Document Zm5yEpYdJ4Lz22Be8gYdV8ML

V of Chrysotile Asbestos Mine and Mill Workers of Quebec J. Corbett McDonald. MD; Margaret R. Becklake, MD; Graham W. Gibbs, PhD; Alison D. McDonald, MD; Charles El Rossiter, MA, Montreal t IThe results of studies of respiratory n recent years, exposure to as nary function,and respiratory \ symptoms and function, roentgonograph- \ bestos dust has been widely recog symptoms.* The purpose of this report lc changes, and mortality In relation to nized as an important etiological fac is to bring together these and certain y dust exposure In the Quebec chrysotile tor in pulmonary fibrosis, cancer of subsidiary findings and to discuss Industry, which has employed some the lung, and malignant mesothelial their implications for control. 21,000 workers, are brought together and heir Implications for control examined. tumors. The precise nature of the cau !* Breathlessness on exercise, dlmlrv-. sal association has usually been ob Uhed Inspiratory capacity, parenchymal scured by exposure to more than one Outline of Study Records of the mining companies and pleural changes, and respiratory dis form of asbestos and to other mate were used to identify all persons <* use mortality were related to dust expo- rials in the manufacture or appli known to have worked in the industry aura and to each other. Respiratory can- cation of asbestos products. In the since its inception in 1878. Earlier ) oar was also related to dust exposure. mining and milling industry of Que records tended to be incomplete or The overall excess of deaths from res bec, where almost half the world's missing and certain sets had been de ! piratory cancers, Including five with ma supply of chrysotile is produced, expo stroyed, but altogether some 28,000 3 S 4( o lignant mesothelioma, was at most 50% ibovs expectation, baaed on age-apeclftc rates lor Quebec and the mining region. II safety standards are set, they ehould he baaed on epidemiological avldence. From these data lor the chryeotile produc sure to other types of asbestos or to materials used to make asbestos prod ucts has been very slight. To assess the effects of pure exposure to chryso tile, an epidemiological study in this employees, mostly men, were identi fied and information was assembled for this group. Of these, 6,400 were working on Nov 1, 1966, the registra tion date for the study. The informa ? ing Industry ol Quebec, a resonabie fig industry was started in 1966. Our tion collected for each person con ure, bated on a 1 % risk of acquiring clini aims were to relate dust exposure as sisted of date of birth and a full * cally significant dlaaaaa, would lie sessments to mortality, roentgeno- industrial history. The latest chest between 2 to 4 million particles par cubic graphic change, pulmonary function, roentgenogram of each employee, if hot, calculated for a working life ot 50 and respiratory symptoms. Results of available, was provided by the indus i ' T*ars. In the absence of epidemiological these studies have been presented in trial medical clinics and read by an ij fadings bated on liber counts and lack"of * satisfactory means of conversion, parfa1* counts should continue to be used separate papers on mortality, roentgenographic change,1 pulmo international group of six readers using the UICC/Cirieinnati (now *r control In this Industry. Table 1.--Number of Men In Dust Exposure Analyses Submitted for publication July 2, 1973; ac- l>ii July 30. From the Department of Epidemiology and 7 Health, McGill University, Montreal. Mr. Roaai- 'r a presently with the Medical Research Coun- C B *Umooomo*l> b'nit, Penarth. South Wales lepnnt requests to Department of Epidemiol- 'H ind Health. McGill University, 3775 Univer^7 St, "Montreal 112, Quebec, Canada (Dr. McDonald) Analysis Mortality study Roentgenographic study Lung function and respiratory symptoma Smokers Nonsmokers <10 2.BIO 1.537 Dust Exposure Level, mpcf-yr 10 3.329 1.522 100 1.124 1,133 200 1.007 912 400 837 718 800+ 585 707 Total 9.692 6,529 91 455 159 134 109 69 381 144 117 96 22 74 15 17 13 67 1.015 59 866 8 149 *'cb Environ Heaith/Vol 28, Feb 1974 Chrysotile Asbestos Workers/McDonald et al 61 h CT i U > f ST003U5U7 Table 2.