Median AQI
43
Good
California · County air quality · 2025
Merced County, California
Median AQI 43 (Good). 64.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 64.4%
- Good-air days
- 0
- Unhealthy days
- 22%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Merced County's air is good on balance, among the more polluted 10% of US counties, a median AQI of 43 with 64.4% of 90 monitored days in the Good band, driven mainly by PM2.5.
- 43
- median AQI · Good
- 64.4%
- days rated Good
- PM2.5
- main pollutant
- 22%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 78.
Data updated July 2026
- Good · 58 d · 64.4%
- Moderate · 32 d · 35.6%
How did Merced County's air break down in 2025?
Across 90 monitored days, 58 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 78 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 90 days monitored. Median AQI: 43 (Good).
Insights: Merced County, California. EPA monitors recorded a median Air Quality Index of 43 across 90 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 58 (64%) fell in the Good band (AQI 0–50), 32 (36%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 68, while the worst single-day peak hit 78 (Moderate). Residents averaged about 19.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (47 days), Ozone (43 days). Across the 2020–2025 trend window, good days shifted by -73 and the median AQI moved -14 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 78, moderate 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
64.4%
58 of 90 monitored days
-10.6pp vs 75% target
Max AQI Recorded
78
Moderate
Health Grade
Moderate
90th-pct AQI 68
Merced County, Good days share 64.4%
National avg
Share of monitored days at AQI 0-50 (Good band).
Merced 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 90 monitored days across EPA AQI categories.
Good
58
64%
Moderate
32
36%
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 Merced County.
- PM2.5 · 47 days · 52.2%
- Ozone · 43 days · 47.8%
Health Impact Assessment
Moderate
Merced County's air quality is rated Moderate based on 64.4% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 64.4% of monitored days in 2025, with a median AQI of 43. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 47 of 90 monitored days.
5-Year Air Quality Trend
Air quality is improving in Merced County. Best year: 2023 (171 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 131 | 203 | 17 | 15 | 57 | 194 |
| 2021 | 155 | 177 | 31 | 2 | 54 | 166 |
| 2022 | 140 | 212 | 13 | 0 | 56 | 143 |
| 2023 | 171 | 179 | 15 | 0 | 51 | 129 |
| 2024 | 168 | 163 | 34 | 1 | 52 | 151 |
| 2025 | 58 | 32 | 0 | 0 | 43 | 78 |
How does Merced County compare?
How Merced County compares
Good-day share worse than US avg
Merced County 64.4%
Amador County, California 84.8%
US average 76.5%
Median AQI worse than US avg
Merced County 43
Amador County, California 40
US average 38
Unhealthy days / yr about average
Merced 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 Merced County.
Allegan County, Michigan
AQI 43 · 71.2% good
Allen County, Ohio
AQI 43 · 65.9% good
Ashtabula County, Ohio
AQI 43 · 81.2% good
Bell County, Kentucky
AQI 43 · 63.5% good
Boise County, Idaho
AQI 43 · 72.9% good
Brazos County, Texas
AQI 43 · 63.4% good
What Merced 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 43 means for exposure. Health guidance
- See how Merced County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Merced County with its California neighbours before drawing a conclusion about the region. All of California
Based on 90 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 Merced County, California? ▼
Merced County has a median Air Quality Index (AQI) of 43, rated "Good." 64.4% of monitored days (58 out of 90) had good air quality in 2025.
What is the main pollutant in Merced County? ▼
The primary pollutant in Merced County is Fine Particles (PM2.5), which was the main AQI driver on 47 days (52% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Merced County have? ▼
In 2025, Merced County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 78.
Is the air quality in Merced County getting better or worse? ▼
Air quality in Merced County is improving based on 5-year trends (2020-2025). Best year: 2023 with 171 good days. Worst year: 2025 with 58 good days.
What does AQI 43 mean for health? ▼
Merced County's median AQI of 43 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 Merced County compare to other counties in California? ▼
Merced County has 64.4% 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 Merced County? ▼
The 90th percentile AQI for Merced County is 68, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 78 (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.
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.