The impact of cigarette smoking on asthma: a population-based international cohort study.

BACKGROUND The prevalence rates of smoking in subjects with asthma have frequently been reported as similar to those in the general population; however, available data are not up-to-date. There is only limited and somewhat conflicting information on the long-term effects of smoking on health outcomes among population-based cohorts of subjects with asthma. We aimed to investigate changes in smoking habits and their effects on forced expiratory volume in 1 s (FEV(1)) in subjects with asthma in comparison with the rest of the population, focusing on the healthy smoker effect. METHODS We studied 9,092 subjects without asthma and 1,045 with asthma at baseline who participated in both the European Community Respiratory Health Survey I (20-44 years old in 1991-1993) and II (1999-2002). RESULTS At follow-up, smoking was significantly less frequent among subjects with asthma than in the rest of the population (26 vs. 31%; p < 0.001). Subjects with asthma who were already ex-smokers at the beginning of the follow-up in the 1990 s had the highest mean asthma score (number of reported asthma-like symptoms, range 0-5), probably as a result of the healthy smoker effect (2.80 vs. 2.44 in never smokers, 2.19 in quitters and 2.24 in smokers; p < 0.001). The influence of smoking on FEV(1) decline did not depend on asthma status. Smokers had the highest proportion of subjects with chronic cough/phlegm (p < 0.01). CONCLUSION One out of 4 subjects with asthma continues smoking and reports significantly more chronic cough and phlegm than never smokers and ex-smokers. This stresses the importance of smoking cessation in all patients with asthma, even in those with less severe asthma.