There are many types of air pollutants regulated under the Clean Air Act, including six “criteria" or generally pervasive pollutants – carbon monoxide, nitrogen oxides, particulate matter, sulfur dioxide, lead and ozone - as well as 187 hazardous air pollutants specifically listed in the statute. These pollutants are responsible for a wide range of adverse health and environmental effects.

Enforcement is a critical component of regulatory air quality programs that helps ensure a level playing field across all companies in an industrial sector. Appropriate methods of enforcement ensure compliance with regulations which helps improve human health and the environment by ensuring that required reductions in air pollution actually occur.

Global warming is the warming of the earth's atmosphere due to increased concentrations of greenhouse gases emitted from human and other activities.  Human activities have substantially increased the amount of greenhouse gases (e.g., carbon dioxide and methane) in the atmosphere, causing the atmosphere to trap more heat and leading to changes in the Earth’s climate.

Industrial sources of air pollution, including power plants, manufacturing facilities and agricultural operations, emit a variety of pollutants that cause or contribute to substantial health and environmental problems throughout the country.

As state and local air pollution control agencies work to achieve their clean air goals, they must, among many other things, measure levels of pollution in the air, issue "permits" for facilities to operate, inspect sources to ensure they comply with clean air regulations, hire and train staff to carry out these activities and seek adequate funding to run clean air programs.

Passenger cars, trucks, buses, ships, locomotives, aircraft and other vehicles and engines, as well as the fuel that run them, represent a principal source of air pollution in the U.S. contributing to serious air quality and environmental problems, such as elevated levels of smog, soot, toxic air pollutants, climate change and acid rain.