These changes are caused by changes in the nation's economy, industrial activity, technology improvements, traffic, and by many other factors. Air pollution regulations and emission controls also have an effect. The report also discusses emission evaluation and prediction methodologies. EPA calls this set of principal air pollutants, criteria pollutants. There are also a large number of compounds which have been determined to be hazardous which are called air toxics.
Sources There are many sources of emissions.
Point sources include things like factories and electric power plants . Mobile sources include cars and trucks, of course, but also lawn mowers, airplanes and anything else that moves and puts pollution into the air. Biogenic sources include trees and vegetation, gas seeps, and microbial activity. Area sources consist of smaller stationary sources such as dry cleaners and degreasing operations. Since then additional laws and regulations have been added including the 1990 Amendments to the Clean Air Act.