Amends the Clean Air Act to revise a required provision for State implementation plans. Permits the construction or modification of a major stationary source in a nonattainment area, if, at the time of application for a permit, this will result in the reduction of emissions of such pollutant from an existing stationary source. Allows such construction or modification even if the emissions from such facility will cause or contribute to concentrations of any pollutant for which a national ambient air quality standard is exceeded in such area.