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at high cumulative exposure. Both of these measures of risk are larger when the Colorado population is
used as a standard, with the SRR rising from 0.7 to over 5.0.
Table 3:
Lung Cancer (ICD 162-163) Mortality by Cumulative Exposure; White Male Cadmium
Workers Hired on or After 1/1/26 Compared to Colorado Death Rates, 1950-79
(Adapted from Thun et al. 1985, Table 8).
Cumulative
exposure
(mg-days/m 3 )
Person-years
at risk
Deaths SMR SRR
≤384 7005 2 76 .70
385-1920 5825 7(6)* 212(182)* 2.29(l.96)*
≥1921 2214 7 387 5.09
Colorado white males 100 1.00
* Numbers in parentheses exclude one lung cancer death which was originally miscoded as being
due to another cause.
Thun et al. (1985) calculated the standardized rate ratio (SRR) for each of 3 exposure groups. (The
person-years at risk, rather than individual workers, were classified by cumulative exposure to that point
in time.) The SRR is suitable for subgroup comparisons, but not for external comparisons. A
regression of the SRRs yielded a slope of 7.33 × 10 -7 which differed from zero with a probability of
0.0001.
Selection criteria described by Thun et al. (1985) appear to have been unbiased: all retired, deceased,
and active employees who had worked a minimum of 6 months in production areas of the plant were
included in the cohort. In calculating cumulative exposure, dates of interruption of employment were
accounted for. Since more than 80% of the workers were followed for 20 or more years it is likely that
the follow-up was sufficient for many latent cadmium-induced cancers to become manifest and lead to
death. Trained nosologists evaluated the death certificates. As indicated by Thun et al., one lung cancer
death was originally miscoded as being due to another cause. Removal of this death from the lung
cancer deaths (i.e. restoring it to the original, but incorrect coding) is necessary in order that the
comparison with general population rates be unbiased (since miscodings also occur in the general
population). However, the findings are not altered in any substantial way by this reclassification.
Exposure categories were chosen prior to the analysis. The cumulative exposure for all person-years
was miscalculated by Thun et al. (1985) because they included non-workdays. This does not cause
bias for purposes of. inference since the misclassification was equivalent for all exposure categories. It
would, however, alter the dose-response relationship, and therefore DHS staff adjusted for this error in
conducting their risk assessment, since an overestimate of exposure would result in an underestimation
of potency. The corrected exposures are shown in Tables 2 and 3.
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