Potentially toxic elements in indoor and outdoor settled dust of houses and kindergartens in Tehran: characteristics, sources, and health risks for children

Abstract

This study characterized physicochemical properties, elemental contamination, and health risks of size-fractionated settled dust in child-relevant microenvironments in Tehran, Iran. Paired indoor and outdoor samples were collected from 50 residential houses and 50 kindergartens (February–July 2023). The < 150 μm fraction was analyzed for potentially toxic elements (PTEs) by microwave plasma-atomic emission spectroscopy. Mineralogy, morphology, and water-soluble ions were determined by XRD, FE-SEM/EDX, and ion chromatography, respectively. Particle size distributions were dominated by coarse material (D50: 29.6–53.3 μm), consistent with a quartz–calcite–clay assemblage. Mean concentrations followed the order Al > Fe > Zn > Mn; Zn (622–1,129 µg/g) and Pb (72–139 µg/g) were the most enriched anthropogenic elements. Contamination factor analysis identified very high contamination by Zn and Pb, while the pollution load index remained below unity (0.24–0.34). Outdoor concentrations explained 82.4% of indoor variability (r² = 0.824; β ≈ 1.09), confirming dominant outdoor influence. Ingestion contributed over 99% of total average daily dose. Non-carcinogenic hazard indices remained below unity but were highest in indoor kindergartens (total HI ≈ 0.73; Pb HI ≈ 0.53). The total carcinogenic risk approached or exceeded 10⁻⁴ in indoor houses, with Ni and estimated Cr(VI) as the principal contributors. Results highlight kindergartens as priority environments for pediatric exposure management and support targeted mitigation strategies, including traffic and construction control near child-occupied locations and enhanced dust-reduction practices.