Effect of outdoor and indoor nitrogen dioxide on respiratory symptoms in schoolchildren.

Nitrogen dioxide (NO(2)), an oxidant gas that contaminates both outdoor and indoor air, is considered to be a potential risk factor for asthma. We investigated concurrently the effects of outdoor and indoor NO(2) on the prevalence and incidence of respiratory symptoms among children. A cohort study was carried out over 3 years on 842 schoolchildren living in seven different communities in Japan. Indoor NO(2) concentrations over 24 hours were measured in both winter and summer in the homes of the subjects, and a 3-year average of the outdoor NO(2) concentration was determined for each community. Respiratory symptoms were evaluated every year from responses to questionnaires. The prevalence of bronchitis, wheeze, and asthma significantly increased with increases of indoor NO(2) concentrations among girls, but not among boys. In neither boys nor girls were there significant differences in the prevalence of respiratory symptoms among urban, suburban, and rural districts. The incidence of asthma increased among children living in areas with high concentrations of outdoor NO(2). Multiple logistic regression analysis showed that a 10 parts per billion (ppb) increase of outdoor NO(2) concentration was associated with an increased incidence of wheeze and asthma (odds ratios [OR] = 1.76, 95% CI : 1.04-3.23 and OR = 2.10, 95% CI : 1.10-4.75, respectively), but that no such associations were found with indoor NO(2) concentration (OR = 0.73, 95% CI : 0.45-1.14 and OR = 0.87, 95% CI : 0.51-1.43, respectively). These findings suggest that outdoor NO(2) air pollution may be particularly important for the development of wheeze and asthma among children. Indoor NO(2) concentrations were associated with the prevalence of respiratory symptoms only among girls. Girls may be more susceptible to indoor air pollution than boys.