2.1. Participants
This is a cross-sectional study performed in kindergarten children and school students from China. Medical examination at child care department or school health school students, aged 3 to 17 ages healthy children. Informed consent was obtained from the parents or legal guardians of the children. Exclusion criteria: ① Respiratory tract infection occurred within 4 weeks before inclusion; ②patients with congenital heart disease, respiratory system, neuromuscular system, immune system and other diseases; ③ a family history of asthma; ④ cannot cooperate with the completion of pulse oscillation lung function detection; ⑤ abnormal pulmonary ventilation function; ⑥ There are smokers in the family. A total of 7000 children were enrolled in the study. This study was approved by the Ethics Committee of Shanghai Children’s Medical Center (Ethics No: SCMCIRB-K2017007), and all branches followed the ethics of the main research unit. The age of enrolled children ranged 4 to 17 years and their body height ranged 100 to 189 cm.