EPA Air Quality System
The counties with the worst air
“Worst” needs a definition, and the one used here is the number of days the Air Quality Index passed 100 - the point at which the EPA considers the air unhealthy for sensitive groups, which includes anyone with asthma or COPD.
Two rankings follow, because they answer different questions. The first is the single worst year, which wildfire smoke can dominate. The second averages five years, which is where the chronically polluted valleys and basins show up instead.
2025
Most days above moderate in a single year
EPA Air Quality System annual summaries by county, 2025. A county appears only if a monitor reported. Rankings from a single year move a long way between years, particularly in the West.
2021-2025 average
Most persistent pollution across five years
The same measure averaged over five years of monitor data. Averaging is what separates a county that had one catastrophic smoke season from a county where the air is reliably poor.
| # | County | Avg days above moderate | Years | 2025 |
|---|---|---|---|---|
| 1 | San Bernardino County, CA | 129.6 | 5 | 110 |
| 2 | Riverside County, CA | 126.4 | 5 | 109 |
| 3 | Los Angeles County, CA | 101.8 | 5 | 98 |
| 4 | Maricopa County, AZ | 100.0 | 5 | 38 |
| 5 | Tulare County, CA | 96.8 | 5 | 43 |
| 6 | Kern County, CA | 95.8 | 5 | 65 |
| 7 | Fresno County, CA | 64.6 | 5 | 34 |
| 8 | Pinal County, AZ | 47.6 | 5 | 34 |
| 9 | Imperial County, CA | 43.4 | 5 | 44 |
| 10 | Harris County, TX | 37.0 | 5 | 38 |
| 11 | El Paso County, TX | 36.8 | 5 | 63 |
| 12 | Kings County, CA | 35.0 | 5 | 11 |
| 13 | San Diego County, CA | 34.2 | 5 | 33 |
| 14 | Fairbanks North Star Borough, AK | 31.6 | 5 | 20 |
| 15 | Jefferson County, CO | 30.8 | 5 | 13 |
| 16 | Doña Ana County, NM | 30.8 | 5 | 48 |
| 17 | Clark County, NV | 30.4 | 5 | 26 |
| 18 | Salt Lake County, UT | 30.2 | 5 | 16 |
| 19 | Plumas County, CA | 30.0 | 5 | 10 |
| 20 | Inyo County, CA | 27.8 | 5 | 13 |
| 21 | Stanislaus County, CA | 26.8 | 5 | 3 |
| 22 | Eddy County, NM | 26.8 | 5 | 25 |
| 23 | Tarrant County, TX | 24.6 | 5 | 30 |
| 24 | Denton County, TX | 24.0 | 5 | 19 |
| 25 | Placer County, CA | 21.8 | 5 | 2 |
| 26 | Bernalillo County, NM | 21.8 | 5 | 21 |
| 27 | Douglas County, CO | 21.4 | 5 | 7 |
| 28 | Fairfield County, CT | 20.6 | 5 | 21 |
| 29 | Larimer County, CO | 19.8 | 5 | 11 |
| 30 | Cook County, IL | 19.8 | 5 | 25 |
Averaged across the years each county reported, 2021-2025. Counties with fewer than four reported years are excluded, because a two-year average is not comparable to a five-year one.