Document Xz3jxdNRz3jD0jbnrKz2vRXDg

WRGO01485 4Ps * * ten British Journal of Industrial Medicine 1986:43:445-449 pec- ujue ir>6r- nifyf Radiological survey of past and present vermiculite of jblk miners exposed to tremolite rane nion jc McDonald, psebastien, and b Armstrong 1 Inti From the School of Occupational Health. McCill University. Montreal, Quebec, Canada ally- TlOf abstract *Chest radiographs'taken by a standard technique were obtained from 173 current* sure ' employees (164 men, 9 women) of a vermiculite mine in Montana, from 80 of 110 past employees > --60. resident within 200 miles; and from 47 men from the same area without known exposure to dust. In and 43 of the 80 and 24 of the 47 an earlier chest x ray film was retrieved from the hospital archives. All don: 367 films were assessed blind and independently by three experienced readers using the ILO 1980 classification. Median radiographic assessment scores were analysed in relation to estimated cumu- 'ibrt and lative exposure to the amphiboie fibres that contaminate the vermiculite. Logistic regression anal* .bee. yses showed independent effects of age, smoking, and exposure on the prevalence of small opacities wuiJ and of age and probably of exposure on pleural thickening." Overall, the data suggest that by ^ retirement age the increase in prevalence of small opacities (> 1/0) lies between 5% and 10% per lOOf/ml ycars. This gradient may be somewhat steeper than for chrysotile miners and millers, but ' not much so. y The survey described in the present paper is one of a senes of studies undertaken to assess the health effects of work in a vermiculite mine in Libby. Montana. Two complementary programmes of research were initiated by NIOSH and ourselves, pnmanly because of contamination of the ore body with fibrous amphiboie deposits in the tremolite senes. A history and description of the mining and milling processes were given in our report on a cohort mortality study in this group of workers.' These data showed sub stantial excess mortality compared with United States white men from respiratory cancers (SMR 2 45). non-malignant respiratory disease (SMR 2-55). and malignant mesothelioma (proportional mortality = 2-4%). The NIOSH mortality study of a more broadly defined cohort obtained similar results from independently assembled data.1 Vermiculite. a micaceous mineral that expands on heal treatment has many uses in construction indus tries. agriculture, and packaging. The largest known '"vermiculite deposit, at Libby, Montana, has been commercially mined since 1923. At first it was on a small scale but. after 1939, when two companies merged, production increased: 20000 tons were shipped in 1940, 150000 in 1950. and, since 1972, about 200000 annually. The Libby deposit is mined using the open pit Accepted 30 September 1933 mining technique incorporating rubber tyre loaders and trucks for both ore and waste removal. Waste is discharged at the perimeter of the mined area whereas the ore is hauled to a transfer point for blending and wet beneficiaiion. During early years of the oper ation. all functions of mining and milling were per formed in a dry state, leading to very dusty condi tions. A senes of process improvements resulted in present mine personnel operating from within enclosed air controlled cabs and a totally wet beneficiation process. Vermiculite concentrate is graded into five size fractions before shipment to expansion plants in vanous pans of the United States. Fibrous tremolite has little commercial value and only one or two small mines in northern Italy produce it. On the other hand, it is a fairly common con taminant in vanous exploited minerals, notably some talc deposits of New York State and in the Quebec chrysotile mines where it is present in trace amounts. There has been no previously published radiological survey of tremolite miners as such but several studies of talc miners and millers exposed to varying concen-.^ (rations of mineral fibre and silica.1 Pleural thick ening was frequently found in these workers but ' parenchymal opacities much less so. These obser vations are not easily interpreted in terms of tremolite fibre because of the other components in the airborne dust. 445 I 446 Material and methods STUDY CROUPS Chest radiographs were obtained from three groups. The first comprised all 164 men and nine women employed by the company on 1 July 1983. The second was selected from 110 past male employees resident within 200 miles of the mine. This information was known as all had been traced as surviving members in the mortality cohort, the criteria for inclusion of which were employment of at least one year and date of hire before I January 1963. Eighty of the 110 thus identified accepted (he company's invitation to come to the hospital in Libby for a chest xray film. The third group comprised 47 men without known exposure to dust who attended the hospital during