This is a multi-center, prospective, cohort study. A total of 10,000 people aged 40～75 without lung disease will be recruited and followed for 3 years. By measure the rate of decline in forced expiratory volume at one second（FEV1） and baseline inflammatory biomarkers in exhaled breath condensate and peripheral blood, we aim to explore the predictive model for chronic obstructive pulmonary disease（COPD） in China.

This is a multi-center, prospective, cohort study. A total of 10,000 subjects aged 40～75 without history of lung disease will be recruited and followed for 3 years. The Department of Respiratory and Critical Care of Peking University First Hospital is responsible for this research. Other 9 units participating in the study, include the The Second Hospital of Hebei Medical University, Henan Provincial People's Hospital, The Second Hospital of Jilin University, The First Affiliated Hospital of Xi'an Jiaotong University, Shandong Provincial Hospital Affiliated to Shandong University,Shanxi Dayi Hospital, Tianjin Medical University General Hospital, The Affiliated Hospital of Inner Mongolia Medical University,The First Hospital of Qinhuangdao. Some questionnaire such as St George's Respiratory Questionnaire (SGRQ), income class, educational level, comorbidity, smoking habit and biomass smoke exposure history will be collected. The baseline level of Interleukin 6 (IL-6), high-sensitivity C-reactive Protein (hs-CRP), microRNAs-23a (miR-23a) in peripheral blood and pH value in exhaled breath condensate (EBC) will be measured, lung spirometry will be tested in the first, second and fourth years. Primary outcome is the incidence of COPD, multivariate regression analysis will be used to establish the predictive model for COPD in China.

The study protocol has been approved by the Peking University First Hospital Institutional Review Board (IRB) (2018-31). Any protocol modifications will be submitted for the IRB review and approval.

Inclusion Criteria:

1. aged at 40-75 years old;
2. FEV1/FVC>70% after inhaled bronchodilator
3. have willing to participate in this study, follow the research program and have the ability to sign the informed consent;
4. lived in a community for more than 1 years and has no plans to move out in the next 4 years
5. can be contacted

Exclusion Criteria:

1. history of asthma, COPD, lung cancer, active pulmonary tuberculosis, bronchiectasis, diffuse lung disease (interstitial pneumonia, pulmonary sarcoidosis, occupational lung disease, sarcoidosis et al) and pleural disease;
2. history of lobectomy and / or lung transplantation;
3. predicted life expectancy less than 3 years;
4. history of severe psychiatric illnesses, mental disorders, neurological disorders, malignant tumors, chronic liver disease, heart failure, autoimmune diseases, chronic kidney disease;
5. Alcoholism, drug abuse or abuse of toxic solvents;
6. Cannot finish long term follow-up or poor compliance;
7. Do not provide consent

A total of 10,000 subjects aged 40～75 without history of lung disease will be recruited and followed for 3 years.