Study Questionnaire
The information on doctor diagnosed asthma among participants was collected by the Chinese version of Epidemiologic Standardization Project Questionnaire of the American Thoracic Society (ATS-DLD-78-A), which were completed by parents or guardians. Doctor diagnosed asthma was defined by the question “Has a doctor ever diagnosed this child with asthma?” In addition, asthma for family members was also collected by the questions “Has this child’s father ever had asthma?”, “Has this child’s mother ever had asthma?”, “Has this child’s paternal grandfather ever had asthma?”, “Has this child’s paternal grandmother ever had asthma?”, “Has this child’s maternal grandfather ever had asthma?” and “Has this child’s maternal grandmother ever had asthma?”. According to the total number of family members affected by asthma, participants were classified into four groups, no family member affected, 1 family member affected, 2 family members affected and ≥ 3 family members affected.