text: Air pollution is a broad term applied to any chemical, physical (e.g. particulate matter), or biological agent that modifies the natural characteristics of the atmosphere. The atmosphere is a complex, dynamic natural system that is essential to support life on planet earth. Stratospheric ozone depletion due to air pollution has long been recognized as a threat to human health as well as to the earth's ecosystems.\n\nWorldwide air pollution is responsible for large numbers of deaths and cases of respiratory disease. Setting of air quality standards, such as the U.S. Clean Air Act, has begun to improve air quality for some pollutants. While major stationary sources are often identified with air pollution, the greatest source of emissions are actually mobile sources, principally the automobile. There are many available air pollution control technologies and urban planning strategies available to reduce air pollution; however, worldwide costs of addressing this issue are high. The most immediately available method of improving air quality is adoption of greater use of ethanol fuel (and other biofuels) and increased reliance upon hybrid vehicle technology.