‘exposed’ into two exposure groups: ‘substantial exposure’
was assigned to subjects who had been exposed to
medium or high concentrations, during more than 5% of
their work week, and for 5 years or more, whereas ‘nonsubstantial
exposure’ was assigned to the remaining exposed
subjects. Exposures having occurred less than five
years previous to the index date were discounted on
latency grounds. Other cumulative exposure indices were
calculated using different combinations of weights to the
exposure dimensions frequency, concentration, duration
and latency. None of these indices showed better goodnessof-fit
than the simple categories described above so they
are not presented here.
Besides treating smoking as an a priori confounder, we
explored potential effect modification by smoking. Since
the number of never smokers among cases was very low,
the non-smokers category was supplemented with lifetime
low intensity smokers. Operationally, we defined lifetime
low intensity smokers as individuals having a CSI value
below the 25th percentile on this scale. Because of the way
it is constructed [28], the CSI index does not translate easily
onto the duration or daily amount of pack-year scale.
We can illustrate the amount of smoking in these categories
by showing two smoking profiles that would fall on the
25th percentile of the CSI scale, namely: a current smoker
who smoked three cigarettes per day during 40 years (with
lifetime cumulative exposure of 6 pack-years), or a former
smoker who smoked six cigarettes a day for 30 years and
quit 10 years ago (with cumulative exposure of 9.8 pack
years). Smokers with CSI values above the 25th percentile
were considered medium/heavy smokers. To evaluate the
statistical significance of the difference in ORs between
the two strata of smokers, we carried out an analysis based
on all subjects including the two variables, smoking status
(binary) and exposure to wood dust (binary), by testing
their cross-product term. The continuous CSI variables
were maintained as a covariate in the models to avoid any
residual confounding within the smoking status strata.
The associations between wood dust and the most
prevalent histologic types of lung cancer, namely squamous
cell, adenocarcinoma, small cell and large cell, were
also evaluated.
Results
Table 1 shows the distribution of cases and controls according
to different socio-demographic characteristics. In
both studies, compared to controls, cases were more likely
to have French ancestry, had fewer years of education,