42 U.S. Code § 7513 - Classifications and attainment dates
Every area designated nonattainment for PM–10 pursuant to section 7407(d) of this title shall be classified at the time of such designation, by operation of law, as a moderate PM–10 nonattainment area (also referred to in this subpart as a “Moderate Area”) at the time of such designation. At the time of publication of the notice under section 7407(d)(4) of this title (relating to area designations) for each PM–10 nonattainment area, the Administrator shall publish a notice announcing the classification of such area. The provisions of section 7502(a)(1)(B) of this title (relating to lack of notice-and-comment and judicial review) shall apply with respect to such classification.
For a Moderate Area, the attainment date shall be as expeditiously as practicable but no later than the end of the sixth calendar year after the area’s designation as nonattainment, except that, for areas designated nonattainment for PM–10 under section 7407(d)(4) of this title, the attainment date shall not extend beyond December 31, 1994.
For a Serious Area, the attainment date shall be as expeditiously as practicable but no later than the end of the tenth calendar year beginning after the area’s designation as nonattainment, except that, for areas designated nonattainment for PM–10 under section 7407(d)(4) of this title, the date shall not extend beyond December 31, 2001.
Upon application by any State, the Administrator may extend the attainment date for a Serious Area beyond the date specified under subsection (c), if attainment by the date established under subsection (c) would be impracticable, the State has complied with all requirements and commitments pertaining to that area in the implementation plan, and the State demonstrates to the satisfaction of the Administrator that the plan for that area includes the most stringent measures that are included in the implementation plan of any State or are achieved in practice in any State, and can feasibly be implemented in the area. At the time of such application, the State must submit a revision to the implementation plan that includes a demonstration of attainment by the most expeditious alternative date practicable. In determining whether to grant an extension, and the appropriate length of time for any such extension, the Administrator may consider the nature and extent of nonattainment, the types and numbers of sources or other emitting activities in the area (including the influence of uncontrollable natural sources and transboundary emissions from foreign countries), the population exposed to concentrations in excess of the standard, the presence and concentration of potentially toxic substances in the mix of particulate emissions in the area, and the technological and economic feasibility of various control measures. The Administrator may not approve an extension until the State submits an attainment demonstration for the area. The Administrator may grant at most one such extension for an area, of no more than 5 years.
The Administrator may, on a case-by-case basis, waive any requirement applicable to any Serious Area under this subpart where the Administrator determines that anthropogenic sources of PM–10 do not contribute significantly to the violation of the PM–10 standard in the area. The Administrator may also waive a specific date for attainment of the standard where the Administrator determines that nonanthropogenic sources of PM–10 contribute significantly to the violation of the PM–10 standard in the area.
 So in original. Probably should be “subsection”.