The layer of the atmosphere closest to the earth’s surface where humans live and breathe (the troposphere) is made up of about 78 percent nitrogen, 21 percent oxygen, and trace (small) amounts of water vapor and other gases including argon and carbon dioxide. The term “air quality” refers to the quality of this mixture of invisible and odorless gases. Good air quality refers to air that is free from harmful contaminants. Poor air quality refers to air that contains harmful levels of unhealthy gases and airborne particles.
The quality of the air around us can change from day-to-day, depending on the level of pollutants in it, the rate at which sources emit pollutants, and the effects of weather conditions such as wind direction and speed, humidity, temperature, and solar radiation. Stagnant air can lead to a buildup of chemicals and particles from local emissions sources. Windy conditions can cause pollutants to disperse. Pollutants can move hundreds of miles on the wind and affect areas far away from the original source.
Although natural events such as volcanoes and wildland fires can contribute to poor air quality, air pollution comes primarily from human sources. Scientists use the term “anthropogenic” to refer to human sources. Categories of human sources include stationary sources and mobile sources. Stationary sources are sources that are fixed in place (do not move) and include a wide variety of manufacturing, industrial, and commercial facilities. Mobile sources include a wide variety of gasoline and diesel-powered cars, trucks and buses, marine vessels, locomotives, aircraft, heavy duty off-road vehicles and equipment, recreational vehicles, and small engines and tools.
Various regulations exist for common (criteria) pollutants, hazardous air pollutants, ozone-depleting substances, and greenhouse gases. Citizens and businesses can help improve air quality through voluntary pollution prevention.