hazard in medical labor atories. New Engl J Med 261:1002 Nov 12, 1959. 24. Buchwald, H. Exposure of dental workers to mercury. Am Ind Hyg Assoc J 33:492 July 1972. 25. Monkman, J.L.; Maffett, P.A.; and Doherty, T.F. The deter mination of mercury in air samples and biological materials. Am Ind Hyg Q 17:418 Dec 1956. 26. Linstedt, G. A rapid method for the determination of mer cury in urine. Analyst 95:264 March 1970. 27. Jacobs, M.B.; Ladd, A.C.; and Goldwater, L.J. Absorption and excretion of mercury in man. VI. Significance of mercury in urine. Arch Environ Health 9:454 Oct 1964. 28. Molyneux, M.K. Observations on the excretion rate and concentration of mercury in urine. Ann Occup Hyg 9:95 July 1966. 29. State of California Department of Public Health, Bureau of Occupational Health. Medical supervision of employees in mer cury mines and mills. Tech Bull for Physicians. Berkeley, Cal ifornia State Department of Health, Aug 1967. 30. TLVs— threshold limit values for chemical substances and physical agents in the workroom environment with intended changes for 1972. Cincinnati, American Conference on Govern mental Industrial Hygienists. 31. Criteria for a recommended standard occupational expo sure to inorganic mercury. HSM 73-11024. DHEW, National Institute for Occupational Safety and Health, 1973.
An environmental study of mercury contamination in dental offices M e ie r S c h n e id e r, M S, Los A n g e l e s B B H H B H B
The U S dental profession uses more than 200,000 lb of mercury metal a year in dental amalgams to restore decayed teeth.1 The knowledge of the toxicity of this metal to humans dates to ancient Greece. Although reports in the literature indicate that inorganic metallic mercury can be absorbed by inhalation of its vapor or through the unbroken skin, insufficient data exist to establish the latter route as a significant hazard for those who handle this metal. The most common form of occupa tional exposure of humans to mercury vapor is chronic exposure.210 D ental office personnel may be exposed to mercury vapor when they prepare and handle dental amalgam or are in the immediate vicinity of such preparation and handling operations; spilled mercury or improperly stored mercurycontaining wastes also are sources of exposure. Concern about environmental pollution has fo cused attention on the use of mercury in den tistry. Questions regarding the potential hazards to dental personnel and patients from mercury in dental offices continue to be raised, although several studies have been made of this subject. The literature on hazards from the use of mer cury in dental offices consists of the popular type of articles as well as those of a scientific nature. The conclusions of the authors range from casual dismissal of the hazard of mercury to dental pa tients and office employees to dire predictions of damage to health.11’22 A recent publication10 reports a fatality in volving a dental assistant with a 20-year history 1092 ■ JADA, Vol. 89, November 1974
of exposure to dental amalgams containing about 40% mercury. The dental assistant died from acute renal failure attributed to mercury intox ication. The patient apparently had .no indication of chronic mercury poisoning until she became suddenly ill with vomiting, passing of dark urine, pain in the right lumbar region of the abdomen, and edema of the face and legs. The manner in which death occurred is considered somewhat uncommon for those exposed to mercury va pors. Unfortunately, the description of this case omitted environmental data pertaining to the handling of mercury by this individual. The relatively voluminous data published about mercury in dental offices generally con clude that no danger of systemic poisoning ex ists for patients who have their teeth restored with amalgam. However, individuals in medical and dental practices who handle mercury can suffer either acute or chronic mercury intoxica tion because of inhalation of mercury vapor. The extent of exposure to mercury vapor depends on several factors. These include the quantity of metal handled, ventilation, and the manner in which the mercury is handled.23 A thorough review of the literature on haz ards of mercury in dental offices revealed sev eral field studies conducted in the United States and some foreign countries. F or the most part these studies provided little environmental in formation relating to details of mercury handling in dental offices. It was believed that a more searching study, with greater attention to the manner in which m ercury and the resultant amai-
gam are handled by dentists and their assistants, would provide information for the control of mercury vapor exposures in dental offices. One such study was conducted in Alberta, Canada, during the summer of 1968.24 Inasmuch as clim atic conditions and practices in the m anner of preparing amalgam can vary considerably from country to country, as well as from one part of a country to another, it was decided to repeat the Alberta study in Southern California.
