ARE THE APPARENT EFFECTS OF CIGARETTE-SMOKING ON LUNG AND BLADDER CANCERS DUE TO UNCONTROLLED CONFOUNDING BY OCCUPATIONAL EXPOSURES

It has been suggested that the well known associations between smoking and cancer may in part: reflect inadequately controlled confounding d ue to occupational exposures. The purpose of the present. analysis is to describe the association between cigarette smoking and both lung an d bladder cancers, taking into account the potential confounding effec ts of over 300 covariates, most of which represent occupational exposu res. A population based case-control study was undertaken in Montreal to investigate the associations between a large variety of environment al and occupational exposures, on the one hand, and several types of c ancer, on the other. Interviews were carried out: with male incident c ases of several sites of cancer, including 857 lung cancers and 484 bl adder cancers. A group of non-smoking-related cancers, comprising 1,70 7 interviewed subjects, was used as one control group. Additionally, 5 33 population controls were interviewed and constituted a second contr ol group. Interview information included detailed lifetime smoking his tories, job histories, and other potential confounders. Each job histo ry was reviewed by a ream of experts who translated it into a history of occupational exposures. These occupational exposures, as well as no noccupational covariates, were treated as potential confounders in the analysis of cigarette smoking effects. Regardless of whether populati on controls or cancer controls were used, the odds ratio (OR) between smoking and lung cancer (ranging from 12 to 16 for ever vs never smoke rs) was not materially affected by adjustment for occupational exposur es. The odds ratios for bladder cancer (ranging from 2 to 3) were also unaffected by confounding due to occupational exposures. Because of t he extensive information available on occupational exposures, it is po ssible to affirm that the reported associations between smoking and bo th lung and bladder cancer do nor result from inadequate control for o ccupational confounders. The consistency of; findings when using two d ifferent control groups further diminishes the possibility that the re sults represent an artifact of inadequate research design.