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equivalent to the screening level every day (350 days/year) for a total of 30 years. While most residential exposures would not occur at the default levels used to derive these screening levels, the defaults are designed to be protective for this hypothetical scenario. Note that site concentrations that exceed the screening levels do not indicate unacceptable human health risks with regards to these pathways; rather, an exceedance may indicate that a site-specific evaluation of human health risk is warranted. 2.2 Chemicals Considered Risk-based soil screening levels were developed for benzene, ethylbenzene, naphthalene and PAHs. These constituents are considered the primary risk-driving compounds at petroleum-impacted sites. Total petroleum hydrocarbons (TPH) were not considered as a chemical of concern. The stakeholders chose not to include TPH in policy for the following reasons: • TPH consists of a mixture of more than 2000 chemicals. Once in soil, the TPH starts weathering immediately changing its composition through time and from one site to the next. Bulk TPH measurements, such as those obtained by analytical method 8015M, are not suitable for risk assessment because they do not provide information about the composition with respect to chemical toxicity and fate and transport properties. None of the regulatory agencies in California that are responsible for requiring risk assessment have an approved analytical method for evaluating TPH for purposes of risk assessment (such as a fractionation method). In fact, most analytical labs in California are not familiar with TPH fractionation. Benzene, ethylbenzene and naphthalene more accurately capture the risk that TPH poses for human health concerns. Methyl tert-butyl ether (MTBE) was not considered as a chemical of concern for the following reasons:3 For benzene, the USEPA RSL in soil for residential land use is 1.1 mg/kg, which is approximately 1,650-times lower than its soil saturation concentration. For MTBE, the residential soil RSL is 43 mg/kg, which is about 200-times lower than its soil saturation concentration. Even though the MTBE content of gasoline may be 10- to 15-times that of benzene, potential risks from direct contact with soil will still be driven by benzene, which is about 60-times more toxic than MTBE. Currently, USEPA does not evaluate MTBE as a potential human carcinogen. The State of California has developed a cancer slope factor for MTBE based on a combination of data from two animal studies, one study by the inhalation route and the other study by the oral route. Numerous 3 Written communication with the Department of Toxic Substances Control, Human and Ecological Risk Office. 3