Study Characteristics and Quality Assessment
Table S3, S4 summarizes the detailed characteristics and quality assessment of all studies, respectively. Most articles were case-control studies. Geographical location included: Europe (12 studies), America (2 studies), and Asia (1 study). The total number in the 15 studies was 18,813 for cases (having T1DM) and 3,018,809 for controls (not having T1DM). The prevalence of asthma was 8.51% in the cases and 6.84%, in the controls. A moderate risk of bias was observed in the NOS assessment. The selection domain was mostly related to a case-control design; consecutive or representative sampling was not used. The exposure domain was assessed using questionnaire, and thus subjected to recall bias.