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4 criteria pollutants. Descriptions of individual HAPs and their a function of exposure time, and the concentration of a pol- affects on human health have been published in recent docu- lutant, which is determined by the quantity emitted from the ments (URS 2003; FAA 2005). There are many sources that various sources and how these emissions disperse and react emit HAPs--ground transportation, construction, power in the air, as depicted in Figure 1. generation, dry cleaning, and aviation--to name a few. At The emission sources and pollutants that present the great- airports there are several sources of HAP emissions. A partial est risk to the health of a baggage tug operator may differ greatly list of "airside" sources (vehicles that do not leave the airport) from those that affect the health of a nearby resident. Consider includes baggage tugs and other ground support equipment a few exposure groups related to an airport (Figure 2). (GSE), solvent use, and the aircraft themselves. There are A baggage tug operator's exposure to airport pollutants is additional "roadside" sources, also called ground access vehi- presumably greatly influenced by those emission sources that cles (GAVs), that consist mainly of on-road vehicles (cars, she or he is closest to, for example, baggage tugs, belt loaders, buses, shuttles, etc.). and possibly idling aircraft engines and auxiliary power units. This report focuses on gas-phase HAPs, which in the air- Residential Neighborhood 3 is sandwiched between the air- port context is mainly a subcategory of volatile organic com- port runways and the ocean. Since there are few busy roads pounds (VOCs). A related report, ACRP Report 6: Research nearby, the emission source to which these residents experi- Needs Associated with Particulate Emissions at Airports, ence the most exposure might be the aircraft themselves addresses airport particulate matter emissions. during the various phases of the landing take-off (LTO) cycle. Residential Neighborhood 1 is near the airport ground traf- fic, the downtown business district, and a number of busy 1.4 Approach Used for Identifying roads. Residents of Neighborhood 1 are presumably exposed Information Gaps Associated to a wider range of emission sources--airport and non- with Airport-Related Hazardous airport related. Furthermore, the local meteorology (e.g., Air Pollutants wind direction and speed) is another parameter that greatly What is the risk presented by airport-related hazardous air affects the relative importance of these various airport emis- pollutants? Information gaps preclude a definitive answer to sion sources. this question; however, knowledge of which airport sources As evident by the example of just this one airport and a few matter the most and which hazardous air pollutants are most exposure groups, there are many parameters that determine important will surely assist in addressing that question. This risk. A full health risk assessment that is relevant for the clos- section discusses the approach used to determine which est exposure groups must be based on exposure and toxicity, pieces of information are most crucial to determining the rel- which must be done on a case-by-case basis and may be ative risk presented by airport-related HAPs. Note that this different from airport to airport. This is beyond the scope of report is not a human health risk assessment and therefore this report. An example of such an undertaking is the 2003 does not evaluate knowledge gaps based on exposure risk, supplemental environmental impact report for Oakland which is explained in more detail further in this section. International Airport (OAK) (CDM 2003), which explicitly In this report airport emission sources are divided into calculated the risk presented to a number of exposure groups four main categories: due to emissions from OAK. This report provides a preliminary assessment of which 1. Aircraft, airport sources and which individual hazardous air pollutants 2. GSE (baggage tugs, belt loaders, etc.) and auxiliary power should be closely considered in future research. This assess- units (APU), ment is based on both the toxicity of the individual pollutants 3. GAV (cars, buses, shuttles, etc.), and and the amount emitted. It is not possible to identify human 4. Stationary sources (power generation, HVAC systems, health risk generically, as such effects require an extensive solvent use, etc.) amount of information regarding exposure that is uniquely site-specific. Thus, the approach deployed in this evaluation Quantification of the "risk" posed to a particular exposure resulted in the identification of airport-related sources, their group by the emissions of HAPs by these numerous emission associated HAPs, and the relative toxicity of the individual sources at an airport is complicated. The risk presented to HAPs. With this approach, key HAPs and associated sources human health by any particular compound is a function of were identified. The rationale for using this approach is that exposure and toxicity. Exposure and toxicity are in turn both HAPs inventories can be quantified much more easily than a function of the exposure pathway (e.g., ingestion, dermal exposure, and thus the results of the analysis presented herein contact, inhalation). For the inhalation pathway, exposure is can be considered relevant to most commercial airports. The