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

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10 Table 3. Toxicity-emissions weighting of HAPs emitted by airport sources. Compound Aircraft GSE Aircraft GSE RBC RBC Min RBC EI(x)/EI(HCHO) EI(x)/EI(HCHO) Relative Relative noncancer cancer (mg/m3) Impact Impact methane 0.227 ethane 0.042 1 1.256 ethene 2.1E+03 9.2E-01 9.2E-01 1 propane 0.006 acetylene 0.32 propene 3.1E+03 3.1E+03 0.368 0 1-butene 3.1E+03 3.1E+03 0.142 0 1,3-butadiene 2.1E-03 8.1E-05 8.1E-05 0.137 0.059 1689 727 c2-butene 0.017 1-pentene 3.1E+03 3.1E+03 0.063 0 n-pentane 1.9E+01 1.9E+01 0.016 0 2,2,4-trimethylpentane 1.9E+01 1.9E+01 0 to 0.1 0 C5-ene 0.029 2-methyl-2-butene 0.015 C5-ene 0.022 2-methylpentane 1.9E+01 1.9E+01 0.033 0 1-hexene 0.06 6.5E-01 6.5E-01 0 benzene 1.0E-02 3.1E-04 3.1E-04 0.137 0.378 439 1210 1-heptene 0.036 n-hexane 7.3E-01 7.3E-01 ~0.01 0.062 0 0 n-heptane 1.9E+01 1.9E+01 0.005 0 toluene 5.2E+00 5.2E+00 0.052 0.450 0 0 hexanal 0.014 1-octene 0.022 n-octane 1.0E-01 1.0E-01 0.005 0 ethylbenzene 1.0E+00 9.7E-04 9.7E-04 0.014 0.119 14 122 m,p-xylene 1.0E-01 1.0E-01 0.023 0.403 0 4 styrene 1.0E+00 1.0E+00 0.025 0.007 0 0 o-xylene 1.0E-01 1.0E-01 0.023 0 1-nonene 0.02 n-nonane 1.0E+00 1.0E+00 0.005 0 phenol 2.1E-01 2.1E-01 0.016 0 1-decene 0.015 n-decane 1.0E+00 1.0E+00 0.026 0 C4-benzene 0.018 n-undecane 1.0E+00 1.0E+00 0.036 0 C5-cyclohexane 0.021 C5-benzene 0.017 naphthalene 3.1E-03 2.4E-05 2.4E-05 0.044 1808 naphthalene 7.2E-05 7.2E-05 0.044 615 n-dodecane 1.0E+00 1.0E+00 0.035 0 C13-alkane 1.0E+00 1.0E+00 0.014 0 C14-alkane 1.0E+00 1.0E+00 0.014 0 2-methyl naphthalene 2.1E-01 2.1E-01 0.017 0 1-methyl naphthalene 2.1E-01 2.1E-01 0.02 0 formaldehyde 0.01 1.9E-04 1.9E-04 1 0.150 5342 801 formaldehyde 0.01 4.4E-01 1.0E-02 1 0.150 100 15 acetaldehyde 4.1E-01 1.1E-03 1.1E-03 0.347 0.029 314 27 acetaldehyde 9.4E-03 1.1E-03 1.1E-03 0.347 0.029 314 27 acrolein 2.1E-05 2.1E-05 0.293 0.002 14048 99 propanal 0.059 0.072 acetone 3.2E+01 3.2E+01 0.03 0 butanal 4.1E-01 1.1E-03 1.1E-03 0.078 71 benzaldehyde 0.038 glyoxal 0.148 methylglyoxal 0.122 dimethylnapthalene 2.1E-01 2.1E-01 0.01 0 other PAHs 4.4E-05 4.4E-05 0.001 23 crotonaldehyde 0 to 0.1

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

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