The effect of outdoor air pollution on the risk of hospitalisation for bronchiolitis in infants: a systematic review

Objective To systematically review the evidence around the effect of ambient levels of particulate and gaseous pollutants, and the risk of hospitalisation with bronchiolitis for infants under two years of age. Design Systematic review of observational epidemiological studies including cohort, time series, case crossover and case control study designs. Data sources Medline, Scopus, and Web of Science searched to November 2017 with no language restrictions. Eligibility criteria Studies investigating impact of air pollution levels on particulate pollutants (diameter <2.5 μm (PM2.5) or <10 μm (PM10) and gaseous pollutants (nitrogen dioxide (NO 2 ), sulphur dioxide (SO 2 ), carbon monoxide (CO), ozone (O 3 )) on hospital admission for bronchiolitis. Main outcome measure Risk of hospitalisation from bronchiolitis. Results Eight studies were eligible for review. Long term exposure to PM2.5 may be associated with increased risk of hospitalisation for bronchiolitis. SO 2 may also be associated with hospitalisation, but results for other pollutants are inconsistent between studies. In three of the five studies that showed a positive association between air pollutants and hospitalisation, measured concentrations were below World Health Organization (WHO) recommended levels. Conclusions Certain particulate and gaseous pollutants may have a clinically relevant effect on hospital admissions for bronchiolitis in children below age two years old. Large cohort or time series studies are needed to examine this possible association. Protocol The protocol can be found at PROSPERO (CRD42017080643).