2. The SECOND STEP is a community-based nested case-control study to make sure whether the significant biomarkers in first part can predict rapid decline of lung function. The study will be a community based, multicenter prospective cohort designed, which including a total of 2,400 study subjects. EBC and blood are collected at the baseline and followed up for two years. Case group is defined as subjects with rapid decline of lung function and new COPD patients. Control group is selected according to 1:1 matching with age and gender.