Variation in Nordic Work-Related Cancer Risks after Adjustment for Alcohol and Tobacco

Background: Alcohol and tobacco strongly increases the risk of cancers of the tongue, mouth, pharynx, larynx, and oesophagus, and are also established risk factors for cancer of the liver, colon, and rectum. It is well documented that these habits are unequally distributed among occupational groups. Most occupational cohort studies lack information on these potentially important confounders, and may therefore be prone to bias. Aim: The aim of the study is to present Nordic standardized incidence ratios (SIRs) for alcohol and tobacco related cancer by occupation, after adjustment for alcohol and tobacco, and to compare to the unadjusted SIRs. Material and Methods: The study is based on the Nordic Occupational Cancer (NOCCA) database. We used confirmatory factor analysis models for simultaneous analysis of the cancer sites related to alcohol and tobacco, to obtain factors that allow for computation of adjusted expected numbers from the reference rates. We then calculated adjusted SIRs for the relevant cancer sites for each occupation. Results: For some occupations and cancers, the changes of risk estimates were striking, from significantly high to significantly low and vice versa. Among Nordic farmers, unadjusted SIRs for cancer of the mouth and oesophagus were 0.56 (95% confidence interval (CI) 0.51⁻0.61) and 0.67 (CI 0.63⁻0.70), respectively. After adjustment, estimates changed to 1.10 (CI 1.01⁻1.21) and 1.16 (CI 1.10⁻1.22). Unadjusted SIR for pharynx cancer among wood workers was 0.83 (CI 0.75⁻0.91), adjusted SIR was 1.14 (CI 1.03⁻1.25). For larynx cancer, results in the opposite direction were seen: unadjusted SIR for economically inactive was 1.38 (CI 1.31⁻1.46) while the adjusted SIR was 0.91 (CI 0.86⁻0.96). Conclusions: Adjustment for the latent indicators of alcohol and tobacco consumption changed risk estimates for several occupations, gave a less confounded description of risk, and may guide in the identification of true risk factors.