PRINCIPLES OF TOXICOLOGY

484 EXAMPLE OF RISK ASSESSMENT APPLICATIONS
toluene, xylenes, and hexane. The Amerian Society for Testing and Materials (ASTM) has prepared a
thorough guidance document for conducting risk assessments of petroleum mixtures using the
indicator chemical approach.
Examples of the surrogate chemical risk assessment approach for petroleum hydrocarbons include
the Massachusetts Department of Environmental Protection and Total Petroleum Hydrocarbon Committee
Working Group methods. These methods identify specific carbon ranges for both aliphatic and
aromatic hydrocarbons and assign a reference dose to each fraction. The primary difference between
the two methods is the number of separate petroleum fractions identified (MADEP method, 6;
TPHCWG method, 13) and the manner in which toxicological surrogates are assigned. The MADEP
method uses single chemicals to represent the toxicity of a petroleum fraction whereas the TPHCWG
method uses petroleum-fraction-specific toxicological data as available.
We illustrate the use of the TPHCWG method to assess the risks posed by weathered diesel fuel
in an industrial exposure scenario. In this example, a railyard worker is assumed to be exposed to
diesel fuel in soil via incidental ingestion of dust and absorption of petroleum hydrocarbons from
soil into the skin. Air monitoring did not detect the presence of petroleum hydrocarbons that could
be attributed to site sources.
Table 19.1 presents the soil concentrations of diesel fuel constituents by petroleum fraction, the
reference doses (RfDs) used to assess the toxicity that may result from exposure to these fractions, and
the target organ or critical effect associated with exposure to each fraction. Animal toxicity data is the
basis for the RfD for each petroleum fraction.
The USEPA defines the RfD to be an estimate of the daily exposure that is likely to be without adverse
health effects. The exposure (in milligrams of chemical intake per kilogram of body weight per day) divided
by the RfD is termed the “hazard quotient” or HQ. The sum of the HQ values for different routes of exposure
or chemicals is termed the “hazard index” (HI) (see also Chapter 18 for a discussion of HQ and HI). If the
TABLE 19.1 Example—Petroleum Hydrocarbon Risk Assessment Concentrations of Petroleum
Hydrocarbon Fractions in Soil, Reference Doses, Critical Effects
Petroleum Hydrocarbon
Fraction
Concentration Detected in
Soil (mg/kg)
Oral Reference Dose
(mg/kg-day) Critical Effect
Aliphatics
C5–C6 ND a
5 Neurotoxicity
C>6–C8 ND 5 Neurotoxicity
C>8–C10 ND 5 Liver and hematologic
changes
C>10–C12 ND 0.1 Liver and hematologic
changes
C>12–C16 2,200 0.1 Liver and hematologic
changes
C>16–C21 18,000 2 Liver granuloma
C>21–C35 6,600 2 Liver granuloma
Aromatics
C>7–C8 ND 0.04 Decreased body weight
C>8–C10 ND 0.04 Decreased body weight
C>10–C12 ND 0.04 Decreased body weight
C>12–C16 1,500 0.04 Decreased body weight
C>16–C21 9,300 0.03 Kidney toxicity
C>21–C35 9,100 0.03 Kidney toxicity
a Not detected.