Ann Occup Hyg 1997 41:231-236


Lung Fibre Content for Mesothelioma in the 1891-1920 Birth Cohort of Quebec Chrysotile Workers: A Descriptive Study.

Bruce W. Case, Andrew Churg, André Dufresne,

                Patrick Sébastien, Alison McDonald and J.C. McDonald.
                                                  (Department of Epidemiology, McGill University, Montreal, Canada.)

            Since the discovery of the crocidolite/ mesothelioma relationship by Wagner et al. (1960) human studies have indicated that the amphibole forms of asbestos, and not chrysotile, are responsible for asbestos-related mesothelioma with rare, if any, exceptions (Elmes, 1994; and the accompanying workshop discussion reported by Churg A. et al., 1994).   A recent paper by Stayner et al. (1996), reaffirms that “...the differences in mesothelioma response observed among chrysotile- and amphibole (primarily crocidolite)- exposed workers are so striking that alternative explanations for these differences appear unlikely.”  There remain observers (including Stayner and colleagues) who are nonetheless unconvinced that amphibole co-exposure (to tremolite and commercial amphiboles) explains mesotheliomas observed in the 1891-1920 birth cohort of Quebec chrysotile miners and millers.  This is true even though they can otherwise document only thirteen cases in eleven other cohorts  of “...workers  exposed to predominantly chrysotile asbestos” (Stayner et al., 1996, Table 1, page 180).  This, the chrysotile hypothesis for mesothelioma, has implications for risk assessment both for chrysotile and for tremolite (Case BW, 1991).     

            The 1891-1920 birth cohort consists of  11,000 men employed in the Quebec chrysotile industry for at least one month, including over 700 who were first employed in an asbestos products factory at the town of Asbestos, where commercial amphiboles were used.  Miners and millers at the local Jeffrey Mine replaced sick factory workers, and visited the factory building on a regular basis.  Lung-retained fibre analysis for workers with and without mesothelioma at Asbestos (Case and Sébastien 1987, 1988; Dufresne et al. 1995, 1996) showed excesses of crocidolite, and to a lesser extent amosite asbestos, in about 75% of lungs analysed, in addition to tremolite and chrysotile.   In the separate region around Thetford and Black Lake,  a number of mines were exploited, but there were no products factories and no known exposure to commercial amphiboles among men.  Recently the first full analysis of mesotheliomas from these two areas has been performed (McDonald et al., 1996).  A maximum of 38 possible mesotheliomas among 8009 cohort deaths were identified, with highest proportional mortality (1.08%) in the factory and lowest (0.21%) in miners and millers at Asbestos.  Miners and millers at Thetford Mines had most mesothelioma deaths but an intermediate rate (25 of 4125 observed deaths; 0.62%; p <.01 vs. Asbestos).  Internal analyses confirmed the previous observation by McDonald and McDonald (1995) that mesothelioma deaths in Thetford were largely confined to those who had worked in five central mines.  These appeared both from historical geologic data and from lung fibre analysis to be mineralogically  distinct, particularly in terms of an increased intrapulmonary tremolite burden.   In the current study we analysed lung fibre content in both Asbestos (N=7) and Thetford Mines (N=15) workers with mesothelioma.  Subgroups examined included (1)  miners and millers in the "central mines" of Thetford, near the town site (N=9), (2) those who worked more peripheral to the town of Thetford (N=1); (3) Thetford miners and millers who had worked in the central area but also had some exposure in the peripheral mines and mills (N=5); (4) men who worked in the asbestos products factory at Asbestos (N=2); and (5) those who worked exclusively in the mine and mill at Asbestos (N=5). Data on cumulative exposure allowed us for the first time to relate external exposure measurement to lung fibre content.  


            We contacted scientists who had published lung fibre analyses for workers in the Quebec chrysotile industry and asked to obtain results from any cases of mesothelioma, together with pathology accession numbers. We then matched the latter to those obtained for our own list of 38 possible mesotheliomas among cohort members.  Altogether, 17 male cohort members with mesothelioma and available results were identified.  Lung analyses produced in the past by four investigators (AC, AD, BC and PS) were used.   Five additional cases were obtained from pathologists and analysed in our laboratory.  Detailed methods of fibre analysis are available in the references cited.  All analyses but one included fibres of "all lengths"; fibre definition (aspect ratio) was greater than 3:1; and results were expressed as fibres/ mg  dry lung.  Both “wet” (formalin-fixed) and “block” (paraffin-embedded) tissues were used.  All analyses included filtered bleach digestion; low-temperature ashing had also been used in specimen preparation in the McGill laboratory.  Fibre identification was performed using energy dispersive x-ray spectrometry (EDS) coupled with transmission electron microscopy (TEM).  For analysis, results from all laboratories were combined:  where results were available from more than one laboratory we chose our own (McGill) laboratory first (AD, BC, PS):  where results were available from more than one investigator within our own laboratory we chose the analysis performed by AD.  Ultimately 17 of the 22 cases came from the McGill laboratory and 16 were analyzed by AD; the remaining five cases were analysed by AC in his Vancouver laboratory. 