--Roentgenographic Change Prevalence Percentages Roentgenographic Charge Irregular small opacities VO 2/1 Pleural thickening Grade 1 Grade 2 Irregular small opacities I/O 2/1 Pleural thickening Grade 1 Grade 2 Dust Exposure Level, mpcf-yr ,____________ ----------------------------------------------- , <10 to 100 200 400 800+ Thetford Mines Average 1.8 3.3 63 8.7 12.0 172 7.5 0 0.2 2.2 0.9 2.4 9.2 2.0 2.4 4 6 1.3 0 7 Asbestos 6.5 03 58 12 8.3 10.5 6 4 1 3 20 1 5.4 3.7 6.3 5.0 0 0.4 1.5 0.8 6.8 7.0 5.1 0.6 3.9 0.8 2.9 2.6 3.0 3.0 0 0 0 6 1.0 OS 4.7 5.8 3.2 1 4 0.7 0.7 Rates are standardized for age and years In industry. Males were 36 to 65 years of age at time of roentoenogram. Table 3 --Age-Corrected Death Rates Per 1,000 Men Born 1891-1920. Deaths to December 1969 Dust Exposure Level, mpcf-yr Causa All causes Respirator/ cancers' Abdominal cancers Pneumoconiosis <10 365 10.3 18 0 16 10 355 13 1 13 6 15 100 354 13.4 18 7 08 200 313 15.5 11 6 4.9 400 323 21.4 26.3 4.9 Includes malignant pleural mesothelioma. 800+ 395 32.1 28.7 23.6 Table 4.--Relative Risk of Death From Respiratory Cancer in Men by Dust Group, Estimated From Retrospective Analysis Dust Exposure Level, mpcf-yr < to 10 100 200 400 800-r Total No. of Casas 32 41 13 14 15 19 134 No. of Controls 166 188 39 61 40 22 538 Relative Risk 1.0 1.3 1.9 1.3 22 50 Table 5.--Age-Corrected Mortality Per 1,000 Men From Respiratory Cancer and Pneumoconiosis at Thetford Mines and Asbestos Respiratory cancer Thetford Mines Asbestos Pneumoconiosis Thetford Mines Asbestos Dust Exposure Level, mpcf-yr <10 10 100 200 400 800+ 9 12 18 15 24 31 11 13 8 18 13 43 4 1 1 7 6 24 0 0 0 5 3 24 I'C/ILO) classification. : A follow-up of ex-employees was set up and death certificates and autopsy reports were examined for those who had died. One section of this article deals with re sults in women; the rest is concerned with men only. Dust Exposure.-Dust exposure lev els were determined for each recorded job within.the industry, and for each year of employment. These levels were used to calculate a total dust ex posure index for each worker. Gibbs and Lachance" described the tech niques used and Gibbs" dealt further with certain qualitative aspects. The main index was based on particle counts from midget impinger sam ples, expressed as millions of particles per cubic foot (mpcf). A total dust ex posure index was calculated for each worker by multiplying the measure ment of dust exposure by the number of years of exposure at that level, arc was expressed as mpcf-years. The results which follow are pre- . sented in terms of range of total dus: ' exposure based on particle counts (Tabe 1). Table 1 also lists the number of men included in the main analyses considered in this report. ~ Since disease is probably caused by T asbestos fibers rather than dust par- * tides, it has been recommended tha: ' standards for occupational exposure i; be based on fiber counts obtained by J the membrane filter technique. ' !:' there were a reasonably consistent relationship between particle count: - by midget impinger and fiber counts by membrane filter, the results of our * surveys could also be presented anc 5 interpreted in terms of fiber counts. J A study was carried out by Gibbs anc 4 JLachance" in which 87 side-by-side I midget impinger and membrane lik ter measurements locations in each were made at of five mines nine and i mills of the Quebec chrysotile indus try. The correlation between the two types of measurement was too poor w l justify the use of any single corner- I sion factor. I Roentgenographic Changes.--The main roentgenographic indices of re sponse to asbestos exposure are ir regular small opacities and pleura j thickening. The prevalences of these and of all other roentgenograph 62 Arch Environ Health/Vol 28, Feb 1974 Chrysotile Asbestos Workers/McOonald el * follow-up :nd death orts were died. One with reoncerned (sure levrecorded for each se levels I dust exer. Gibbs :he tech:t further ects. The particle ger samparticles ! dust exfor each measurei number evel, and s. are preotal dust e counts i number analyses aused by iust par ked that exposure ained by que.M If insistent e counts r counts :s of our ted and counts, bbs and -by-side rane file at nine ;nes and '.e industhe two o poor to