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.

Power plants are currently the dominant emitters of mercury (50 percent), sulfur dioxide (60 percent), acid gases (over 75 percent) and arsenic (62 percent) in the United States. They are also a significant source of emissions of greenhouse gases (34 percent) and nitrogen oxides (13 percent). Most of these emissions come from coal- and oil-fired power plants.

Manufacturing facilities are among the industries in the United States that are responsible for emissions of air pollution. These include factories and other operations, large and small, that produce goods that will be used by individual consumers (e.g., automobiles, appliances, clothing, food). They also include facilities that make intermediate goods that are destined for other manufacturing plants, such as building materials and parts to be used in the creation of end-use items.

Agriculture comprises many types of activities, from crop production to raising animals for food. Agricultural activities contribute directly to emissions of air pollutants through a variety of processes: enteric fermentation in domestic livestock (i.e., methane gas produced during digestion), manure management, rice cultivation, agricultural soil management, machines used in agricultural operation (e.g., diesel irrigation pumps) and burning of agricultural residues.