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FILE NAME Household Contact HC DATE 1992 DOC HC017 DOCUMENT DESCRIPTION Journal Article - Changing Risk Groups for Malignant Mesothelioma | 2704 34.70 34.70 Changing Risk Groups for Malignant Mesothelioma Michael Huncharek MD MPH Although malignant mesothelioma is a relatively rare tumor its incidence is rising Much of the increase is attributable to widespread exposure to asbestos in past decades in asbestos industries In recent years it has be- Newly Recognized Occupational Risk Groups Brake Mechanics come increasingly clear that mesothelioma risk is no longer confined to workers in the asbestos industry This article reports a variety of recently identified risk groups and highlights the need for increased surveil- lance of these groups to document the occurrence of as- related malignancy and the institution of mea- sures for disease prevention Cancer 1992 2704 2711 Friction products once constituted one of the major markets in the United States for asbestos and it is esti- mated that approximately 900,000 workers in the Unites States are or at one time were employed as automo- bile mechanics and garage workers It is uncertain what proportion of these workers regularly service brake lin- a The incidence of mesothelioma previously relatively rare tumor has experienced a recognized increase during the last 2 decades Data from the National Cancer Institute's SEER Surveillance Epidemiology and End Results program of population regis- tries show that the annual adjusted incidence for the years 1973 to 1978 versus 1979 to 1980 increased by almost 10 per year in males Information from Brit- ish Columbia indicates that for the year 1982 the inci- dence in that Canadian province was nearly six times higher than it was in period 1969 to 1975. ings and are therefore at risk for asbestos mesothelioma In addition it is also unclear what the health risks are to coworkers not directly involved in brake repair A major problem with epidemiologic studies of this work force is the difficulty in tracing a large nonunion- ized group of workers Estimation of disease risk has been impeded by lack of quantitative data on exposure levels among individuals with term exposure However several studies examining this occupational setting show the existence of a potentially serious health hazard Clearly mesothelioma is becoming an increasingly common In 1976 investigators at the Mount Sinai School of clinical entity Medicine studied asbestos exposure among brake repair The etiologic link between asbestos and mesotheli- workers in New York city. Both clinical examinations oma is well documented The upsurge in incidence of and fiber counts produced by various operations in e this tumor closely parallels the widespread commercial- brake maintenance workers were analyzed Samples lization of asbestos which is responsible for the major- taken at a distance of 3 to 5 feet from brake drums nn ity of cases Unfortunately disease risk is no longer during periods of blowing dust showed fiber concen- ae confined to workers in the asbestos industry This arti- trations of 6.6 to 29.4 fibers with a mean of 15.9 e n cle highlights recent trends in incidence by discussing fibers In addition ten samples of brake drum dust some newly recognized risk groups With increased un- were analyzed by contrast optical microscopic derstanding of changes in disease risk more effective examination and transmission electron microscopic ex- disease prevention strategies can be developed amination to determine the percentage of short fibrils From the Massachusetts General Hospital Harvard Medical School and the Institute for Cancer Epidemiology Boston Massa- chusetts angstroms i.e. 250 to 500 angstroms ^ 750 to 3750 Eighty percent of all chrysotile fibers were in this category and almost 20 of the total mass of ten sam- ples was chrysotile determined by electron diffraction Address for reprints Michael Huncharek MD MPH Institute or Cancer Epidemiology Suite 2513 One Devonshire Place Boston MA 02109 Accepted for publication September , 1991 Throughout the examination by electron microscopy attention was given to the morphology of the fibers A majority of fibers showed little alteration in the typical chrysotile fiber Malis 1 al and fiber linin curre ~-lart fiber brakt 10 to brak tile in fiber F posu an ar work ing e a 55 tire < repoi repla tos e ra chrys by el fiber point sotilt denc used from studi tos d mort 1 mech a 47 know repai 41 v differ Duri and ... trans asbes and braki work work year 150CO quen Malignant Mesothelioma Risk Huncharek 2705 - In an additional report from Mount Sinai Rohl et al analyzed residual dust recovered from brake linings and made direct measurements of the free asbestos fiber content of workroom air in areas in which brake lining maintenance and brake shoe installation oc- curred Airborne asbestos dust concentrations were sim- ilar to those cited by Lorimer et al.i.e. mean airborne fiber concentrations during compressed air blowing of