conver- -The two es of re* are ir! pleural of these, ographic naid et ^ changes were h:gner :n the Thetford tion would be low, should differ from the Quebec chrysotile industry as a Mines area than in the Asnestos area those for mill workers, in whom the whole has been at most 50% above ex of Quebec, although the two areas are fiber proportion would be higher. Al pectation1 and probably about 25%. only 80 km apart and the asbestos together. there were 506 workers who The primary method of analysis mined is geologically similar. The as had worked for ten or more years en used in our report on deaths up to sociation of the changes with dust ex tirely in mining or entirely in mill 1966' had two main weaknesses. The posure was also stronger at Thetford ing. Analysis of roentgenographic first was that length of exposure Mines than at Asbestos. changes in these men showed that might well have been related to Table 2 shows that at Thetford mill workers had a slightly higher length of survival and thus might Mines the prevalence of irregular prevalence of irregular small opac tend to obscure differences in mor small opacities of category 1 or more ities. but the differences between tality between exposure groups. We rose steadily with increasing dust ex Thetford Mines and Asbestos re looked for evidence of such an inter posure whereas for category 2 or mained. No other consistent differ action using the parametric approach more the relation to dust exposure ences could be detected between the of Berry,11 and though we failed to was less marked, except for the con mine and mill workers. find a significant effect, the possi siderable rise in the most exposed Mortality.-This study was limited bility remained. The second weakness workers. Even with allowance for dif to those who had worked for one lay in the fact that deaths accumu ferences in dust exposure, the preva month or more, and who were born lated over many years were used in a lence of category 1 or more was 1891 to 1920. This age group was se single calculation of mortality. It is markedly age-related, rising to 14.3% lected because the records of older reassuring that our subsequent anal for those 61 to 65 years old. This age persons were frequently incomplete ysis/ which dealt with deaths over a effect was much lower for category 2 and few of the younger ones had died. three-.,-ear period, 1967 through 1969, or more, and amounted to only about An initial analysis1 was limited to gave essentially the same results. To one third of the change associated deaths before Nov 1, 1966, but the fol some extent these problems are some with dust exposure. low-up will continue until at least what academic, since the total excess At Asbestos, the prevalence of ir 1974. By the end of December 1969, mortality in the industry from res regular small opacities of category 1 87.5% of the 11,572 persons in the piratory cancer, compared with mor or more rose to 9.2% for those aged 61 cohort and 99% of those who had tality of the general population, was to 65, and there was virtually no rela worked ten years or more had been so small. The data are of practical im tion to dust exposure. With respect to traced.1 Of these, 3-270 had died, com portance nevertheless in defining the only category 2 or more, the relation prising 65.4% of those born 1891 to form of the dose-response relation to age was mucn lower and preva 1895 but only 9.8% of those born 1916 ship, essential for the setting of lence increased among those with the to 1920. safety standards. highest dust exposure. The age-standardized mortality per For this reasoson, when the follow For pleural thickening, the effect of 1.000 men for certain causes of death up is eventually completed, an analy- age was slightly greater than the ef are given in Table 3. Cancer of the sis based on man-years of exposure 1 fect of exposure on the prevalences of lung showed a rising rate wdth in is planned to minimize these possible grade 1 or more, and of grade 2 or creasing dust exposure, particularly errors. Meantime, another method of more, for both mining areas. in the two highest dust exposure analysis (G. Eyssen, MSc, and F. D. The biggest difference among all groups. Cancers of the gastrointesti