BACKGROUND: There are few data on the quantitative effects of cigarette smoking on lung function in young adults. These effects are important in the understanding of the early stages of chronic airflow obstruction. METHODS: A longitudinal study over eight years was carried out to estimate quantitatively the effect of cigarette smoking on ventilatory lung function in young adults and to examine the possibility that the effect is modified by other factors. The study population were 15 to 40 years of age at initial examination, when they underwent spirometry and completed an interviewer administered questionnaire on respiratory health. Eight years later 391 of the subjects were re-examined (38% response rate). The quantitative effect of cigarette smoking during the study period on the average change of forced expiratory volume in one second (FEV1) over time (delta FEV1) was estimated in two linear regression models that included potential confounders and other determinants of outcome. RESULTS: The first model showed a significant dose-response relation between the average rate of smoking during the study period and delta FEV1, giving an estimate of annual change in FEV1 of -0.42 ml for each cigarette smoked per day (-8.4 ml for each pack) (p = 0.04). In the second model, which took smoking before the study period as a potential confounder, the effect of smoking during the study period was slightly smaller (-0.33 ml/year for each cigarette smoked per day). This indicated that smoking before the study period had a marginal latent effect on delta FEV1 during the study. However, neither the effect of smoking before the study nor that of smoking during the study was significant, presumably because of collinearity. Interactions between cigarette smoking and gender, wheezing, atopy, and exposure to environmental tobacco smoke during the growth period were not significant with respect to their effect on the relation between cigarette smoking and delta FEV1. CONCLUSION: Cigarette smoking has a dose related adverse effect on the evolution of ventilatory lung function in young adulthood.
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