BEYOND TPH - HEALTH-BASED EVALUATION OF PETROLEUM HYDROCARBON EXPOSURES

The term ''total petroleum hydrocarbons'' (TPH) is a widely used, but loosely defined, parameter quantified by a number of different methodo logies for expressing the aggregate amount of petroleum hydrocarbon co mpounds (PHCs) in a sample. Because of the shortcomings associated wit h comparing data from different methods, and the difficulty of assessi ng potential toxicities of complex mixtures of hydrocarbons, a new app roach at more fully and explicitly defining the PHC composition of sam ples and predicting human noncancer health risks from those exposures has been developed. This new approach is the subject of this paper. Th is method can be used to perform site-specific risk assessments or to develop health-based cleanup standards for petroleum hydrocarbons. The technique divides the broad chemical classes of PHC (i.e., saturated versus unsaturated) into subgroups of compounds based on numbers of ca rbon atoms in the compounds within each subgroup. The mass of compound s in each subgroup is then translated into discrete estimates of healt h risk for specified exposure scenarios. The subgroups were identified from qualitative and quantitative changes in the nature of noncancer toxicities recorded in the literature. For saturated compounds, toxici ty changes as carbon chain length increases (measured by numbers of ca rbon atoms). A ''reference compound'' was chosen for each range of com pounds, usually because its toxicity was relatively well characterized . A published oral reference dose (RfD) was identified for these compo unds, or in the absence of a published value, an oral dose-response va lue was developed from available toxicity information. For saturated P HCs (alkanes, cycloalkanes, and isoalkanes) the subgroups' reference c ompounds and assigned toxicity value used are C5 to C8 (n-hexane, 0.06 mg/kg/day); C9 to C18 (n-nonane, 0.6 mg/kg/day); and C19 to C32 (eico sane, 6.0 mg/kg/day). For unsaturated compounds (aromatics), one refer ence RfD was identified for all compounds: C9 through C32 (pyrene, 0.0 3 mg/kg/day). Dependent upon the analytical technique used for separat ion of compounds, the unsaturated alkenes may be grouped and subsequen tly quantified with either the saturate or unsaturate groups. The impl ications of possible association with either group and contributions t o risk estimates are probably not significant. Alkenes make up a small fraction of most fuel products, and they bear structural similarity t o the alkanes and are not particularly toxicologically active. If grou ped analytically with the aromatics the alkene contribution to toxicit y estimates would likely be minor and not be an underestimate of its t rue toxicity. The mass of PHC in each segment of a chromatogram is qua ntified and converted to a medium-specific concentration which is then entered into standard medium intake equations to arrive at a daily do se of PHC. This dose is then used with the toxicity value identified f or the particular segment of the chromatogram to derive a hazard quoti ent. The quotients can then be summed across fractions to yield a tota l hazard index. The noncancer health risks from the aromatics benzene, toluene, and xylenes are evaluated separately using standard risk ass essment techniques. (C) 1996 Academic Press, Inc.