Statistical treatment and study methods The association between personnel groups in dental offices and their exposures to mercury vapor was assessed. Urinary mercury levels were used to determine exposure to mercury vapor. The P values were based on the chisquare test with one degree of freedom and the application of the Yates’s correction for contin uity. The study participants worked in 19 dental offices. F or the study 101 dentists, 88 dental as sistants, 13 dental hygienists, 20 dental labor atory technicians, 9 dental X-ray technicians, 6 maintenance men, and 47 receptionist-clerical personnel participated. The survey included small offices as well as large clinics. M ost dentists practiced general dentistry, and a few restricted their practice to pedodontics. The pedodontists use significantly larger quanti ties of amalgam than most practitioners of gen eral dentistry. D ata were collected on contamination of sur faces, by taking wipe samples, and the atmos phere, by use of a direct-reading instrument (Mercury Sniffer*), as well as by scrubbing of the dental office air through an absorbing solu tion that was subsequently analyzed in the labor atory for mercury in vapor or particulate form or both. D ata for excreted mercury were obtained by analysis of collected urine samples.25,26 These data were analyzed to determine environ mental factors primarily responsible for absorp tion of mercury by dental office personnel. ■ Urine samples: The literature27,28 dealing with excretion of mercury in urine indicates no consistent correlation between excretion and duration of exposure for individuals. However, on a group basis, a good correlation between mercury exposure levels- and levels of urinary excretion apparently exists. M oreover, wide fluctuations are found in hourly, as well as daily,
excretion of mercury in the urine of exposed per sons. One study28 indicated a rhythmic pattern to the excretion of mercury. This pattern seems more related to some physiologic mechanism rather than to an immediate environmental ex posure. A considerable time lag exists between increased exposure and a corresponding in crease in urinary mercury excretion, as noted by Molyneux28 in 1966 and in a letter of March 1971. During the planning phase of this study, a de cision was made to use urinary mercury values to identify groups of individuals in dental offices who exhibit urinary mercury levels in excess of those for the average nonoccupationally ex posed population. F o r the most part, all urine voidings during a normal workday were collect ed from personnel in the dental offices surveyed. A few spot-voidings were collected in instances in which it was impractical to obtain a full day’s voidings. All participants in this study were involved with some aspect of dental practice and were subject to some degree of exposure to mercury vapor. The participants were classified into groups according to functions performed with respect to mercury handling in the dental prac tice. Urinary mercury concentrations were separ ated into two classes. Those concentrations equal to or less than 30/ug/liter were placed in the normal category. Those exceeding the upper limit of this arbitrarily chosen value were con sidered abnormal in that they were indicative of increased mercury absorption over previously established “ normal” limits for nonoccupation ally exposed individuals. The literature on uri nary mercury excretion of nonoccupationally exposed individuals indicates an upper limit of about 10/xg/liter of urine.3,29 The arbitrarily chosen upper limit, used here for the average “ normal” category, is higher than the generally accepted value because personnel employed in dental offices cannot be considered nonoccupa tionally exposed, regardless of their job func tion. M ercury urine values are not considered abso lute indicators of exposure to mercury vapor. However, they can be useful on a group basis for the evaluation of human exposure to mer cury, provided that the individuals in the groups surveyed exhibit normal kidney function. ■ Am bient air concentrations: During this study, air in the dental offices was analyzed dir ectly as well as indirectly for mercury vapor. Schneider: MERCURY CONTAMINATION ■ 1093
T a b le 1 ■ S u r fa c e c o n ta m in a tio n b y m e rc u ry . A v e ra g e d s u rfa c e c o n ta m in a tio n ( f i g H g/100 c m 2 s u rfa c e )
D e nta l o ffic e
1
24 557 2 171 13 93 6 247 41 37 2 8 2 21 43 8
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
T ab le 2 ■ Urine mercury levels for functional groups in the dental offices. U rin e m e rc u ry le vels ( m 3 H g /lite r) G rou p D e n tis ts H a n d lin g m e rc u ry N o t h a n d lin g m e rc u ry D e nta l a s s is ta n ts H a n d lin g m e rc u ry N o t h a n d lin g m e rc u ry O th e r p e rs o n n e l N o n o p e ra to ry
No.
