The Clean Air Act and the first haze rules were introduced in 1977. In developing a strategy to control emissions, it was assumed that older facilities, built before 1962, would quickly become obsolete and retire, or would need to install newer, less polluting technologies to stay competitive. Modifications to older sources trigger regulations and pollution limits under the Clean Air Act.

It was also assumed that emissions from facilities built after 1977 would be regulated and controlled under the many provisions of the Clean Air Act. In addition, these newer facilities would use newer, less polluting technologies and would therefore be cleaner than older facilities.

Therefore, the regional haze rules focus on facilities built between 1962 and 1977 that would not be regulated by the Clean Air Act or that would not have newer, less polluting technologies.