Median AQI
21
Good
California · County air quality · 2025
Marin County, California
Median AQI 21 (Good). 100.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 21
- Median AQI
- 100.0%
- Good-air days
- 0
- Unhealthy days
- 96%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Marin County's air is good on balance, among the cleanest 5% of US counties, a median AQI of 21 with 100.0% of 20 monitored days in the Good band, driven mainly by PM2.5.
- 21
- median AQI · Good
- 100.0%
- days rated Good
- PM2.5
- main pollutant
- 96%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 32.
Data updated July 2026
- Good · 20 d · 100.0%
How did Marin County's air break down in 2025?
Across 20 monitored days, 20 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 32 (Good).
EPA AQS →Air quality data from EPA Air Quality System (2025). 20 days monitored. Median AQI: 21 (Good).
Insights: Marin County, California. EPA monitors recorded a median Air Quality Index of 21 across 20 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 20 (100%) fell in the Good band (AQI 0–50), 0 (0%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 29, while the worst single-day peak hit 32 (Good). Residents averaged about 30.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (20 days). Across the 2020–2025 trend window, good days shifted by -252 and the median AQI moved -15 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 32, good but never Unhealthy. Nearby counties show comparable readings: Amador (84.8% good, AQI 40); Butte (66.1% good, AQI 47); Calaveras (73.9% good, AQI 43); Colusa (84.9% good, AQI 39); Del Norte (81.0% good, AQI 33); El Dorado (85.8% good, AQI 39).
Air Quality Summary
Good Days
100.0%
20 of 20 monitored days
25.0pp vs 75% target
Max AQI Recorded
32
Good
Health Grade
Excellent
90th-pct AQI 29
Marin County, Good days share 100.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Marin County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 20 monitored days across EPA AQI categories.
Good
20
100%
Moderate
0
0%
USG
0
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Marin County.
- PM2.5 · 20 days · 100.0%
Health Impact Assessment
Excellent
Marin County's air quality is rated Excellent based on 100.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 100.0% of monitored days in 2025, with a median AQI of 21. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 20 of 20 monitored days.
5-Year Air Quality Trend
Air quality is improving in Marin County. Best year: 2023 (315 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 272 | 85 | 4 | 4 | 36 | 231 |
| 2021 | 267 | 98 | 0 | 0 | 38 | 88 |
| 2022 | 275 | 90 | 0 | 0 | 37 | 91 |
| 2023 | 315 | 48 | 0 | 0 | 34 | 99 |
| 2024 | 308 | 49 | 0 | 0 | 28 | 86 |
| 2025 | 20 | 0 | 0 | 0 | 21 | 32 |
How does Marin County compare?
How Marin County compares
Good-day share better than US avg
Marin County 100.0%
Amador County, California 84.8%
US average 76.5%
Median AQI better than US avg
Marin County 21
Amador County, California 40
US average 38
Unhealthy days / yr about average
Marin County 0
Amador County, California 0
US average 0
Other Counties in California
Air quality comparison for nearby counties in California.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Marin County.
Cowlitz County, Washington
AQI 21 · 91.6% good
Juneau County, Alaska
AQI 21 · 85.1% good
Kitsap County, Washington
AQI 21 · 96.2% good
Mason County, Washington
AQI 21 · 92.6% good
Pitkin County, Colorado
AQI 21 · 91.6% good
Putnam County, Florida
AQI 21 · 96.0% good
What Marin County's air means for you
The air here stays in the breathable range year-round, with PM2.5 the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 21 means for exposure. Health guidance
- See how Marin County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Marin County with its California neighbours before drawing a conclusion about the region. All of California
Based on 20 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Marin County, California? ▼
Marin County has a median Air Quality Index (AQI) of 21, rated "Good." 100.0% of monitored days (20 out of 20) had good air quality in 2025.
What is the main pollutant in Marin County? ▼
The primary pollutant in Marin County is Fine Particles (PM2.5), which was the main AQI driver on 20 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Marin County have? ▼
In 2025, Marin County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 32.
Is the air quality in Marin County getting better or worse? ▼
Air quality in Marin County is improving based on 5-year trends (2020-2025). Best year: 2023 with 315 good days. Worst year: 2025 with 20 good days.
What does AQI 21 mean for health? ▼
Marin County's median AQI of 21 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Marin County compare to other counties in California? ▼
Marin County has 100.0% good air days. Nearby counties in California: Amador (84.8% good), Butte (66.1% good), Calaveras (73.9% good), Colusa (84.9% good), Del Norte (81.0% good), El Dorado (85.8% good).
What is the 90th percentile AQI in Marin County? ▼
The 90th percentile AQI for Marin County is 29, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 32 (Good).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.
Every figure on PlainAir 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.