brake drums ranged from 2.6 fibers at a distance of 10 to 20 feet to 16.0 fibers at 3 to 5 feet Samples of brake drum dust showed that the proportion of chrysotile in this material averaged % to % as both free fiber and as particulates in pulverized binder Regarding the health effects resulting from the ex- posures described Langer and McCaughey published an analysis of a case of mesothelioma in a brake repair worker the report included lung fiber studies includ- ing electron microscopic examination The subject was a year man who had worked in the used car tire and car repair business since the age of 19 years He reported routinely servicing automobiles including the replacement of brake linings No other source of asbes- tos exposure was found Analysis of lung tissue showed the presence of chrysotile fibers no amphibole was found confirmed by electron diffraction techniques Ten percent of the \ fibers found were longer than 10 microns The authors point out that Controversy over the potential of chrysotile to cause mesothelioma has continued despite evi- dence from asbestos textile fabricators thought to have used only chrysotile from workers making brake pads from chrysotile miners and millers and from animal studies They also state The risk of malignant asbes- tos disease among these workers seems to be low but mortality data have yet to be thoroughly evaluated The most recent report of mesothelioma in a brake mechanic reviews a pleural mesothelioma occurring in a year male automobile mechanic whose only known exposure to asbestos was from clutch and brake repair work during an year period From age 30 to 41 years the man worked as a brake mechanic in five different automobile service stations and dealerships During his employment he primarily performed brake and clutch repairs engine overhauls and automotive transmission and electrical system repairs He reported asbestos exposure from clutches fly wheels and brakes and said that he often used compressed air to blow out brake drums which he described as creating dusty working conditions Another case of mesothelioma in a brake repair worker was recently published In this report a 56- year male elevator mechanic was noted to have anisocoria on routine physical examination Subse- quently right upper quadrant pain and chest discom- fort developed Medical evaluation with computed tomography showeda right paracervical mass that on biopsy was found to be pleural mesothelioma The pa- tient reported working as an elevator mechanic for 30 years He reported asbestos exposure primarily from elevator brake linings that he routinely cut fitted and removed during elevator installation and maintenance Several recent studies from Scandinavia on this topic also deserve mention Hansen from the Institute of Community Medicine in Denmark completed a his- torical cohort study examining the mortality of car mechanics from ischemic heart disease and malignant neoplasms The study cohort was identified using re- cords of a nationwide census carried out in Denmark in November 1970. Comparison was made with another cohort of skilled male workers who were not exposed to asbestos or petrochemical substances Of 583 observed deaths one case of pleural mesothelioma was found Likewise Jarvholm and Brisman in a 1988 report used the Swedish Death Register and the census of 1960 to study the occurrence of associated tumors in car mechanics One hundred eighty deaths attributable to cancer were observed whereas 154 were expected Thirty were caused by lung cancer whereas only 23 were expected Again one death from pleural mesothelioma was found Analyses of mesothelioma mortality in the United States which rely on death certificate data have a major flaw 1.e. misclassification of pleural tumors as deaths caused by lung cancer or other tumors of the thorax Because the diagnosis of mesothelioma often can be difficult from a pathologic standpoint death certificate studies may re- sult in gross underestimation of incidence It is unclear how this problem may have influenced the results of the two reports detailed here It has been estimated that 20,000 deaths from as- related cancer will occur during the next 40 years among automobile maintenance workers in the United States With the many difficulties faced by epidemiologists studying this work force it is unclear how accurate this estimate will prove to be Clearly what is needed is better information on duration and intensity of exposure to respirable asbestos fibers in this occupational group Additional study is needed to accurately determine the incidence of mesothelioma among members of this work force Railroad Workers The 1960 report by Wagner et " of occupational me- sothelioma risk included reports of several workers exposed to asbestos through employment in the railroad 2706 CANCER June 1 1992 Volume 69 No. 11 industry Recently Schenker et al.12 published the results of a large scale study in this occupational group In a control analysis the authors examined mesothe- lioma deaths among current and