Role of Air Pollution in the Development of Asthma among Children with a History of Bronchiolitis in Infancy

Background: Infants experiencing bronchiolitis are at increased risk for asthma, but few studies have identified modifiable risk factors. We assessed whether early life air pollution influenced child asthma and wheeze at age 4-6 years among children with a history of bronchiolitis in the first postnatal year. Methods: Children with caregiver-reported physician-diagnosed bronchiolitis were drawn from ECHO-PATHWAYS, a pooled longitudinal cohort from six US cities. We estimated their air pollution exposure from age 1 to 3 years from validated spatiotemporal models of fine particulate matter (PM_2.5), nitrogen dioxide (NO₂), and ozone (O₃). Caregivers reported children's current wheeze and asthma at age 4-6 years. We used modified Poisson regression to estimate relative risks (RR) and 95% confidence intervals (CI), adjusting for child, maternal, and home environmental factors. We assessed effect modification by child sex and maternal history of asthma with interaction models. Results: A total of 224 children had caregiver-reported bronchiolitis. Median (interquartile range) 2-year pollutant concentrations were 9.3 (7.8-9.9) µg/m³PM_2.5, 8.5 (6.4-9.9) ppb NO₂, and 26.6 (25.6-27.7) ppb O₃. RRs (CI) for current wheeze per 2-ppb higher O₃were 1.3 (1.0-1.7) and 1.4 (1.1-1.8) for asthma. NO₂was inversely associated with wheeze and asthma whereas associations with PM_2.5were null. We observed interactions between NO₂and PM_2.5and maternal history of asthma, with lower risks observed among children with a maternal history of asthma. Conclusion: Our results are consistent with the hypothesis that exposure to modest postnatal O₃concentrations increases the risk of asthma and wheeze among the vulnerable subpopulation of infants experiencing bronchiolitis.