Aircraft and Airport-Related Hazardous Air Pollutants: Research Needs and Analysis (2008)

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9The central goal of this project is to produce a prioritized research agenda that addresses shortcomings in our knowl- edge related to airport-related HAPs. The current ability of researchers, airport operators, consultants, and others to assess the health risks associated with human exposure to aviation-related HAPs relies on a number of criteria includ- ing the sources, toxicity, and chemistry of HAPs. One activity that will greatly assist in this goal is to identify which pollutants to consider as aviation-related HAPs and rank their relative importance. In general terms, this report intends to provide a screening-level analysis of the volatile or- ganic compounds (VOCs) emitted at airports in order to help prioritize compounds that might pose an inhalation risk to people at or near the airport. This risk is a function of the tox- icity of each molecular species and the extent of exposure to this species. Total human exposure is a complex function of emissions, chemistry, dispersion, length of exposure, location, and affected population and is not easily condensed into a simple formula (see Figure 1 in Section 1.4). To determine which sources are the most important, one must know what exposure group is of interest. Airline pas- sengers are in closest proximity to GAV emissions while trav- eling to and from the airport, and spend a few hours inside the airport buildings and several hours inside the aircraft cabin itself. Residents of nearby communities are likely in closest proximity to either GAV or to aircraft themselves, depending on the geographic configuration of the roads, airfield, and housing. Airport employees can be in close prox- imity to virtually all airport-related sources. As shown previ- ously in Figure 2’s map of a hypothetical airport, some communities near the hypothetical airport are closest to the airfield, while other nearby communities are closest to the roadside traffic. Evaluation of which source is most impor- tant for each possible exposure group must be done on a case- by-case basis, and this report does not attempt to do so. Instead this report only examines overall airport-related emissions. The FAA adopted a related but simpler approach to de- velop a list of airport-related HAPs compounds in a recent review of HAPs emissions (URS 2003). FAA selected the HAPs compounds that are emitted in largest quantity in airport-related activities. This approach resulted in their tab- ulation of 10 aviation-related HAP compounds on the basis of mass emission rates (see Table 1, second column). The rel- ative toxicity of the compounds was not directly considered. The difficulty with that approach is that the relative toxicities of the various compounds have the potential to vary far more than their emission rates. A strength of that approach is that the methodology is simpler since many of the toxicity factors are uncertain. The approach used in this report is to weigh the impor- tance of HAPs by both their relative emission rate and toxicity, even though the toxicological data are clearly in- complete. Estimates of relative HAP emissions can be made as a function of molecular species for the various sources present at airports. As discussed in the next section, the most important source of most airport gas-phase HAPs is the aircraft themselves. Furthermore, the vast majority (> 90%) of aircraft HAP emissions occur when aircraft are taxiing or idle. This report therefore focuses primarily on this HAPs source (aircraft idle/taxi emissions) to generate a list of the most important aviation-related HAPs com- pounds. This list is supplemented where appropriate with information regarding emissions from airport stationary sources, GAV, or GSE. In Table 3, the relative priority of more than 50 organic species that are emitted by aircraft turbine engines and GSE are calculated. The first column identifies the compounds. The second and third columns give risk-based concentrations (RBC) for each species if available. The RBCs in the second column reflect noncarcinogenic health risks while those in the third column are derived from estimates of cancer risk. There is a detailed discussion of the sources and methods used to estimate the RBCs later on in Section 4. The RBCs S E C T I O N 2 Integration of Emission Rates with Toxicology—Prioritization of Airport Hazardous Air Pollutants

