Median AQI
50
Good
California · County air quality · 2025
Merced County, California
Median AQI 50 (Good). 51.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 50
- Median AQI
- 51.0%
- Good-air days
- 0
- Unhealthy days
- 7%
- Cleaner than US counties
Merced County's 2025 annual median AQI is 50 (Good), with 51.0% Good days
According to the U.S. EPA Air Quality System, Merced County, California posted a 2025 median AQI of 50 (Good) across 365 monitored days, with 51.0% 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
- Merced County's 51.0% Good-day share places it cleaner than 7% of US counties in 2025.
- Closest median-AQI peer
- Denver County (50.4% Good, median AQI 50) is the closest nationwide median-AQI peer to Merced County.
- 2020–2025 trend
- Merced County's floor-qualifying Good-day share is improving from 2020 to 2025 (35.8% → 51.0%).
- Median AQI
- Median AQI: 50
The verdict
According to EPA Air Quality System monitoring data, Merced County's air is good on balance, among the most polluted counties nationally, a median AQI of 50 with 51.0% of 365 monitored days in the Good band, driven mainly by Ozone.
- 50
- median AQI · Good
- 51.0%
- days rated Good
- Ozone
- main pollutant
- 7%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 100.
Annual release change
What changed in the 2025 EPA annual release
- Merced County's peak single-day AQI fell from 151 to 100 between the 2024 and 2025 EPA annual releases, a change of -51 points.
- Merced County's Good-day share rose from 45.9% to 51.0% between the 2024 and 2025 EPA annual releases, a change of +5.1 percentage points.
- Merced County's Unhealthy-category days fell from 1 to 0 between the 2024 and 2025 EPA annual releases, a change of -1 days.
Data updated August 2026
- Good · 186 d · 51.0%
- Moderate · 179 d · 49.0%
How did Merced County's air break down in 2025?
Across 365 monitored days, 186 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 100 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 50 (Good).
Insights: Merced County, California. EPA monitors recorded a median Air Quality Index of 50 across 365 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 186 (51%) fell in the Good band (AQI 0–50), 179 (49%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 77, while the worst single-day peak hit 100 (Moderate). Residents averaged about 15.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (228 days), PM2.5 (131 days), PM10 (6 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +15.2 points and the median AQI moved -7 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 100, 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
51.0%
186 of 365 monitored days
-24.0pp vs 75% target
Max AQI Recorded
100
Moderate
Health Grade
Moderate
90th-pct AQI 77
Merced County, Good days share 51.0%
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 365 monitored days across EPA AQI categories.
Good
186
51%
Moderate
179
49%
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.
- Ozone · 228 days · 62.5%
- PM2.5 · 131 days · 35.9%
- PM10 · 6 days · 1.6%
Health Impact Assessment
Moderate
Merced County's air quality is rated Moderate based on 51.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 51.0% of monitored days in 2025, with a median AQI of 50. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 228 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Merced County. Highest Good-day share: 2025 (51.0%, 186 of 365 monitored 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 | 186 | 179 | 0 | 0 | 50 | 100 |
How does Merced County compare?
How Merced County compares
Good-day share worse than typical
Merced County 51.0%
Denver County, Colorado 50.4%
Typical US county 76.0%
Median AQI worse than typical
Merced County 50
Denver County, Colorado 50
Typical US county 39
Unhealthy days / yr better than typical
Merced County 0
Denver County, Colorado 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Merced County (50), not geographic neighbors.
Denver County, Colorado
Median AQI 50 · 50.4% Good-day share
Larimer County, Colorado
Median AQI 50 · 50.7% Good-day share
Madera County, California
Median AQI 50 · 50.4% Good-day share
New York County, New York
Median AQI 50 · 50.1% Good-day share
Philadelphia County, Pennsylvania
Median AQI 50 · 50.4% Good-day share
San Luis Obispo County, California
Median AQI 50 · 52.3% 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
Ventura County, California
52.0% Good-day share · median AQI 50
Stanislaus County, California
49.9% Good-day share · median AQI 51
Eddy County, New Mexico
53.7% Good-day share · median AQI 49
Yuma County, Arizona
54.0% Good-day share · median AQI 48
Sacramento County, California
54.3% Good-day share · median AQI 49
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
What Merced 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 50 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 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 Merced County, California? ▼
Merced County has a median Air Quality Index (AQI) of 50, rated "Good." 51.0% of monitored days (186 out of 365) had good air quality in 2025.
What is the main pollutant in Merced County? ▼
The primary pollutant in Merced County is Ozone (O3), which was the main AQI driver on 228 days (62% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
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 100.
Is the air quality in Merced County getting better or worse? ▼
Air quality in Merced 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 51.0% (186 of 365 days). Lowest qualifying Good-day share: 2020 at 35.8% (131 of 366 days).
What does AQI 50 mean for health? ▼
Merced County's median AQI of 50 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Merced County compare to other counties in California? ▼
Merced County has 51.0% 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 Merced County? ▼
The 90th percentile AQI for Merced County is 77, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 100 (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 Merced 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.