Inclusion of Additional Significant Influencing Factors. Additional significant influencing factors were included in adults' FeNO norms [9–24]: race, ethnicity, atopy, allergy, total IgE, serum eosinophil cationic protein, smoking status, interaction between sex and smoking habits, asthma diagnosis, ambient NO, and upper respiratory tract infection symptoms. Here, IGHE is linked to Recurrent upper respiratory tract infections.