K. Liddell, MA, unpublished data) has the roentgenographic features was in nal tract also showed a rise in the two been employed that, we believe, elim- _2 pleural calcification, for which the highest, and pneumoconiosis in the inates certain of these problems, -- prevalence of grade 1 or more was highest categories of dust exposure. though it does not make full use of v-~ 0.4% at Asbestos and 5.2% at Thetford Of 134 deaths in men from respira the data available and provides only Mines. In spite of the relationship to tory cancer, five were from pleural estimates of relative rather than ab dust exposure, no correlation ex mesothelioma. These cases showed no solute risk. For this analysis, the dust ceeded 0.3. This is largely a reflec clear relationship with dust exposure. exposures of the 134 men included in tion of the very high proportion of There were no peritoneal mesothe the 1969 analysis of respiratory can men who showed no roentgenographic liomas. Mortality from all causes fell cer mortality were compared with a change. Some of the differences be-' with increasing dust exposure up to sample of men, four for each case, se tween the two areas were not surpris the highest dust group, probably be lected at random among persons liv ing, as overall dust exposure levels at cause those who died young could not ing at the time of the death of the Asbestos were considerably lower attain a high dust exposure. On the respiratory cancer case and born in than at Thetford Mines, but others basis of Quebec death rates, the ex the same year. The distribution of cannot yet be explained in terms of pected number of respiratory cancer cases and controls by dust exposure dust exposure or geology. If variation deaths in the cohort was 139, or about category is presented in Table 4. It in fiber content of dust were an im 93 on the basis of estimated rates in can be seen that the pattern of rela portant factor, the results in mine the mining region. This suggests that tive risk obtained by this approach is workers, for whom the fiber propor mortality from respiratory .cancer in quite similar to that for respiratory Afch Environ Health Vol 28. Feb 1974 Chrysotile Asbestos Workers/McDonald et al 61 Table 6.-- Mortality From All Causes and Roentgenograohic Changes at Thet'ord M nes Roentgenographic Changes Parenchymal changes only Pleural changes only Both parenchymal and pleural changes 1691*1895 Obs Exp 17 10 6 M 10 3 14 1' 4 No. of Observed and Expected Deaths by Year of Birth Cohort* 1896-1900 1901-1905 1906-1910 1911-1915 Obs Exp Obs Exp Obs Exp Obs Exp 7 78 4 3 9 11 4 5 7 1.6 15 16 8 10 9.2 5 52 2 3 4 17 12 7 8 66 6 2 2 3 06 ' Expected number was calculated from death rates In men without roentoenographic change. 1916-1920 Obs Exp 2 15 2 2.4 02 Total Obs 46 45 49 cancer mortality shown in Table 3. It seemed possible that the roent genographic differences between Thetford Mines and Asbestos might also be reflected in mortality. There was little evidence of this. The agestandardized mortality per 1.000 pop ulation, for all causes, all malignant neoplasms, all respiratory diseases, and all circulatory diseases were 342, 54, 20, and 120, respectively, at As bestos and 362, 61, 22, and 121. respec tively, at Thetford Mines. All detailed comparisons showed the same sim ilarity. For example, in Table 5, the age-standardized mortality by dust exposure is presented for respiratory cancer and pneumoconiosis. The only apparent difference is that the res piratory cancer rate rose in relation to dust slightly earlier but to a less extent at Thetford Mines. Mortality and Roentgenographic Changes.-Of 10,120 persons traced in the mortality study, 9,692 were men and of these 5,082 had chest roent genograms and 785 had died. At Thetford Mines, there were 354 deaths in 2,448 men traced. Death rates were calculated by date of birth, dust exposure, and the presence or absence of parenchymal or pleural roentgenographic changes. The rate from all causes combined was in creased in the highest dust exposure group but, with allowance for expo sure and date of birth, those whose roentgenograms showed parenchymal changes had a higher mortality (220/1,000) than those without roentgenographic changes (131/1.000) or with pleural changes only (126/1,000). In Table 6, the observed number of deaths by year of birth and roentgen ographic change is compared with the expected number based on those without roentgenographic change. Of 97 deaths in those with parenchymal changes, 33 were in excess of the expected figure. Considering onlydeaths from respiratory disease, in cluding tuberculosis and cancer, we calculated in a similar way that there were 32 deaths in those with paren chymal change compared with eight expected, an excess of 24. Thus of the 33 excess deaths in this group at Thetford Mines, 24 were attributed to Table 7.