R a nge
> 3 0 (% )
M ean
75 25
2-110 2-60
26.3 20.0
24.7 19.3
20.1 17.4
74 33
2-300 2-92
40.6 12.1
37.6 16.8
44.5 19.6
95
2-224
11.1
18.3
29.3
±
SE
T able 3 ■ Comparison of urinary mercury levels of dental office personnel. H Q Hg/liter
G ro u p
<30
>30
132
56
188 j I
N o n o p e ra to ry
85
10
95 !
D e n tis ts H a n d lin g m e rcu ry
56
20
I 761►
20
5
44
30
N o t h a n d lin g m e rc u ry
29
4
D e n tis ts h a n d lin g m e rc u ry
56
20
76 iI
D e n ta l a s s is ta n ts h a n d lin g m e rcu ry
44
30
74!
A ll d e n tis ts
76
25
D e nta l a s s is ta n ts h a n d lin g m e rc u ry
44
30
O p e ra to ry
T o ta l
P
< 0 .0 1
N o t h a n d lin g m e rc u ry D e n ta l a s s is ta n ts H a n d lin g m e ru ry
N ot s ig n ific a n t
25 !
74 I» J
< 0 .0 1
33
-
101
N o t s ig n ific a n t
}i N o t s ig n ific a n t
74 !
Since most offices surveyed had ambient air con centrations less than the currently accepted threshold limit value30 (TLV) of 0.05 mg Hg/m3 of air, it was decided that ambient air mercury concentrations would not be suitable for estab lishing levels of mercury contamination in these offices. (This value of 0.05 mg Hg/m3 of air is an eight-hour day, 40-hour workweek concentration that most persons can be exposed to without ad verse effect.) The average ambient air concen tration obtained for 19 dental offices was 0.03 mg Hg/m3, with a range of 0.00 to 0.16 mg Hg/m3. ■ Contamination o f surfaces: Mercury drop lets oxidize rapidly in air, with the result that the metal is coated with oxide or sulfide. This oxi1094 ■ JADA, Vol. 89, November 1974
dation coating, as well as grease and dust, sup presses the vaporization of the underlying metal if the droplet is undisturbed. After various sur faces such as bench tops, carpets, flooring tile, and operatory units were rubbed, a mercury va por meter was used to detect mercury vapor a few inches above the disturbed surface. The rub bing action not only broke the coating, expos ing bare metal, but also raised the surface tem perature slightly because of frictional heating. The latter served to increase the vaporization of the mercury contaminating the surface. Wipe samples were taken from surfaces in the dental offices to obtain an indication of the degree of mercury contamination.
Results
Table 1 contains a list of those offices surveyed where wipe samples were obtained. About six wipes were obtained for each office, and the in dividual samples were averaged for each office surveyed. Urine mercury levels for the functional groups surveyed are summarized in Table 2. A rank or dering of functional groups according to risk of absorbing mercury reveals that dental assistants who handle amalgam have the greatest risk and that nonoperatory personnel have the least risk; this is based on the percentage of individuals in each functional group who exhibited urinary mercury levels greater than 30/ug/liter. Table 3 shows a comparison of urinary mer cury levels by functional groups. A significant difference in urinary mercury levels was found between dental operatory personnel and dental nonoperatory personnel, and between dental assistants who handle mercury amalgams and dental assistants who do not. N o significant dif ferences were found for dentists who handle mercury compared with dentists who do not, or between dentists who handle mercury and dental assistants who also handle amalgams.
