Wildfire smoke, PM_2.5 exposure and respiratory morbidity in a densely populated urban setting: Spatial mapping and...

Wildfires have intensified in Brazil, raising concerns about respiratory health in densely populated regions. This study evaluated the association between wildfire occurrence, PM_2.5 exposure, and respiratory hospital admissions in São Paulo State (2018-2025), using hospitalization data from DATASUS, wildfire records from INPE, and daily PM_2.5, temperature, and relative humidity data from CETESB monitoring stations. Spatial analyses used population-standardized hospitalization rates (per 100,000 inhabitants) to assess alignment between wildfire hotspots and respiratory burden. The temporal association between PM_2.5 and admission rates was examined using a Generalized Additive Model (GAM) applied to weekly aggregated data, adjusted for temperature, relative humidity, calendar month, and nonlinear temporal trend. A total of 1,467,172 respiratory hospital admissions were recorded; children aged 0-10 years and males were the most affected groups. Population-standardized rates revealed higher per capita burdens in northern and northwestern regions, coinciding with wildfire hotspots, whereas absolute counts were concentrated in metropolitan areas. Between 2018 and 2025, 20,446 wildfire events were documented, predominantly in northern and northeastern areas. The GAM explained 82.8% of the deviance in hospitalization rates (adjusted R² = 0.808). PM_2.5 showed a positive exposure-response association after full adjustment (p = 0.054), with each 10 μg/m³ increase in weekly mean PM_2.5 associated with a 3.4% increase in admission rates (RR: 1.034; 95% CI: 0.889-1.204), strongest within the same weekly exposure window. A strong positive Spearman correlation between active fire counts and PM_2.5 concentrations (ρ = 0.732; p < 0.001) suggests that wildfire activity was a major contributor to atmospheric particulate burden in the region. Spatial and temporal evidence support a positive wildfire-respiratory morbidity association, highlighting the need for expanded environmental monitoring and integrated public health surveillance in densely populated regions facing recurrent wildfire seasons.