--Age-Corrected Prevalence Percentages by Dust Exposure and Cumulative Smoking Habits Bronchitis Never smoked Up to 100 cigaretteyears 100-499 500-999 1.000 Breathlessness Never smoked Up to 100 cigarette- years 100-499 500-999 1 000 <10 10 18 41 32 100 0 0 16 9 0 Dust Exposure Laval, mpef-yr 10 100 200 400 800+ 19 19 46 21 49 0 12 0 46 28 21 33 42 38 44 28 47 47 45 55 54 0 43 58 35 14 24 31 13 44 12 39 21 14 8 20 19 21 19 23 27 31 41 43 42 18 14 39 34 46 respiratory causes. At Asbestos, the differenc tween the mortality of those w;-. without roentgenographic cr. was less. Calculations similar used for Table 6 showed that seven of the 431 total deaths w-: sociated with parenchymal of and of these 2.4 were attnbura respiratory causes. Dust exposure levels a: ThMines were much higher than . bestos; thus, in the highest ext group there were 386 and 69 me spectivelv, on whom roentgenot had been performed and wn< been subsequently traced. Thou, excess death rate in this higne. posure group was about 9% at places, the numbers of excess : were therefore 35 and 6 respect-. Calcified pleura! plaques wer related to mortality as there w< deaths in persons with p plaques compared with an ext 59 deaths." Pulmonary Function.--A tot 1,015 current employees unde pulmonary function studies c the summers of 1967 and 196S.' sample chosen for study was fied to include a higher proper older than younger workers, _ former were more likely to changes in function. Those : comprised 83% of the sample selt The variation in some of the function indices by dust lev smokers and nonsmokers is she Fig 1, 2, and 3. The results of test were standardized to at years, height 170 cm, and weig kg. Total lung volume fell -h with increasing dust exposure exposure had little effect on - 64 Arch Environ Health/Vol 28, Feb 1974 Chrysotile Asbestos Workers/McDonan Total )bs Exp 18 JOJ) 45 47.3 49 33.7 lifference be.hose with and .phic changes imilar to those ved that only eaths were asnymal change, attributable to s at Thetford ner than at Asghest exposure ind 69 men, re-oentgenograms and who had ;ed. Though the this highest exout 9% at both d excess deaths : 6 respectively. 3ques were not as there were 60 with pleural ith an expected ,on.-A total of wees underwent studies during 7 and 1968.* ' The tudy was stratiher proportion of - workers, as the likely to show ,n. Those tested e sample selected, some of the lung iy dust level in -okers is shown in ne results of each -dized to age 60 m, and weight "0 .ume fell slightly ast exposure, but * effect on either kers/McDonald et al S T or 0 65 0 functional residual capacity (FRC) or residual volume (RV). Thus exposure tochrysotile affected only the inspira tory capacity portion of the total lung volume (TLV). Forced vital capacity (FVC) and forced expiratory volume in one second (FEVJ both declined with increasing exposure, the FVC vi tal capacity falling by about 18% in the highest dust group and the FEV rather less. Neither steady state nor single-breath diffusing capacities showed any effect of exposure at rest: on exercise, the steady state diffusion declined slightly. Analysis of the relation between o lung function and roentgenographic u. changes* showed that in those with small opacities of category 2 or more there was an average reduction of , 20% in three indices: FRC, RV, and single-breath diffusing capacity at "l rest. Most indices showed a progres- i sive reduction with increasing cate gory of small opacities, but only VC and FVC showed significant reduc tions in association with the earliest roentgenographic changes. Pleural changes were also associated with re ductions in pulmonary function, which ranged from about 3% in those without parenchymal change to about 6% in those with advanced parenchy mal change. An additional small sur Oust Group, mpcf-yr vey showed that changes in pulmo Fig 1.