Emissions is the term used to describe the gases and particles which are put into the air or emitted by various sources. Sources include factories and electric power plants, cars, trucks, lawn mowers, airplanes and anything else that moves and puts pollution into the air.
The amounts and types of emissions change every year. These changes are caused by changes in the economy, industrial activity, technology improvements, traffic and many other factors. Air pollution regulations and emission controls also have an effect. The EPA is mainly concerned with emissions which are or could be harmful to people, specifically: carbon dioxide (CO2), carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM) and sulfur dioxide (SO2).
In 1970, the U.S. Congress passed Clean Air Act (CAA) Amendments which set into motion a nationwide effort to improve the country's air quality. Since then, other laws and regulations have been added, along with efforts and policies developed and enacted by countries worldwide. (Source: EPA / Photo: milehightraveler/iStockphoto)