Statistics
The prevalence of wheeze/asthma was calculated as the ratio of cases to total number of responders. The Mantel-Haenzel extension of chi-square test for trends was applied to evaluate prevalence tendency among surveys. Prevalence rate ratios (i.e. prevalence rate changes) between consecutive surveys were calculated by logistic regression analysis with adjustment for sex. Spirometric parameters were compared with one-way ANOVA with Tukey’s post-hoc test for multiple comparisons. Statistical analyses were performed in IBM SPSS version 25 (IBM Corp., Armonk, NY).