Multi-hazard exposure in public schools of a medium-sized city: outdoor road dust contamination, and indoor Rn risk

Road dust is a major carrier of potentially toxic elements (PTEs) in urban environments. This study characterizes road dust samples collected in the vicinity of public schools (n = 17) to assess their physicochemical properties, mineralogy, chemical composition, magnetic susceptibility, particle morphology, contamination indices, and gastric bioaccessibility of some PTEs (< 250 µm fraction). Samples were mainly composed of quartz, with feldspars, Σphyllosilicates, carbonates, and magnetite-maghemite, revealing geogenic and anthropogenic sources, e.g., tire wear, brake abrasion, and resuspended pavement debris. The fraction < 106 µm was enriched in PM₁₀, suggesting high potential for inhalation and ingestion. The geoaccumulation index was generally < 1, indicating low contamination, although isolated Zn hotspots reached moderate levels. Pollution Index identified Zn as the main pollutant (max. 11.6), while Pollution Load Index classified the area as slightly polluted. Bioaccessibility tests showed negligible gastric solubility for As and Cr, while Ni, Cu, and Pb exhibited moderate to high bioaccessible fractions, with increased ingestion risk despite moderate total concentrations. SEM-EDS analysis revealed carbonaceous soot, Fe-oxide brake debris, alloy fragments, Ti-rich paint particles, and occasional As-sulfide grains in respirable sizes. Road dust showed moderate contamination but included some hotspots with bioaccessible PTEs fractions relevant to children exposure, highlighting the need for targeted dust-control measures and periodic monitoring. Simultaneously, indoor Rn concentrations in classrooms were high (mean 1540 ± 921 Bq/m³), posing a cancer risk by inhalation. The combined assessment of road dust and Rn exposure provided an integrated evaluation of inhalation and ingestion pathways in school environments, underscoring the importance of multi-hazard air-quality monitoring.