The relationship of bronchitic symptoms to ambient particulate matter and to particulate elemental and organic carbon (OC), nitrogen dioxide (NO 2 ), and other gaseous pollutants was examined in a cohort of children with asthma in 12 Southern California communities. Symptoms, assessed yearly by questionnaire from 1996 to 1999, were associated with the yearly variability of particulate matter with aerodynamic diameter less than 2.5 m (odds ratio [OR] 1.09/g/m 3 ; 95% confidence interval [CI]

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... -1.17), OC (OR 1.41/ g/m 3 ; 95% CI 1.12-1.78), NO 2 (OR 1.07/ppb; 95% CI 1.02-1.13), and ozone (OR 1.06/ppb; 95% CI 1.00-1.12). The ORs associated with yearly within-community variability in air pollution were larger than the effect of the between-community 4-year average concentrations. In two pollutant models, the effects of yearly variation in OC and NO 2 were only modestly reduced by adjusting for other pollutants, except in a model containing both OC and NO 2 ; the effects of all other pollutants were reduced after adjusting for OC or NO 2 . We conclude that OC and NO 2 deserve greater attention as potential causes of the chronic symptoms of bronchitis in children with asthma and that previous cross-sectional studies may have underestimated the risks associated with air pollution.