retired US railroad em- ployees Of 15,059 deaths reported by the Railroad Re- tirement Board death certificates were obtained for 87 which identified 20 deaths caused by mesothelioma Among workers who had worked in jobs with potential asbestos exposure a significant odds ratio of 7.2 was found based on a 10 matched analysis with railroad workers dying of nonmalignant nonaccident causes Comparison of groups used in skilled trades e.g. locomotive repair with regular exposure to asbestos yielded a statistically significant odds ratio of 21.4 A nonsignificant odds ratio of 2.3 was found for workers in occupations with intermittent exposure Although this study is important in that it reports the existence of a mesothelioma risk among a previ- ously unexamined occupational group these data must be viewed as representing an underestimate of the occurrence of this disease This is because of the reliance on death certificate data which in the case of mesotheli- oma often results in misclassification Additional support for a high risk of mesothelioma among railroad employees is provided by Mancuso.,, This study explored the occurrence of mesothelioma deaths among machinists in the railroad and other in- dustries Among a cohort of 197 machinists employed at the same railroad before 1935 and observed until 1982 causes of death were identified for 132. Of 29 deaths attributable to cancer 9 were caused by mesothe lioma The author comments that based on other data available in the literature because of the small size of the cohort it is remarkable to have detected such a large number of mesotheliomas In addition in 1987 Huncharek and Muscat re- viewed a series of confirmed cases of pleural mesothelioma with documented occupational exposure to asbestos Of 37 cases reviewed 4 subjects had been railroad employees In two instances specific job titles were unavailable the other two had been a brake repair worker and a machinist These data provide additional support for the existence of a mesothelioma hazard in this occupational setting particularly in light of the death being documented pathologically rather than relying on death certificate information Construction Trades Use of containing materials ACM is particularly widespread in building construction including commercial and noncommercial structures The US En- vironmental Protection Agency EPA estimates that ap- proximately 733,000 buildings in the United States contain ACM.15 Of particular concern is that thousands of these structures are public and private school buildings Obviously with such widespread use of ACM in multitude of capacities in buildings across the country employees in the various construction trades constitute another mesothelioma risk group Estimates suggest that as many as 7,500,000 con- struction workers have been exposed to elevated levels of respirable asbestos in the past These exposures are the result of such activities as installation of metal duct work ventilation systems and manual removal of as- bestos fireproofing Bystander exposure also is thought to play a role because sheet metal workers often work in contaminated areas secondary to other related construction work e.g. boiler removal Zoloth and Michaels analyzed 385 deaths that oc- curred between January 1976 and April 1983 among members of a sheet metal workers union in the New York metropolitan area A proportional mortality analysis showed elevated proportional mortality ratios PMR for lung colon and rectal cancer and an increased incidence of mesothelioma In 1987 this study was extended to include deaths occurring before Oc- tober 1986 resulting in the increase of the study population by 331 subjects Statistically significant elevated PMRs were observed for all malignant neoplasms PMR = 148 and for cancers of the stomach PMR = 259 liver PMR260 PMR260 PMR260 and lung PMR = 186 Six deaths attributable to mesothelioma were documented on death certificates The authors conclude that the 1.8 of the deaths in this study attributable to mesothelioma 6 of 331 demonstrates that this disease continues to be an important cause of mortality in this population Recent data from Sweden using the Swedish Environment Registry also documents in- creased mesothelioma risk among workers in the con- struction industry Malker et 19.20 examined cancer incidence data from that National Swedish Cancer Re- gistry for 1961 and 1979. Thirty confirmed cases of peritoneal mesothelioma were found 13 cases among employees in the construction industry three of the em- ployees were insulators In addition textile workers painters and pulp and paper industry workers showed a nonsignificant elevated risk of peritoneal mesothe lioma Also suggesting the existence of a mesothelioma risk in this occupational group is a recent report pleural mesothelioma in an electrician who did not work in a shipyard This patient worked as an electri cian from 1959 until retirement in 1974. He reported working in buildings in which acoustic insulation was being sprayed In 1971 he became