Air pollution is a mix of hazardous substances from both human-made and natural sources. The sources of air pollution can be categorized into four main types:

1. Mobile sources: These include cars, buses, planes, trucks, and trains. The primary mobile source of air pollution is the automobile, which accounts for more than half of all the air pollution in the United States.

2. Stationary sources: These include power plants, oil refineries, industrial facilities, and factories. Stationary sources emit large amounts of pollution from a single location and are also known as point sources of pollution.

3. Area sources: These include agricultural areas, cities, and wood-burning fireplaces. Area sources are made up of lots of smaller pollution sources that arent a big deal by themselves but can be significant when considered as a group.

4. Natural sources: These include wind-blown dust, wildfires, and volcanoes. Natural sources can sometimes be significant but do not usually create ongoing air pollution problems like the other source types can.

Other sources of air pollution include traffic-related air pollution, which is a mixture of gases and particles emitted by vehicles. Chemical production, fuel oils, and natural gas used to heat homes, and by-products of manufacturing and power generation, particularly coal-fueled power plants, are also primary sources of human-made air pollution. Nature also releases hazardous substances into the air, such as smoke from wildfires, ash and gases from volcanic eruptions, and gases emitted from decomposing organic matter in soils.

It is important to note that air pollutants may have a natural, anthropogenic, or mixed origin, depending on their sources or the sources of their precursors.