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.

66.8%
Non-Good days · #1 Butler, Ohio
36
Counties ranked

Butler County, Ohio has the worst 2025 Good-day share among 36 ranked U.S. counties (33.1% Good)

According to the U.S. EPA Air Quality System, this ranking orders 36 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
Butler County, Ohio ranks #1 of 36 for non-Good days at 66.8% (cohort median Good 77.5%).
#1 median AQI
#1 median AQI: 55
Counties ranked
Counties ranked: 36
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 33.1%. 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.

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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 36 matching Ohio 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 (Butler, Ohio) records 33.1% Good days - 44.4 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 Butler, Ohio 66.8%
  2. 2 Hamilton, Ohio 65.8%
  3. 3 Franklin, Ohio 59.7%
  4. 4 Cuyahoga, Ohio 56.7%
  5. 5 Lucas, Ohio 52.6%
  6. 6 Stark, Ohio 46.9%
  7. 7 Jefferson, Ohio 46.6%
  8. 8 Scioto, Ohio 44.1%
  9. 9 Montgomery, Ohio 39.7%
  10. 10 Trumbull, Ohio 39.7%
  11. 11 Clark, Ohio 38.6%
  12. 12 Belmont, Ohio 37%

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 Butler Ohio 33.1% 55 Moderate 151
2 Hamilton Ohio 34.3% 54 Moderate 154
3 Franklin Ohio 40.3% 53 Moderate 156
4 Cuyahoga Ohio 43.3% 53 Moderate 150
5 Lucas Ohio 47.4% 52 Moderate 154
6 Stark Ohio 53.1% 49 Good 124
7 Jefferson Ohio 53.4% 49 Good 101
8 Scioto Ohio 55.9% 46 Good 238
9 Montgomery Ohio 60.3% 46 Good 129
10 Trumbull Ohio 60.3% 46 Good 130
11 Clark Ohio 61.4% 45 Good 126
12 Belmont Ohio 63.0% 42 Good 123
13 Mahoning Ohio 63.4% 44 Good 122
14 Summit Ohio 63.6% 44 Good 126
15 Allen Ohio 67.0% 42 Good 129
16 Lake Ohio 67.2% 42 Good 133
17 Preble Ohio 67.7% 43 Good 115
18 Warren Ohio 67.9% 44 Good 126
19 Medina Ohio 69.0% 43 Good 126
20 Lawrence Ohio 71.0% 42 Good 90
21 Clermont Ohio 76.9% 41 Good 119
22 Portage Ohio 77.5% 43 Good 150
23 Madison Ohio 79.1% 43 Good 112
24 Ashtabula Ohio 81.2% 43 Good 122
25 Geauga Ohio 82.9% 42 Good 129
26 Clinton Ohio 83.1% 41 Good 112
27 Athens Ohio 83.4% 31 Good 75
28 Miami Ohio 84.2% 40 Good 101
29 Delaware Ohio 84.3% 40 Good 119
30 Wood Ohio 85.3% 39 Good 108
31 Greene Ohio 85.3% 40 Good 105
32 Knox Ohio 85.3% 39 Good 159
33 Licking Ohio 85.7% 40 Good 119
34 Noble Ohio 85.9% 37 Good 90
35 Lorain Ohio 87.3% 39 Good 119
36 Washington Ohio 88.4% 40 Good 105

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.