employed as main- in tenance electrician at an airport and reported being contact with old asbestos insulation and fireprooting yar car rec -- Ina Kn exp en ref par bui tic and cal for Su pre cia of 19 tro tal ing Th US tain tie pu bo as of tra USt EP ing mc Sa 510 23 Kit ast MESES MESES MESES MESES E Malignant Mesothelioma Risk Huncharek 2707 material He stated that insulators and other coworkers often would remove asbestos insulation in his presence Although electricians who do not work in ship- yards are known to have an increased risk of lung cancer the risk of mesothelioma in these workers has received little attention Increased surveillance of this occupational group may be justified. Indoor Asbestos Exposure Knowledge of the health risks associated with asbestos exposure in the work place has led to concerns that environmental i.e. nonoccupational exposure could represent a serious health hazard These concerns are particularly acute with regard to exposure to ACM in buildings Of all indoor pollutants asbestos represents a particularly difficult problem from a health risk assessment and risk management perspective This is largely be- cause of its widespread use as fireproofing in buildings for acoustic control and as pipe and boiler insulation Surveys by the US EPA estimate that friable ACM is present in approximately 733,000 public and commer- cial buildings in the United States representing 20 of 3.6 million buildings examined Five percent 192,000 of buildings were found to have sprayed or troweled asbestos surfacing material representing a to- tal of 1.2 billion square feet In addition 563,000 build- ings contain asbestos in the form of thermal insulation The EPA also conducted a national telephone survey of US schools and found that approximately 35,000 contain friable ACM representing 35 of all school facili- ties Unfortunately little information is available in the published literature evaluating ambient levels of airborne asbestos fibers in buildings In addition data that assess exposure risks given a building's function type of ACM present and condition of these materials are not available Few studies exist that document airborne concen- tration of asbestos fibers in buildings However some useful information is available such as that from 1987 EPA report of 43 containing federal build- ings.23 Air sampling was done at sites containing the most seriously damaged ACM within each building Samples were subsequently analyzed using transmis- sion electron microscopic examination In addition data were obtained from 35 buildings in the United Kingdom and Canada The EPA report indicates that Prevalent airborne asbestos levels . . . ranged from 0.0 to 0.2 fibers per cubic centimeter cc with an arithmetic mean of 0.006 microscopy cc.23 In a related transmission electron TEM measurement of airborne fibers in US Canadian and British school buildings fiber levels ranged from 0.0 to 0.1 cc with an arithmetic mean of 0.03 cc The EPA qualified these findings by stating These data . .provide .provide an indication of prevalent lev els which may be found in schools and in nonschool buildings However they are insufficient to generalizations about the total support or the relative airborne population of buildings concentrations in schools as buildings contrasted with public and commercial In addition the EPA presents the caveat that main- tenance and cleaning activities in such structures can result in elevated airborne asbestos levels a fact not reflected in air sampling data Data such as that discussed have led some to gest that asbestos sug- exposure in buildings poses an ex- a tremely low cancer risk Unfortunately a major limita- tion of using short air sampling as basis for risk assessment is that it does not necessarily reflect longterm airborne fiber levels Sampling generally takes place under nonroutine conditions i.e. under cir- cumstances of curtailed activity Thus the data do not reflect contamination levels that may occur with rou- tine building use For instance in a review of several hundred cases of pleural mesothelioma conducted by the author in 1986 to 1987,14 two cases of pleural mesothelioma in school teachers were found Both of the teachers were exposed to friable ceiling and pipe insulation during the course of many years Analysis by an independent labo- ratory of the insulation material in both environments showed the presence of predominantly chrysotile as- bestos with in one case small amounts of amosite A study from the United Kingdom reports pleural mesothelioma in a female office worker the disease re- sulted from exposure to amosite in the building of the patient's employment The ceiling of this building was sprayed with amosite insulating material approximately 1 inch thick covered by a layer of paint This material was in disrepair for some time causing dust release into the indoor environment Analysis of the autopsy ceiling material showed 70 amosite asbestos This tient died of mesothelioma 9 months after it was dipaag-- nosed The