INTRODUCTION After intensive tobacco control efforts in recent decades, the prevalence of active smoking has decreased. However, the hazardous effect of indirect exposure to cigarette smoke is often underestimated, especially in children. We aimed to investigate the effect of parental smoking on the respiratory morbidity of the children of parents who smoke by evaluating the relationship between parental smoking behavior and children's respiratory symptoms.

METHODS We conducted a cross-sectional follow-up study of 31,584 children aged 6-11 in an urban community in Anyang City, Korea. The children's parents were asked about their smoking status and completed questionnaires regarding their children's symptoms related to asthma and other upper or lower respiratory illnesses. Our analysis focused on a comparison of the frequency of respiratory and ocular symptoms according to parental smoking status, whether it was non-smoking (Non-S), indirect passive smoking (third-hand smoking, THS) or direct passive smoking (second-hand smoking, SHS).

RESULTS The children with Non-S patients were 40.9%, THS group 40.6%, and SHS group 18.5%. THS group showed lower ORs for most respiratory symptoms when compared with those of SHS group, however, THS group revealed increased ORs compared with Non-S in cough-related symptoms. There was a linear trend in frequencies of cough and sputum-related symptoms according to the degree of exposure to cigarette smoke (P < 0.05).

CONCLUSION The prevalence of respiratory symptoms increased in children exposed to parental smoking including SHS and THS. To avoid the risk of respiratory and allergic disease by environmental tobacco smoke, absolute smoking cessation by parents is strongly recommended.