Congress established much of the basic structure of the Clean Air Act in 1970, and made major revisions in 1977 and 1990. Dense, visible smog in many of the nation's cities and industrial centers helped to prompt passage of the 1970 legislation at the height of the national environmental movement.  The subsequent revisions were designed to improve its effectiveness and to target newly recognized air pollution problems such as acid rain and damage to the stratospheric ozone layer.

tates are required to adopt enforceable plans to achieve and maintain air quality

States are required to adopt enforceable plans to achieve and maintain air quality meeting the air quality standards.   State plans also must control emissions that drift across state lines and harm air quality in downwind states.