ObjectiveTo 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.DesignSystematic review of observational epidemiological studies including cohort, time series, case crossover and case control study designs.Data sourcesMedline, Scopus, and Web of Science searched to November 2017 with no language restrictions.Eligibility criteriaStudies investigating impact of air pollution levels on particulate pollutants (diameter <2.5 μm (PM2.5) or <10 μm (PM10) and gaseous pollutants (nitrogen dioxide (NO₂), sulphur dioxide (SO₂), carbon monoxide (CO), ozone (O₃)) on hospital admission for bronchiolitis.Main outcome measureRisk of hospitalisation from bronchiolitis.ResultsEight studies were eligible for review. Long term exposure to PM2.5 may be associated with increased risk of hospitalisation for bronchiolitis. SO₂may 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.ConclusionsCertain 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.ProtocolThe protocol can be found at PROSPERO (CRD42017080643).