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11 In addition to PM, measurements during APEX and from older military engines indicate the presence of hazardous air pollutants (HAPs), alternatively referred to as air toxics. HAPs are regulated by the EPA based on the cancer and non- cancer risk they pose with acute or chronic exposure. Volatile organic compounds (e.g., toluene), chlorinated volatile organic compounds (e.g., tetrachloroethylene), and metals (e.g., nickel) are three classes of HAPs. As dictated by the Clean Air Act, the EPA maintains a list of HAPs. Additionally, for mobile source emissions, the EPA maintains a âMaster List of Compounds Emitted by Mobile Sources.â Measurements of ambient HAP concentrations are not as widespread as those of the criteria pollutants. Descriptions of individual HAPs and their sources and emissions at airports have been provided in recent documents (URS 2003; FAA 2005). For more informa- tion on the human health effects of HAPs, see EPAâs Inte- grated Risk Information System (http://www.epa.gov/iris). In addition to aviation, many sources emit HAPs, including ground transportation, construction, power generation, and dry cleaning. At airports, several sources contribute to HAPs emissions. A partial list of âairsideâ sources includes baggage tugs, solvent use, and the aircraft themselves. Benzene and formaldehyde are two commonly known aircraft engine HAPs. Airport âroadsideâ sources include on-road vehicles (cars, buses, shuttles, etc.). A separate ACRP study has ex- amined the issue of airport HAPs emissions and provides the results in ACRP Report 7: Aircraft and Airport-Related Haz- ardous Air Pollutants: Research Needs and Analysis. C H A P T E R 2 Primer on Hazardous Air Pollutants
