Baseline characteristics of the study sample, presented as mean±sd for continuous and percentage for categorical variables.

§BMI: body mass index.*FEV1: Forced expiratory volume in 1 s.##Season was defined as winter (December-March), spring (April-May), summer (June-September), and autumn (October–November).#Exacerbations requiring either hospitalisation or treatment with oral antibiotics or oral steroids.§§PaO2: arterial oxygen tension.**Associations were tested with t-test and Chi-square.