--Variation in lung volumes with dust group, standardized for age, heignt. ar nary mechanics possibly precede weight. roentgenographic and other function changes.* Detailed analysis of those to about 50% in both the highest dust were both decreased to a similar : aged 61 to 65 showed that those with and highest smoking categories. tent with increasing dust exposu an obstructive lung function profile, Thus, the effect of heavy exposure but only FEV was lower in smoke ie, RV and TLC greater than ex and heavy smoking together were the As mentioned above, smoking a pected and flow rates less than ex same as that of either separately. The dust exposure were both related pected, had had heavier dust expo same relationships have been ob the prevalence of bronchitis, r sure, more symptoms of bronchitis, served in other dusty trades." In con smoking was not associated and more irregular small opacities trast, the prevalence of breath breathlessness. than those withjiormal or restrictive lessness on exercise was unaffected Attempts are now being made patterns of pulmonary function." by smoking, but increased steadily collect smoking histories in the coni Respiratory Symptoms.-Each of the with dust exposure. study of mortality, both for those -a 1,015 workers in the function survey Effect of Smoking.--Smoking habits are still alive and for those who ; answered a slightly modified version were originally determined only for dead. This may eventually prov of the Medical Research Council employees in the pulmonary function prospective and retrospective e (MRC) respiratory questionnaire survey. Figures 1,2, and 3 show that, dence on the interrelationship given in French or English by a bi with many of the physiological in smoking and asbestos exposure, lingual interviewer. Both persistent dices, the effect of smoking was controlled retrospective study10 cough and phlegm (bronchitis) and greater than the effect of exposure to been made based on deaths from h breathlessness on exercise were re asbestos dust. This was most obvious cancer in the cohort study. Each lated to exposure.' Table 7 shows, in FRC, RV, and perhaps in the tient was matched with a death fi however, that after standardizing for steady-state diffusing capacity at lung cancer from the same hosp a?e, the prevalence of bronchitis rose rest. Forced vital capacity and FEV, records in a man of approxima Arch Environ Health/Vol 28, Feb 1974 Chrysotile Asbestos Workers/McDonald et al ST006 Dust Group, mpct-yr Fig 2 --Variation in forced expiratory volumes with dust group, standardized for age. height, and weight. the same age and year of death but who had never been employed in the industry. There was a higher propor tion of nonsmokers in patients than controls, which suggests that pulmo nary cancer may be caused by as bestos exposure in the absence of smoking. The controls were on aver age heavier smokers, which reinforces the point but does not rule out the possibility of synergism between as bestos exposure and cigarette smok ing. Clear evidence was obtained from a survey of all known cases of malignant mesothelioma in Canada that this disease was not related to smoking.; ` Women.-The number of women ever employed in the Quebec chrysotile industry is small. In the mortality study, 428 of 465 women in the cohort were traced and of these 54 had died. Only 79 women had worked for more than ten tears, and few had been ex posed heavily to asbestos dust. One death was ascribed to lung cancer and none to pneumoconiosis. Chest roentgenograms were avail able for 294 women, most of whom were 30 years old or less. The only roentgenographic changes recorded were one subcategory 0/1 rounded small opacities, one subcategory 0/1 irregular small opacities, and one grade 1 ill-defined cardiac outline. Comment Although death is the most definite and serious manifestation of expo sure to asbestos, it is not the most sensitive. Excess deaths related to ex posure altogether were probably in total no more than 2% of the 