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.

89.3%
Non-Good days · #1 Riverside, California
46
Counties ranked

Riverside County, California has the worst 2025 Good-day share among 46 ranked U.S. counties (10.7% Good)

According to the U.S. EPA Air Quality System, this ranking orders 46 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
Riverside County, California ranks #1 of 46 for non-Good days at 89.3% (cohort median Good 77.5%).
#1 median AQI
#1 median AQI: 80
Counties ranked
Counties ranked: 46
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 10.7%. 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 46 matching California 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 (Riverside, California) records 10.7% Good days - 66.9 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 Riverside, California 89.3%
  2. 2 Los Angeles, California 89.3%
  3. 3 San Bernardino, California 87.4%
  4. 4 Kern, California 82.5%
  5. 5 San Diego, California 82.5%
  6. 6 Tulare, California 75.9%
  7. 7 Imperial, California 72.3%
  8. 8 Fresno, California 71.2%
  9. 9 Kings, California 65.5%
  10. 10 Inyo, California 62.7%
  11. 11 Orange, California 62.2%
  12. 12 Santa Barbara, California 55.1%

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 Riverside California 10.7% 80 Moderate 365
2 Los Angeles California 10.7% 74 Moderate 201
3 San Bernardino California 12.6% 77 Moderate 207
4 Kern California 17.5% 70 Moderate 200
5 San Diego California 17.5% 67 Moderate 161
6 Tulare California 24.1% 64 Moderate 147
7 Imperial California 27.7% 64 Moderate 426
8 Fresno California 28.8% 61 Moderate 164
9 Kings California 34.5% 57 Moderate 133
10 Inyo California 37.3% 55 Moderate 411
11 Orange California 37.8% 53 Moderate 159
12 Santa Barbara California 44.9% 52 Moderate 87
13 Placer California 45.8% 52 Moderate 143
14 Stanislaus California 49.9% 51 Moderate 115
15 Madera California 50.4% 50 Good 136
16 Merced California 51.0% 50 Good 100
17 Ventura California 52.0% 50 Good 131
18 San Luis Obispo California 52.3% 50 Good 119
19 Plumas California 52.9% 45 Good 120
20 Sacramento California 54.3% 49 Good 122
21 Yolo California 58.4% 47 Good 100
22 San Joaquin California 60.0% 46 Good 93
23 Sutter California 63.2% 47 Good 100
24 Butte California 65.2% 46 Good 96
25 Trinity California 69.6% 31 Good 164
26 Mendocino California 73.7% 38 Good 81
27 Shasta California 75.6% 44 Good 154
28 Tehama California 77.8% 38 Good 108
29 Humboldt California 78.1% 38 Good 87
30 Siskiyou California 78.2% 37 Good 166
31 Colusa California 79.2% 40 Good 96
32 Nevada California 79.8% 42 Good 108
33 Glenn California 80.0% 40 Good 94
34 Mariposa California 80.6% 44 Good 90
35 Del Norte California 81.8% 32 Good 70
36 Calaveras California 82.2% 40 Good 100
37 Monterey California 86.0% 38 Good 62
38 Tuolumne California 87.0% 39 Good 105
39 Amador California 87.4% 38 Good 97
40 El Dorado California 87.7% 40 Good 115
41 Santa Cruz California 87.7% 35 Good 71
42 Mono California 90.1% 21 Good 704*
43 San Benito California 91.5% 39 Good 71
44 Solano California 99.2% 33 Good 61
45 Lake California 99.2% 31 Good 74
46 Sonoma California 100.0% 9 Good 39

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.