cause of death was confirmed at In addition to building occupants another group that must be evaluated for possible health risks is main- tenance and repair workers Oliver et al.26 reported find- ing radiographic abnormalities and impaired pulmo- nary function associated with asbestos exposure among school custodians Little information concerning these groups is available although it is easy to envision how periodic intense exposure could occur among such workers Short air sampling cannot evaluate this type of potential hazard Projections of expected disease incidence among building occupants and building maintenance personnel may grossly underestimate such occurrences 2708 CANCER June 1 1992 Volume 69 No. 11 Other Risk Groups As mentioned mesothelioma risk was once thought to be limited to industry workers During the last 2 decades it has become increasingly clear that this risk is quite diverse A recent study by Muscat and Wynder highlights this point In a hospital control study an attempt was made to determine the role cigarette smoking plays in the etiology of malignant mesothelioma and to quantify the risk of asbestos exposure in this malignancy One hundred five male and 19 female patients with histologically confirmed malignant mesothelioma and 265 matched control subjects without tobacco conditions were in- cluded in the study Table I shows the occupations of 68 male patients with mesothelioma and 1 female patient with mesothelioma with probable or likely asbestos exposure As shown 38 patients were employed as construction maintenance workers including such oc- cupations as carpenter painter electrician welder and sheet metal worker Table 2 lists patients with mesothelioma from the same study who were not employed in asbestos industries Of particular interest were the cases of mesothelioma in a potter who reported using a kiln with asbestos insulation and a teacher whose exposure was from friable asbestos insulation in a classroom during a year period Both females listed in Table 2 had histo- ries of domestic asbestos exposure Table 3 lists the occupations of 24 male and 16 female patients with mesothelioma and without known _ or reported asbestos exposure It is unclear how many of these individuals were exposed to respirable fibers because exposure in nonasbestos industry set- tings often goes unnoticed For this reason it is unknown what proportion of all mesothelioma cases are nonasbestos induced In the discussion section of their article Muscat and Table 1. Occupations of 68 Men With Mesothelioma And 1 Woman With Probable or Likely Asbestos Exposure Occupation Shipyard workers n = 18 Worker Electrical engineer No. of yr of asbestos exposure 2 20 Wynder comment on a report by Zielhuis et 28 who reported a female patient with mesothelioma the woman had worked as an ironer in a laundry where deteriorating asbestos cloths were used that contaminated the surrounding environment Another patient was found to have worked for 5 years as a stagehand This patient reported often being covered with painted asbestos flakes that originated from stage scenery which had been covered with asbestos as a fire retar- Maintenance construction 39.16 39.16 dant Mechanic 2 Welder 2 Metal shipyard worker 23 Construction maintenance workers n = 38 The medical literature is filled with reports of other patients with mesothelioma attributable to unusual as- bestos exposurFoer instance Otte et al reported three family members who died of mesothelioma sec- Carpenter construction worker Construction worker Construction manager Electrician Electrical engineering technician Gasket winder 20 25 36 14 38 3 60 38 47 35 36 33 1 54 38 2 30 Insulator Maintenance supervisor worker Painter 32. 26 38 40 13 31 Plumber pipe layer Plumber pipe sheet metal worker Sheet metal worker Other metal worker Weider Other construction worker Other construction / painter 24 44 11 2 33 41,30 41,30 3 31 13 15 19,5 40 7 ~ Data are summaries of patients in Muscat J Wynder EL Cigarette smoking asbestos exposure and malignant mesothelioma Cancer Res 1991 2263 2267 Used with permission ondary to exposure from an asbestos cement product manufactured by family members in their home The family manufactured a material called Rollfix which was used to fix screws in drilled holes Amosite gyp- sum and sand were the major components of the product the material was produced in the basement of the family home for 17 years As the author reports The basement had no ventilation and cleaning was undertaken by the mother No protective equipment was used . . and no special work clothes were used - After production stopped the family stayed in the same residence and no attempts were made to remove asbestos from the house The father mother and son all died of malignant mesothelioma they were three of the six family members involved in the manufacturing pro- cess al.30 Another report was published by Driscoll et in 1988. These investigators reported five patients with malignant mesothelioma in a Native American pueblo of 2000 