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r �... S E C O R �..c reflect a target hazard quotient (HQ) of 1, ESLs based on noncarcinogenic effects reflect a target HQ of 0.2 (or 0.5 for TPHg). It should be noted that not all PRGs, CHHSLs, and ESLs for soil are risk-based levels. They can be based on saturation levels (denoted"sat"), or ceiling limit of 100,000 mg/kg(denoted "max") (USEPA, 2004a; Cal-EPA, 2005b; and RWQCB, 2005), or background levels (RWQCB, 2005). Per the USEPA (USEPA, 1991), the acceptable multi-chemical and multi-pathway ECR range is from 1E- 04 to 1E-06, with 1E-06 being point of departure; and the acceptable multi-pathway noncarcinogenic HQ for a single chemical or multi-chemical and multi-pathway hazard index (Hl) (segregated by toxic effects) for all COPCs is 1.0 (the HI is calculated by summing the chemical-specific and/or pathway-specific HQs). Therefore, use of ESLs can account for up to five risk-driving chemicals with an HQ of 0.2 to meet the cumulative HI of 1.0. In California, under the Proposition 65 program, the "no significant risk levels" represent the daily intake level calculated to result in a cancer risk not exceeding one excess case of cancer in 100,000 individuals exposed over a 70-year lifetime (Cal-EPA, 2006c). As such, Cal-EPA uses a target ECR of 1E-05 for individual carcinogenic chemicals to warn the public of potential carcinogens in every day products. Cumulative ECR of 1E-04 must not be exceeded to the exposed populations, including sensitive subgroups. According to Cal-EPA (Cal-EPA, 2001), an individual's actual risk of contracting cancer from exposure to a chemical is often less than the theoretical risk to the entire potentially exposed population estimated in a risk assessment. This may be due to difference in intake rate or a shorter time of occupancy in the area of concern. Moreover, an individual's risk not only depends on the individual exposure to a specific chemical, but also on his/her genetic background, health, diet, and lifestyle choice. Cal-EPA states that ECRs in a risk assessment are often compared to the overall background risk of cancer in the general U.S. population (about 250,000 cases for every one million people) or to the risk posed by all harmful chemicals in a particular medium, such as the air. For example, the ECR from breathing current levels of pollutants in California's ambient air over a 70-year lifetime is estimated to be 760 in one million (or 8E- 04) (Cal-EPA, 2001). 12