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Otitis media is the leading reason young children receive antibiotics or visit a physician. We evaluated the impact of ambient air pollution on outpatient physician visits for otitis media in a population-based birth cohort.All children born in southwestern British Columbia during 1999-2000 were followed until the age of 2 years. Residential air pollution exposures were estimated for the first 24 months of life by inverse-distance weighting of monitor data (CO, NO, NO2, O3, PM2.5, PM10, SO2), temporally adjusted land-use regression models (NO, NO2, PM2.5, black carbon, woodsmoke), and proximity to roads and point sources. We used generalized estimating equations to longitudinally assess the relationship between physician visits for otitis media (ICD-9) and average pollutant exposure in the 2 months prior to the visit, after adjustment for covariates.Complete exposure and risk-factor data were available for 45,513 children (76% of all births). A total of 42% of subjects had 1 or more physician visits for otitis media during follow-up. Adjusted estimates for NO, PM2.5, and woodsmoke were consistently elevated (eg, relative risk of 1.10 [95% confidence interval = 1.07-1.12] per interquartile range [IQR] increase in NO; 1.32 [1.27-1.36] per IQR increase in days of woodsmoke exposure). No increased risks were observed for the remaining pollutants (eg, 1.00 [0.98-1.03] per IQR increase in PM10; 0.99 [0.97-1.01] per IQR increase in black carbon).Modest but consistent associations were found between some measures of air pollution and otitis media in a large birth cohort exposed to relatively low levels of ambient air pollution.