When the study population was stratified according to smoking status, the following factors were not significantly different between never smokers and former/current smokers: the frequencies of UACS, asthma, chronic laryngitis, abnormal chest radiographs, hypertension, diabetes, hyperlipidemia, stroke, cardiac disease, and history of tuberculosis. However, the prevalence of COPD was higher in current smokers (Table 2 and Table S1).