the association of air pollution with the prevalence of chronic lower respiratory tract symptoms among children with a history of asthma or related symptoms was examined in a cross - sectional study . 
 parents of a total of 3,676 fourth , seventh , and tenth graders from classrooms in 12 communities in southern california completed questionnaires that characterized the children 's histories of respiratory illness and associated risk factors . 
 the prevalences of bronchitis , chronic phlegm , and chronic cough were investigated among children with a history of asthma , wheeze without diagnosed asthma , and neither wheeze nor asthma . 
 average ambient annual exposure to ozone , particulate matter ( pm(10 ) and pm(2.5 ) ; [ less than / equal to ] 10 microm and < 2.5 microm in aerodynamic diameter , respectively ) , acid vapor , and nitrogen dioxide ( no(2 ) ) was estimated from monitoring stations in each community . 
 positive associations between air pollution and bronchitis and phlegm were observed only among children with asthma . as pm(10 ) increased across communities , there was a corresponding increase in the risk per interquartile range of bronchitis [ odds ratio ( or ) 1.4/19 microg / m(3 ) ; 95% confidence interval ( ci ) , 1.1 - 1.8 ) . increased prevalence of phlegm was significantly associated with increasing exposure to all ambient pollutants except ozone . 
 the strongest association was for no(2 ) , based on relative risk per interquartile range in the 12 communities ( or 2.7/24 ppb ; ci , 1.4 - 5.3 ) . 
 the results suggest that children with a prior diagnosis of asthma are more likely to develop persistent lower respiratory tract symptoms when exposed to air pollution in southern california.imagesfigure 1figure 2