Air pollution and the risk of bronchiolitis-related hospital admission in a birth cohort of over 400,000 children born in London

Air pollution exposure in early life may be associated with an increased risk of bronchiolitis in children, but evidence for long-term exposures (over weeks or months) is limited. We estimated associations between fine particulate matter (PM 2.5 ) and nitrogen dioxide (NO 2 ) exposure during pregnancy and in the first year of life and bronchiolitis-related hospital admissions in a London birth cohort. We used a national birth cohort to identify London-resident mothers whose children were born in London from 2010 to 2013 and extracted information from birth and death registrations and maternal and child longitudinal Hospital Episode Statistics. We linked modeled PM 2.5 and NO 2 data to residential postcode histories during pregnancy and infancy. We applied a landmark approach with Cox proportional hazard models, adjusted for sociodemographic characteristics and housing energy efficiency, to estimate associations between time-varying monthly PM 2.5 or NO 2 exposure and first bronchiolitis admission. Among 415,311 children, we found inconclusive evidence overall, with suggestive signals of increased risk associated with pregnancy and exposures in the final month of infancy to PM 2.5 and NO 2 and time to first bronchiolitis-related hospital admission. There were modest increases in risk in the first month after birth, corresponding to prenatal exposures, for PM 2.5 (adjusted Hazard ratio HR a = 1.07, 95% confidence interval CI: 0.70, 1.61 per 5 ug/m 3 ) and NO 2 (HR a = 1.15, 95% CI: 0.98, 1.35 per 10 ug/m 3 ). We found a similar estimated increased risk in the last month of follow-up (PM 2.5 HR a = 1.12, 95% CI: 0.94, 1.34; NO 2 HR a = 1.31, 95% CI: 1.00, 1.71), corresponding to late infancy exposures. These findings highlight the uncertainty about critical windows of susceptibility during infancy. Future studies should examine the association between air pollution and bronchiolitis in emergency departments and primary care settings.