Median AQI
38
Good
California · County air quality · 2025
Monterey County, California
Median AQI 38 (Good). 86.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 38
- Median AQI
- 86.0%
- Good-air days
- 0
- Unhealthy days
- 72%
- Cleaner than US counties
Monterey County's 2025 annual median AQI is 38 (Good), with 86.0% Good days
According to the U.S. EPA Air Quality System, Monterey County, California posted a 2025 median AQI of 38 (Good) across 365 monitored days, with 86.0% 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
- Monterey County's 86.0% Good-day share places it cleaner than 72% of US counties in 2025.
- Closest median-AQI peer
- Adams County (94.7% Good, median AQI 38) is the closest nationwide median-AQI peer to Monterey County.
- 2020–2025 trend
- Monterey County's floor-qualifying Good-day share is improving from 2020 to 2025 (71.3% → 86.0%).
- Median AQI
- Median AQI: 38
The verdict
According to EPA Air Quality System monitoring data, Monterey County's air is good on balance, cleaner than the median US county, a median AQI of 38 with 86.0% of 365 monitored days in the Good band, driven mainly by Ozone.
- 38
- median AQI · Good
- 86.0%
- days rated Good
- Ozone
- main pollutant
- 72%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 62.
Annual release change
What changed in the 2025 EPA annual release
- Monterey County's Good-day share rose from 79.8% to 86.0% between the 2024 and 2025 EPA annual releases, a change of +6.2 percentage points.
Data updated August 2026
- Good · 314 d · 86.0%
- Moderate · 51 d · 14.0%
How did Monterey County's air break down in 2025?
Across 365 monitored days, 314 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 62 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 38 (Good).
Insights: Monterey County, California. EPA monitors recorded a median Air Quality Index of 38 across 365 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 314 (86%) fell in the Good band (AQI 0–50), 51 (14%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 53, while the worst single-day peak hit 62 (Moderate). Residents averaged about 25.8 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (175 days), PM2.5 (161 days), PM10 (29 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +15.3 points and the median AQI moved -1 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 62, 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
86.0%
314 of 365 monitored days
11.0pp vs 75% target
Max AQI Recorded
62
Moderate
Health Grade
Good
90th-pct AQI 53
Monterey County, Good days share 86.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Monterey 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
314
86%
Moderate
51
14%
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 Monterey County.
- Ozone · 175 days · 47.9%
- PM2.5 · 161 days · 44.1%
- PM10 · 29 days · 7.9%
Health Impact Assessment
Good
Monterey County's air quality is rated Good based on 86.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 86.0% of monitored days in 2025, with a median AQI of 38. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 175 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Monterey County. Highest Good-day share: 2025 (86.0%, 314 of 365 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 259 | 92 | 4 | 8 | 39 | 275 |
| 2021 | 261 | 104 | 0 | 0 | 41 | 80 |
| 2022 | 299 | 66 | 0 | 0 | 39 | 80 |
| 2023 | 274 | 90 | 1 | 0 | 42 | 115 |
| 2024 | 292 | 74 | 0 | 0 | 39 | 71 |
| 2025 | 314 | 51 | 0 | 0 | 38 | 62 |
How does Monterey County compare?
How Monterey County compares
Good-day share better than typical
Monterey County 86.0%
Adams County, Illinois 94.7%
Typical US county 76.0%
Median AQI better than typical
Monterey County 38
Adams County, Illinois 38
Typical US county 39
Unhealthy days / yr better than typical
Monterey County 0
Adams County, Illinois 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Monterey County (38), not geographic neighbors.
Adams County, Illinois
Median AQI 38 · 94.7% Good-day share
Albemarle County, Virginia
Median AQI 38 · 81.0% Good-day share
Alexander County, North Carolina
Median AQI 38 · 93.5% Good-day share
Amador County, California
Median AQI 38 · 87.4% Good-day share
Anoka County, Minnesota
Median AQI 38 · 74.1% Good-day share
Avery County, North Carolina
Median AQI 38 · 92.3% 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).
Dorchester County, Maryland
86.0% Good-day share · median AQI 39
Noble County, Ohio
85.9% Good-day share · median AQI 37
Bucks County, Pennsylvania
86.2% Good-day share · median AQI 36
Door County, Wisconsin
85.9% Good-day share · median AQI 38
Apache County, Arizona
86.2% Good-day share · median AQI 33
Carson City County, Nevada
86.2% Good-day share · median AQI 42
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
What Monterey 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 38 means for exposure. Health guidance
- See how Monterey County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Monterey 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 Monterey County, California? ▼
Monterey County has a median Air Quality Index (AQI) of 38, rated "Good." 86.0% of monitored days (314 out of 365) had good air quality in 2025.
What is the main pollutant in Monterey County? ▼
The primary pollutant in Monterey County is Ozone (O3), which was the main AQI driver on 175 days (48% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Monterey County have? ▼
In 2025, Monterey County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 62.
Is the air quality in Monterey County getting better or worse? ▼
Air quality in Monterey 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 86.0% (314 of 365 days). Lowest qualifying Good-day share: 2020 at 71.3% (259 of 366 days).
What does AQI 38 mean for health? ▼
Monterey County's median AQI of 38 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Monterey County compare to other counties in California? ▼
Monterey County has 86.0% 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 Monterey County? ▼
The 90th percentile AQI for Monterey County is 53, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 62 (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 Monterey 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.