Here is a top-ten list of Most and Least polluted cities (metro areas) in United States:

Areas with the highest CO emissions:

1. Lexington-Fayette, Ky.
2. Indianapolis
3. Cincinnati-Middletown, Ohio-Ky.-Ind.
4. Toledo, Ohio
5. Louisville
6. Nashville-Davidson-Murfreesboro
7. St. Louis
8. Oklahoma City
9. Harrisburg-Carlisle, Pa.
10. Knoxville

Areas with the lowest CO emissions:

1. Honolulu
2. Los Angeles-Long Beach-Santa Ana
3. Portland-Vancouver-Beaverton, Ore.-Wash.
4. New York-Northern New Jersey-Long Island, N.Y-N.J.-Pa.
5. Boise-Nampa, Idaho
6. Seattle-Tacoma-Bellevue, Wash.
7. San Jose-Sunnyvale-Santa Clara
8. San Francisco-Oakland-Fremont
9. El Paso
10. San Diego-Carlsbad-San Marcos

Source: Brookings Institution

Read: The complete report