Smoking and new-onset asthma in a prospective study on Italian adults

Background: The existence of a cause-effect relation between active smoking and new-onset asthma in adults, although supported by several studies, has not been proven yet. Aims: We aimed to prospectively study asthma incidence as a function of smoking habits in the Italian adult population. Methods: A population-based cohort of 5,241 non-asthmatics was enrolled in Verona and Sassari in 1998-2000. The cohort was contacted again in 2007-2009 within the Gene-Environment Interactions in Respiratory Diseases study, and 3,187 subjects (60.8%) answered a screening questionnaire on smoking habits and respiratory disorders. The relation between smoking habits and self-reported new-onset asthma, defined as asthma attacks/use of medicines for asthma, was investigated by a multivariable logistic model. Results: During follow-up, 145 new cases of asthma were observed, yielding a cumulative incidence of 4.6% (95% CI 3.9-5.4); cumulative incidence of asthma did not significantly differ among never-smokers (76/1,666 = 4.6%), ex-smokers (30/554 = 5.4%) and current smokers (39/883 = 4.4%) (p = 0.641). In a multivariable analysis, the most important risk factor for asthma onset was allergic rhinitis (OR = 4.00, 95% CI 3.68-4.35). Compared to never-smokers, the risk of asthma onset was slightly increased in ex-smokers (OR = 1.28, 1.09-1.49) but not in current smokers (OR 1.01, 0.66-1.53). Current smoking became a significant predictor only when both new-onset wheezing and new-onset asthma were considered as the outcome (OR = 2.03, 1.35-3.05). Conclusions: In this prospective study, current smoking was not a risk factor for new-onset asthma, unless new-onset wheezing was also considered. The increase in asthma incidence among ex-smokers was likely due to reverse causation.