Probabilistic eco-human health risk assessment of heavy metals in soils from Northwestern and Northcentral Nigeria using Monte Carlo simulation

Urban soil contamination by potentially toxic elements (PTEs) poses increasing ecological and public health concerns in rapidly industrializing environments. This study assessed PTE concentrations, contamination status, ecological risks, and human health implications of soils from Abuja and Kano, Nigeria, using deterministic and probabilistic risk assessment approaches. Sixteen topsoil samples (0–15 cm) collected from residential, commercial, and industrial locations were analyzed for Pb, Cd, Ni, Cr, and Al using atomic absorption spectrophotometry following aqua regia digestion. Mean concentrations of Pb (9.60 ± 4.29 vs 6.38 ± 3.08 mg/kg), Cd (1.53 ± 0.53 vs 1.12 ± 0.45 mg/kg), Ni (7.66 ± 1.71 vs 5.85 ± 1.42 mg/kg), Cr (9.86 ± 1.35 vs 8.23 ± 1.61 mg/kg), and Al (364.75 ± 96.47 vs 291.88 ± 80.39 mg/kg) were significantly higher in Kano than Abuja. Contamination assessment showed moderate Cd contamination (CF = 3.73–5.10), PLI values of 1.42 (Kano) and 1.16 (Abuja), Igeo values indicating unpolluted-to-moderately polluted soils, and enrichment factors suggesting moderate anthropogenic enrichment of Cd and Pb. Ecological risk indices revealed Cd as the dominant contributor, with Eᵣ values of 111.9–153.0 and total RI values of 176.4–229.8, indicating moderate-to-considerable ecological risk. Estimated daily intake followed the order ingestion > dermal > inhalation, with children exhibiting higher exposure levels than adults. Non-carcinogenic risks remained acceptable (HQ and HI < 1), with maximum HI values of 0.38 for children and 0.21 for adults in Kano. Incremental lifetime cancer risks values were highest for Cd (6.39 × 10⁻⁶ in Kano children and 4.38 × 10⁻⁶ in Kano adults) and lowest for Ni (4.41 × 10⁻⁹ in Kano children and 3.02 × 10⁻⁹ in Kano adults), with lower corresponding values in Abuja (9.78 × 10⁻⁷–1.71 × 10⁻1⁰ for children and 6.72 × 10⁻⁷–1.17 × 10⁻1⁰ for adults), all remaining within acceptable risk thresholds. Monte Carlo simulation confirmed consistently higher exposure probabilities in Kano. Continuous monitoring, stricter emission control, and targeted child-focused exposure mitigation are recommended for sustainable urban environmental management.