the same period for a '`routine" chest x ray film, mostly in connection with other local employment. The latter group was not selected or matched in any way with the mine workers and was intended only to control the reading process. All 291 men (and nine women) in the three groups were examined radiologically by a standard technique, based on that outlined in appendix A of the 1980 ILO guidelines.4 In 43 of the 80 ex-employees and in 24 of the 47 unexposed sub jects an earlier chest x ray film was located in the hos pital film archives. The earlier films for the ex employees had usually been taken shortly before leaving the company and by a generally less satis factory radiological technique. In total, 367 films were thus assembled for classification. McDonald. Sebastian, and Armstror EXPOSURE ESTIMATES Work histories were used to obtain the dates ar duration of work in various locations. The mean a> borne fibre concentrations for each locauon opc anon, year by year, were estimated from all availab midget impinger and membrane filler measuremen after a site visit and detailed review by one of us (PS The methods of estimation and results are describe fully in the mortality paper.1 statistical analysis Radiographic readings from the three readers we: combined as a median score. TWo methods were use to investigate relations between xray changes ar. exposure to fibrous dusL Firstly, the prevalence t abnormalities was tabulated in five cumulate exposure groups, after stratification in three ag groups (20-39, 40-39, >60) to reduce confoundim Age standardised prevalence rates were then coir, puted for each exposure group, the combined popi lation being taken as standard. Secondly, logist. regression analyses for selected abnormalities we: carried out in which age, smoking, and cumulati' exposure were entered into the regression cquatioi Age was considered as a continuous variable an smoking in three groups: never smoked, ex-smoker and current smokers. The logistic regression analys and the fitting of linear exposure response lines to th grouped prevalences were carried out using th GLIM computer package.1 Results READING PROCEDURES The films, with identity obliterated, were renumbered in random order and classified independently by three experienced readers. The ILO 1980 International Classification and standard sets of films were used. The readers were told only that the films were mainly from mineworkers with varying exposure to mixed dusts and that about 20% of the senes were unexposed subjects. Preliminary tabulations after the readings were completed showed that, overall. 36% of the recent films were considered of good quality, 36% fair, 7% poor, and 0-4% unreadable. The corre sponding ratings for the older films were 22%, 33%. 23%, and 2%. Although (he readers were experienced and had often worked together, interobserver vari ation remained. Agreement between all three readers on the presence or absence of changes was achieved in about 90% of readings for pleural calcification, costophrenic angle obliteration, and pleural thickening on the diaphragm. The rate fell to about 80% for small parenchymal opacities and for pleural plaques or diffuse thickening. The prevalence of the main radiographic changes i the current male employees and the sample of pa: employees, based on their most recent films, : presented in table 1. The overlap between the tw< groups is confined to men aged 40 and over. Even i: these older men the rates for both parenchymal an pleural changes differ considerably but perhaps nc more than might be explained by differences in mca: levels of cumulative exposure. A steep age gradient u both groups is also evident, probably attributable i: part to exposure. Of the 47 unexposed subjects, 3: were aged 40 or more: small opacities were read ir one. pleural thickening on the chest wall in fou (8 5%), and pleural calcification in two (2-7%). Table 2 gives the age standardised prevalences o small opacities and pleural thickening by cumulativ exposure. Because of small numbers and, in particu lar, since few young men had achieved high cumu lative exposures, these standardised rates are subjee to rather large standard errors in the higher exposun groups. Notwithstanding these problems, the trend o increasing prevalence of small opacities witl increasing exposure is unlikely to be due to chano urer- ble ms S). >ed ere ied ind of ive ige ng- pu- Stic ive on. ind 3*5* *SIS the the ; in ast wo i in rnd BOt an tm : in 32 1 in aur . of 3ve U- FCt re lofath ce Radiological survey of past and present vtrmicultte miners exposed to tremoiite Tii hie I Percemiage ofradiographic changes by age At4 fi.'itest 20-.'9 40-59 All Current employees No Mean Small cumulative opacities exposure ( pi 0: tfiml y) SO ' I3i 69 57-3 f 15 103 S 164 401 25 9 130 ' 26-7 91 Pleural thickening (chess wall i 75 x-y 400 15 9 Pleural calcification Past rmptovees No fimi v /3 . . 