3,270 deaths in the cohort study. Most of these were attributed to respiratory cancer or pneumoconiosis and almost all were in the two highest dust expo sure categories. Thus, excess mortal- :ty was virtually confined to men with exposure equivalent to at least 400 mpef-yr. Detailed examination of mortality showed no particular cause, except pneumoconiosis, with a rate above that of the general population of Quebec. An investigation was made of the 236 known cases of malignant meso thelioma in Canada, 1960 through 1970. The manner of death certifica tions of these patients were exam ined/' and the pathological findings reviewed in detail by the Mesothe lioma Panel of the Canadian Tumour Reference Centre."" Epidemiologi cal inquiries showed that 23% of men. and 1% of women had definite or* probable occupational exposure to as-^ bestos, and a further 1% of men and 6% of women had lived in the home o(^ an asbestos worker. In the remaining cases, none had ever lived within 32 km of an asbestos mine or mill. Of all known cases of mesothelioma in Can ada, only nine have been associated in any way with the Quebec chrysotile mining and milling industry. Seven (five in the cohort) had been employed and two were women whose fathers had worked in the industry. These cancer rates are very low in comparison with the studies of New York insulation workers" and the London crocidolite factory workers,:J in both of which there were much higher rates of malignant neoplasms, particularly of mesothelioma. Other studies in the Soviet Union" and Italy" support the view that only high levels of exposure to chrysotile during mining and milling have an appreciable effect on mortality. Although roentgenographic changes had some relationships to dust expo sure, the clinical significance of the minor changes which were 'also re lated strongly to age is uncertain. Ir regular small opacities of category 2 and pleural thickening of grade 2 are usually considered of clinical impor tance, and these were much less influ enced by age. Changes of this order occurred in 1.0% and 0.7% of the en tire working population. In men aged 61 to 65. employed an average of 20 years in the industry, the rates were ( 5.0% and 2.5% at an average exposure level of 13 mpef. These rates are very 66 Arch Environ Health/Vol 28. Feb 1974 Chrysotile Asbestos Workers/McDonald et III II similar to those found in the chryso- tile mining and milling industry in Cyprus,but are much lower than those in the New York insulation workers," or in the British Royal Na val Dockyards." " For pulmonary function and res piratory symptoms, the effect of smoking was generally greater than the effect of exposure to asbestos. However, for inspiratory capacity, forced flow rates, and breathlessness on exercise, relationships to dust ex posure were found. With respect to these indices, a 10% reduction in non- smokers occurred after a total expo sure in excess of 100 mpcf-yr. This is close to the lower limit of the dust category at which 1% of subjects had grade 2 roentgenographic changes. In the Quebec studies, information on dust exposure was obtained, where as in most other studies quantitative information was not available. Mur phy et al'1 found 11 cases of asbes- tosis in 101 New England shipyard workers with an average exposure of 120 mpcf-years, compared with one in a nonexposed control series. These workers appear to show a larger ef fect of asbestos exposure than those engaged in chrysotile production. However, the main constituent of the insulating materials used was amo- Dust Group, mpcf-yr site, with some chrysotile and no cro- Fig 3 --Variation in diffusing capacity with dust group, standardized for age, height, cidoiite. and weight. Dust exposure assessments were also made for the Cyprus mining and milling industry," and the rates of change. However, the roentgen averaged over three months, is based roentgenographic change for the ograms were all taken during work on the concept that a 1% risk of ac same exposure levels were quite sim ing life, so the correlation between quiring clinically significant disease ilar to those in Quebec Exposure in total dust and years of exposure or in a 50-year working lifetime of expo dices have also been assessed