Outdoor Air Pollutants and Asthma Risk in Adolescents: Evidence from a Systematic Review and Meta-Analysis

Wenwen ShiSupapon KaewsanmungWuttipat KiratipaisarlRatana Sapbamrer^*

• Department of Community Medicine, Faculty of Medicine, Chiang Mai University, Chiang Mai, Thailand

Background: Existing epidemiological evidence linking outdoor air pollution to asthma incidence in adolescents remains inconclusive, due to methodological heterogeneity in study design, exposure assessment, and asthma case definitions. Methods: This systematic review and meta-analysis synthesized observational studies examining associations between outdoor air pollutants and adolescent asthma. Comprehensive searches of PubMed, Embase, and Scopus were conducted using Medical Subject Headings (MeSH) and free-text terms. Eligible studies reported quantitative estimates, odds ratio (OR), hazard ratio (HR), relative risk (RR), prevalence ratio (PR), incidence rate ratio (IRR), or prevalence odds ratio (POR), with corresponding 95% confidence interval (CI). Pooled estimates were calculated using inverse-variance weighted fixed-and random-effects models. Heterogeneity was assessed using Cochran's Q and I² statistics. Results: Of 51 eligible studies, 40 were incorporated into the meta-analysis. Statistically significant associations were identified for each 10 μg/m³ increase in nitrogen dioxide (NO₂) (adjusted odds ratio [aOR] = 1.18; 95% CI: 1.08–1.29) and ozone (O₃) (aOR = 1.01; 95% CI: 1.00–1.03), as well as per 1 ppm increase in carbon monoxide (CO) (aOR = 1.31; 95% CI: 1.08–1.53). Traffic-related air pollution (TRAP) was also significantly associated with an elevated risk (aOR = 1.15; 95% CI: 1.10–1.21). In contrast, the pooled estimate for particulate matter with diameter ≤ 2.5 micrometers (PM2.5), particulate matter with diameter ≤ 10 micrometers (PM₁₀), and sulfur dioxide (SO2) did not reach statistical significance, These non-significant results, however, were accompanied by considerable between-study heterogeneity (I² = 32.3% for PM₂.₅, 76.5% for PM₁₀, and 53.6% for SO₂), reflecting substantial inconsistency across the included studies and precluding a definitive conclusion regarding the absence of an association. Following adjustment for potential publication bias, the association for NO₂ remained statistically significant (aOR = 1.21; 95% CI: 1.10–1.33). Conversely, the point estimate for PM2.5 shifted toward the null and remained non-significant (aOR = 0.89; 95% CI: 0.70–1.12), while SO₂ continued to show no significant association. Conclusion: This meta-analysis highlights significant associations between adolescent asthma and specific combustion-related air pollutants. Future research should prioritize standardized exposure metrics, life-course cohort designs, and multipollutant modeling to inform targeted prevention and public health strategies.