[Confounding of occupational cancer risk in epidemiological studies due to ignorance of smoking data as exemplified by bladder and lung cancer in painters].

Studies on occupational risk factors for cancer are frequently carried out without information on smoking habits. However, the proportion of smokers may vary considerably across occupational groups. Thus it is unclear, whether an observed increased cancer risk in an occupational group is at least partly the result of a higher proportion of smokers in that group. The paper demonstrates the possible extent of such confounding by using hypothetical and empirical examples of studies concerning the lung cancer and bladder cancer risk in painters. A formula for the indirect control of such confounding is applied. Relative risks greater than 1.2 for bladder cancer and 1.3 for lung cancer should thus not be considered solely the result of confounding. The degree of confounding is less than generally assumed. However, the evaluation of this degree remains questionable in case of an interaction between smoking and occupational exposure.