58 67 ' J7 i JO 49 SO 60 160 2 96 1 1189 Small opacities < P HO) -- JJJ 408 J7 5 447 Pleural thickening i chest nail) -- 400 61 1 52-5 Pleural __ 16 7 36 7 28 8 Table 2 Age standardised prt valence ofabnormalities by cumulative exposure . Cumulative exposure fflml vi Croup Mean k No in group Small opacities t > U0) > No Prevalence % \0^ '"lO-CO \ :o-< ioo 100- <200 >:oo 41 17 5 5) 9 144 4 495 8 92 64 53 16 19 7 12 10 8 8 Tout 65 9 244 45 Directly age standardised, toul study group taken as standard. 10 6 18 4 15 4 31 3 27 9 18-4 Pleural thiekenmg of chest mad > No Prtraitnct % 15 2)) 14 21-7 22 lt-7 6 22 2 II 38 5 68 27-9 Table 3 Prevalence ofabnormalities in men aged 60 or more by cumulative exposure Cumulative exposure (fiml vj Group Mean <10 ,, l0-< 20 20-< 100 100-< 200 >200 Toul 6-5 190 56-7 153-4 4634 97 9 Vo tn group 15 17 16 8 8 64 Small opacities ( P ItOj So Prevalence % 2 UJ 6 35 J 6 17 5 6 75 0 4 50 0 24 37 5 Pleural thickening of chest mad No Prevalence V# 7 46 7 9 37* 10 47 3 5 62 5 5 621 36 563 Table 4 Logistic regression analysis of exposure response relations Test foe In the pretence of Small opacities / P 1:01 Pleural thickening of chest wait ll <<*f> P I1 <df) p Cumulative exposure Age, smoking 109(1) p < 0 001 5 9(1) Smoking Age. cumulative exposure 8 2 (2) p < 0025 4 6(2) Age Cumulative exposure 27 2 (1) p < 0 001 28 I (t) smoking Estimated coefficients: p Small opacities: Log ------- -7 27 + 0 0035 (cum exp) + OOS4(age) {0-82 if ex-smoker 1 "p p {1.72 if current smoker Pleural thickening: Log ------ = -- 5*62 + 0-0024 (cum exp) 0-088 (age) {-0-41 if ex-smoker _________________________ ~ p __________________ [0 42 if current smoker p < 0-025 pa 0-10 p < 0-001 448 McDonald. Sebastien, and Armstrong Lop'S>C Ojrrrr* yrcsn ogra Among the 43 ex-employees for whom an earlier xray film had been obtained, small opacities (^ 1/0) were read in 18 (42%) of the 1983 films and nine (21%) of the films taken on average 8-9 years earlier. This difference derived from 10 men whose readings had deteriorated and two whose readings had improved. In the 24 pairs from unexposed men on average seven to eight years apan, no abnormality was read in the earlier film and one only in the later film. Since there was virtually no change in the cumu lative exposure of the 43 ex-employees between their two x ray films, the substantial increase in prevalence was presumably due in part to aging and in part to differences in radiographic technique. Discussion Prevalence of small opacities I H/0) and cumulative exposure (f/ml years). ix1 (Idf) = 5-4. p a 0 02). The corresponding trend for prevalence of pleural thickening of the chest wall is less strong (x1 (Idf) - 2-6, p a 010). Table 3 gives the prevalence of abnormalities by cumulative exposure for men aged 60 and over. Here, too, there is an association of small opacities with exposure (y3 (Idf) = 6-4, p i 0 01) and a weaker one for pleural change (xl (ldO = 0-9, p > 0-20). Results of the logistic regression analyses of abnor malities (table 4) show the independent effects of each of the three factors examined (cumulative exposure, age. and smoking), after allowing for the other two. Clear effects of cumulative exposure and age on the prevalence of small opacities are apparent and there is also some indication of a smoking effect (p = 0 02). The prevalence of pleural thickening on the chest wall is affected by age and more weakly by cumulat ive exposure (p a 0 02). There is no consistent evi dence of a smoking effect. The data are insufficient to determine whether there are interactions between the factors. The estimated exposure response relations for small opacities are graphed in the figure. The graph shows the linear relations computed from tables 2 and 3. and that predicted from the logistic regression equation for age 65. The latter includes two curves, one for current smokers and one for non-smokers. The curve for ex-smokers would be intermediate. For ah these relations, standard errors of slopes and intercepts are large. The usual criticism levelled at cross sectional morbidity studies in working groups is that selection may have removed a disproportionate number of employees who had suffered the ill effects of occupational exposure. It would be equally wrong to study only ex-employees. We tried in this survey to achieve a balance by including both present and past employees and analysing their joint experience. In addition to the logistic regression analysis presented in uble 4 another analysis was made in which a fourth variable for ex or current employment was included. This showed that, having allowed for the other three factors, rates of change were higher in past than present employees but, statistically, not significantly so. The coefficient for the relation to exposure was, in fact, the same whether or not the fourth variable was included. We believe, therefore, that it is reasonable to base our estimate