4. Discussion
Emergency visits for asthma increased with increased annual levels of 3 related pollutants (CO, NO2, and PM10) in children but not for any pollutants in adults in the
present study. Diesel- and gasoline-powered vehicular engines and coaland oil-fired power plants are the main sources of ambient NO2 emissions, which typically result from the fixation of nitrogen in the air during high-temperature combustion [9]. Several studies have shown that NO2 is a risk factor for exacerbation of asthma in children. The risks of respiratory tract symptoms were associated with increased ambient NO2 in London and Japan [10,11].