Air pollution and hospitalization risk in infants with bronchiolitis: A systematic review and meta-analysis

Bronchiolitis is one of the leading causes of hospitalization among infants. Established risk factors include young age, prematurity, and exposure to tobacco smoke. Emerging evidence suggests that air pollution may also contribute to the burden of respiratory diseases. However, its link with bronchiolitis hospitalizations remains debated. To address this, we conducted a systematic review and meta-analysis to assess whether exposure to air pollutants is associated with an increased risk of hospitalization for bronchiolitis in infants. A systematic review and meta-analysis were conducted following the PRISMA guidelines. PubMed, Embase, and Web of Science were searched up to May 2024. Eligible studies examined the relationship between air pollutants and bronchiolitis hospitalizations in infants up to 2 years of age. Meta-analyses were performed to estimate the association between pollutant levels and hospitalization risk. Out of 788 identified studies, 23 met the inclusion criteria. Studies were heterogeneous regarding design, adjustment for confounders, and statistical approaches. Particulate matter with diameter ≤2.5 μm (PM_2.5) or ≤10 μm (PM₁₀) and nitrogen dioxide (NO₂) were the most studied pollutants, with positive associations found between short-, medium-, and long-term exposure and increased hospitalization risk. Meta-analyses showed a 2%-9% increase in hospitalization risk for exposure to PM_2.5, PM₁₀, and NO₂; however, statistical significance was reached only for short-term exposure to PM₁₀. In contrast, data on sulfur dioxide, carbon monoxide, ozone, and black carbon were sparse and inconsistent. PM_2.5, PM₁₀, and NO₂ are likely relevant risk factors for an increased risk of hospitalization for bronchiolitis in infants. Further research using a standardized approach is needed to clarify the role of other pollutants in bronchiolitis.