Sources of air pollution are divided up into four categories:
- Point Sources
- Non-Point Sources
- On-Road Mobile Sources
- Non-Road Mobile Sources
Point sources are large, stationary sources of emissions. Examples of point sources are natural gas compressor stations, petroleum refineries and grain processing or storage facilities. Non-Point sources are smaller, generally more numerous sources whose individual emissions do not qualify them as point sources. Although area sources release relatively small amounts of air pollutants on an individual basis, because of the numbers of these sources, their emissions as a whole are significant. Examples include household solvents and paints, motor vehicle refueling and residential fuel combustion.
On-Road Mobile Sources are sources of air pollution that are not stationary, and can typically be driven on a highway such as cars, trucks, buses and motorcycles. Non-Road Mobile Sources are also not stationary, but typically are not driven on highways. Examples of Non-Road Mobile Sources include lawnmowers, locomotives, and tractors.