Median AQI
38
Good
California · County air quality · 2025
Mendocino County, California
Median AQI 38 (Good). 73.7% of monitored days rated Good, direct from EPA Air Quality System stations.
- 38
- Median AQI
- 73.7%
- Good-air days
- 0
- Unhealthy days
- 41%
- Cleaner than US counties
Mendocino County's 2025 annual median AQI is 38 (Good), with 73.7% Good days
According to the U.S. EPA Air Quality System, Mendocino County, California posted a 2025 median AQI of 38 (Good) across 365 monitored days, with 73.7% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Mendocino County's 73.7% Good-day share places it cleaner than 41% of US counties in 2025.
- Closest median-AQI peer
- Adams County (94.7% Good, median AQI 38) is the closest nationwide median-AQI peer to Mendocino County.
- 2020–2025 trend
- Mendocino County's floor-qualifying Good-day share is improving from 2020 to 2025 (66.1% → 73.7%).
- Median AQI
- Median AQI: 38
The verdict
According to EPA Air Quality System monitoring data, Mendocino County's air is good on balance, more polluted than the median US county, a median AQI of 38 with 73.7% of 365 monitored days in the Good band, driven mainly by PM2.5.
- 38
- median AQI · Good
- 73.7%
- days rated Good
- PM2.5
- main pollutant
- 41%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 81.
Annual release change
What changed in the 2025 EPA annual release
- Mendocino County's Good-day share rose from 66.7% to 73.7% between the 2024 and 2025 EPA annual releases, a change of +7.0 percentage points.
- Mendocino County's median AQI fell from 43 to 38 between the 2024 and 2025 EPA annual releases, a change of -5 points.
Data updated August 2026
- Good · 269 d · 73.7%
- Moderate · 96 d · 26.3%
How did Mendocino County's air break down in 2025?
Across 365 monitored days, 269 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 81 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 38 (Good).
Insights: Mendocino County, California. EPA monitors recorded a median Air Quality Index of 38 across 365 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 269 (74%) fell in the Good band (AQI 0–50), 96 (26%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 59, while the worst single-day peak hit 81 (Moderate). Residents averaged about 22.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (217 days), Ozone (147 days), PM10 (1 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +8.1 points and the median AQI moved -2 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 81, moderate but never Unhealthy. Nearby counties show comparable readings: Alameda (53.0% good, AQI 49); Amador (87.4% good, AQI 38); Butte (65.2% good, AQI 46); Calaveras (82.2% good, AQI 40); Colusa (79.2% good, AQI 40); Contra Costa (53.5% good, AQI 48).
Air Quality Summary
Good Days
73.7%
269 of 365 monitored days
-1.3pp vs 75% target
Max AQI Recorded
81
Moderate
Health Grade
Moderate
90th-pct AQI 59
Mendocino County, Good days share 73.7%
National avg
Share of monitored days at AQI 0-50 (Good band).
Mendocino 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 365 monitored days across EPA AQI categories.
Good
269
74%
Moderate
96
26%
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 Mendocino County.
- PM2.5 · 217 days · 59.5%
- Ozone · 147 days · 40.3%
- PM10 · 1 days · 0.3%
Health Impact Assessment
Moderate
Mendocino County's air quality is rated Moderate based on 73.7% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 73.7% of monitored days in 2025, with a median AQI of 38. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 217 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Mendocino County. Highest Good-day share: 2025 (73.7%, 269 of 365 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 240 | 103 | 10 | 10 | 40 | 716 |
| 2021 | 210 | 147 | 6 | 2 | 46 | 185 |
| 2022 | 129 | 234 | 2 | 0 | 54 | 103 |
| 2023 | 144 | 216 | 5 | 0 | 53 | 140 |
| 2024 | 244 | 122 | 0 | 0 | 43 | 83 |
| 2025 | 269 | 96 | 0 | 0 | 38 | 81 |
How does Mendocino County compare?
How Mendocino County compares
Good-day share worse than US avg
Mendocino County 73.7%
Adams County, Illinois 94.7%
US average 76.0%
Median AQI better than US avg
Mendocino County 38
Adams County, Illinois 38
US average 39
Unhealthy days / yr better than US avg
Mendocino County 0
Adams County, Illinois 0
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Mendocino County (38), not geographic neighbors.
Adams County, Illinois
Median AQI 38 · 94.7% Good-day share
Albemarle County, Virginia
Median AQI 38 · 81.0% Good-day share
Alexander County, North Carolina
Median AQI 38 · 93.5% Good-day share
Amador County, California
Median AQI 38 · 87.4% Good-day share
Anoka County, Minnesota
Median AQI 38 · 74.1% Good-day share
Avery County, North Carolina
Median AQI 38 · 92.3% Good-day share
Same primary pollutant: PM2.5
PM2.5 led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Eau Claire County, Wisconsin
73.7% Good-day share · median AQI 38
Escambia County, Florida
73.7% Good-day share · median AQI 43
Flathead County, Montana
73.7% Good-day share · median AQI 41
Rapides County, Louisiana
73.7% Good-day share · median AQI 37
Monroe County, Indiana
73.6% Good-day share · median AQI 36
Tuscaloosa County, Alabama
73.6% Good-day share · median AQI 41
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
What Mendocino 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 38 means for exposure. Health guidance
- See how Mendocino County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Mendocino County with its California neighbours before drawing a conclusion about the region. All of California
Based on 365 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in Mendocino County, California? ▼
Mendocino County has a median Air Quality Index (AQI) of 38, rated "Good." 73.7% of monitored days (269 out of 365) had good air quality in 2025.
What is the main pollutant in Mendocino County? ▼
The primary pollutant in Mendocino County is Fine Particles (PM2.5), which was the main AQI driver on 217 days (59% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Mendocino County have? ▼
In 2025, Mendocino County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 81.
Is the air quality in Mendocino County getting better or worse? ▼
Air quality in Mendocino County is improving across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2025 at 73.7% (269 of 365 days). Lowest qualifying Good-day share: 2022 at 35.3% (129 of 365 days).
What does AQI 38 mean for health? ▼
Mendocino County's median AQI of 38 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Mendocino County compare to other counties in California? ▼
Mendocino County has 73.7% good air days. Nearby counties in California: Alameda (53.0% good), Amador (87.4% good), Butte (65.2% good), Calaveras (82.2% good), Colusa (79.2% good), Contra Costa (53.5% good).
What is the 90th percentile AQI in Mendocino County? ▼
The 90th percentile AQI for Mendocino County is 59, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 81 (Moderate).
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 Mendocino County, California · 2020–2025
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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.