National ranking · most polluted air

50 Counties with the Worst Air Quality

Ranked by the highest median AQI among counties with at least 100 monitored days in the year, counties where residents face the most polluted air days each year, from EPA Air Quality System monitoring.

63
#1 Harris, Texas
50
Counties ranked

According to the U.S. Environmental Protection Agency (EPA), 977 counties reported Air Quality Index data through the federal Air Quality System (AQS) for the 2025 reporting year. The ranking below draws on those official records; see the methodology for how median AQI is computed.

Counties below the 100-day monitoring floor are excluded from this ranking: with only a handful of readings, a single bad-air stretch can push a county's median AQI far higher than a full year's weather would produce. For comparison, the cohort median across all 896 floor-qualifying counties is 41, well below the top-ranked counties shown here, so the gap reflects a real, persistent pollution pattern rather than a thin sample.

Sensitive group advisory

These counties have elevated exposure risk. People with asthma, COPD, heart disease, children, pregnant women, and adults 65+ should consult airnow.gov for real-time conditions and follow EPA AQI alerts before outdoor activity.

About AQI readings above 500

EPA's standard AQI scale caps at 500 (Hazardous category 301-500). Several counties below show max AQI readings exceeding 500, these represent extreme events (typically wildfire smoke) extrapolated beyond the index's design range. The EPA labels values above 500 as "Beyond AQI." See EPA wildfire smoke guidance.

Top 12 most-polluted counties

Median Air Quality Index, 2025 (higher is worse)

  1. 1 Harris, Texas 63
  2. 2 Riverside, California 87
  3. 3 Los Angeles, California 77
  4. 4 San Bernardino, California 90
  5. 5 El Paso, Texas 74
  6. 6 Maricopa, Arizona 71
  7. 7 Dona Ana, New Mexico 70
  8. 8 Bernalillo, New Mexico 59
  9. 9 Kern, California 61
  10. 10 Pinal, Arizona 66
  11. 11 Clark, Nevada 64
  12. 12 Imperial, California 64

Median AQI · EPA Air Quality System

# County State Median AQI Max AQI
1 Harris Texas 63 Moderate 172
2 Riverside California 87 Moderate 365
3 Los Angeles California 77 Moderate 201
4 San Bernardino California 90 Moderate 207
5 El Paso Texas 74 Moderate 2122*
6 Maricopa Arizona 71 Moderate 366
7 Dona Ana New Mexico 70 Moderate 528*
8 Bernalillo New Mexico 59 Moderate 228
9 Kern California 61 Moderate 143
10 Pinal Arizona 66 Moderate 1215*
11 Clark Nevada 64 Moderate 138
12 Imperial California 64 Moderate 190
13 Tulare California 59 Moderate 140
14 Santa Barbara California 54 Moderate 186
15 Montgomery Texas 55 Moderate 133
16 Tarrant Texas 56 Moderate 162
17 Cameron Texas 55 Moderate 139
18 Hamilton Ohio 54 Moderate 154
19 Pulaski Arkansas 54 Moderate 239
20 Butler Ohio 55 Moderate 151
21 Nueces Texas 55 Moderate 137
22 Hidalgo Texas 54 Moderate 140
23 Jefferson Alabama 53 Moderate 112
24 Pima Arizona 55 Moderate 357
25 Salt Lake Utah 54 Moderate 175
26 Lake Indiana 53 Moderate 219
27 Bexar Texas 53 Moderate 122
28 Kings California 53 Moderate 105
29 Franklin Ohio 54 Moderate 156
30 Fresno California 54 Moderate 157
31 Larimer Colorado 54 Moderate 119
32 Cook Illinois 53 Moderate 168
33 Inyo California 54 Moderate 163
34 Duval Florida 52 Moderate 136
35 Travis Texas 53 Moderate 126
36 Dallas Texas 53 Moderate 304
37 Lucas Ohio 53 Moderate 154
38 Jefferson Texas 52 Moderate 93
39 San Diego California 54 Moderate 161
40 Weld Colorado 52 Moderate 119
41 Marion Indiana 53 Moderate 170
42 Hinds Mississippi 52 Moderate 84
43 McLennan Texas 52 Moderate 142
44 Cuyahoga Ohio 53 Moderate 150
45 San Luis Obispo California 51 Moderate 119
46 Denver Colorado 54 Moderate 115
47 Orange California 52 Moderate 159
48 Tulsa Oklahoma 51 Moderate 508*
49 Wayne Michigan 52 Moderate 156
50 Jefferson Colorado 51 Moderate 119

Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS). Median AQI is calculated from daily AQI readings across all available monitoring days. Unhealthy Days includes Unhealthy, Very Unhealthy, and Hazardous categories U.S. Environmental Protection Agency (EPA), Air Quality System (AQS). Median AQI is calculated from daily AQI readings across all available monitoring days. Unhealthy Days includes Unhealthy, Very Unhealthy, and Hazardous categories

Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.

Informational only, not professional advice. Data from EPA AQS. See our disclaimer.