Discussion
Every dental office surveyed during this study was contaminated with mercury. The extent of contamination was governed by many variables including the number of amalgam restorations prepared, the number of operations in the office, the method of mixing the amalgam, the care in the handling of mercury and dental amalgam, the
type of floor covering in the dental office, house keeping procedures, and the type of ventilation. Because finely divided mercury can form a covering of either oxide, sulfide, dust, or grease as a result of contact with air, the presence of such longstanding contamination may not pose a health hazard to personnel in the dental office unless this residual contamination is disturbed mechanically or by heating. If ambient temper atures are relatively high, the vapor pressure of the mercury can increase to such an extent that vapor can penetrate and escape through the coating of the mercury droplet. In this situation hazardous concentrations of mercury may result in closed spaces with insufficient ventilation to dilute the mercury vapor to safe levels. Thus, the lack of association between elevated urine mer cury levels and surface contamination levels in the offices is not surprising. Of all the offices surveyed, only one showed an association between urine mercury levels of personnel and surface contamination. This of fice had a heavy patient load composed of union members associated with a prepaid dental plan. The building housing the various operatories was completely air-conditioned; about 80% of the air was recirculated. Puddles of mercury were noted on a ceramic tile floor, and dental office personnel were observed stepping on the liquid mercury and tracking it throughout the building. Shoe soles of a few dental assistants were checked with the Mercury Sniffer for evi dence of contamination. In all instances the me ter went off scale. This office was so badly con taminated that the air in the waiting room mea sured 0.07 mg Hg/m3, which is in excess of the current TLV of 0.05 mg Hg/m3. The major cause of the mercury contamination in this office was the misuse of amalgamators. The amalgamators used in this office were designed for the prepar ation of only single batches of amalgam, that is, one measure of alloy plus one measure of mer cury. However, the dentists instructed the den tal assistants to prepare triple-size batches of amalgam. Since this amalgamator is unable to accommodate such large amounts of alloy and mercury, excess mercury was discharged from the machine during the initial amalgamation pro cedure before the final mulling procedure. This excess mercury spilled on to bench tops and rolled off the bench to the tile floor. Dental office personnel whose functions in volve the full-time use of the dental operatory where amalgam is used are more likely to be ex posed to mercury vapor than those whose pri
mary duties are conducted outside of the dental operatory. Therefore, elevated mercury levels in urine of dental operatory personnel, compared with the more normal mercury levels in urine of nonoperatory personnel, are to be expected. During amalgam preparation, the mercury and alloy mixture is subjected to severe vibratory motion to permit maximum contact between the liquid mercury and metal alloy. This intense vi bration occurs while the amalgam ingredients are enclosed in a small plastic capsule. Frictional heating occurs during this operation and is large ly responsible for increasing the vapor pressure of the mercury above that associated with nor mal ambient temperatures extant in the dental operatory. Thus, when the dental assistant opens the triturating capsule and drops the plastic amalgam onto a squeeze cloth or into a glass cup, before handing it to the dentist, she encounters a relatively high, but short (in the order of sec onds) exposure to mercury vapor. This phenomenon was noted by sampling of the breathing zone of the dental assistant during all phases of amalgam preparation, including the act of transferring amalgam from the amalgam ator area to the dental unit. Air sampling with the Mercury Sniffer during amalgam prepara tion and handling revealed values ranging from about twice the current TLV for mercury vapor in the workroom (0.05 mg Hg/m3) to readings that were so high that they went off the scale of the meter. The highest readings on the Mercury Sniffer were noted during the time the triturating capsule was opened and just before the transfer of the amalgam to the dental unit. Moreover, dental assistants receive greater mercury vapor exposures because they work in the immediate vicinity of the amalgamator, whereas dentists usually do not. Generally, the greatest contam ination of mercury occurs within a radius of 1 foot of the bench area where the amalgamation of the alloy and mercury is conducted. Mercury vapor is released as a result of spills of mercury metal and improper storage of waste amalgam and recovered mercury metal. The constant activity in this area of the dental operatory dis turbs mercury globules and very finely divided mercury on various surfaces; this exposes fresh metal surfaces that give off vapor. Many dentists prefer a more splashy amalgam, which necessi tates the use of an excess of mercury during amal gamation, rather than the recommended 1:1 ratio of alloy and mercury. Much of this excess mer cury must be expressed by hand squeezing in a squeeze cloth. The excess mercury usually is Schneider: MERCURY CONTAMINATION ■ 1095
deposited in some type of container such as aja r, flat dish or glass, paper, or plastic cup. Most storage containers for excess mercury and waste amalgam were not secured with lids. During the squeezing operation, the dental assistant receives an additional vapor exposure. The squeezing is normally done in the immediate vicinity of the amalgamator. With the exception of the report of the fatal ity ,10 the literature on mercurialism lacks these data as they pertain to individuals in dentistry. O f all the personnel involved in the handling of dental amalgams in the dental operatory, the risk of absorption of the metal by the body is greatest for the dental assistant. The lack of data relating to mercurialism among dental assistants may be explained in that many dental assistants are women in their early 20s who tend to leave the profession after a few years as a result of mar riage, and because the vapor exposures are inter mittent and are of relatively short duration. N o data are available regarding the effects of these intermittent exposures to mercury vapor on human health. However, since there may be danger to the fetus, pregnant women should not be exposed to mercury vapor for any prolonged period.
