State air quality · 2025

California Air Quality Ranking

Across 47 monitored counties, the typical county in California posts a median AQI of 46 (Good) and 61.2% of monitored days were rated Good, from EPA AQS.

46
Avg median AQI
61.2%
Good-air days
47
Counties monitored
2%
Cleaner than US states

California's typical county median AQI is 46 (Good) across 47 monitored counties

According to the U.S. EPA Air Quality System, California monitors 47 counties with a day-weighted Good-day share of 61.2%. Cleanest: Sonoma County (100.0% Good). Most polluted: Riverside County (10.7%). These are annual summaries, not live AirNow readings.

Data source EPA Air Quality System (AQS)

Cleaner than US states
California's day-weighted Good-day share of 61.2% places it cleaner than 2% of US states.
In-state spread
Within California, Sonoma County leads at 100.0% Good days while Los Angeles County sits at 10.7% across 47 monitored counties.
2024→2025 release
California's Good-day share moved +4.0 points vs the 2024 EPA annual release.
Typical median AQI
Typical median AQI: 46

Verify with AirNow → · EPA AQS →

EPA state monitoring brief

AQS state key CALIFORNIA

California · 2025 EPA AQS · 47 counties · med AQI 46 · 61.2% Good · Sonoma→Riverside spread

  • GOOD 46 Median AQI · Good
  • SHARE 61.2% Good-day share
  • COUNT 47 Counties monitored
  • UNH 147 Unhealthy+ days
  • RANK 2% Cleaner than states
  • O3 49.2% Main pollutant share

Good-day share peers

Utah65.4%Georgia65%California (this)61.2%Arizona60.7%
Good-day share neighbourhood · nearest ABS peers vs California

California's 61.2% Good-day share is cleaner than 2% of monitored US states; Sonoma and Riverside County bracket the in-state range. Open Sonoma County →

According to EPA AQS annual summaries on PlainAirData, California ranks #51 of 52 by day-weighted good-day share (61.2%) and #52 by mean county median AQI (46.1). Hawaii is #1 of 52 by day-weighted good-day share (93.8% of monitored days) at mean county median-AQI #4 (27.3) ≠ Alaska #1 lowest mean county median AQI (20.3) at good-day share #8 (84.4%).

Data updated August 2026

What changed, what drives it, and how thick is the sample

Three derived views WPR-style word count does not substitute for: release-over-release movement from county annual rows, the EPA main-pollutant day mix, and how many counties sit below the 100-day ranking floor.

Release change

61.2%

Good-day share in 2025, up 4.0 percentage points vs 2024. County-mean median AQI 46, 1.8 points lower (cleaner) than 2024.

Pollutant mix

Ozone

Main driver on 49.2% of pollutant-assigned days.

  • Ozone 49.2%
  • PM2.5 45.6%
  • PM10 5.0%
  • NO2 0.2%

Monitoring coverage

356

Average monitored days per county in the latest release. 1 of 47 counties sit below the 100-day floor used for cleanest/most-polluted claims.

Counties Monitored

47

Good Days %

61.2%

Avg Median AQI

46

Total Unhealthy Days

147

Best Air Quality

Sonoma County

Worst Air Quality

Riverside County

Where California sits nationally

COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. California's day-weighted Good-day share is 61.2% - cleaner than 2% of that comparison set.

California Good-day share vs. every US state

Day-weighted share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release

61.2% 2nd percentile higher than 2% of 52 US states

0.0%–5.0%: 0 US states (0%). Below this entry. 5.0%–10.0%: 0 US states (0%). Below this entry. 10.0%–15.0%: 0 US states (0%). Below this entry. 15.0%–20.0%: 0 US states (0%). Below this entry. 20.0%–25.0%: 0 US states (0%). Below this entry. 25.0%–30.0%: 0 US states (0%). Below this entry. 30.0%–35.0%: 0 US states (0%). Below this entry. 35.0%–40.0%: 0 US states (0%). Below this entry. 40.0%–45.0%: 0 US states (0%). Below this entry. 45.0%–50.0%: 0 US states (0%). Below this entry. 50.0%–55.0%: 0 US states (0%). Below this entry. 55.0%–60.0%: 0 US states (0%). Below this entry. 60.0%–65.0%: 2 US states (4%). This entry sits in this band. 65.0%–70.0%: 9 US states (17%). Above this entry. 70.0%–75.0%: 11 US states (21%). Above this entry. 75.0%–80.0%: 11 US states (21%). Above this entry. 80.0%–85.0%: 13 US states (25%). Above this entry. 85.0%–90.0%: 5 US states (10%). Above this entry. 90.0%–95.0%: 1 US states (2%). Above this entry. 95.0%–100.0%: 0 US states (0%). Above this entry. 100.0%–105.0%: 0 US states (0%). Above this entry. This state 0.0% 100% every floor-qualified US state, bucketed by value

Each bar is a band; taller bars hold more US states. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.

Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025

Which counties in California have the cleanest and most polluted air?

Cleanest counties in California

Share of monitored days rated Good (AQI 0–50)

  1. 1 Sonoma County 100%
  2. 2 Lake County 99.2%
  3. 3 Solano County 99.2%
  4. 4 Marin County 91.6%
  5. 5 San Benito County 91.5%
  6. 6 Mono County 90.1%
  7. 7 San Mateo County 88.1%
  8. 8 Santa Cruz County 87.7%
  9. 9 El Dorado County 87.7%
  10. 10 Amador County 87.4%
  11. 11 Tuolumne County 87%
  12. 12 Monterey County 86%

Good-day share · EPA Air Quality System

County Good % Median AQI
Sonoma 100.0% 9
Lake 99.2% 31
Solano 99.2% 33
Marin 91.6% 21
San Benito 91.5% 39
Mono 90.1% 21
San Mateo 88.1% 27
El Dorado 87.7% 40
Santa Cruz 87.7% 35
Amador 87.4% 38
Tuolumne 87.0% 39
Monterey 86.0% 38
Calaveras 82.2% 40
Del Norte 81.8% 32
Mariposa 80.6% 44
Glenn 80.0% 40
Nevada 79.8% 42
Colusa 79.2% 40
Siskiyou 78.2% 37
Humboldt 78.1% 38
Tehama 77.8% 38
San Francisco 76.0% 33
Shasta 75.6% 44
Mendocino 73.7% 38
Trinity 69.6% 31
Butte 65.2% 46
Sutter 63.2% 47
San Joaquin 60.0% 46
Yolo 58.4% 47
Santa Clara 55.4% 47
Sacramento 54.3% 49
Contra Costa 53.5% 48
Alameda 53.0% 49
Plumas 52.9% 45
San Luis Obispo 52.3% 50
Ventura 52.0% 50
Merced 51.0% 50
Madera 50.4% 50
Stanislaus 49.9% 51
Placer 45.8% 52
Santa Barbara 44.9% 52
Orange 37.8% 53
Inyo 37.3% 55
Kings 34.5% 57
Fresno 28.8% 61
Imperial 27.7% 64
Tulare 24.1% 64
Kern 17.5% 70
San Diego 17.5% 67
San Bernardino 12.6% 77
Los Angeles 10.7% 74
Riverside 10.7% 80

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025. See our methodology for how AQI and median figures are computed.

Coverage: 47 counties in California with active monitoring stations. Unmonitored counties are not represented.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files, aggregated to the state level · 2020–2025

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.