Daily outdoor PM2.5 exposure characterization and road edge effect during a 2022 - 2023 monitoring campaign in Gambia, Kenya and Mozambique

Abstract

Air pollution is a major contributor to mortality and adverse health outcomes, particularly in pregnant women and children, with vehicular emissions as a key source. This study aimed to characterize long-term exposure to fine particulate matter (PM₂.₅), evaluate potential health risks, and quantify the contribution of road transport and proximity to roads in The Gambia, Kenya, and Mozambique. Multi-temporal personal exposure data were interpolated into continuous spatial surfaces and aggregated into daily averages, which were then compared with World Health Organization (WHO) guidelines to assess potential health impacts. Multi-distance buffers at 10 m intervals up to 1000m from major roads were created to examine pollutant variation with road proximity. Overlay analyses and linear regression models were applied to quantify the road-edge effect on pollution exposure. The Inverse Distance Weighting interpolation technique produced accurate PM₂.₅ field in all sites (R2>0.55, MAE<4 μg/m3 and RMSE<10 μg/m3) based on leave-one-out cross-validation. PM₂.₅ concentrations were highest in The Gambia across all land use and land cover (LULC) types, reflecting localized industrial and vehicular emissions. Elevated pollutant levels were observed in herbaceous wetlands, bare land, and built-up areas. Most sites across all three countries exceeded WHO recommended thresholds, indicating significant health risks. PM₂.₅ decreased with distance from roads in Rabai (Kenya) and Mozambique, while in other sites it increased away from roads. These findings highlight the combined influence of multiple local sources, including traffic, fire smoke, wetlands, and industry, on pollutant exposure. The study demonstrates the utility of WHO thresholds for health risk assessment and shows how road proximity affects pollutant dispersal. The results provide critical insights for modeling air pollution exposure and informing mitigation strategies in sub-Saharan Africa.