Mortality data from 9609 workers at two asbestos mining areas in Quebec were analyzed to assess the effects of the intensity and timing of exposure on lung cancer risk. Summary exposure measures based on differing assumption were computed for lung cancer cases and matched controls and were fitted to the data using conditional logistic regression. A non-linear relationship between intensity and risk fit both mining areas, but risk was greater at one area than the other. At the mine with lower risk, exposure occurring more than 30 years prior to death had little effect, while at the other mine risk did not vary with time since exposure and men starting employment before 1924 were at elevated risk. The results point to differences in dust composition at the two areas and illustrate the difficulties in estimating risk.
We report a further follow-up of a birth cohort of 11 379 workers exposed to chrysotile. The cohort consisted of 10 939 men and 440 women, born 1891-1920, who had worked for at least a month in the mines and mills of Asbestos and Thetford Mines in Quebec. For all subjects, length of service and estimates of accumulated dust exposure were obtained, with a smoking history for the vast majority. Three methods of analysis, two based on the "man-years" methods, the other a "case-and-multiple-controls" approach, gave results consistent with one another and with previous analyses. By the end of 1975, 4463 men and 84 women had died. Among men, the overall excess mortality, 1926-75 was 2% at Asbestos and 10% at Thetford Mines, much the dustier region. The women, mostly employed at Asbestos, had a standardised mortality ratio (SMR) all causes, 1936-75) of 0.90. Analysis of deaths 20 years or more after first employment showed that in men with short service (less than five years) there was no discernible correlation with dust exposure. Among men employed at least 20 years, there were clear excesses in those exposed to the heaviest dust concentrations. Reanalysis in terms of exposure to age 45 showed definite and consistent trends for SMRs for total mortality, for lung cancer, and for pneumoconiosis to be higher the heavier the exposure. The response to increasing dose was effectively linear for lung cancer and for pneumoconiosis. Lung cancer deaths occurred in non-smokers, and showed a greater increase of incidence with increasing exposure than did lung cancer in smokers, but there was insufficient evidence to distinguish between multiplicative and additive risk models. There were no excess deaths from laryngeal cancer, but a clear association with smoking. Ten men and one woman died from pleural mesothelioma. If the only subjects studied had been the 1904 men with at least 20 years' employment in the lower dust concentrations, averaging 6.6 million particles per cubic foot (or about 20 fibres/cc), excess mortality would not have been considered statistically significant, except for pneumoconiosis. The inability of such a large epidemiological survey to detect increased risk at what, today, are considered unacceptable dust concentrations, and the consequent importance of exposure-response models are therefore emphasised.
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Among 4463 deaths in a cohort of 10,939 men, born between 1891 and 1920, who had worked in the Quebec chrysotile mines and mills for at least a month, 244 were due to lung cancer. The interval from first asbestos exposure to death averaged 39.6 years; not unexpectedly, this interval was related to age at death and to age at first employment. However, the interval did not appear to be associated in any important way with other measures of exposure. All of the problems of interpretation pointed out by Enterline (1978) are acknowledged, and there is no guarantee of generalization to those exposed to asbestos in other circumstances. Nevertheless, among these 244 cases, there was no evidence to support the hypothesis that the latent period of asbestos-induced lung cancer depends on the dose.
The study examines differential leucocyte counts in blood of asbestos miners and quarriers identified during the course of a community health survey comprising 693 men over 25 years of age of all occupations. Subjects in the asbestos mining and quarrying occupational groups were present in the lowest total leucocyte quintile in significantly greater numbers than expected on the basis of a age-specific uniform leucocyte distribution based upon the total population of male workers. This relative leucopenia was not seen in their wives nor in any of the 22 other major occupational groups examined after their leucocyte counts were adjusted for age and sex variations.
A review of 15,689 chest radiographs of Quebec chrysotile miners and millers, representing the latest film prior to November 1, 1966, for all such persons ever x-rayed, identified 206 men with pleural calcification. Of these, 198 had worked in the Thetford Mines area, 6 at Asbestos, and 2 at St. Remi de Tingwick; 2.5%, 0.08%, and 1% of the films from these areas, respectively. A series of case-control studies revealed that pleural calcification was concentrated in men employed at a small group of mines in Thetford Mines and occurred more often among miners and maintenance personnel than among millers. Calcification was not related to past history of illness or injury, place of residence, or employment in other industries. The distribution of pleural calcification in this Quebec industry suggests that it is related to some characteristic of airborne dust or mineral closely associated with the chrysotile that is encountered during mining in Thetford Mines but not in other mining areas. Possible minerals include mica, talc, and breunnerite.
