Background: Permanent smoking cessation reduces loss of pulmonary function. Less is known in the long term about individuals who give up smoking temporarily or quitters with lower initial pulmonary function. Little is known also about the relationship between decline in pulmonary function and mortality. We examined these aspects and the association between smoking, decline in pulmonary function, and mortality.
Methods: Two middle aged male Finnish cohorts of the Seven Countries Study and their re-examinations on five occasions during a 30 year period of follow up were analysed.
Results: During the first 15 years (n=1007) adjusted decline in forced expiratory volume in 0.75 seconds (FEV(0.75)) was 46.4 ml/year in never smokers, 49.3 ml/year in past smokers, 55.5 ml/year in permanent quitters, 55.5 ml/year in intermittent quitters, and 66.0 ml/year in continuous smokers (p<0.001 for trend). Quitters across the entire range of baseline FEV(0.75) had a slower decline in FEV(0.75) than continuous smokers. Among both continuing smokers and never smokers, non-survivors had a significantly (p<0.001) more rapid decline in FEV(0.75) than survivors. The adjusted relative hazard for total mortality was 1.73 (95% confidence interval (CI) 1.41 to 2.11) and 1.24 (95% CI 1.02 to 1.52) in the lowest and middle tertiles of decline in FEV(0.75). Never smokers, past smokers, and quitters had significantly lower total mortality than continuous smokers, partly because of their slower decline in FEV(0.75).
Conclusion: These results highlight the positive effect of smoking cessation, even intermittent cessation, on decline in pulmonary function. Accelerated decline in pulmonary function was found to be a risk factor for total mortality. The beneficial effect of smoking cessation on mortality may partly be mediated through a reduced decline in pulmonary function.