Numerous epidemiological studies have suggested a relationship between nitrogen dioxide (NO2) and impaired respiratory function or asthma symptoms. One of main source of NO2 is considered to be traffics. However, according to Japanese Social and Demographic Statistics, the prevalence of pediatric asthma patients is higher in the northern regions of Japan than regions with much traffics. Asthma related factors also include temperature and solar radiation. We examined the association between prevalence rate of asthma, concentration of outdoor NO2 and weather conditions in Japan. A temporary decrease in temperature and a decrease in solar radiation were associated with an increase in NO2. Temperature and solar radiation do not directly affect NO2, but affect nitrous acid which is in equilibrium with NO2. These results suggest that the change of the temperature and the solar radiation effects the concentration of NO2 in the environment via nitrous acid in the atmosphere.