asthma is the most common chronic illness of childhood and its prevalence is increasing , causing much concern for identification of risk factors such as air pollution . 
 we previously conducted a study showing a relationship between asthma visits in all persons < 65 years of age to emergency departments ( eds ) and air pollution in seattle , washington . in that study the most frequent zip codes of the visits were in the inner city . 
 the seattle - king county department of public health ( seattle , wa ) subsequently published a report which showed that the hospitalization rate for children in the inner city was over 600/100,000 , whereas it was < 100/100,000 for children living in the suburbs . 
 therefore , we conducted the present study to evaluate whether asthma visits to hospital emergency departments in the inner city of seattle were associated with outdoor air pollution levels . 
 ed visits to six hospitals for asthma and daily air pollution data were obtained for 15 months during 1995 and 1996 . 
 the association between air pollution and childhood ed visits for asthma from the inner city area with high asthma hospitalization rates were compared with those from lower hospital utilization areas . 
 daily ed counts were regressed against fine particulate matter ( pm ) , carbon monoxide ( co ) , sulfur dioxide , and nitrogen dioxide using a semiparametric poisson regression model . 
 significant associations were found between ed visits for asthma in children and fine pm and co. a change of 11 microg / m3 in fine pm was associated with a relative rate of 1.15 [ 95% confidence interval ( ci ) , 1.08 - 1.23 ] . 
 there was no stronger association between ed visits for asthma and air pollution in the higher hospital utilization area than in the lower utilization area . 
 these findings were seen when estimated pm2.5 concentrations were below the newly adopted annual national ambient air quality standard of 15 microg / m3.imagesfigure 1figure 2