2.1. Data Source

The National Health Insurance (NHI) program of Taiwan was established in 1995 and provides universal coverage to over 99% of the residents of Taiwan. The NHIRD is a comprehensive database that includes information on hospitalization, emergency care and medical visits. We conducted this population-based retrospective cohort study using data from the Longitudinal Health Insurance Database (LHID), a subset constructed from original registration files and original claims data in the NHIRD. The LHID contains the data of one million enrollees randomly sampled individuals from the NHIRD. Diagnoses were coded with International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnostic codes. The NHIRD ensures the encryption of patient information; therefore, informed consent is not required. This study was approved by the Institutional Review Board of China Medical University (CMUH104-REC2-115[CR-4]). We also used the TAQMD for obtaining information on the air pollutants PM2.5, PM10 and SO₂. We combined and stratified the LHID and the TAQMD data by linking the residential areas of enrollees with nearby air quality monitoring stations.