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5 Figure 1. Schematic depiction of the factors that determine risk. Emissions of pollutants from various sources undergo chemistry and dispersion in the atmosphere to determine a concentration. Risk is determined by the toxicity of these pollutants and the total exposure. This report focuses on emissions and toxicity, rather than exposure and toxicity, since to a first approximation total exposure due to a given source is proportional to emission rates, which are much easier to quantify and less situation-specific. There is also a "background" concentration of most pollutants that is determined by regional to global processes. disadvantage of the approach is that exposure is essential for sources, and GSE) and the toxicity of these emissions are determining true risk of any population, and thus, the discussed. approach identified in this report would not enable identifi- cation of absolute human health risk at any airport. Rather, 1.5 Main Findings this preliminary assessment is intended to guide the identifi- cation of information gaps and the formation of a prioritized The research team reported the following findings: research agenda, as noted below. The greatest source of airport-related source gas-phase 1. Aircraft emissions at idle/taxi power are the predominant HAPs is aircraft operations, as described in Section 3 of this source of significant gas-phase HAP emissions at an airport. report. As a result, this report focuses on the current state of The actual emissions can vary by more than a factor of two knowledge regarding HAPs emitted by aircraft and the associ- depending on the actual power thrust levels used (e.g., real- ated information gaps. In Section 2 the emissions and toxicity- world ground idle versus International Civil Aviation Orga- weighted relative rankings of HAPs emitted by aircraft and nization [ICAO] 7% idle), time spent idling, and ambient GSE are described. In subsequent sections the state of knowl- conditions, especially temperature. (Yelvington, Herndon et edge regarding emissions from the various airport-related al. 2007). It is likely that many current airport emission sources (e.g., aircraft, auxiliary power units, GAVs, stationary inventories underestimate aircraft HAP emissions as the
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6 Figure 2. Map of a hypothetical airport. The factors that determine risk for any given exposure group depend on numerous factors such as the proximity of emission sources to the location of exposure. required emissions and dispersion modeling system portance. This ranking reflects that some HAPs emitted in (EDMS) assumes use of 7% thrust during the idle and taxi great quantity have relatively low toxicity, while other phase (HAP emission rates are higher at lower thrust levels). HAPs emitted in lower quantities have high toxicity. The 2. Consideration of both the quantity of emissions and the "rank" of formaldehyde on the list prepared for this report toxicity of the individual pollutants yields a list that ranks depends on which of two values is used for its inhalation the relative importance of airport-related HAPs. This list is unit risk (IUR). Use of the value currently listed on EPA's similar to those produced by two previous studies for many Integrated Risk Information System (IRIS) database results compounds (acrolein, benzene, 1,3-butadiene, etc.), but in the placement as shown in Table 1 (i.e., second only to differs significantly for others (e.g., toluene, xylene). The acrolein in importance). Use of the value by the Chemical compounds in Table 1 are listed in order of relative im- Industry Institute of Toxicology (CIIT), which is used by Table 1. Comparison of aviation-related HAPs lists. This ACRP Review FAA 2003 ORD 2005 Acrolein Formaldehyde Acrolein Formaldehyde* Acetaldehyde 1,3-Butadiene 1,3-Butadiene Benzene Formaldehyde Naphthalene Toluene Benzene Benzene Acrolein Acetaldehyde Acetaldehyde 1,3-Butadiene Naphthalene Ethylbenzene Xylene Toluene Lead Naphthalene Propanal (Propionaldehyde) Notes: HAP hazardous air pollutant FAA Select Resource Materials and Annotated Bibliography on the Topic of Hazardous Air Pollutants (URS 2003) ORD O'Hare Modernization Environmental Impact Statement (FAA 2005) * Using EPA IRIS value