of exposure response on the experience of the combined sample. Even so, the relations illustrated in the figure entail great uncer-' tainty, for which there are several reasons. Firstly, the exposure estimates, especially for the important period before about 1972, were approximate. Sec ondly, radiographic techniques and radiograph read ings are major sources of variation. Thirdly, age, 'smoking, and probably other environmental factors all contribute to radiographic change. Finally, the study sample was quite small. In the absence of other information our data provide the only indication of risk in relation to exposure. Of the four lines shown in the figure, the two based on logistic regression and the one for men aged 60 or more are roughly parallel, and probably more valid than that based on age standard isation. These three lines suggest that by retirement age the increase in prevalence of small parenchymal opacities (1/10) lies between 5% and 10% per 100 f/ml years. At present exposure levels in the Libby operation, reported by the company to average trong arlier 9 I/O) nine irlier. dings had :n on nality later umutheir .lence trt to ional ction :r of s of ng to ey to past e. In sued urth led. three than mily a. in : was able the . the icer\ the riant Sectadge. aors . the her a of n in fithe .and tardnent anal per tbby rage Radiological suryry ofpast and present vermtcultte miners exposed to tremohie 449 0-1 f/ml. no excca of radiological change should be detectable after a working life of 40 years. The imprecision associated with radiological technique and reading error would be far greater than the expected level of risk. There are few other data sets against which these findings may be compared. Our original studies of Quebec chrysolite miners* gave a slope of about 0-6% per 100 fibre years and a later analysis.7 confined to men aged 60-69 with an average gross service of 42-3 years, gave a lower estimate (0-2*/.). Both these esti mates were based on men still at work and it was concluded that the true gradients were probably steeper. A more recent study of the Quebec workers (JC McDonald et at, paper presented at XXI inter national congress on occupational help. Dublin. 1984) was based on current and past employees, used .rray films taken with a similar technique to that at Libby, and one of the three readers took part in both studies. That study gave a slope of 4% per 100 fibre years, about half that in the present survey. The radiographic study by NIOSH was based on the most recent chest x ray films (188 in all) available in l ie local hospital for men employed five or more years and at some time since 1974.2 These men were all current employees at the time of the x ray exam ination, were somewhat younger than those in our sample, but had been employed nearly four years longer and at a slightly higher average level of exposure. Despite this, and the fact that one of the readers was common to both studies, the prevalence rates of both parenchymal and pleural changes were substantially higher in our study than theirs. The slope of the logistic regression line for small opacities ( ^ 1/0) and cumulative exposure was also somewhat steeper in our data. We believe that these differences were due to better film quality and perhaps to the inclusion of ex-employees in our survey This under lines the problems and imprecision of radiographic estimates of exposure response in dust exposed workers. This research was supported by a grant to McGill University from WR Grace and Co. The chest x ray films were taken at St John's Lutheran Hospital. Libby, Montana, under the supervision of Dr T Gill and read by Dr J Gilson. Dr R Jones, and Dr G Sheers. The active participation of past and present employees and the company is gratefully acknowl edged. References 1 McDonald iC. McDonald AD. Armiiron| 8. Sebuucn P Cotan study of mortality of vermtculue minen exposed to tremobte. Br J tnd Med 1986;43:436-64- * Amandus HE. Wheeler R Morbidity and mortality of vcrmtculite mmen and miller* exposed to iremohte-ecunoUta. Pvt .11: Mortality. Am J Ind Med (in press). 1 Dement JM^ Zumwalde RD. Gamble IF. etai. Occupational exposure to laic mntauunf asbestos- Washington. DC: US Government Printing Office. 1980. (National Institute for Occupational Safety and Health technical report.) 4 International Labour Office. Guidelines for the use of fLO miee* national classification of radiographs ofpneumoconioses. Geneva: ILO. 1980 (Occupational Safety and Health tenet 22 Rev 80.) * Baker RJ. Nelder JA. The GUM System- release J. Oxford*. Numerical Algorithm Group. 1978. * Rossiter CE. Bristol LI. Cartier PH. etal. Radiographic changes in chrysolite mine and mill workers of Quebec. Arch Snrtron Health 1972.24.368-400. ' Liddell FDK. Gibbs GW, McDonald JC. Radiological changes and fibre exposure in chrysolite workers aged 60-69 yean at Thelford Mines. Ann Occup H\% 1982:26:889-98.