Smoking as a confounder in case-control studies of occupational bladder cancer in women

Background: In studies in men, risk estimates on occupation and bladder can cer are distorted by about 10% when not adjusting for smoking. We examined the degree to which occupational risk estimates for bladder cancer in women are confounded by smoking, and the degree of residual confounding by inade quate control of this effect. Methods: Primary data of 11 case-control studies on occupation and bladder cancer from Denmark, France, Germany, Greece, Italy, and Spain were pooled. Information for smoking and lifetime occupational history for 700 female c ases and 2,425 female controls ages 30-79 was abstracted and recoded. Logis tic regression was used to calculate odds ratios (OR) by occupation, applyi ng five models which differed in their degree of adjustment for smoking. Results: In major occupational groups risk estimates were distorted by less than 10% when not adjusting for smoking. A statistically significant exces s risk for bladder cancer was found in 13 specific occupations and industri es. In most occupations, adjustment for smoking led the ORs towards the nul l value, but all statistically significant associations were maintained aft er adjustment. In three occupations (lathe operators, field crop workers, a nd wood manufacturers), a statistically significant excess risk was masked when not adjusting for smoking. In six occupations, estimates were distorte d by more than 10% (-22% up to +40%). In occupations where smoking acted as a positive confounder, the proportion of confounding removed using a dicho tomous smoking variable (ever/never) was around 60%. In one occupation (buy ers), controlling for smoking status (ever, never) led to over-adjustment, because the percentage of smokers was high but the quantity smoked was low. Conclusions: Tobacco smoking was not found to be a major confounder for the association between occupation and bladder cancer in women Most of this co nfounding effect could be removed by adjustment by smoking status (ever/nev er),. without consideration of amount or duration of smoking. Am.. Ind. Med . 36:75-82, 1999. (C) 1999 Wiley-Liss, Inc.