Recommendations These precautions are recommended for the safe handling of mercury by dental personnel. ■ Education: All personnel involved in the handling of mercury and amalgam should be made aware of the toxic nature of these sub stances and should be instructed in safe handling practices. ■ Storage: All mercury should be kept in tight ly sealed containers of glass, plastic, or glazed ceramic when not being used. W aste mercury and amalgam should be stored in similar contain ers that have tight-fitting covers. If a tightly cov ered container is not available for stored waste amalgam, this material should be covered with about 14 inch of water. All mercury and waste amalgam should be stored in a cool place. All stored mercury and waste amalgam should be labeled. The label should indicate the contents of the container, as well as its hazardous nature. ■ Disposal: W aste
mercury
1096 ■ JADA, Vol. 89, November 1974
and
amalgam
should be accumulated in tightly sealed contain ers. These materials should not be flushed down the sink, dropped on the floor, thrown into waste baskets, or allowed to accumulate in open trays, beakers, or on bench tops. Operatory units should be equipped with suit able traps to trap waste amalgam expectorated into sinks by patients. The dentist should use a rubber dam when filling a tooth with amalgam or when removing amalgam restorations. Waste amalgam accumulated on the rubber dam can be suctioned off by the dental assistant. All articles such as paper tissues and squeeze cloths that have become contaminated with amalgam should be placed in plastic bags that are kept sealed until ready for disposal. ■ Operatory: All amalgam preparation should be conducted in a ventilated enclosure. The en closure should be under slight negative pressure with respect to the ambient air, and air discharged from the enclosure should be passed through iodized activated charcoal to remove mercury vapor. The enclosure should be constructed of smooth metal or plastic, with no blind comers where mercury can collect. This unit should be equipped with a removable tray to catch any spilled mercury or amalgam. The unit should be cleaned each day of all free mercury and amal gam. The interior surfaces and removable tray should be washed with a water solution contain ing a mercury decontaminant. Mercury filters should be checked with a mercury meter at least once each six months to ensure that no break through of mercury vapor has occurred. During removal of amalgam restorations, den tists should wear a tight-fitting oral-nasal dust mask capable of filtering out particulates as small as 0.5/im in diameter to prevent inhalation of finely divided amalgam particulates. The use of a drill equipped with a cooling water spray pre vents frictional heating of the amalgam and sub sequent release of mercury vapor during remov al of the restoration from the patient’s mouth. All surfaces where mercury and amalgam are used should be free of cracks or comers where mercury can be trapped. Stainless steel or For mica with concave comers and edges are suitable for surface coverings. Floors should be onepiece, with a vinyl coating; comers and abutting walls should be covered. Carpets should not be used in operatories because it is almost impos sible to decontaminate them after a mercury spill. Furthermore, vacuuming of mercury-con taminated carpets can create a temporary severe
inhalation hazard because the mercury, passing through the heated exhaust of the vacuum clean er, is readily vaporized. Operatory chairs should be covered with a smooth vinyl upholstery material. Cloth cover ings should be avoided. Excess amalgam should not be permitted to fall from the patient’s clothing because it can contaminate operatory floors, dental chairs, cor ridors, and waiting-room chairs and floors. Waste amalgam and mercury can be accum ulated for eventual disposal to a metal reclaim er. If the latter procedure is not feasible, the mer cury should be placed in a container that can be sealed. The waste container should be labeled as to the nature of its contents, and then disposed of through a waste chemical collection service for ultimate burial in a legal dump designated for hazardous chemicals. M ercury contamination can be eliminated during handling of the metal if encapsulated pre proportioned materials are used when the amal gam is being prepared. M anufacturers’ recommendations with re spect to maximum quantities of materials to be used in their amalgamators should be followed. Mulling cups that contain old amalgam should never be héated to remove the amalgam unless this procedure is carried out in a chemical fume hood equipped with mechanical exhaust ventila tion to carry the fumes outside of the building. All fumes vented to the outside should be passed through an iodized charcoal filter. ■ Spills: Spilled mercury should be cleaned up immediately by use of a suction device that can trap the collected metal. Other mechanical mer cury-gathering devices may be obtained from a chemical supply house. A vacuum cleaner should never be used to clean up mercury spills unless it is specially designed for that purpose. It must have a mercury filter on the discharge. If mercury has spilled into places that are in accessible to a clean-up device, vaporization of the mercury can be suppressed by covering of the metal with a thin coating of flowers of sulfur. A commercial, water-soluble mercury decon taminant can be used to wash down table tops, benches, and equipment that are contaminated. Dental instruments that are contaminated with mercury in the course of tooth restoration can be soaked in this decontaminant solution for an hour before the washing and sterilizing of these instruments. If the commercial decontaminant is not available, technical grade photographic
