Short-term exposure to urban air pollution and asthma-related morbidity in children under five: a PRISMA-guided systematic review with narrative synthesis (2021-2025).
INTRODUCTION Urban air pollution is a critical environmental determinant of pediatric respiratory health. This PRISMA 2020-guided systematic review synthesizes evidence on the association between short-term exposure to urban ambient air pollutants and asthma-related morbidity in children under five years of age. METHODS PubMed and Google Scholar were searched for urban time-series or case-crossover studies published between 2021 and 2025 (final search: 15 January 2026). Eligible outcomes included hospital admissions, emergency department visits, or urgent consultations in children aged 0-5 years. Two reviewers independently screened records, extracted data, and assessed study quality. Due to methodological heterogeneity, a structured narrative synthesis was performed. RESULTS Four studies met the inclusion criteria. Across settings, short-term increases in particulate matter (PM_{2.5}/PM_{10}) and nitrogen dioxide (NO_2) were consistently associated with increased asthma-related hospitalizations or urgent visits. Stronger effects were reported during cold seasons and in high-traffic environments. CONCLUSIONS Short-term exposure to traffic-related and particulate pollutants exacerbates asthma morbidity in vulnerable preschool children. However, the certainty of evidence was rated as low due to residual confounding, potential exposure misclassification inherent to observational designs, and the limited number of eligible studies. These findings support strengthening urban air-quality policies. PROTOCOL REGISTRATION PROSPERO (Identifier: CRD420251251275; Available from: https://www.crd.york.ac.uk/PROSPERO/view/CRD420251251275.