Most studies on tobacco smoking have focused on daily-smokers. Occasional smokers, who have never smoked daily, have often been included in the reference group of never-smokers. We have investigated the association between occasional smoking and cancer of the bladder, kidney, pancreas, upper aero-digestive tract and lung.
The study population consisted of 158,488 persons, who provided information on occasional smoking, within the European Prospective Investigation into Cancer and Nutrition (EPIC), 780 of whom developed a smoking-related cancer. We used Cox proportional hazard model, stratified by gender and country to estimate incidence rate ratios (IRR) for smoking-related cancers.
The results suggest that occasional smokers have a higher risk of bladder cancer (IRR: 1.92, 95% confidence interval (CI) 0.93–3.98) and of the major smoking-related cancers combined (IRR: 1.24, 95% CI 0.80–1.94) than true never-smokers. Including occasional smokers in the reference group resulted in a lower risk estimate for former and current smokers.
Occasional smoking should be discouraged.