Performance of road-traffic-based exposure proxies against personal PM2.5 measurements in three Sub-Saharan African countries

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Particulate Matter (PM 2.5 ) exposure contributes to the global disease burden, yet its monitoring remains sparse and uneven, with limited ground sensor infrastructure. Road-traffic proxy indicators can provide indirect estimates of PM 2.5 where measurements are limited but require context-specific validation. We evaluated three PM 2.5 road-traffic-related proxies: (i) population-Weighted Road Network Density (wRND), (ii) Euclidean (straight line) distance from highways (EH), and (iii) Euclidean distance from main roads (EM). We validated these proxies using high-resolution outdoor filtered PM 2.5 personal exposure measurements collected over 1 year from 343 postpartum participants in The Gambia, Kenya, and Mozambique. Proxy-PM 2.5 associations were assessed using Spearman correlation, and predictive utility was tested using country-specific and global Random Forest (RF) models (3-fold cross-validation), reporting R 2 , RMSE, and feature importance. Spatial mapping showed heterogeneous proxy–PM 2.5 relationships across and within sites, with elevated PM 2.5 occurring in both low- and high-proxy contexts. wRND–PM 2.5 correlations were weak overall and statistically significant only in Mozambique ( r = 0.351; p = 0.005), with non-significant associations in Kenya ( r = − 0.041; p = 0.673) and The Gambia ( r = − 0.020; p = 0.909). EH–PM 2.5 correlations were positive in The Gambia ( r = 0.335; p = 0.053) and Mozambique ( r = 0.292; p = 0.020) but negative and significant in Kenya ( r = − 0.224; p = 0.018). Single-variable RF models performed poorly across all countries (R 2 < 0.45) and the Global model (R 2 = 0.42). Combining proxies improved performance in Kenya (R 2 = 0.52; RMSE = 31.7 µg/m 3 ) and Mozambique (R 2 = 0.60; RMSE = 8.9 µg/m 3 ), Global R 2 = 0.46; RMSE = 29.1 µg/m 3 ), although in The Gambia, the combined model (R 2 = 0.53; RMSE = 37.6 µg/m 3 ) did not exceed the best single-proxy model. Road-network proxies provided limited but context-dependent signals of personal PM₂.₅ exposure. Their performance varied substantially across countries, indicating that road-based indicators should not be used as stand-alone exposure measures in heterogeneous sub-Saharan African settings. Instead, they are most defensible as locally validated components of hybrid exposure models that also incorporate meteorology, land use, biomass burning, household energy, and other non-traffic sources.

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