All human activities add gases and particles to the air we breathe. When these gases and particles, or air pollutants, accumulate in the air in high enough concentrations, they can harm us and our environment. In response to the identification of air pollution as a national problem that needed a comprehensive program to address it, the Clean Air Act was passed and the Environmental Protection Agency was established to implement it and other environmental laws.
Human-generated (also called anthropogenic) sources of air pollution can be broken down into two categories: mobile and stationary sources. Mobile sources of air pollution include most forms of transportation such as onroad vehicles like automobiles and trucks, as well as nonroad equipment including tractors, backhoes, trains, and airplanes. Stationary sources of air pollution consist of non-moving sources, such as power plants, industrial facilities, gas stations, or paint booths.
Stationary sources are further divided into two classifications: point sources and area sources. Large stationary sources of emissions that have specific locations and release pollutants in quantities above a certain emission threshold are known as point sources. Those facilities or activities whose individual emissions do not qualify them as point sources are called area sources. Area sources represent numerous facilities or activities that individually release small amounts of a given pollutant, but collectively can result in significant amounts of emissions. For example, dry cleaners, vehicle refinishing, gasoline dispensing facilities, and residential heating will not typically qualify as point sources, but collectively the various emissions from these sources are classified as area sources.