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5 RISK CHARACTERIZATION <br />5.1 CARCINOGENIC CHEMICAL RISK <br />The SCAQMD CEQA Air Quality Handbook (1993) states that emissions of toxic air contaminants <br />(TACs) are considered significant if a HRA shows an increased risk of greater than ten in one <br />million. Based on guidance from the SCAQMD in the document Health Risk Assessment Guidance <br />for Analyzing Cancer Risks from Mobile Source Diesel Idling Emissions for CEQA Air Quality <br />Analysis ( (3), for purposes of this analysis, ten (10) in one million is used as the cancer risk <br />threshold for the proposed Project. <br />Excess cancer risks are estimated as the upper -bound incremental probability that an individual <br />will develop cancer over a lifetime as a direct result of exposure to potential carcinogens over a <br />specified exposure duration. The estimated risk is expressed as a unitless probability. The cancer <br />risk attributed to a chemical is calculated by multiplying the chemical intake or dose at the human <br />exchange boundaries (e.g., lungs) by the chemical -specific cancer potency factor (CPF). A risk level <br />of 1 in a million implies a likelihood that up to one person, out of one million equally exposed <br />people would contract cancer if exposed continuously (24 hours per day) to the levels of toxic air <br />contaminants over a specified duration of time. This risk would be an excess cancer risk that is in <br />addition to any cancer risk borne by a person not exposed to these air toxics. <br />Health risks associated with exposure to carcinogenic compounds can be defined in terms of the <br />probability of developing cancer as a result of exposure to a chemical at a given concentration. <br />Under a deterministic approach (i.e., point estimate methodology), the cancer risk probability is <br />determined by multiplying the chemical's annual concentration by its unit risk factor (URF). The <br />URF is a measure of the carcinogenic potential of a chemical when a dose is received through the <br />inhalation pathway. It represents an upper bound estimate of the probability of contracting <br />cancer as a result of continuous exposure to an ambient concentration of one microgram per <br />cubic meter (µg/m3) over a 70 year lifetime. The URFs utilized in the assessment and <br />corresponding cancer potency factors were obtained from the Consolidated Table of OEHHA/ARB <br />Approved Risk Assessment Health Values. <br />Notwithstanding, it is the intent of the HRA to provide risk estimates from near -field on -road <br />sources that are reflective of anticipated exposures experienced at a given residential occupancy. <br />As such, a review of relevant guidance was conducted to determine applicability of the use of <br />early life exposure adjustments to identified carcinogens. For risk assessments conducted under <br />the auspices of The Air Toxics "Hot Spots" Information and Assessment Act (AB 2588, Connelly, <br />Statutes of 1987; Health and Safety Code Section 44300 et seq.) a weighting factor is applied to <br />all carcinogens regardless of purported mechanism of action. However, for this assessment, the <br />HRA relied upon U.S. Environmental Protection Agency guidance relating to the use of early life <br />exposure adjustment factors (Supplemental Guidance for Assessing Susceptibility from Early -Life <br />Exposure to Carcinogens, EPA/630/R-003F) whereby adjustment factors are only considered <br />when carcinogens act "through the mutagenic mode of action." The U.S. Environmental <br />Protection Agency has identified 19 compounds that elicit a mutagenic mode of action for <br />