Median AQI
44
Good
California · County air quality · 2025
Mariposa County, California
Median AQI 44 (Good). 80.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 44
- Median AQI
- 80.6%
- Good-air days
- 0
- Unhealthy days
- 58%
- Cleaner than US counties
Mariposa County's 2025 annual median AQI is 44 (Good), with 80.6% Good days
According to the U.S. EPA Air Quality System, Mariposa County, California posted a 2025 median AQI of 44 (Good) across 340 monitored days, with 80.6% 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
- Mariposa County's 80.6% Good-day share places it cleaner than 58% of US counties in 2025.
- Closest median-AQI peer
- Allegan County (63.9% Good, median AQI 44) is the closest nationwide median-AQI peer to Mariposa County.
- 2020–2025 trend
- Mariposa County's floor-qualifying Good-day share is improving from 2020 to 2025 (54.9% → 80.6%).
- Median AQI
- Median AQI: 44
The verdict
According to EPA Air Quality System monitoring data, Mariposa County's air is good on balance, cleaner than the median US county, a median AQI of 44 with 80.6% of 340 monitored days in the Good band, driven mainly by Ozone.
- 44
- median AQI · Good
- 80.6%
- days rated Good
- Ozone
- main pollutant
- 58%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 90.
Annual release change
What changed in the 2025 EPA annual release
- Mariposa County's Good-day share rose from 69.7% to 80.6% between the 2024 and 2025 EPA annual releases, a change of +10.9 percentage points.
- Mariposa County's peak single-day AQI fell from 133 to 90 between the 2024 and 2025 EPA annual releases, a change of -43 points.
Data updated August 2026
- Good · 274 d · 80.6%
- Moderate · 66 d · 19.4%
How did Mariposa County's air break down in 2025?
Across 340 monitored days, 274 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 90 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 340 days monitored. Median AQI: 44 (Good).
Insights: Mariposa County, California. EPA monitors recorded a median Air Quality Index of 44 across 340 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 274 (81%) fell in the Good band (AQI 0–50), 66 (19%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 64, while the worst single-day peak hit 90 (Moderate). Residents averaged about 24.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (338 days), PM2.5 (2 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +27.3 points (measured against 366 monitored days in 2020 and 340 in 2025, so raw day counts are not comparable) and the median AQI moved -5 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 90, 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
80.6%
274 of 340 monitored days
5.6pp vs 75% target
Max AQI Recorded
90
Moderate
Health Grade
Good
90th-pct AQI 64
Mariposa County, Good days share 80.6%
National avg
Share of monitored days at AQI 0-50 (Good band).
Mariposa 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 340 monitored days across EPA AQI categories.
Good
274
81%
Moderate
66
19%
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 Mariposa County.
- Ozone · 338 days · 99.4%
- PM2.5 · 2 days · 0.6%
Health Impact Assessment
Good
Mariposa County's air quality is rated Good based on 80.6% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 80.6% of monitored days in 2025, with a median AQI of 44. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 338 of 340 monitored days.
5-Year Air Quality Trend
Air quality is improving in Mariposa County. Highest Good-day share: 2025 (80.6%, 274 of 340 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 195 | 123 | 27 | 10 | 49 | 1073 |
| 2021 | 193 | 140 | 25 | 7 | 49 | 162 |
| 2022 | 211 | 125 | 8 | 21 | 48 | 198 |
| 2023 | 217 | 131 | 10 | 5 | 47 | 264 |
| 2024 | 255 | 105 | 6 | 0 | 45 | 133 |
| 2025 | 274 | 66 | 0 | 0 | 44 | 90 |
How does Mariposa County compare?
How Mariposa County compares
Good-day share better than typical
Mariposa County 80.6%
Allegan County, Michigan 63.9%
Typical US county 76.0%
Median AQI worse than typical
Mariposa County 44
Allegan County, Michigan 44
Typical US county 39
Unhealthy days / yr better than typical
Mariposa County 0
Allegan County, Michigan 2
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Mariposa County (44), not geographic neighbors.
Allegan County, Michigan
Median AQI 44 · 63.9% Good-day share
Allen County, Indiana
Median AQI 44 · 60.0% Good-day share
Baker County, Florida
Median AQI 44 · 69.5% Good-day share
Bay County, Florida
Median AQI 44 · 66.4% Good-day share
Blount County, Tennessee
Median AQI 44 · 71.5% Good-day share
Cache County, Utah
Median AQI 44 · 66.0% Good-day share
Same primary pollutant: Ozone
Ozone 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).
Creek County, Oklahoma
80.6% Good-day share · median AQI 42
Loudoun County, Virginia
80.5% Good-day share · median AQI 40
Minnehaha County, South Dakota
80.8% Good-day share · median AQI 36
Brazoria County, Texas
80.8% Good-day share · median AQI 35
Crittenden County, Arkansas
80.8% Good-day share · median AQI 39
McKenzie County, North Dakota
80.8% Good-day share · median AQI 39
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
What Mariposa County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 44 means for exposure. Health guidance
- See how Mariposa County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Mariposa County with its California neighbours before drawing a conclusion about the region. All of California
Based on 340 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 Mariposa County, California? ▼
Mariposa County has a median Air Quality Index (AQI) of 44, rated "Good." 80.6% of monitored days (274 out of 340) had good air quality in 2025.
What is the main pollutant in Mariposa County? ▼
The primary pollutant in Mariposa County is Ozone (O3), which was the main AQI driver on 338 days (99% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Mariposa County have? ▼
In 2025, Mariposa County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 90.
Is the air quality in Mariposa County getting better or worse? ▼
Air quality in Mariposa 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 80.6% (274 of 340 days). Lowest qualifying Good-day share: 2021 at 52.9% (193 of 365 days).
What does AQI 44 mean for health? ▼
Mariposa County's median AQI of 44 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Mariposa County compare to other counties in California? ▼
Mariposa County has 80.6% 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 Mariposa County? ▼
The 90th percentile AQI for Mariposa County is 64, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 90 (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 Mariposa 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.