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uncertainties have been identified in the animal studies, including severe mortality and lack of histopathological criteria. In addition, the mechanism of MTBE carcinogenicity is not known. Given the uncertainties associated with MTBE carcinogenicity, benzene will be the risk-driving chemical of concern associated with fuel-related hydrocarbons, especially considering that benzene is a known human carcinogen with a known mechanism of action. The soil screening level for "PAH" is appropriate for comparison with the total concentration of the seven carcinogenic PAHs, as benzo(a)pyrene equivalents (BaPe)4 . The carcinogenic PAHs typically analyzed during site investigations are: benz[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene, dibenz(a,h)anthracene, and indeno(1,2,3-cd)pyrene. The toxicity value used for the entire group of carcinogenic PAHs is California's Office of Environmental Health Hazard Assessment (OEHHA) cancer potency value for benzo(a)pyrene (OEHHA 2010). This is a conservative assumption because the few PAHs that are more carcinogenic than benzo(a)pyrene are typically not found in petroleum mixtures. 2.3 Requirements for Using Screening Levels There is only one "model" used in calculation of the Screening Levels. This model assumes the following: The area of impacted soil where a particular exposure occurs is 25 by 25 meters (approximately 82 by 82 feet) or less. This does not mean that the site or the property has to be less than 25 by 25 meters. If the area of impacted soil where a particular exposure is larger, a site-specific risk analysis may be warranted. The receptor is located at the downgradient edge for inhalation exposure. For residential exposures, it is assumed that the receptor is located on site for 24 hours/day for the entire exposure duration. For industrial and utility workers, it is assumed that the worker is located onsite for 8 hours/day. The wind speed is assumed to equal 2.25 meters per second (m/s) on average. If the average wind speed is lower, a site-specific risk analysis may be warranted. The default input parameters for all exposure scenarios were obtained from DTSC defaults for California hazardous waste sites. If the exposure scenarios are different, a site-specific risk analysis may be warranted. 4 The Office of Environmental Health Hazard Assessment (OEHHA) of Cal/EPA has developed potency equivalency factors (PEFs) for carcinogenic PAHs based on their potential toxicity when compared to benzo(a)pyrene [B(a)P]. To estimate B(a)P toxicity equivalents (TEQs; referred to as BaPe), the concentration of each carcinogenic PAH detected in soil is multiplied by the appropriate PEF developed by OEHHA [benz[a]anthracene (0.1), benzo[a]pyrene (1.0), benzo[b]fluoranthene (0.1), benzo[k]fluoranthene (0.1), chrysene (0.01), dibenz(a,h)anthracene (0.34), and indeno(1,2,3-cd)pyrene (0.1). The sum of BaPe concentrations for a mixture of PAH results in a total BaPe for each sample; the total BaPe concentrations should be compared to the soil screening level for PAHs. 4