Association of Outdoor Allergens with Respiratory Allergic Diseases: A Systematic Review

The global burden of respiratory allergic diseases linked to outdoor allergens remains poorly quantified despite increasing evidence of the allergens' impacts on respiratory allergic diseases. This systematic review synthesizes epidemiological evidence on the association between exposure to outdoor airborne allergens and respiratory disease incidence.

Following PRISMA 2020 guidelines, we systematically searched three databases, PubMed, Embase, and the Cochrane Library, until December 2024. Observational studies reporting effect estimates for respiratory outcomes (asthma, allergic rhinitis, COPD exacerbations, and cough) associated with quantified outdoor allergen exposure were eligible. Two reviewers independently conducted screening, data extraction, and quality assessment using an adapted Newcastle-Ottawa Scale (NOS). Random-effects models were employed to pool odds ratios (ORs), with subgroup analyses by sex. Publication bias was evaluated using Begg's and Egger's tests, respectively.

From 6,551 identified records, 7 articles with 70 studies (46,325 participants across 3 countries) met the inclusion criteria. High-quality studies (NOS ≥7) constituted 86% of the included articles. Pooled analyses revealed significant associations between outdoor allergen exposure and respiratory outcomes: OR = 1.03 (95% confidence interval [CI]: 1.01-1.04; p = 0.008). Egger's publication bias was non-significant (Egger's p = 0.21), and Begg's publication bias was also non-significant (Begg's p = 0.31). Trim-and-fill funnel plots suggested possible missing small-null studies, but the statistical analysis was non-significant. Sensitivity analyses confirmed robustness when excluding studies with potential residual confounding (pooled OR range: 1.02-1.06). Subgroup analyses demonstrated that the female subgroup, rather than the male subgroup, got stronger effects significantly: OR = 1.04 (95% CI: 1.01-1.07), compared with the male subgroup OR = 1.02 (95% CI: 0.99-1.04).

This comprehensive synthesis provides high-quality evidence that outdoor allergen exposure significantly increases risks of respiratory allergic disease. The female is more likely to get respiratory morbidity compared with the male. Further research should prioritize harmonized exposure metrics and evaluate interactive effects with anthropogenic air pollutants.