Prior surveys of malignant mesothelioma in Québec have noted that almost all the excess in occupational exposure related mesothelioma was in the manufacture and industrial application of asbestos rather than in the mining and milling operations. To evaluate the current status of malignant pleural mesothelioma in the Québec workforce, we reviewed all cases of pleural mesothelioma seen and accepted by the Québec Workman's Compensation Board (CSST) for work related compensation of industrial disease. We identified 120 cases, 7 of whom were females. They were of an average age of 59 +/- 8.5 yrs (sd) (range 42-84); they were exposed to asbestos dust in the workplace for an average of 26 +/- 14.3 yrs (range 0.5-50). The cases were subdivided into 3 groups according to workplace asbestos exposures. There were 49 cases originating in the mines and mills of the Québec Eastern Township region (primary industry, group 1), 50 cases from the manufacture and industrial application sector (secondary industry, group 2), and 21 cases from industries where asbestos was not a major work material, often an "incidental" material (tertiary industry, group 3). Group 1 was of an average age of 62 +/- 8 years, exposed to asbestos dust 31 +/- 14 years and the distribution of exposure time was as follows: 15% cases with or = 25 year-exposure. In group 2, the age was significantly lower at 57 +/- 9 years; the exposure time was also significantly lower at 22 +/- 14 years, and the distribution of exposure time differed from the above (29% cases with or = 25 year-exposure). In group 3, the average age was 58 +/- 7 years, the exposure time was also significantly lower at 28 +/- 12 years and the distribution of exposure time differed from the above (33% cases with or = 25 year-exposure). Analyses of the yearly incidence of new cases in each group documented the general incremental trend in all groups, with the sharpest rises in group 3. In the mining towns of Thetford and Asbestos, the incidence of mesothelioma was proportional to the workforce, thus suggesting that the tremolite air contamination, which is 7.5 x higher in Thetford, may not be a significant determinant of the disease in these workers.(ABSTRACT TRUNCATED AT 400 WORDS)
Comment In: Am J Ind Med. 1993 Aug;24(2):245-88213852
To describe the small area system developed in Finland. To illustrate the use of the system with analyses of incidence of lung cancer around an asbestos mine. To compare the performance of different spatial statistical models when applied to sparse data.
In the small area system, cancer and population data are available by sex, age, and socioeconomic status in adjacent "pixels", squares of size 0.5 km x 0.5 km. The study area was partitioned into sub-areas based on estimated exposure. The original data at the pixel level were used in a spatial random field model. For comparison, standardised incidence ratios were estimated, and full bayesian and empirical bayesian models were fitted to aggregated data. Incidence of lung cancer around a former asbestos mine was used as an illustration.
The spatial random field model, which has been used in former small area studies, did not converge with present fine resolution data. The number of neighbouring pixels used in smoothing had to be enlarged, and informative distributions for hyperparameters were used to stabilise the unobserved random field. The ordered spatial random field model gave lower estimates than the Poisson model. When one of the three effects of area were fixed, the model gave similar estimates with a narrower interval than the Poisson model.
The use of fine resolution data and socioeconomic status as a means of controlling for confounding related to lifestyle is useful when estimating risk of cancer around point sources. However, better statistical methods are needed for spatial modelling of fine resolution data.
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Beginning in the 1930s, the Canadian asbestos industry created and advanced the idea that chrysotile asbestos is safer than asbestos of other fiber types.
We critically evaluate published and unpublished studies funded by the Quebec Asbestos Mining Association (QAMA) and performed by researchers at McGill University.
QAMA-funded researchers put forth several myths purporting that Quebec-mined chrysotile was harmless, and contended that the contamination of chrysotile with oils, tremolite, or crocidolite was the source of occupational health risk. In addition, QAMA-funded researchers manipulated data and used unsound sampling and analysis techniques to back up their contention that chrysotile was "essentially innocuous."
These studies were used to promote the marketing and sales of asbestos, and have had a substantial effect on policy and occupational health litigation. Asbestos manufacturing companies and the Canadian government continue to use them to promote the use of asbestos in Europe and in developing countries. Am. J. Ind. Med. 44:540-557, 2003.