Urbanized area
Urbanized area means an area with a minimum residential population of at least 50,000 people and which generally includes core census block groups or blocks that have a population density of at least 1,000 people per square mile and surrounding census blocks that have an overall density of at least 500 people per square mile. The Census Bureau notes that under certain conditions, less densely settled territory may be part of each Urbanized Area.
Source
40 CFR § 58.1
Scoping language
As used in this part, all terms not defined herein have the meaning given them in the Clean Air Act.