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.)
Introduction:
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.
Methods:
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.
Statistics:
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.
RESULTS:
(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.
TABLE 1:
SUBJECT
CHARACTERISTICS & EXPOSURE PROFILES IN FIVE SUBGROUPS (median values)
Median Demographic and Exposure variables: |
N |
Year Start |
Age Start |
Net Years Worked |
Latency (years) |
Exposure (MPCFY) 1 |
Cessation Interval 2 |
|
Asbestos Miners/Millers |
5 |
1936 |
18 |
37 |
49 |
373 |
11 |
|
Asbestos
Factory |
2 |
1944 |
25 |
16 |
37 |
42* |
22 |
|
Thetford: both areas |
5 |
1935 |
22 |
38 |
48 |
188 |
6 |
|
Thetford
Central |
9 |
1935 |
18 |
32 |
50 |
319 |
16 |
|
Thetford
Peripheral |
1 |
1947 |
31 |
34 |
38 |
325 |
4 |
|
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 |
N |
Tremolite |
Chrysotile |
Crocidolite |
Amosite |
MPCFY 2 |
|
|
|
|
|
|
|
|
|
|
|
Asbestos Miners/ Millers |
5 |
7.50 |
4.35* |
1.69 |
0.25 |
126 |
|
|
Asbestos Factory Workers |
2 |
0.47‡ |
2.12 |
6.34 |
0.32 |
41† |
|
|
ALL ASBESTOS MEN |
7 |
3.40** |
3.54* |
2.47** |
0.27 |
92 |
|
|
|
|
|
|
|
|
|
|
|
Thetford Central Area |
9 |
119 |
10.5 |
n.d.3 |
n.d. |
298 |
|
|
Thetford Peripheral Area |
1 |
101 |
32.0 |
n.d. |
n.d. |
325 |
|
|
Thetford: Both Areas |
5 |
85 |
15.7 |
n.d. |
n.d. |
196 |
|
|
ALL THETFORD MEN |
15 |
105 |
12.9 |
n.d. |
n.d. |
260 |
|
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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