42 U.S.C. § 7508 : The Public Health And Welfare — Air Pollution Prevention And Control — Programs And Activities — Plan Requirements For Nonattainment Areas — Nonattainment Areas In General — Guidance Documents

The Administrator shall issue guidance documents under section 7408 of this title for purposes of assisting States in implementing requirements of this part respecting the lowest achievable emission rate. Such a document shall be published not later than nine months after August 7, 1977, and shall be revised at least every two years thereafter.