National ranking · most polluted air

Worst Air Quality in the US (2025)

Ranked by the lowest share of Good air days among counties with at least 100 monitored days in 2025, with higher median AQI breaking ties, from EPA Air Quality System monitoring.

91.2%
Non-Good days · #1 Maricopa, Arizona
50
Counties ranked

Maricopa County, Arizona has the worst 2025 Good-day share among 50 ranked U.S. counties (8.8% Good)

According to the U.S. EPA Air Quality System, this ranking orders 50 counties with at least 100 monitored days in 2025 by the lowest share of days in the Good AQI band, using median AQI only to break ties. It is an annual comparison of exposure patterns, not a live advisory. For current conditions and alerts, use AirNow.

Data source EPA Air Quality System (AQS)

#1 non-Good share
Maricopa County, Arizona ranks #1 of 50 for non-Good days at 91.2% (cohort median Good 77.5%).
#1 median AQI
#1 median AQI: 73
Counties ranked
Counties ranked: 50
Cohort median Good
Cohort median Good: 77.5%

from the compiled EPA AQS release in this database.

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 collapse a county's Good-day share. Across all 951 floor-qualifying counties, the cohort median Good-day share is 77.5%; the first-ranked county records 8.8%. The gap therefore reflects a sustained annual 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.

Filter this ranking

Filters keep the Good-day-share ranking contract. A state filter ranks that state's qualifying counties; pollutant filters keep rows whose annual day-type count is led by ozone, PM2.5, or PM10. Raising the monitored-days floor tightens the sample beyond the default 100-day Module-34 guard.

Compare up to three counties

Where the national worst cohort concentrates

Among the unfiltered national top 50 (lowest Good-day share, ≥100 monitored days), California contributes 13 counties , followed by Texas (11). Use the state filter above to rank every qualifying county inside one jurisdiction instead of truncating the national list.

Top contributing states in the worst-50

What leads the bad-air days

On each county row, EPA reports how many monitored days were ozone-, PM2.5-, or PM10-classified. In the national worst-50 cohort, PM2.5-led days are the plurality (32 of 50). Counts: ozone 16 · PM2.5 32 · PM10 2 . Filter by driver to keep only counties whose annual day-type count is led by that pollutant.

Ozone-led vs particle-led counties

Ozone-led rows (16) often sit in long warm-season photochemical corridors; PM2.5-led rows (32) more often track wildfire or winter particle episodes. Neither label is a health diagnosis - it is only which pollutant-day count dominated the annual monitor record.

How far below the cohort median

Across all 951 floor-qualifying counties, the median Good-day share is 77.5%. The #1 county here (Maricopa, Arizona) records 8.8% Good days - 68.8 percentage points below that median. That gap is why a thin sample is excluded: short monitor series can invent false extremes.

Annual record vs live advisory

This page answers “which counties had the worst annual Good-day share in 2025?” It does not answer “is the air safe today?” For current conditions and alerts use AirNow. State hubs at /states roll the same annual contract up one geography level.

Compare any two counties

Need a side-by-side annual record (Good-day share change + median AQI endpoints)? Use the compare form above or the county compare tool - it searches the full EPA AQS county set, not just this ranking slice.

Also useful nearby

Largest share of days outside the Good band

Largest share of days outside the Good band

100% minus Good-day share, 2025 (higher is worse)

  1. 1 Maricopa, Arizona 91.2%
  2. 2 Riverside, California 89.3%
  3. 3 Los Angeles, California 89.3%
  4. 4 Harris, Texas 89%
  5. 5 San Bernardino, California 87.4%
  6. 6 Pinal, Arizona 83%
  7. 7 Kern, California 82.5%
  8. 8 San Diego, California 82.5%
  9. 9 El Paso, Texas 81.9%
  10. 10 Tulare, California 75.9%
  11. 11 Bernalillo, New Mexico 73.4%
  12. 12 Dona Ana, New Mexico 72.6%

Non-Good monitored days · EPA Air Quality System

Ranked counties

Reading the table columns

Good Days % is the ranking key. Median AQI breaks ties. Max AQI flags extreme days (values above 500 are beyond EPA’s standard scale). Day driver is the pollutant with the most classified days on that county’s annual row.

# County State Good Days % Median AQI Max AQI
1 Maricopa Arizona 8.8% 73 Moderate 366
2 Riverside California 10.7% 80 Moderate 365
3 Los Angeles California 10.7% 74 Moderate 201
4 Harris Texas 11.0% 62 Moderate 172
5 San Bernardino California 12.6% 77 Moderate 207
6 Pinal Arizona 17.0% 67 Moderate 1215*
7 Kern California 17.5% 70 Moderate 200
8 San Diego California 17.5% 67 Moderate 161
9 El Paso Texas 18.1% 70 Moderate 2122*
10 Tulare California 24.1% 64 Moderate 147
11 Bernalillo New Mexico 26.6% 58 Moderate 204
12 Dona Ana New Mexico 27.4% 61 Moderate 528*
13 Imperial California 27.7% 64 Moderate 426
14 Fresno California 28.8% 61 Moderate 164
15 Cameron Texas 29.9% 57 Moderate 139
16 Clark Nevada 31.0% 61 Moderate 138
17 Pulaski Arkansas 31.2% 55 Moderate 138
18 Tarrant Texas 31.8% 56 Moderate 162
19 Salt Lake Utah 33.1% 57 Moderate 175
20 Butler Ohio 33.1% 55 Moderate 151
21 Pima Arizona 33.7% 54 Moderate 357
22 St. Louis City Missouri 34.3% 55 Moderate 738*
23 Hamilton Ohio 34.3% 54 Moderate 154
24 Kings California 34.5% 57 Moderate 133
25 Wayne Michigan 36.2% 54 Moderate 156
26 Inyo California 37.3% 55 Moderate 411
27 Orange California 37.8% 53 Moderate 159
28 Montgomery Texas 39.1% 53 Moderate 133
29 Marion Indiana 40.0% 54 Moderate 170
30 Franklin Ohio 40.3% 53 Moderate 156
31 Dallas Texas 40.5% 53 Moderate 304
32 Nueces Texas 40.5% 53 Moderate 137
33 Cook Illinois 40.8% 53 Moderate 168
34 Lake Indiana 41.6% 53 Moderate 219
35 Union Arkansas 41.8% 52 Moderate 73
36 Kent Michigan 41.9% 52 Moderate 163
37 Bibb Georgia 42.7% 52 Moderate 119
38 Cuyahoga Ohio 43.3% 53 Moderate 150
39 Bexar Texas 43.3% 52 Moderate 154
40 Travis Texas 43.6% 53 Moderate 129
41 Jefferson Alabama 43.6% 52 Moderate 117
42 Hidalgo Texas 44.4% 52 Moderate 140
43 Weld Colorado 44.7% 52 Moderate 119
44 Santa Barbara California 44.9% 52 Moderate 87
45 Placer California 45.8% 52 Moderate 143
46 Hinds Mississippi 45.8% 52 Moderate 84
47 Davidson North Carolina 45.8% 51 Moderate 93
48 Allegheny Pennsylvania 46.0% 52 Moderate 129
49 McLennan Texas 46.6% 51 Moderate 142
50 Richmond Georgia 46.9% 52 Moderate 130

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

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Every figure on PlainAirData 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.