The New Source Review Permitting Improvement Act amends the Clean Air Act to clarify when changes at industrial facilities trigger strict permitting requirements. Specifically, the bill narrows the definition of what counts as a "modification" requiring new permits by establishing that emissions increases are only counted if they exceed the maximum hourly emission rate achieved during the previous ten years, and by exempting certain pollution control, efficiency, safety, and reliability improvements from modification requirements. The legislation also limits permitting requirements to physical construction directly involving emissions units rather than other on-site activities, even if those activities are costly or permanent. The bill applies these clarifications to both air quality attainment and non-attainment areas. No new federal funding or implementation timeline is specified in the legislation.