Risk of hospital admission for COPD following smoking cessation and reduction: a Danish population study

BACKGROUND: Little is known about the effects of changes in smoking habits on the subsequent risk of chronic obstructive pulmonary disease (COPD). The aim of this study was to investigate the relationship between smoking cessation and reduction and admission to hospital for COPD in a general population sample. METHODS: A total of 19,709 participants from three prospective population studies in Copenhagen were followed with record linkage for date of first hospital admission for COPD until 1998 (mean follow up 14 years). Heavy smokers (>/=15 cigarettes/day) who reduced their tobacco consumption by at least 50% between the two initial examinations without quitting and smokers who stopped smoking during this time were compared with continuous heavy smokers using a Cox proportional hazards model. RESULTS: During the follow up period 1,260 subjects (741 men and 519 women) were admitted to hospital for COPD. After multivariate adjustment, quitting smoking was associated with a significant reduction in the risk of hospital admission. The relative hazard (HR) was 0.57 (95% confidence interval (CI) 0.33 to 0.99). Those who reduced smoking did not show a significantly lower risk of hospitalisation than continuing heavy smokers (HR 0.93 (95% CI 0.73 to 1.18)). Exclusion of events during the first 5 study years, detailed adjustment for lung function, or restriction of analyses to participants with impaired pulmonary function did not reverse the observed trend. CONCLUSIONS: Self-reported smoking cessation is associated with a reduction in the risk of COPD morbidity of approximately 40%; the benefit of smoking reduction is questionable.