Dermal Exposure to Heavy Metals in Urban Green Space Soils: A Review of Bioavailability, Toxic Mechanisms, and Precision Risk Assessment

Urban green spaces (UGSs) provide essential ecological services but also accumulate heavy metals (HMs) in their soils, posing a paradoxical health risk through dermal exposure. Traditional risk assessments, based solely on total HM concentrations, often overestimate threats by ignoring bioavailability (the fraction actually absorbed by organisms) and dynamic skin microenvironment factors. This review synthesizes recent advances to propose a precision assessment framework that integrates bioavailability. The framework advocates for the incorporation of bioaccessibility (the fraction of pollutants dissolved in body fluids)-driven exposure metrics (e.g., physiologically based extraction tests), mechanistic dermal permeation models (e.g., Franz diffusion cells, 3D skin constructs), and population-specific susceptibility factors (e.g., children, individuals with compromised skin). We elucidate how soil properties (pH, organic matter) and metal speciation (e.g., Cr(III)/Cr(VI)) modulate cutaneous uptake, and detail toxicological mechanisms including oxidative stress, ferroptosis/cuproptosis, immunotoxicity, and pigmentation disorders. Case studies reveal heterogeneous HM hotspots in high-traffic and densely populated areas, while in vitro–in vivo extrapolation highlights the potential for misestimation in traditional models. Consequently, we discuss the limitations and future directions of this framework, aiming to shift UGS risk management from over-conservative assessment to bioavailability-based precision governance, thereby supporting the health security of sustainable urban habitats.