home edit2 whole TSD November 2002 PDF format - OEHHA

If the cadmium-exposed workers included a disproportionate number of individuals with exposures to
other agents responsible for lung cancer, then the observed association might be spurious. The potential
confounders with regard to lung cancer mortality in this cohort were smoking and arsenic exposure.
(a) Smoking:
Indirect evidence that smoking was not a confounder in this cohort is provided by the cardiovascular
death rate in this cohort, which was 35% lower than expected based on U.S. white male death rates. If
this cohort included a higher proportion of total smokers or heavy smokers as compared to the general
population of white males in the same age categories, then one would expect an increase (or at least not
a deficit) in the cardiovascular death rate as well.
Data on the smoking habits of these workers were provided to Thun et al. (1985) by the company.
The data came from company medical records and from a questionnaire survey mailed to surviving
workers or the next-of-kin in 1982. The results of this survey have elicited differing interpretations
depending on the choice of measure of smoking and on the choice of the comparison group. The 1985
paper by Thun et al. reported data on 70% of the workers. For these workers, the data indicated that
as of 1982, 77.5% were current or former smokers compared to 72.9% current or former smokers
among U.S. white males 20 years or older reported in the 1965 Health Interview Survey (HIS)
conducted by the National Center for Health Statistics. It is clear that these 2 figures are not
comparable since data from 1982 for the exposed group were compared with data from 1965 for the
control group.
In the updated report by Thun et al. (1986), presented at the Fifth International Cadmium Conference
in San Francisco, February 6, 1986, the authors provided a more meaningful comparison by limiting the
smoking analysis to the 49% of the cohort for whom lifetime smoking histories were available. These
data indicated that as of 1965 a larger percentage of the cadmium-exposed cohort were nonsmokers
and a smaller percentage were heavy smokers compared to general population rates available from the
HIS. The year 1965 was chosen since this was the midpoint of the study.
The percent who "ever smoked" was 77.5% in the cadmium-exposed cohort, and 76% in the total HIS
sample. The data on the cadmium workers represented information from only 36% of the cohort.
Given that the cohort under study was considerably older than the HIS sample, that the HIS survey was
done about 10 years earlier than the survey of the cadmium cohort, and that different information was
reported from these two surveys, the differences between the smoking habits of the total HIS sample
and those of the cadmium-exposed workers do not appear to be very large.
The magnitude of confounding from differential smoking habits can be assessed. A method to estimate
the contribution of smoking to lung cancer mortality in the cohort is described by Axelson (1978). The
method is applied to the lifetime smoking histories summarized by Thun et al. (1985). The calculations
(summarized in Table 4) are based on information regarding smoking habits in the exposed group,
smoking habits in the comparison group, and the relative risk for lung cancer at each level of smoking.
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