Objectives To assess the effects of long-term variations in ambient air pollutants on longitudinal changes in exhaled nitric oxide (FeNO), a potentially useful biomarker of eosinophilic airway inflammation, based on data from the southern California Children's Health Study.

Methods Based on a cohort of 1211 schoolchildren from eight Southern California communities with FeNO measurements in 2006–2007 and 2007–2008, regression models adjusted for short-term effects of air pollution were fitted to assess the association between changes in annual long-term exposures and changes in FeNO.

Results Increases in annual average concentrations of 24-h average NO₂ and PM_2.5 (scaled to the IQR of 1.8 ppb and 2.4 μg/m³, respectively) were associated with a 2.29 ppb (CI 0.36 to 4.21; p=0.02) and a 4.94 ppb (CI 1.44 to 8.47; p=0.005) increase in FeNO, respectively, after adjustments for short-term effects of the respective pollutants. In contrast, changes in annual averages of PM₁₀ and O₃ were not significantly associated with changes in FeNO. These findings did not differ significantly by asthma status.

Conclusions Changes in annual average exposure to current levels of ambient air pollutants are significantly associated with changes in FeNO levels in children, independent of short-term exposures and asthma status. Use of this biomarker in population-based epidemiological research has great potential for assessing the impact of changing real world mixtures of ambient air pollutants on children's respiratory health.