in a years since first exposure is high, and sure is acceptable. Considering all study of two asbestos-cement plants the separation of the contributions facets of disease--death, roentgen in the United States by Enterline and from these two factors, duration and ographic changes, pulmonary func Weill.where the main exposure has concentration, would be very diffi tion changes, and respiratory symp- been to chrysotile and silica, but the cult. The cohort study also showed toms-the 1% risk is reached by men results are not yet available. that respiratory cancer mortality was in our third dust exposure category' The limit standards for occupa greater in those who took more years (100 to 200 mpcf-yr). tional exposure to asbestos are based to reach a given dust level. Therefore, The relationships that exist be solely on the airborne concentration in assessing the overall effect of as tween measurements of total dust of dust or fiber, averaged over a life bestos exposure, duration of exposure and fiber exposure are thus of critical time's work, and the results in this re should not really be ignored. For this importance if standards are to be 3et port have been presented from this reason, it would be unwise to apply for asbestos production and other as point of view. There was some evi the results of this or other descriptive bestos industries. Such standards dence in our studies that longer expo epidemiological studies to widely dif should be based on dose-response re sures for the same total dust levels ferent exposure patterns. lationships established by sound epi are associated with slightly higher The British dust standard for demiological inquiries. To date, very prevalence rates of roentgenographic chrysotile of 2 fibers per milliliter, few of these have incjuded any quan i. t i I i stnnn'i ry5 Arch Environ Health/Vol 28. Feb 1974 Chrysotile Asbestos Workers/McDonald et al 67 t;tat;ve assessments of dust, let alone of fiber exposure. Though safety stan dards expressed :n fiber concentra tions have theoretical and conceptual attractions, there is as yet little or no direct epidemiological evidence on which to base them. Our studies ap pear to provide a reasonable basis for establishing safety standards for chrysotile mining and milling in terms of dust concentration. Without more evidence on the conversion fac tor that should be applied in different parts of the industry, we cannot ex press our results confidently in fiber counts. Further efforts to find a satis- factory means of converting rr.. impinger to fiber counts 3re mcr but, until new epidemiological dence based on fiber counts has : assembled, it appears unwise to continue use of particle counts control in this industry. References 1 McDonald JC. et ai Mortality in the chryso'.iie asbestos mines and mills of Quebec Arch En\iron Health 22:677-686. 1971. 2 McDonald JC, et ai. Mortality in the chryso* tile producing industry of Quebec: A progress re port. Read-before the Fourth International Pneumoconiosis Conference. Bucharest, Hun gary. 1971. 3. McDonald AD. et al. Epidemiology of pri mary malignant mesothelial tumours in Canada. Cancer 26514-919. 1970. 4 McDonald AD. McDonald JC Epidemiologic surveillance of malignant mesothelioma in Can ada. Can Med Assoc J 109:359-362, 1973. 5 Rossiter CE. et ai: Radiographic changes to chrysotile asoestos mine and mill workers of Queoec Arch Environ Health 24 388-400, 1972. 6. Becklake MR. et ai: Lung function in rela tion to chest radiographic changes in Quebec a$oestos workers. Bud Physiopathol Resp 6:6376.59. 1970 7 Becklake MR. et ai. Lung function in chrysotile asbestos mme and mill workers of Quebec. Arch Fninrcm Health 24:401-409, 1972. 3. Jodoin G. et al. Early effects of asbestos ex posure on lung function. Am Rev Resp Dvs 104.525-535. 1971. 9 McDonald JC. et ai. Respiratory symptoms in chrysotile asbestos mine and mill workers of Quebec. Arch Tnnr^n Health 24:358-363, 1972. 10. UlCC Cincinnati classification of the ra diographic appearances of pneumoconiosis. Chest 53.57-67, 1970. 11. Gibbs GW. Lachance M Dust exposure in the chrysotile asbestos mines and mills of Que bec. Ar'-h Environ Health 24'189-197, 1972. 12. 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