Statistical Analyses
We analysed asthma, FENO, allergen-specific IgE level, and eosinophils, in a four-variate twin model. In this model, the covariance between variables within individuals, as well as the covariance between twins in pairs, are explicitly modelled in structural equation models. Asthma was analyzed as a binary variable and adjusted for age and sex. The data for FENO, allergen-specific IgE level, and eosinophils were all log-transformed to obtain a distribution closer to normal, and adjustments were done for ICS, age and sex. Due to the sampling scheme, the population may have a skewed distribution (compared to the source population) of investigated variables; thus, we re-weighted all analyses according to sampling probability.