             We limit our observations in this paper to fibre concentration data and parameters of exposure. Fibre concentration data are expressed as geometric mean fibres per microgram dry lung in the groups and subgroups of interest:  comparisons for statistical significance were made using two-sample t tests having independent variance.   All values were log-transformed and where fibre concentration was “not detected” a value of  0.1 fibres/ µg dry lung was  used.  Parameters of exposure were compared to lung fibre concentrations  using Pearson and Spearman rank correlation coefficients.  All comparisons were made using Minitab® version 11.12 for Windows 95.


(1)  Subject characteristics and exposure history:

            Median values for the five subgroups are provided in Table 1.   With the exception of the two factory workers in Asbestos, who began work in 1940 and 1948, most men started  in the 1930s (N=11) or prior to 1930 (N=7).  The factory workers performed three and 28 years of factory work (much less than the average total work performed by the other groups); the man with only three years also worked in a shipyard for one year.  Their cumulative exposures derived from  historical midget impinger counts were an order of magnitude lower than those in other groups.  The median period of time between the end of their employment and their death from mesothelioma was twice that in other groups, but the latency period for the appearance of their tumors (37 and 38 years) was 10 years shorter, on average.  Other subgroups were not distinguishable from one another by demographic, disease or exposure variables.

(2)        Fibre Concentrations:

            Geometric mean fibre type concentrations for all Thetford men combined and for all Asbestos men combined, and for each of the five exposure subgroups, are presented in Table 2.  Crocidolite concentrations were significantly higher in workers at Asbestos, and when individual values are examined (10.1 and 4.0 fibres / mg in the Asbestos factory workers; 14.7, 7.0, 2.0, 0.7 and 0.0 fibres / mg in the Asbestos miners and millers) six of seven men had levels we consider diagnostic of significant occupational exposure to commercial amphiboles.  Three of the six also had elevated intrapulmonary amosite levels. Men in Thetford Mines had strikingly high levels of intrapulmonary tremolite.  This excess of tremolite was not explained by cumulative exposure levels, which were not significantly different from those in Asbestos miners and millers.   In addition, division of  intrapulmonary concentrations for crocidolite and tremolite by total cumulative exposure left a twenty-fold excess of tremolite fibres/  mg dry lung/ unit MPCFY in the 15 Thetford  men (p <.01)  and a 25-fold excess for crocidolite/ unit MPCFY in lungs of the seven subjects from Asbestos (p <.01). Chrysotile content was  higher in the lungs of miners and millers in Thetford, regardless of subgroup, and lowest in Asbestos factory workers.  However, in this instance adjustment of intrapulmonary chrysotile fibre levels through division by total cumulative exposure left no residual differences between the two areas.

            In Asbestos, it is notable that crocidolite not only formed 53% of all asbestos fibres in the lungs of factory workers, but also more than 20% of asbestos fibres in the five miners and millers studied.  Thus, while the tremolite per cent fraction was significantly lower in Asbestos chrysotile workers, their total amphibole concentrations constituted about the same fraction of asbestos lung burden as those observed in workers in Thetford Mines.

