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.

89.0%
Non-Good days · #1 Harris, Texas
44
Counties ranked

Harris County, Texas has the worst 2025 Good-day share among 44 ranked U.S. counties (11.0% Good)

According to the U.S. EPA Air Quality System, this ranking orders 44 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
Harris County, Texas ranks #1 of 44 for non-Good days at 89.0% (cohort median Good 77.5%).
#1 median AQI
#1 median AQI: 62
Counties ranked
Counties ranked: 44
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 11.0%. 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

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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

Showing 44 matching Texas counties . Back to the national top 50.

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 (Harris, Texas) records 11.0% Good days - 66.6 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 Harris, Texas 89%
  2. 2 El Paso, Texas 81.9%
  3. 3 Cameron, Texas 70.1%
  4. 4 Tarrant, Texas 68.2%
  5. 5 Montgomery, Texas 60.9%
  6. 6 Dallas, Texas 59.5%
  7. 7 Nueces, Texas 59.5%
  8. 8 Bexar, Texas 56.7%
  9. 9 Travis, Texas 56.4%
  10. 10 Hidalgo, Texas 55.6%
  11. 11 McLennan, Texas 53.4%
  12. 12 Jefferson, Texas 52%

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 Harris Texas 11.0% 62 Moderate 172
2 El Paso Texas 18.1% 70 Moderate 2122*
3 Cameron Texas 29.9% 57 Moderate 139
4 Tarrant Texas 31.8% 56 Moderate 162
5 Montgomery Texas 39.1% 53 Moderate 133
6 Dallas Texas 40.5% 53 Moderate 304
7 Nueces Texas 40.5% 53 Moderate 137
8 Bexar Texas 43.3% 52 Moderate 154
9 Travis Texas 43.6% 53 Moderate 129
10 Hidalgo Texas 44.4% 52 Moderate 140
11 McLennan Texas 46.6% 51 Moderate 142
12 Jefferson Texas 48.0% 51 Moderate 160
13 Webb Texas 49.9% 51 Moderate 121
14 Kleberg Texas 51.7% 49 Good 107
15 Bowie Texas 55.0% 48 Good 107
16 Denton Texas 57.8% 47 Good 179
17 Navarro Texas 59.2% 47 Good 107
18 Galveston Texas 59.7% 45 Good 179
19 Orange Texas 60.2% 45 Good 90
20 Maverick Texas 63.0% 42 Good 90
21 Harrison Texas 64.4% 44 Good 100
22 Bell Texas 66.0% 45 Good 140
23 Kaufman Texas 66.0% 44 Good 126
24 Brazos Texas 68.5% 40 Good 78
25 Ector Texas 72.3% 39 Good 183
26 Brewster Texas 74.7% 43 Good 164
27 Collin Texas 75.8% 40 Good 156
28 Atascosa Texas 76.0% 35 Good 92
29 Brazoria Texas 80.8% 35 Good 143
30 Parker Texas 81.8% 38 Good 147
31 Randall Texas 82.8% 41 Good 97
32 Hood Texas 83.4% 38 Good 122
33 Johnson Texas 85.5% 37 Good 101
34 Gregg Texas 86.6% 36 Good 115
35 Victoria Texas 86.7% 34 Good 100
36 Ellis Texas 89.2% 35 Good 93
37 Smith Texas 90.4% 35 Good 93
38 Hunt Texas 91.7% 34 Good 119
39 Lubbock Texas 92.9% 24 Good 167
40 Potter Texas 94.1% 16 Good 85
41 Polk Texas 94.8% 33 Good 105
42 Rockwall Texas 96.8% 31 Good 108
43 Wilson Texas 100.0% 8 Good 41
44 Karnes Texas 100.0% 6 Good 32

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.