Heavy metals profiling of road dust from metro stations: spatial distribution, source apportionment, and Monte Carlo simulation-based probabilistic human health assessment.

Road dust in urban areas is a significant carrier of heavy metals (HMs), posing serious environmental and health threats to humans. This study investigates the concentration, sources, and associated health risks of HMs (Cr, Mn, Ni, Cu, Zn, Cd, and Pb) in road dust collected from 27 metro bus stations in Lahore, Pakistan. The mean concentrations (mg/kg) of Cr, Mn, Ni, Cu, Zn, Cd, and Pb in road dust were 51.83, 364.79, 22.36, 93.57, 421.50, 3.37, and 50.64, respectively, following the descending order: Zn > Mn > Cu > Cr > Pb > Ni > Cd. Pollution indices revealed severe contamination by Cd (CF > 6), significant contamination by Zn, and a high ecological risk index (RI = 329.90), while the pollution load index (PLI = 2.49) indicated overall high pollution. Pearson correlation analysis showed strong associations among traffic-related metals (Ni-Cr, r = 0.92; Cu-Zn, r = 0.91). PCA and HCA grouped the metals into three clusters, and PMF identified three major sources: industrial traffic mixed emissions, non-exhaust vehicular emissions, and mixed urban deposition. Health risk assessment indicated ingestion as the dominant exposure pathway. Non-carcinogenic risks (HI < 1) were within safe limits; however, carcinogenic risks for Cr (2.84 × 10-4) and Ni (4.16 × 10-4) in children exceeded the acceptable threshold. This study proposes that non-exhaust traffic emissions are the dominant contributors to HMs loading in metro-corridor dust, with children facing elevated long-term carcinogenic risks, highlighting the need for targeted mitigation strategies in urban transport environments.