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2 . 4 Sources of Air Contaminants in Orange County There are two primary categories of sources of air pollutants - stationary and mobile . Stationary sources include power plants , manufacturing facilities , gas stations , incinerators, etc . Large stationary sources , such as power plants and large manufacturing facilities , must have permits from the State to operate if their emissions exceed certain threshold amounts . Smaller sources may also require permits , but the requirements for application and reporting are less stringent than for larger emitters . However, some sources , e . g . , dry cleaners , generally do not require permits due to the small volume of emissions . Mobile sources include cars , trucks, buses , planes , trains , motorcycles , and gasoline -powered lawn mowers . Mobile sources may be classified as on-road (e . g . , cars and trucks) and off-road (trains , lawn mowers , tractors , and construction equipment) . In addition to the stationary and mobile sources , air pollutants , for example particulate matter, may be generated from agriculture activities , construction activities , or open burning . 2 . 4 . 1 Stationary Sources Stationary sources may emit criteria pollutants (e . g . , S02 from a power plant) or hazardous air pollutants (HAPs) . The Clean Air Act requires the EPA to address 188 HAPs . The State of North Carolina is responsible for issuing permits to facilities that emit air pollutants . Each facility prepares an application for a permit with a list of emitted pollutants and estimates of annual emissions . Actual emissions are reported by the facility . For this report, the NC Division of Air Quality provided the NC Emissions : Facility Detail report for 1997 for the 20 permitted facilities in Orange County . These data are also available from the EPA Aerometric Information Retrieval System (AIRS ) and can be accessed via the Internet at http : //www . epa . gov/enviro/html/airs/airs_query java . html . The first observation that can be made about permitted sources in Orange County is the small number . Compared to adjacent counties , Orange County has very few permitted facilities . The number of permitted facilities listed in AIRS for adjacent counties is as follows : • Orange 20 • Alamance 159 • Chatham 69 • Durham 163 • Person 36 • Wake 309 The mass of air pollutants emitted in a County may not be directly related to the number of facilities , but the number of permitted facilities is a good indicator of the relative significance of stationary sources in the adjacent counties . Data on the mass of pollutants emitted by each permitted facility is available , but compilation of the data was beyond the scope of this report . 6