CFR > Title 40 - Protection of Environment > CHAPTER I—ENVIRONMENTAL PROTECTION AGENCY > SUBCHAPTER C—AIR PROGRAMS (Parts 50–99) > PART 81—DESIGNATION OF AREAS FOR AIR QUALITY PLANNING PURPOSES > SUBPART D—Identification of Mandatory Class I Federal Areas Where Visibility Is an Important Value (§81.400 to §81.437) > 81.424—Oklahoma.