It turns out they’re mostly in California. But for the full list we go to Environment America’s clean air report (PDF).
| Rank | Metropolitan Area | Population | Smog Days in 2010 | Red-Alert Days |
| 1 | Riverside-San Bernardino, CA | 3,254,821 | 110 | 24 |
| 2 | Visalia-Tulare-Porterville, CA | 368,021 | 78 | 6 |
| 3 | Bakersfield, CA | 661,645 | 69 | 10 |
| 3 | Los Angeles-Long Beach, CA | 9,519,338 | 69 | 3 |
| 5 | Fresno, CA | 922,516 | 42 | 6 |
| 6 | Baltimore, MD | 2,552,994 | 33 | 6 |
| 6 | Washington, DC-MD-VA-WV | 4,923,153 | 33 | 3 |
| 8 | Philadelphia, PA-NJ | 5,100,931 | 29 | 3 |
| 9 | Houston, TX | 4,177,646 | 27 | 1 |
| 10 | Atlanta, GA | 4,112,198 | 23 | 1 |
| 10 | Sacramento, CA | 1,628,197 | 23 | 4 |
