Changes in smoking habits and risk of asthma: a longitudinal population based study.

A common statement from exsmokers is that symptoms of asthma develop shortly after smoking cessation. This study, therefore, investigated the relationship between smoking cessation and development of asthma in a large cohort from the Copenhagen City Heart Study (CCHS). The CCHS is a longitudinal, epidemiological study of the general population from the capital of Denmark, conducted between 1976 and 1994. The study population involved the 10,200 subjects who provided information on self-reported asthma and smoking habits from the first two examinations (baseline and 5-yr follow-up), and the 6,814 subjects who also attended the third and last examination (10-yr follow-up). The point-prevalence of smoking cessation as well as the asthma incidence between examinations was estimated, and a multivariate logistic regression model was used to examine the relationship between changes in smoking habits and development of asthma. During the study period, asthma incidence increased from 1.2-4.2%. Between examinations 1,316 subjects quit smoking. Smoking cessation between examinations was significantly related to reported asthma at follow-up. With never-smokers as the reference group and following adjustment for sex, age, chronic bronchitis, level of forced expiratory volume in one second and pack-yrs of smoking, the odds ratio (OR) for developing asthma when quitting smoking between examinations was 3.9 (95% confidence interval (CI) 1.8-8.2) from baseline to first follow-up and 3.1 (95% CI 1.9-5.1) from first to second follow-up. Continuing smoking also increased the risk of asthma significantly (OR 2.6 and 2.0, respectively). The results indicate that exsmokers have a higher incidence of self-reported asthma than never-smokers. It is likely that subjects perceive chronic obstructive pulmonary disease as asthma, hence the relationship between smoking cessation and asthma might be due to misclassification rather than causality.