Friday, April 7, 2023

Total petroleum hydrocarbon (TPH) contamination is present at numerous Superfund sites across the nation. Hydrocarbon products are complex mixtures containing perhaps hundreds of hydrocarbon compounds and compositions vary among different products. Once released, the composition of a hydrocarbon product will change due to differential fate and transport of its components. To estimate the human health hazards and risk associated with TPH exposure at Superfund sites, a fraction-based approach was taken in the Provisional Peer Reviewed Toxicity Value (PPRTV) assessment of these complex mixtures. The TPH mixture was divided into 6 subfractions, defined based on structural attributes and further categorized by carbon and equivalent carbon ranges. The health risk of each fraction can be estimated using a variety mixture assessment approaches, including the indicator chemical method, surrogate mixture method, hazard index method, or relative potency factor method, with method selection driven by data availability. Fraction-specific hazard and risk estimates are then summed using either dose (noncancer) or response (cancer) addition to provide an estimate of mixture hazard or risk. Background on the PPRTV Program, mixture risk assessment methods, and an example calculation will be presented.