What is the air quality in Marin County, California?

EPA monitors reported only 95 days for Marin County in 2025, below the 100-day floor used for annual conclusions. Within that limited sample, the median AQI was 21 and 87 days were rated Good; this does not describe the full year.

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 95 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?

Among 95 reported days in 2025, 0 reached Unhealthy or worse. Because the record is below 100 monitored days, that count cannot establish how many unhealthy days occurred across the full year.

Is the air quality in Marin County getting better or worse?

Air quality in Marin County is improving across floor-qualifying years (2020-2024); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2023 at 86.8% (315 of 363 days). Lowest qualifying Good-day share: 2021 at 73.2% (267 of 365 days).

What does AQI 21 mean for health?

The median of the 95 reported days was AQI 21, in the "Good" category. The limited sample is not enough to assign the county a full-year health grade. See the AQI category guide for day-level meaning and EPA AirNow for current conditions.

How does Marin County compare to other counties in California?

Marin County is not placed in county comparisons because its 95-day record is below the 100-day comparison floor.

What is the 90th percentile AQI in Marin County?

The 90th percentile AQI for Marin County is 49, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 66 (Moderate).

California · County air quality · 2025

Marin County, California

Limited EPA sample: 95 monitored days, below the 100-day floor for a full-year verdict. The figures below describe only those reported days.

Compiled by PlainAirData · August 23, 2026

21
Sample median AQI: 91.6%
Good in sample: 0
Unhealthy in sample: 95
100-day comparison floor

Marin County has only 95 EPA-monitored days in 2025, below the 100-day annual floor

According to the U.S. EPA Air Quality System, Marin County, California reported 95 monitored days in 2025. That sample is too thin for a full-year air-quality grade or peer ranking. Use the figures only as the reported-day record, and check AirNow for current conditions.

Data source EPA Air Quality System (AQS)

Monitored days: Marin County reported 95 monitored days in 2025, below the 100-day floor: sample-only, not a full-year peer rank.
Sample median AQI: Sample median AQI: 21
Annual floor: Annual floor: 100

Marin County's median AQI fell from 34 to 28 between the 2023 and 2024 EPA annual releases, a change of -6 points.
Marin County's peak single-day AQI fell from 99 to 86 between the 2023 and 2024 EPA annual releases, a change of -13 points.

Data updated August 2026

Good · 87 d · 91.6%
Moderate · 8 d · 8.4%

How did Marin County's reported sample break down in 2025?

Across 95 monitored days, 87 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 66 (Moderate).

EPA AQS →

Pollutant Breakdown

Days where each pollutant was the primary driver of AQI readings in Marin County.

PM2.5 · 95 days · 100.0%

Sample Coverage and Health Context

Limited sample

Marin County has no annual health grade because only 95 days were reported, below the 100-day floor. The 91.6% Good-day share describes only that sample.

Reported days do not reveal what happened during unmonitored parts of the year, including seasonal smoke, ozone, or inversion periods. Use these values as a description of the available EPA sample only, and check AirNow.gov for current health guidance.

The dominant pollutant is PM2.5, driving AQI on 95 of 95 monitored days.

5-Year Air Quality Trend

Air quality is improving in Marin County. Highest Good-day share: 2023 (86.8%, 315 of 363 monitored days). The 2025 row is shown for transparency but excluded from the direction because it has fewer than 100 monitored 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	87	8	0	0	21	66

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. Spotted a figure that looks wrong? Report a correction.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Marin County, California · 2020–2025 Accessed 2026-08-23

Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.

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.