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.