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25 Table 7. Chemical structures for propanal (propionaldehyde), butanal, butene, crotonaldehyde, and their corresponding surrogate compounds. HAP without Toxicity Criteria Surrogate Compound O O O H Propanal Butanal Acetaldehyde (Propionaldehyde) Butene Propene O Crotonaldehyde Acrolein CR ATcancer increasing severity, for exposure periods ranging from 10 min RBCcancer (g/m3 ) = to 8 hr. For this analysis we selected AEGL-1 values, which IUR EF ED ATnoncancer correspond with concentrations at which the general popula- RBCnoncancer (g/m3 ) = 100 HQ RfC tion, including susceptible subpopulations, might experience EF ED transient and reversible discomfort or irritation, but not any Where: disabling effects (NRC 2007). Acute MRLs are developed by ATcancer = Averaging time - cancer (25550 days); ATSDR, and represent a concentration that would not be ATnoncancer = Averaging time - noncancer (10950 days); associated with adverse health effects including in sensitive CR = Acceptable cancer risk (1 10-6); individuals (i.e., a "no-effect" concentration), for an expo- ED = Exposure duration (30 years); sure period of 114 days. Acute RELs are developed by EF = Exposure frequency (350 days/year); California Environmental Protection Agency's Office of HQ = Hazard quotient (1); Environmental Health Hazard Assessment. As with the acute IUR = Inhalation unit risk (risk per g/m3 - chemical MRLs, the acute RELs represent a "no-effect" concentration specific); and at which no adverse health effects are expected, including for RfC = Reference concentration (mg/m3 - chemical the most sensitive individuals in an exposed population specific). (CalEPA 1999). Acute RELs are applicable to a 1-hr exposure period unless they are based on reproductive or develop- We used standard assumptions for averaging time, expo- mental endpoint, in which case they are applicable to an sure duration, and exposure frequency, as recommended by exposure period of several hours. Toxicity criteria for evalu- EPA (USEPA 1989). ating acute exposures are presented in Table 8. Table 3 listed RBCs along with relative emissions factors As noted in Section 2, toluene and xylene are emitted in for airport-related HAPs. fairly large quantities, yet what is currently known regarding their toxicity indicates they are much less of a health concern than that of the prioritized HAPs identified in this analysis. 4.4 Evaluation of Acute Exposures As indicated by the values in Table 8, this is true for both for Aviation-Related HAPs chronic and acute toxicity. The determination that toluene For evaluating acute effects the report researchers identi- and xylene airport emissions likely do not present a substan- fied acute exposure guidelines (AEGLs), acute minimal risk tial health concern is supported by an evaluation of acute levels (MRLs), and acute inhalation reference exposure levels human health risks for Oakland International Airport (CDM (RELs). AEGLs are developed through a federal advisory 2003). This evaluation indicates that CalEPA's acute RELs for committee with input from stakeholders for use in chemical toluene and xylene are more than 2,000-fold greater than emergency planning, prevention and response programs, and estimated air concentrations, which provides a very wide include peer review by the National Research Council margin of safety to account for any uncertainties in estimates (USEPA 2007). AEGLs are established at three levels, with of toxicity or exposure.

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26 Table 8. Acute toxicity criteria for airport-related hazardous air pollutants. Toxicity Criterion (ppm) RELa AEGL-1 AEGL-1 MRL HAP 16 Hours 1 Hour 8 Hour 114 Days Acetaldehyde 45 45 Acetone 200 200 26 Acrolein 0.00008 0.03 0.03 0.003 Benzene 0.4 52 9 0.009 1,3-butadiene 45 45 Formaldehyde 0.08 0.9 0.9 0.04 Methanol 530 270 Phenol 1.5 15 6.3 0.02 Propane 5,500 5,500 Styrene 5 20 20 Toluene 10 200 200 1 Xylenes 5 130 130 2 Data Sources: ATSDR 2007a (MRLs); CalEPA 1999 (RELs); USEPA 2007 (AEGLs ). Notes: ppm parts per million HAP hazardous air pollutant REL reference exposure level AEGL acute exposure guideline MRL minimum risk level a Benzene REL is for a 6-hr exposure period, all other RELs are for a 1-hr exposure period.