persons In such a small population far fewer cases were expected to occur during a month period Mali Tabl With With Occu Men Pol Tez Pot a a Malignant Mesothelioma Risk Huncharek 2709 Table 2. Nonasbestos Occupations of 14 Men With Mesothelioma and 2 Women With Mesothelioma With Reported Asbestos Exposure No. of yr of No. of asbestos Occupation patients exposure Men Police officer Teacher Potter Navy ensign Environmental consultant Navy machinist Navy service engineer Stationary engineer Machinist Insurance salesperson Navy seaman Plant manager Supervisor oil company 2 10 1 2t t 12 1 1 1 1 6 1 40 1 36 I 31 1 2 1 3 1 2 I 26 Women Bank teller Executive director I 10 1 5 Reported domestic exposure to asbestos Reported frable asbestos crumbling from classroom ceiling for 2 years and worked in an asbetsos plant for 2 months Used a kiln with asbestos insulation Reported asbetos exposure on a Navy ship Data are summanes of panents in Muscat J. Wynder EL Cigarette asbestos exposure and malignant mesothelioma Cancer Res 1991 2267 Used with permission smoking 31 2263- Chrysotile and Mesothelioma Any discussion of asbestos mesothelioma risk must consider the controversy regarding what role the asbestos fiber type plays in the etiology of mesothelioma This is particularly true because the chrysotile vari- ety of asbestos accounts for about 95 of the world's production of asbestos Early work by Stanton et al.32 demonstrated that fiber dimensions are an important determinant of bio- logic response and carcinogenic potential Recent modifications of those findings have led some such as Moss- of Table 3. Occupations 24 Men and 16 Women With Mesothelioma With no Asbestos Jobs or Reported Exposures Men Accountant Air traffic controller Air Force Bus ironworker Carpet installer Delivery driver recapper Director of merchandise Health officer Insurance sales n = 2 Inspector steel mill Jewelry manufacturer . Lumber business manager News editor Musician telephone coin collector Office administrator April 1984 to September 1985 Additional investigation showed that asbestos was routinely used by Native American craftsman in the production of silver jewelry i.e. Asbestos was used to insulate worktables against the intense heat of brazing torches and molten metal Silver slag periodically built up on these boards which were cleaned by clapping the boards together or rub- bing the working surfaces together This procedure lib- me erated respirable asbestos fibers In addition this population also experienced asbestos exposure through the use of asbestos as a whitening agent for buckskin leggings and moccasins that are used in ceremonial dances These leather items are scrubbed Owner of boat business boat engine repair farmer teacher Real estate sales Retail Stationary engineer Stock clerk driver Truck bakery owner Truck terminal manager Vice president fuel company Women Housewife secretary machine operator Artist Bookkeeper Data entry operator with cakes of asbestos which often is derived from pipe insulation Talcott et al.31 studied a cohort of cigarette filter workers all males involved in manufacturing asbescontaining filters from 1951 to 1957. The primary type of asbestos used was crocidolite Of the 33 men in the cohort 28 had died as of December 31 1988 versus the 8.3 expected deaths The authors state that the threefold increase in mortality was almost entirely attributable to associated diseases i.e. lung cancer malignant mesothelioma and asbestosis Five of the 28 deaths were attributable to mesothelioma Housewife n = 3 Housewife farm worker Housewife nurse Housewife secretary Salesperson n = 2 Secretary Secretary driver Showroom manager Teacher real estate agent of in Muscat ) Wynder EL Cigarette smoking Data are summaries patients Cancer Res 1991. 51 2263- asbestos exposure and malignant mesothelioma 2267 Husband is an auto mechanic ee 2710 CANCER June 1 1992 Volume 69 No. 11 man et al to advocate the amphibole hypothesis Mossman et al suggest that chrysotile is of doubtful carcinogenicity whereas the amphiboles are much more potent carcinogens and are therefore responsible for most asbestos mesotheliomas Unfortu- nately numerous factors complicate analyses of the pathogenic potential of different fiber types These factors include the scarcity for study of occupational cohorts exposed to a single fiber type during considerable time periods Most past exposures have been to mix- tures of fibers In addition accurate exposure data often are lacking or of questionable quality Nicholson points out that wide variations of risk exist among different industrial settings and even within a single industry suggesting that fiber type is not the parameter responsible for these differences Al- though errors in exposure estimates may partially account for such discrepancies it is likely that the altering of the asbestos fibers by industrial processing is responsible For instance processing chrysotile may change fiber dimensions from the majority being long curly fibers to narrow straighter ones