10 Compound RBC noncance r RBC cancer Mi n RBC (mg/m 3 ) Ai rcraft EI(x)/EI(HCHO ) GSE EI(x)/EI(HCHO ) Ai rcraft Relativ e Impact GS E Relative Impact me thane 0.227 ethane 0.042 ethene 1 2.1E+03 9.2E-01 9.2E -0 1 1.256 1 pr opane 0 .006 acet yl ene 0 .3 2 pr open e 3.1E+03 3.1E +0 3 0.368 0 1- buten e 3.1E+03 3.1E +0 3 0.142 0 1,3- butadiene 2.1E-0 3 8 .1E-05 8.1E -0 5 0.13 7 0 .059 1689 72 7 c2-butene 0.017 1- pentene 3.1E+03 3.1E +0 3 0.063 0 n- pentane 1.9E+01 1.9E +0 1 0.016 0 2,2,4-tr im ethy lp entane 1.9E+01 1.9E +0 1 0 to 0. 1 0 C5- ene 0.029 2- me th yl -2 -buten e 0 .015 C5- ene 0.022 2- me th yl pentan e 1.9E+01 1.9E +0 1 0.033 0 1- hexen e 6.5E-01 6.5E -0 1 0.0 6 0 benz ene 1.0E-0 2 3 .1E-04 3.1E -0 4 0.13 7 0 .378 439 1210 1- heptene 0.03 6 n- hexan e 7.3E-01 7.3E -0 1 ~0.01 0.06 2 0 n- heptane 1.9E+01 1.9E +0 1 0.005 0 toluen e 5.2E+00 5.2E +0 0 0.05 2 0 .450 0 hex anal 0.014 1- octene 0.022 n- octane 1.0E-01 1.0E -0 1 0.005 0 ethy lbenz en e 1.0E+00 9.7E-04 9.7E -0 4 0.01 4 0 .119 14 12 2 m, p- xy lene 1.0E-01 1.0E -0 1 0.02 3 0 .403 0 st yr ene 1.0E+00 1.0E +0 0 0.02 5 0 .007 0 0 0 4 0 o- xy le ne 1.0E-01 1.0E -0 1 0.023 0 1- nonene 0.0 2 n- nonane 1.0E+00 1.0E +0 0 0.005 0 phenol 2.1E-01 2.1E -0 1 0.016 0 1- decene 0 .015 n- decan e 1.0E+00 1.0E +0 0 0.026 0 C4- benzene 0.018 n- undecane 1.0E+00 1.0E +0 0 0.036 0 C5- cy clohexane 0.021 C5- benzene 0.017 naphthalen e 3.1E-0 3 2 .4E-05 2.4E -0 5 0.044 1808 naphthalen e 7.2E-0 5 7.2E -0 5 0.044 615 n- dodecane 1.0E+00 1.0E +0 0 0.035 0 C13- alk ane 1.0E+00 1.0E +0 0 0.014 0 C14- alk ane 1.0E+00 1.0E +0 0 0.014 0 2- me th yl naphthalene 2.1E-01 2.1E -0 1 0.017 0 1- me th yl naphthalene 2.1E-01 2.1E -0 1 0.0 2 0 form aldehy de 0.01 1.9E-0 4 1.9E -0 4 1 0 .150 5342 80 1 form aldehy de 0.01 4.4E-0 1 1.0E -0 2 1 0 .150 100 15 acetaldehy de 4.1E-0 1 1 .1E-03 1.1E -0 3 0.34 7 0 .029 314 27 acetaldehy de 9.4E-0 3 1 .1E-03 1.1E -0 3 0.34 7 0 .029 314 27 ac ro lein 2.1E-05 2.1E -0 5 0.29 3 0 .002 1404 8 9 9 pr opanal 0.059 0.07 2 acetone 3.2E+01 3.2E +0 1 0.0 3 0 butanal 4.1E-0 1 1 .1E-03 1.1E -0 3 0.078 71 ben za ldeh yde 0 .038 gly oxal 0.14 8 me th yl gl yo xal 0 .122 dim ethy ln apthalene 2.1E-01 2.1E -0 1 0.0 1 0 other PAHs 4.4E-0 5 4.4E -0 5 0.001 23 crotonaldehy de 0 to 0. 1 Table 3. Toxicity-emissions weighting of HAPs emitted by airport sources.

listed in the fourth column are the smaller (higher risk) of those listed in columns two and three. The fifth column lists the emission ratios for each molecular species for a CFM-56 turbine engine operated at idle. The emission ratio is ex- pressed as the mass emission rate of each species divided by that of formaldehyde. EI(x)/EI(HCHO) means emission index of species “X” divided by the emission index of formaldehyde. For example, the mass emission rate of 1,3- butadiene is equal to 13.7% of the mass emission rate of formaldehyde. The choice of formaldehyde as a reference is due to its abundance in aircraft exhaust and its commonality across different measurement efforts. The emission factors are taken from Spicer, Holdren et al. (1994). More recent datasets (EXCAVATE, APEX, etc.) have mostly confirmed the relative speciation of Spicer and have found that the spe- ciation is fairly constant among the ∼10 engines that have since been characterized. These topics will be discussed in Section 5. The sixth column lists emission factors for GSE HAPs scaled to the aircraft source so that aircraft and GSE emissions may be compared. This scaling is based upon the formaldehyde emission inventories from three airports: Fort Lauderdale-Hollywood International Airport (FLL), Philadelphia International Airport (PHL), and Chicago O’Hare International Airport (ORD), all of which indicate that formaldehyde emissions from GSE are equal to 15% ± 3% of the aircraft formaldehyde source. Thus, the GSE formaldehyde emission ratio was set equal to 0.15 in the table and the values for the other HAPs were set relative to this value using the EPA’s speciation profile for nonroad mobile sources. Finally, in columns seven and eight are the ratios of the source-specific emission factor to the minimum RBC for each species. These quantities are proportional to overall health risk but are not intended to have absolute significance. The result of this exercise is a list of the most significant HAPs species emitted by aircraft and GSE in the immediate vicinity of airports. Similar calculations for GAV and station- ary source emissions, when only considering vehicle miles trav- eled within the airport perimeter, show that these emissions are minor compared to the aircraft emissions (and in some cases GSE emissions). The species with the largest “relative impact” values in columns 7 and 8 are taken to be the most significant aviation-related HAPs species. This estimate is, of course, lim- ited by our incomplete knowledge of the input parameters. This is particularly true for the RBCs, which are uncertain for many species. Nevertheless, the result of this exercise is impor- tant since it challenges the conventional view of which species are of highest priority for future studies. The alternative is to ignore the toxicology, which is equivalent to assuming that all species carry equal risk. By considering the emissions and tox- icity (the combined approach using toxicity values described previously), the ranking of the five most important airport- related gas-phase HAPs is: (1) acrolein, (2) formaldehyde, (3) 1,3-butadiene, (4) naphthalene, and (5) benzene. Of these compounds, the only ones for which non-aircraft sources are important are 1,3-butadiene and benzene, for which GSE emissions may be comparable to the aircraft source. This is only the case for airports in which GSE emissions of VOCs (by mass) are comparable in magnitude to VOC emissions by aircraft—though