- Title 1 - General Provisions
- Title 2 - Grants and Agreements
- Title 3 - The President
- Title 4 - Accounts
- Title 5 - Administrative Personnel
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- Title 18 - Conservation of Power and Water Resources
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- Title 40 - Protection of Environment
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- Title 45 - Public Welfare
- Title 46 - Shipping
- Title 47 - Telecommunication
- Title 48 - Federal Acquisition Regulations System
- Title 49 - Transportation
- Title 50 - Wildlife and Fisheries
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 A—Meaning of Terms (§81.1 to §81.1)