IFNG and allergic disease: In multivariate analysis, exposure to PM2.5 in AR children was significantly associated with the methylation level of the promoter region in IFN-γ after the adjustment for exclusive breastfeeding within 4 months after birth and parental history of allergic disease, which showed significant difference between the AR and the control (β = 1.441, SE = 0.672, p = 0.039).