This Provisional Peer-Reviewed Toxicity Value (PPRTV) assessment document describes a fraction-based approach to risk assessment for complex mixtures of aliphatic and aromatic hydrocarbons. This approach is implemented following a chemical analysis of the total petroleum hydrocarbon (TPH) mixture that is present. The components of TPHs are generally classified into aliphatics and aromatics, and each of these major fractions are then further separated into low, medium, and high carbon range fractions based on the number of carbon (C) atoms in the compounds and/or the compounds’ equivalent carbon (EC) number index. In all, the following six fractions of TPH mixtures are addressed:

• Aliphatic low carbon range TPH fraction
• Aliphatic medium carbon range TPH fraction
• Aliphatic high carbon range TPH fraction
• Aromatic low carbon range TPH fraction
• Aromatic medium carbon range TPH fraction
• Aromatic high carbon range TPH fraction

In this effort, the U.S. EPA is updating the PPRTV assessments for the aliphatic low carbon range TPH fraction (U.S. EPA, 2022a), the aromatic medium carbon range TPH fraction (U.S. EPA, 2022d), the aromatic high carbon range TPH fraction cancer assessment (U.S. EPA, 2022b), the aromatic high carbon range TPH fraction noncancer assessment (U.S. EPA, 2022c), and the TPH mixture assessment (i.e., this document). The U.S. Environmental Protection Agency (U.S. EPA) published its PPRTV assessments for TPHs in 2009. The primary motivation for updating this PPRTV assessment was the release of updated toxicity values from the U.S. EPA’s Integrated Risk Information System (IRIS) program and/or PPRTV assessments for several key constituents of the aliphatic low carbon range fraction and aromatic medium and high carbon range fractions since 2009. U.S. EPA also revised the fraction boundaries for the aromatic medium and high carbon range fractions both to align the fraction definitions with the fractions resulting from current analytical methods and to avoid grouping the generally less toxic substituted benzenes (now in the aromatic medium carbon fraction) with the polycyclic aromatic hydrocarbons (PAHs), naphthalenes, and 1,1-biphenyl (in aromatic high carbon range fraction).

The fraction-based approach examines the noncancer hazards or cancer risks associated with exposure to each of six fractions defined by chemical properties, and then describes the integration of these fraction hazards and risks to evaluate hazards or risks posed by exposures to the mixture. This PPRTV assessment presents toxicity values for the aliphatic and aromatic hydrocarbon fractions, including subchronic and chronic provisional reference doses (p-RfDs) and provisional reference concentrations (p-RfCs), cancer weight-of-evidence (WOE) assessments, provisional oral slope factors (p-OSFs), and provisional inhalation unit risks (p-IURs). This document also presents risk assessment methods for these fractions and chemical mixtures that are intended to replace current approaches used at TPH-contaminated sites.

The assessment follows a data-driven approach and describes methodological options according to the available analytical chemistry data. Tables ES-1 and ES-2 summarize the selected noncancer provisional toxicity values for each fraction under two exposure options (Options 1 and 2, respectively). Option 1 is utilized when exposure data are available for the fraction, rather than individual chemicals in the fraction; Option 2 is utilized when exposure data include measures of individual chemicals in a fraction. For cancer risk assessment, an indicator chemical or surrogate mixture approach is generally employed for each fraction; only a single option generally is utilized, because fewer cancer risk estimates are available for individual chemicals (see Table ES-3). The exception is the cancer risk assessment for the aromatic high carbon range fraction that has three options, depending on the available analytic data. For the cancer assessment for this fraction, Option 1 relies on an indicator chemical approach. Option 2 uses a component approach for selected PAHs (see Table ES-4). Option 3 relies on an integrated additivity approach that accounts for the contributions to carcinogenic risk from the selected PAH, but also the contributions of two other carcinogens that can occur in this fraction (i.e., 1,1-biphenyl and 1-methylnaphthalene).

Table ES-1

Fraction-Specific Noncancer Toxicity Values for Option 1: Exposure Media Analyzed for BTEX and Fractions.

Table ES-2

Fraction-Specific Noncancer Toxicity Values for Option 2: Analytical Data Available for Individual Components and Fractions.

Table ES-3

Fraction-Specific Cancer Toxicity Values.

Table ES-4

RPFs for PAH Carcinogenicity.

Depending on the available information about the chemicals present, the toxicity of each of the six aliphatic or aromatic hydrocarbon fractions is estimated in one or more of the following ways.

• Indicator Chemical Approach: The toxicity value for an individual compound is selected to represent the entire fraction.
• Hazard Index (HI) Approach: A hazard quotient (HQ) is calculated as the ratio of human exposure to a health hazard reference value (RfV) for each mixture component chemical, and HQs are summed generate an HI. This approach is based on dose addition.
• Relative Potency Factor (RPF) Approach: Using RPFs, chemical component doses are scaled relative to the potency of an index chemical (IC) and these scaled doses are summed and expressed as an index chemical equivalent dose (ICED) for the mixture. This approach is based on dose addition.
• Response-Addition Approach: The response-addition approach assumes simple independent action for mixture chemicals that cause the same effect, assuming that each impact is an independent response. The response to the mixture is predicted by summing the risk estimates for the mixture components under the law of statistical independence.
• Integrated Addition Approach: Mixture components are separated into dose-additive groups based on similar mode of action (MOA); risks are calculated separately for each similarity group and summed using response addition. This approach integrates dose and response addition.
• Surrogate Mixture Approach: Chemical mixtures can be generated in a manner considered similar to a mixture (or mixture fraction) that might be encountered in the environment. Health risk values derived from toxicological tests conducted on these mixtures can be used as surrogates for a mixture that was generated by a similar process and encountered in the environment. For fractions with multiple methods available, methodology selection should be driven by the available exposure data.

Section 1 of this document defines the fractions, and provides overviews of the fraction approach and the various mixtures methods used to evaluate risks and hazards associated with the fraction. Section 2 details the literature searched and data reviewed as well as the selection of various mixture approaches. Section 3 reviews the toxicity values defined for the TPH fractions. An overview of how the presented approaches are applied in this PPRTV assessment is described in Section 4.