Exceedance Probability and Toxicity-Value Sensitivity in Health-Risk Assessment of a Pb–Zn Slag Reprocessing Site Under the 2025 IRIS Arsenic Values

Abstract

Screening-level risk assessments report single-point estimates of carcinogenic risk (CR) and hazard index (HI), conveying neither the probability of exceedance nor its sensitivity to revised toxicity values. We examined both at an active lead–zinc slag reprocessing site in Shymkent, Kazakhstan, by reanalysing a published dataset (24 soil samples) with Monte Carlo simulation for an adult outdoor worker, under the toxicity values of the source assessment and under the January 2025 IRIS values for inorganic arsenic. Under the original values, the probability of exceeding CR = 10−4 was 70% (95% bootstrap interval 59–80%) and of exceeding HI = 1 was 82%. Applied to the same concentrations, the 2025 values gave a 21-fold higher mean risk and exceedance probabilities of 99% and 97%. The screening conclusion therefore holds under both sets of values, whereas the quantitative estimate does not. Arsenic concentration dominated output variance. Because ingestion and dermal contact carry over 99% of the worker’s risk, risk reduction requires measures that interrupt these pathways: engineered capping of the heap perimeter, restricted access, and occupational hygiene. Phytostabilization gave no modelled benefit to on-site workers; its value lies in limiting off-site dust, which was not modelled. Arsenic bioaccessibility testing and denser sampling are priorities.