that are known to be more carcinogenic Thus fiber type becomes a far less important parameter in estimating risk than actual fiber dimensions Also supporting the belief in chrysotile's carcinogenicity are data of the unit risk for mesothelioma The unit risk data for mesothelioma closely parallel those found for lung cancer suggesting that the same factors that affect unit risk for lung cancer also influence mesothelioma risk As Nicholson points out the risk of me- sothelioma per fiber exposure in three studies where it can be estimated directly from exposure and incidence data is identical to that for exposure to 98 chrysotile plus % crocidolite 60 chrysotile plus 40 amosite and 100 amosite respectively Moreover in most other studies where the mesothelioma risk cannot be estimated directly the ratio of the number of mesotheli- oma to excess lung cancers is the same for exposures to predominantly chrysotile to 100 amosite and to mix- tures of chrysotile amosite and crocidolite - . The ratio for 100 crocidolite is about twice as great Animal studies provide additional evidence that all types of asbestos can induce mesothelioma Inhalation studies reveal that chrysotile is a potent carcinogen Im- plantation of fibers into the pleural cavity of laboratory animals shows that the carcinogenicity of mineral fibers is directly related to their dimensions and not their chemical composition The debate over the carcinogenic potential of chry sotile also is complicated by the insistence of some in- vestigators that lung fiber studies of exposed workers demonstrates that the amphiboles are responsible for the majority of lung cancers and mesotheliomas This is based on the finding that chrysotile often is present in much smaller quantities in lung tissue than is amphiboles Unfortunately because little is known about the biologic properties of asbestos fibers in the human lung it is difficult to interpret these data It has been found that chrysotile is preferentially cleared from the lung within the first few weeks of exposure in contrast with amphiboles although the ultimate fate of the fibers is unknown i.e. chrysotile may be preferentially cleared to the pleura In summary available data support the idea that all fiber types are capable of inducing malignancy Although crocidolite may be more potent than chrysotile it is not unique in its tumorigenic ability With the wide- spread use of chrysotile in a multitude of products and appiications it would seem unwise to ignore the potential health risks of chrysotile Conclusion Few commercial products have been as extensively studied as asbestos and few commodities have gener- ated as much legal public health and public policy in- terest as this miracle mineral The use of asbestos fibers dates to ancient times but the health concerns related to its use are a recent phenomenon In 1960 Wagner et al provided the first clear evi- dence of a relationship between asbestos exposure in this case crocidolite and mesothelioma Since then hundreds of studies have documented the epidemiologic biologic and clinical characteristics of this disease Initially mesothelioma risk was confined primarily to miners millers and workers in asbestos ufacturing industries Subsequently occupational groups using asbestos products e.g. inculators pipe fit- ters and shipyard workers demonstrated a relatively high incidence of this tumor Today it is clear that the populations at risk for having this malignancy continue : occupational : to expand As pointed out in this article risk is no longer confined to industry workers } Likewise nonoccupational hazard exists such as house- | hold and occupant exposures i - Many questions regarding the biologic behavior of mineral fibers remain unanswered e.g. how does as- bestos cause cancer how are fibers transported in the human respiratory tract and what role does fiber type play in pathogenesis With the widespread use of as- bestos worldwide particularly now in the Third World the epidemic of mesothelioma may continue well into the 21st century.38 It is hoped that with a greater appre ciation of the scope of asbestos exposure occurring out side of the industrial setting more effective strategies for disease prevention can be developed Re 2 20 Malignant Mesothelioma Risk Huncharek 2711 References McDonald IC Health implications of environmental exposure to asbestos Environ Health Perspect 1985 319 328 Churg A Malignant mesothelioma in British Columbia in 1982 Cancer 1982 672-674 Lorimer WV Rohl AN Miller A Nicholson WJ Selikoff ] As- bestos exposure of brake repair workers in the United States Mt Star MedMed 1976 207-218 Rohl AN Langer AM Wolff MS Weisman I. 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Sebastien P Janson X Gaudichet A et al Asbestos retention in human respiratory tissues Comparative measurements in lung parenchyma and in parietal pleura In JC Wagner ed Biological Effects of Mineral Fibers vol 1. Lyon IARC Scientific Publica- tions 1980 38. Sentes R. The asbestos albatros Policy Options 1989 3 8