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5.9 PUBLIC HEALTH To evaluate potential health risks,the measures of these risks are first described in terms of the types of public health effects and the significance criteria and thresholds for those effects. 5.9.4.1 Significance Criteria Significance criteria exist for both carcinogenic and non-carcinogenic risks, and are discussed separately. 5.9.4.1.1 Cancer Risk Cancer risk is the probability or chance of contracting cancer over a human life span (assumed to be 70 years). Carcinogens are assumed to have no threshold below which there would be no human health impact. In other words, any exposure to a carcinogen is assumed to have some probability of causing cancer;the lower the exposure,the lower the cancer risk (i.e.,a linear,no-threshold model). Under state and SJVAPCD regulations,an incremental cancer risk greater than 10-in-one million due to a project is considered to be a significant impact on public health if the emitting units are determined by the District to be using Toxics Best Available Control Technology (T-BACT).13 The 10-in-one-million risk level is also used by the Air Toxics"Hot Spots" (AB 2588)program and California's Proposition 65 as the public notification level for air toxic emissions from existing sources. 5.9.4.1.2 Non-Cancer Health Impact Non-cancer health effects can be either long-term(chronic) or short-term (acute). In determining potential non-cancer health risks from air toxics, it is assumed there is a dose of the TAC below which there would be no impact on human health. The air concentration corresponding to this dose is called the Reference Exposure Level (REL). A non-cancer health impact is measured in terms of a health hazard quotient for each TAC,which is the calculated maximum exposure (concentration) of each TAC divided by its REL. Health hazard quotients for TACs affecting the same target organ are typically summed with the resulting totals expressed as health hazard indices for each organ system. A health hazard index of less than 1.0 is considered to be a less-than-significant health risk. Chronic toxicity is defined as adverse health effects from prolonged chemical exposure, caused by chemicals accumulating in the body. Because chemical accumulation to toxic levels typically occurs slowly, symptoms of chronic effects usually do not appear until long after exposure commences. The lowest no-effect chronic exposure level for a non- carcinogenic air toxic is the chronic REL. Below this threshold,the body is capable of eliminating or detoxifying the chemical rapidly enough to prevent its accumulation. The chronic health hazard index was calculated as the sum of the chronic health hazard quotients,each of which is calculated as the chronic TAC annual concentration divided by the chronic REL of the TAC. Acute toxicity is defined as adverse health effects caused by a brief chemical exposure of no more than 24 hours. For most chemicals,the air concentration required to produce acute effects is higher than the level required to produce chronic effects because the duration of exposure is shorter. Because acute toxicity is predominantly manifested in the upper respiratory system at threshold exposures, all acute health hazard quotients are typically summed to calculate the acute health hazard index. The maximum one-hour average 13 The threshold would be 1 in one million if the emitting units were determined not to be applying T-BACT. 5.9-10 SAC/371322/082340007(LEC_5.9_PUBLIC_HEALTH.DOC)