Environmental determinants of allergic rhinitis and asthma: a two-year public health study in Ikorodu, Lagos, Nigeria

Abstract

Allergic rhinitis and asthma pose an escalating global health burden, particularly in rapidly urbanising tropical regions where environmental determinants are complex and poorly understood. This two-year longitudinal study in Ikorodu, Nigeria, examined the combined environmental associations of airborne bioaerosols and fine particulate matter (PM2.5) with allergic respiratory disease incidence. Using time-series analysis and Generalised Additive Models (GAMs) on a comprehensive dataset of environmental and clinical records, we found that monthly allergy case counts were associated with a bimodal seasonal structure: case counts co-occurred with elevated PM2.5 concentrations during the dry-season peak (December–January) and with elevated fungal spore deposition during the wet-season peak (June–August). Relative humidity was the sole statistically significant independent predictor (IRR = 0.921, 95% CI: 0.876–0.968, p = 0.001), with higher humidity associated with lower monthly case incidence. The Poisson GAM collectively explained 64.1% of deviance. A formally significant multiplicative interaction between bioaerosols and PM2.5 could not be established (Likelihood Ratio Test: p = 0.999); marginal prediction surfaces are therefore presented as exploratory visualisations only. Throughout the study period, we observed a consistent gender disparity, with female cases outnumbering male cases (61.3% vs. 37.9%). These findings provide observational evidence of seasonal, multi-pollutant environmental associations with allergic respiratory disease burdens in a tropical urban setting, which points to the importance of integrated bioaerosol and air quality monitoring and gender-disaggregated health surveillance.