The joint effect of smoking and respiratory symptoms on risk of lung cancer.

In 1962 a cohort of 4604 Finnish men were interviewed about their smoking habits and cardiorespiratory symptoms. The cohort was followed up for deaths and incidence of lung cancer from 1963 to 1980 in order to study the effect of smoking and respiratory symptoms on the risk of lung cancer. The joint effect of smoking, age and respiratory symptoms on the risk of lung cancer was studied using a log-linear modelling technique. When analysed simultaneously with smoking, the symptoms of phlegm, shortness of breath and wheezing were all significantly associated with increased lung cancer risk. The joint effect of smoking and phlegm as well as that of smoking and wheezing was close to being multiplicative. Even if smoking is a causative factor in both these symptoms and in lung cancer, the symptoms seem to have a separate role as predictors of lung cancer risk.