Median AQI
45
Good
California · County air quality · 2025
Stanislaus County, California
Median AQI 45 (Good). 68.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 45
- Median AQI
- 68.3%
- Good-air days
- 0
- Unhealthy days
- 28%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Stanislaus County's air is good on balance, more polluted than the median US county, a median AQI of 45 with 68.3% of 249 monitored days in the Good band, driven mainly by Ozone.
- 45
- median AQI · Good
- 68.3%
- days rated Good
- Ozone
- main pollutant
- 28%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 101.
Data updated July 2026
- Good · 170 d · 68.3%
- Moderate · 78 d · 31.3%
- USG · 1 d · 0.4%
How did Stanislaus County's air break down in 2025?
Across 249 monitored days, 170 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 101 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 249 days monitored. Median AQI: 45 (Good).
Insights: Stanislaus County, California. EPA monitors recorded a median Air Quality Index of 45 across 249 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 170 (68%) fell in the Good band (AQI 0–50), 78 (31%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 71, while the worst single-day peak hit 101 (USG). Residents averaged about 20.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (182 days), PM2.5 (64 days), PM10 (2 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by +39 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 101, usg 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
68.3%
170 of 249 monitored days
-6.7pp vs 75% target
Max AQI Recorded
101
USG
Health Grade
Moderate
90th-pct AQI 71
Stanislaus County, Good days share 68.3%
National avg
Share of monitored days at AQI 0-50 (Good band).
Stanislaus 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 249 monitored days across EPA AQI categories.
Good
170
68%
Moderate
78
31%
USG
1
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 Stanislaus County.
- Ozone · 182 days · 73.1%
- PM2.5 · 64 days · 25.7%
- PM10 · 2 days · 0.8%
- CO · 1 days · 0.4%
Health Impact Assessment
Moderate
Stanislaus County's air quality is rated Moderate based on 68.3% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 68.3% of monitored days in 2025, with a median AQI of 45. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 182 of 249 monitored days.
5-Year Air Quality Trend
Air quality is improving in Stanislaus County. Best year: 2025 (170 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 131 | 198 | 21 | 16 | 60 | 195 |
| 2021 | 84 | 234 | 42 | 5 | 61 | 172 |
| 2022 | 108 | 234 | 22 | 1 | 57 | 166 |
| 2023 | 166 | 176 | 23 | 0 | 54 | 150 |
| 2024 | 166 | 162 | 38 | 0 | 53 | 150 |
| 2025 | 170 | 78 | 1 | 0 | 45 | 101 |
How does Stanislaus County compare?
How Stanislaus County compares
Good-day share worse than US avg
Stanislaus County 68.3%
Amador County, California 84.8%
US average 76.5%
Median AQI worse than US avg
Stanislaus County 45
Amador County, California 40
US average 38
Unhealthy days / yr about average
Stanislaus 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 Stanislaus County.
Adair County, Oklahoma
AQI 45 · 59.2% good
Allen County, Indiana
AQI 45 · 58.2% good
Atascosa County, Texas
AQI 45 · 58.0% good
Baker County, Florida
AQI 45 · 69.1% good
Carson City County, Nevada
AQI 45 · 79.3% good
Clinton County, Iowa
AQI 45 · 59.9% good
What Stanislaus 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 45 means for exposure. Health guidance
- See how Stanislaus County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Stanislaus County with its California neighbours before drawing a conclusion about the region. All of California
Based on 249 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 Stanislaus County, California? ▼
Stanislaus County has a median Air Quality Index (AQI) of 45, rated "Good." 68.3% of monitored days (170 out of 249) had good air quality in 2025.
What is the main pollutant in Stanislaus County? ▼
The primary pollutant in Stanislaus County is Ozone (O3), which was the main AQI driver on 182 days (73% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Stanislaus County have? ▼
In 2025, Stanislaus County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 101.
Is the air quality in Stanislaus County getting better or worse? ▼
Air quality in Stanislaus County is improving based on 5-year trends (2020-2025). Best year: 2025 with 170 good days. Worst year: 2021 with 84 good days.
What does AQI 45 mean for health? ▼
Stanislaus County's median AQI of 45 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 Stanislaus County compare to other counties in California? ▼
Stanislaus County has 68.3% 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 Stanislaus County? ▼
The 90th percentile AQI for Stanislaus County is 71, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 101 (USG).
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.