(3)  Correlation of fibre concentrations with exposure measurements:       As in a previous study of chrysotile miners, millers and textile workers (Sébastien et al. 1989), lung tremolite and chrysotile concentrations were both good indicators of past exposure.  Pearson correlation coefficients across all 22 men between cumulative exposure in MPCFY and lung chrysotile and tremolite content were .64 (p<.01) and .61 (p<.01) respectively, or .57 and .71 for log-transformed values.  Tremolite and chrysotile concentrations were closely correlated (.94, p<.001), principally in the Thetford groups of men, and chrysotile concentration correlated with crocidolite in the five miners and millers from Asbestos (r=.88, p<.05).  Duration of work did not correlate with any lung fibre concentration. Work was broken down in years spent in the mines, mills, factory or as tradesmen, but only the latter showed weak correlation with any aspect of fibre burden, and that with crocidolite asbestos alone (r=.45, p<.01).  Spearman rank correlation coefficients generally showed the same pattern, with especially high rank correlation between tremolite concentration and cumulative exposure (MPCFY) in the 7 Asbestos men (R=.96; p<.05). There was also an apparent “clearance effect” for crocidolite in this group (Cessation interval vs. crocidolite concentration R= -.92; p<.05), possibly due to a greater available time for fibre clearance between cessation of work and death than that observed in the Thetford workers (Table 1).  Interval between date of retirement and date of lung tissue acquisition (cessation interval) was not correlated with any other lung fibre concentration parameter in any group.

DISCUSSION:  In the absence of comparable data on denominators or appropriate referents, questions of risk cannot be properly examined with this data alone.  However, in this cohort of  “chrysotile” workers, crocidolite, amosite, and/ or tremolite were present in substantial concentrations in the lungs of most  mesothelioma cases.  These known biopersistent fibres corresponded well with cumulative exposure, but so did chrysotile - the human pulmonary dynamics of which are perhaps more complex than has heretofore been acknowledged.  Although the tremolite component of exposure in these groups is well described, the past commercial amphibole problem in the region which has lower tremolite (Asbestos) has only recently come to attention (Case and Sébastien 1987, 1988; Dufresne et al., 1995, 1996).  These results cannot be explained by the chrysotile hypothesis of mesothelioma genesis.  They are consistent with variable amphibole exposure to workers in both areas as reported in epidemiological and geological observations in the mining region (McDonald et al. 1995, 1996).   Recent observations indicate increased mesothelioma mortality among women in Thetford Mines (Camus, M.  personal communication), as opposed to Asbestos.  This finding, coupled with previous observations of pleural disease and both airborne and intrapulmonary tremolite excess near the mines of the Thetford/ Black Lake area, support the idea of a biological role for this amphibole (Case and Sébastien 1989; Case BW, 1991). 

            This extended abstract cannot examine all of  the issues involved in the interpretation of lung-retained fibre. Every measure of exposure used in epidemiological studies is subject to errors of measurement, classification and interpretation.  Careful use is nonetheless essential if we are to adequately explore past exposures.  That possible carcinogens which accumulate in lung (such as crocidolite and tremolite) may be less harmful than those which do so to a lesser degree (such as glass fibre, or chrysotile) seems biologically untenable, even given our inadequate knowledge of mechanism.  The idea that “pleural burden” may be  more useful in assessing a pleural tumour appeals until we realise the opportunities for error due to specimen contamination by short chrysotile fibres; opportunities which have been realised in published work (Case BW 1994).  To ignore the overall evidence could lead to overestimation of chrysotile risk for mesothelioma while inadequately addressing the problem of fibrous tremolite.









Median Demographic and Exposure variables:


Year Start

Age Start

Net Years Worked

Latency (years)



Cessation Interval 2

Asbestos Miners/Millers








Asbestos Factory








Thetford:  both areas








Thetford Central








Thetford Peripheral









1.  Median cumulative exposure in (million particles per cubic foot) X (years worked) (see text).

2.  Interval between end of work and time of death from mesothelioma.

*    p < .05 vs. Thetford Central area men but no other significant differences (Mann-Whitney).


TABLE 2:  Asbestos fibre concentrations in lungs of mesothelioma patients in the Québec cohort:                  comparison of two mining regions and five subgroups.



Geometric Mean Fibre Concentrations 1
















Asbestos Miners/ Millers








Asbestos Factory Workers
























Thetford Central Area








Thetford  Peripheral Area








Thetford:  Both Areas















1.   Geometric mean of concentrations for all fibres of given type in fibres of all lengths, aspect ratio greater than  3:1, per microgram dry lung.

2.    Geometric mean of cumulative lifetime exposure in (million particles per cubic foot) X (years worked);  note difference between these values and median values in Table 1.

3.  Below limit of detection.  For geometric mean calculations set at 0.1 fibres/ µg dry lung.


*    p < .01 vs. THETFORD men who worked in central area or in both areas.

**  p < .01 vs. all THETFORD men combined.

     p < .01 vs. THETFORD men in Central Area and p < .05 vs. THETFORD men in Both Areas.

    p < .05 vs. THETFORD men in  Both Areas.





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