Fraction-Based Risk Assessment of Complex Mixtures of Aliphatic and Aromatic Hydrocarbons using Data-Driven Approaches
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Petroleum releases pose unique risk assessment challenges. In recognition of the inapplicability of whole product toxicity data to many contamination scenarios, the impact of differential fate and transport of individual contaminants, the impracticality of chemically analyzing each constituent separately, and the need for risk-based assessments of petroleum hydrocarbons, an approach was developed to assess petroleum hydrocarbon mixtures on the basis of fractions with similar physical and chemical properties. Six fractions were defined based on structural attributes (aliphatic v. aromatic) and further categorized by carbon (C) and equivalent carbon (EC) number ranges that reflect expected environmental transport and exposure: aliphatic low (C5-8, EC5-8); aliphatic medium (C9-18, EC>8-16); aliphatic high (C19-32, EC>16-35); aromatic low (C6-8, EC6-<9); aromatic medium (C9-10, EC9-<11); and aromatic high (C10-32, EC11-35). For each fraction, toxicity data for mixtures and individual components that met fraction definitions were evaluated to select an approach for hazard identification for that fraction. The evaluation considered the availability of mixture toxicity data and whether the mixture was sufficiently representative of the fraction, whether available component toxicity data were likely to encompass the range of potential toxic effects fraction members, the degree to which the component toxicity data suggested that fraction members exert similar effects at similar doses, and analytical data needed for each approach. Using this approach, the toxicity of each of the six fractions can be estimated in one or more of the following ways: indicator chemical method, hazard index (HI) method, relative potency factor (RPF) method, and/or integrated addition method. For fractions with multiple methods available, methodology selection should be driven by the available exposure data. To estimate health risk or hazard for the entire hydrocarbon mixture, the estimates for all six of the fractions are summed using an appropriate additivity method (i.e., dose or response addition.) Noncancer hazard estimation for the mixture is performed using the HI approach and cancer risk assessment is performed using a combination of dose and response addition methods.
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