sodium thiosulfate can be used in a 10% by weight water solution. The occupational health section of the state or local health department should be consulted for detailed information on mercury decontaminants. These materials do not destroy mercury. They merely change its character to prevent the vapor from contaminating the air. ■ Personal hygiene: Mercury will adhere to the skin. This poses the danger of inhalation of the vapor because the body temperature is suffi ciently high to cause vaporization of some of the metal adhering to the skin. M oreover, those who smoke can transfer mercury from the skin to cigarettes, with subsequent vaporization and in halation of the metal. Many individuals bring their hands near their nose from time to time and thus any metal vaporizing from the hands can be inhaled. Some absorption of mercury can take place through the skin. The rate of absorption of metallic mercury through unbroken skin is low compared with that for organomercury com pounds. However, specific data are not avail able for this phenomenon. Personnel handling mercury or amalgam should scrub their hands thoroughly, using soap or detergent, as soon as it is practical after con tact with the metal or its alloy. Smoking should be prohibited in areas where mercury is being handled. Individuals who handle amalgam should not smoke until they have thoroughly scrubbed their hands. ■ Medical: Periodic urine testing should be done of individuals handling mercury and amal gam to determine the adequacy of preventive measures for safeguarding employees’ health. The testing intervals can be established on the basis of the degree of exposure. Advice on med ical monitoring of personnel handling mercury and amalgam, as well as guidelines about the ex posure of pregnant employees to mercury vapor, can be obtained from the occupational health section of the state or local health departments or from a recently published report.31 This doc ument recommends comprehensive medical examinations for all persons subject to exposure to inorganic mercury before employee place ment and annually thereafter. Medical records are available to the medical representatives of the employer, of the Secretary of Labor, to the Secretary of Health, Education and Welfare, and of the employee at his request. Such records should be retained by the employer for at least five years after the employee’s last occupational Schneider: MERCURY CONTAMINATION ■ 1097
exposure to inorganic mercury. Other recom mendations relate to the posting of warning signs and the use of personal protective equipment, work clothing, and respiratory protection for those exposed to mercury vapor.
Conclusions Environmental contamination of dental offices by mercury does not seem to pose an acute health hazard for personnel. The risk of exposure to mercury vapor, measured by urinary mercury concentrations, is greater for dental office oper atory personnel than for nonoperatory person nel. In large offices in which general dentistry is practiced, no significant difference in mercury absorption exists between dentists who handle mercury and dentists who do not. Dental assis tants who handle mercury and amalgam have a greater risk of mercury vapor absorption than dental assistants whose duties do not include the handling of amalgam. N o significant difference in mercury absorption is found between dentists and dental assistants who handle mercury. How ever, compared with all dental office functional groups, dental assistants who handle mercury have the greatest risk of absorption of mercury vapor. The risk of absorption of mercury vapor is least for nonoperatory office personnel. A detailed health study of dental personnel is warranted for those individuals exhibiting higher than normal urinary mercury concentrations for occupationally exposed persons.
The views expressed in this paper are those of the author and are not necessarily the views of the California State Department of Health. This project was sponsored jointly by the department of health science, California State University, Northridge, and by the Occu pational Health Section, California State Department of Health. The author thanks Hector P. Blejer, MD, head, medical unit, Occupational Health Section, California State Department of Health, and Mrs. Ester Baginsky, statistician with the California State Department of Health. Mr. Schneider is an industrial hygiene engineer for the City of Los Angeles, personnel department, medical services division, 1401 W Sixth St, Los Angeles, 90017. ‘ Mercury Sniffer model MV-2, J/W Bacharach Instrument Co., Pittsburgh, 15238. 1. Mineral facts and problems, 1970 edition. Bureau of Mines Bulletin 650. US Department of the Interior, 1970. 2. Bidstrup, P. Toxicity of mercury and its compounds. Elsevier monograph 34. New York, Elsevier Publishing Co., 1964, p 1. 1098 ■ JADA, Vol. 89, November 1974
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