AHR and asthma: Although the IFN-γ content barely declines after nano-SiO2 exposure in all groups (Fig. 6), we are convinced that nano-SiO2 could lead to a tissue IL-4 increase with or without OVA, which may, in turn, accelerate the Th1/Th2 cytokine imbalance and aggravate the symptoms of OVA-induced asthma such as AHR (Fig. 3) and airway remodeling (Fig. 7).