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

A common statement from exsmokers is that symptoms of asthma develop shortl y after smoking cessation. This study, therefore, investigated the relation ship 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 po pulation involved the 10,200 subjects who provided information on self-repo rted asthma and smoking habits from the first two examinations (baseline an d 5-yr follow-up), and the 6,814 subjects who also attended the third and l ast examination (10-yr follow-up). The point-prevalence of smoking cessatio n as well as the asthma incidence between examinations was estimated, and a multivariate logistic regression model was used to examine the relationshi p 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 examina tions was significantly related to reported asthma at follow-up. With never -smokers as the reference group and following adjustment for sex, age, chro nic bronchitis, level of forced expiratory volume in one second and pack-yr s of smoking, the odds ratio (OR) for developing asthma when quitting smoki ng 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 significant ly (OR 2.6 and 2.0, respectively). The results indicate that exsmokers have a higher incidence of self-reporte d asthma than never-smokers. It is likely that subjects perceive chronic ob structive pulmonary disease as asthma, hence the relationship between smoki ng cessation and asthma might be due to misclassification rather than causa lity.