Air pollution is caused by many types of sources of every size. These include biogenic (i.e., natural) sources, such as volcanoes or forest fires, and anthropogenic sources, which are those created by humans. The latter includes mobile sources (related to transportation) and stationary sources (non-moving sources). Within the category of stationary sources, there are point sources (those at a fixed point), such as a factory’s smokestack, and area sources, which include clusters of smaller sources (e.g., dry cleaners, print shops, spray painters and woodstoves) that may not emit very much individually but together contribute significantly to air pollution levels. While industrial sources can be either point or area sources, the term is often associated with larger operations. These can include any type of industry, such as agricultural operations, factories that manufacture goods, power plants, chemical plants, waste disposal incinerators and many others. There are hundreds of thousands of industrial sources in the United States, which are responsible for emissions of various types of pollutants, including “Criteria Pollutants” and hazardous (or toxic) air pollutants, which are regulated under the Clean Air Act.