Median AQI
51
Moderate
California · County air quality · 2025
Stanislaus County, California
Median AQI 51 (Moderate). 49.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 51
- Median AQI
- 49.9%
- Good-air days
- 0
- Unhealthy days
- 6%
- Cleaner than US counties
Stanislaus County's 2025 annual median AQI is 51 (Moderate), with 49.9% Good days
According to the U.S. EPA Air Quality System, Stanislaus County, California posted a 2025 median AQI of 51 (Moderate) across 365 monitored days, with 49.9% 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
- Stanislaus County's 49.9% Good-day share places it cleaner than 6% of US counties in 2025.
- Closest median-AQI peer
- Davidson County (45.8% Good, median AQI 51) is the closest nationwide median-AQI peer to Stanislaus County.
- 2020–2025 trend
- Stanislaus County's floor-qualifying Good-day share is improving from 2020 to 2025 (35.8% → 49.9%).
- Median AQI
- Median AQI: 51
The verdict
According to EPA Air Quality System monitoring data, Stanislaus County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 51 with 49.9% of 365 monitored days in the Good band, driven mainly by Ozone.
- 51
- median AQI · Moderate
- 49.9%
- days rated Good
- Ozone
- main pollutant
- 6%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 115.
Annual release change
What changed in the 2025 EPA annual release
- Stanislaus County's Good-day share rose from 45.4% to 49.9% between the 2024 and 2025 EPA annual releases, a change of +4.5 percentage points.
- Stanislaus County's peak single-day AQI fell from 150 to 115 between the 2024 and 2025 EPA annual releases, a change of -35 points.
Data updated August 2026
- Good · 182 d · 49.9%
- Moderate · 180 d · 49.3%
- USG · 3 d · 0.8%
How did Stanislaus County's air break down in 2025?
Across 365 monitored days, 182 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 115 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 51 (Moderate).
Insights: Stanislaus County, California. EPA monitors recorded a median Air Quality Index of 51 across 365 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 182 (50%) fell in the Good band (AQI 0–50), 180 (49%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 80, while the worst single-day peak hit 115 (USG). Residents averaged about 15.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (229 days), PM2.5 (132 days), PM10 (4 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +14.1 points and the median AQI moved -9 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 115, usg 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
49.9%
182 of 365 monitored days
-25.1pp vs 75% target
Max AQI Recorded
115
USG
Health Grade
Poor
90th-pct AQI 80
Stanislaus County, Good days share 49.9%
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 365 monitored days across EPA AQI categories.
Good
182
50%
Moderate
180
49%
USG
3
1%
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 · 229 days · 62.7%
- PM2.5 · 132 days · 36.2%
- PM10 · 4 days · 1.1%
Health Impact Assessment
Poor
Stanislaus County's air quality is rated Poor based on 49.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 49.9% of monitored days in 2025, with a median AQI of 51. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 229 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Stanislaus County. Highest Good-day share: 2025 (49.9%, 182 of 365 monitored 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 | 182 | 180 | 3 | 0 | 51 | 115 |
How does Stanislaus County compare?
How Stanislaus County compares
Good-day share worse than typical
Stanislaus County 49.9%
Davidson County, North Carolina 45.8%
Typical US county 76.0%
Median AQI worse than typical
Stanislaus County 51
Davidson County, North Carolina 51
Typical US county 39
Unhealthy days / yr better than typical
Stanislaus County 0
Davidson County, North Carolina 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Stanislaus County (51), not geographic neighbors.
Davidson County, North Carolina
Median AQI 51 · 45.8% Good-day share
Duval County, Florida
Median AQI 51 · 48.0% Good-day share
East Baton Rouge County, Louisiana
Median AQI 51 · 49.6% Good-day share
Garland County, Arkansas
Median AQI 51 · 49.2% Good-day share
Jefferson County, Texas
Median AQI 51 · 48.0% Good-day share
McLennan County, Texas
Median AQI 51 · 46.6% 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).
Washoe County, Nevada
50.1% Good-day share · median AQI 50
Denver County, Colorado
50.4% Good-day share · median AQI 50
Madera County, California
50.4% Good-day share · median AQI 50
Larimer County, Colorado
50.7% Good-day share · median AQI 50
Merced County, California
51.0% Good-day share · median AQI 50
Ventura County, California
52.0% Good-day share · median AQI 50
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
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 51 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 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 Stanislaus County, California? ▼
Stanislaus County has a median Air Quality Index (AQI) of 51, rated "Moderate." 49.9% of monitored days (182 out of 365) 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 229 days (63% 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 115.
Is the air quality in Stanislaus County getting better or worse? ▼
Air quality in Stanislaus 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 49.9% (182 of 365 days). Lowest qualifying Good-day share: 2021 at 23.0% (84 of 365 days).
What does AQI 51 mean for health? ▼
Stanislaus County's median AQI of 51 falls in the "Moderate" category. See the AQI category guide for what each range means for health.
How does Stanislaus County compare to other counties in California? ▼
Stanislaus County has 49.9% 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 Stanislaus County? ▼
The 90th percentile AQI for Stanislaus County is 80, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 115 (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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Stanislaus 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.