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21 <br /> No trend data are available for PM2 , 5 because data are just starting to be collected . <br /> Fortunately, there is a monitoring location in Chapel Hill that will generate data for future trend <br /> analyses . <br /> 2 . 4 Sources of Air Contaminants in Orange County <br /> There are two primary categories of sources of air pollutants - stationary and mobile . <br /> Stationary sources include power plants , manufacturing facilities , gas stations , incinerators , etc . <br /> Large stationary sources , such as power plants and large manufacturing facilities , must have <br /> permits from the State to operate if their emissions exceed certain threshold amounts . Smaller <br /> sources may also require permits , but the requirements for application and reporting are less <br /> stringent than for larger emitters . However, some sources , e . g . , dry cleaners , generally do not <br /> require permits due to the small volume of emissions . Mobile sources include cars , trucks , <br /> buses , planes , trains , motorcycles , and gasoline-powered lawn mowers . Mobile sources may be <br /> classified as on-road (e . g . , cars and trucks ) and off-road (trains , lawn mowers , tractors , and <br /> construction equipment) . In addition to the ' stationary and mobile sources , air pollutants , for <br /> example particulate matter, may be generated from agriculture activities , construction activities , <br /> or open burning . <br /> 2 . 4 . 1 Stationary Sources <br /> Stationary sources may emit criteria pollutants (e . g . , S02 from a power plant) or <br /> hazardous air pollutants (HAPs) . The Clean Air Act requires the EPA to address 188 HAPs . <br /> The State of North Carolina is responsible for issuing permits to facilities that emit air pollutants . <br /> Each facility prepares an application for a permit with a list of emitted pollutants and estimates of <br /> annual emissions . Actual emissions are reported by the facility. <br /> For this report , the NC Division of Air Quality provided the NC Emissions : Facility <br /> Detail report for 1997 for the 20 permitted facilities in Orange County. These data are also <br /> available from the EPA Aerometric Information Retrieval System (AIRS ) and can be accessed <br /> via the Internet at http : //www . epa . gov/enviro /htinl/ airs/airs query Java . html . <br /> The first observation that can be made about permitted sources in Orange County is the <br /> small number . Compared to adjacent counties , Orange County has very few permitted facilities . <br /> The number of permitted facilities listed in AIRS for adjacent counties is as follows : <br /> a) Orange . 20 <br /> b) Alamance 159 <br /> c) Chatham 69 <br /> d) Durham 163 <br /> e) Person 36 <br /> f) Wake 309 <br /> The mass of air pollutants emitted in a County may not be directly related to the number of <br /> facilities , but the number of permitted facilities is a good indicator of the relative significance of <br /> 7 <br />