PBDE-to-OPE Substitution in Urban Greenspace Soils: OPE-Dominated Burdens and Endpoint-Specific Risk Priorities

The phaseout of polybrominated diphenyl ethers (PBDEs) and expanded use of organophosphate esters (OPEs) have reshaped flame-retardant contamination, yet how legacy PBDEs and emerging OPEs coexist in urban soils, and how this coexistence reshapes risk priorities, remains unclear. Here, we analyzed 230 surface soil samples from 49 greenspace sites in Shenzhen, China, for 12 PBDEs and 14 OPEs and evaluated their occurrence, sources and risks. Median ΣOPEs (233 ng/g dry weight, dw) was 22-fold higher than median ΣPBDEs (10.6 ng/g dw), indicating strong OPE dominance in present-day soil burdens. OPE distributions were positively correlated with human activity indicators, whereas PBDEs were more closely associated with soil properties, particularly organic matter. Despite the concentration dominance of OPEs, risk priorities differed by endpoint. OPEs dominated ecological risk and contributed 99.8% of total carcinogenic risk, whereas residual PBDEs contributed 66.9% of total noncarcinogenic hazard despite accounting for only 4.2% of total concentration. Source-resolved analysis further showed that the mass-dominant Alkyl-OPE-rich source was not the dominant contributor to health risk: noncarcinogenic hazard was driven mainly by a legacy PBDE source, whereas carcinogenic risk was dominated by a TDCIPP-rich foam-related factor. Although estimated human health risks remained within commonly used risk threshold, these results show that PBDE-to-OPE substitution can produce endpoint-specific risk priorities, highlighting the need for source-resolved and toxicity-weighted assessment to prioritize contaminants and sources when managing legacy and emerging contaminants in urban soils.