the trend of decreasing GSE emissions due to fleet improvements (see Section 5.2.2) is making this scenario less common. Emissions due to stationary sources and GAV within the airport perimeter are minor compared to the air- craft, as discussed in Section 3. Formaldehyde is included as one of the most important HAPs when using the inhalation unit risk (IUR) currently listed in the EPA’s IRIS database; while use of the value developed by the CIIT, which is used by the EPA’s National Air Toxics Assessment, results in a surpris- ingly less significant role for formaldehyde. That is, the relative importance of formaldehyde depends greatly on which value is considered for its IUR. Several rows in Table 3 are highlighted. Compounds high- lighted in light grey are found to be significant airport- related HAPs and have been generally regarded as such in other studies. “Significant” in this context means these com- pounds have a high relative importance when using this combined emissions-toxicity weighting. Compounds high- lighted in dark grey—glyoxal, methylglyoxal, and croton- aldehyde (butenal)—have the potential to be significant airport-related HAPs but are not generally included in other lists. None of these three compounds is currently included on the EPA’s list of 188 hazardous air pollutants (as dictated by the Clean Air Act), but are included in the EPA’s “Master List of Compounds Emitted by Mobile Sources” (www.epa. gov/otaq/toxics.htm). It is noted that the emission factors for these three species (especially crotonaldehyde) are more un- certain than the other compounds since they are difficult to measure, and the relevant toxicological information is in- complete. Their importance depends on their toxicity and confirmation that they are emitted at the levels estimated in Table 3. Finally, two compounds—toluene and the xylene isomers—highlighted in the table with a dotted background, are found to have a surprisingly low relative ranking by this emissions-toxicity weighting but are included on other lists of aviation related HAPs. These compounds are emitted in large quantities, yet existing toxicity information indicates they present much less of a health risk than other HAPs with comparable emissions, such as acrolein or benzene. The de- tails of their toxicology are discussed below in Section 4. This prioritization does not account for acute effects (Section 4.4), which can be very compelling in terms of lost work days and accidents. These differences were summarized in Table 1 where the most significant HAPs species identified in this review are compared to two other reports. The first report is a recent FAA 11

review (UCR 2003) that compiled a list of aviation-related HAPs ranked in order of overall emission rate (without regard to toxicity). The other source is a recent Chicago O’Hare In- ternational Airport (ORD) Environmental Impact Statement (EIS) (FAA 2005). The ORD document used an approach similar to the one adopted in this report, using toxicity and emission rate as the key criteria. As discussed above, the approach recommended by this ACRP project team is that prioritized aviation-related HAPs be determined by comparing estimated relative emissions with RBCs. Using this prioritization scheme, HAPs with greater emissions and lower RBCs (i.e., higher toxicity) are given greater weight. Although the ORD EIS considered tox- icity, it used a different approach for weighting relative tox- icity than that used in this review. For example, the ORD EIS gave carcinogens classified as “known” or “probable” human carcinogens more weight than “possible” carcinogens, whereas all carcinogens were weighted equally (relative to their potency) in our approach. Neither this report nor the ORD EIS relied exclusively on toxicity criteria available on EPA’s IRIS database. Finally, the ORD EIS used a different approach for weighting carcinogens versus noncarcinogens. Of note on our list of high-priority HAPs is the absence of toluene. Although toluene emissions are relatively high, the toxicity is relatively low. EPA considers that the confidence in the reference concentration for toluene, as well as the under- lying database, is high (USEPA 2005b). Carcinogenicity of toluene has been evaluated by the National Toxicology Pro- gram in both rats and mice, with no convincing evidence that toluene is carcinogenic. In addition, toluene does not appear to be genotoxic (i.e., it does not directly cause DNA damage). As discussed previously, glyoxal, methylglyoxal, and cro- tonaldehyde may be important HAPs for further evaluation as well. These HAPs have not undergone a formal toxicity evaluation. However, a comparison with structurally similar HAPs indicates they may be important in terms of relative toxicity and emissions. This analysis indicates they may be among the most important airport HAPs pending determi- nation of their toxicity. Subsequent sections contain more details concerning the state of the toxicological data and a detailed rationale for our decision to focus on HAP emissions from aircraft turbine engines operating near idle as the dominant source of HAPs at commercial airports. 12