Median AQI
40
Good
California · County air quality · 2025
Monterey County, California
Median AQI 40 (Good). 76.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 76.9%
- Good-air days
- 0
- Unhealthy days
- 43%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Monterey County's air is good on balance, more polluted than the median US county, a median AQI of 40 with 76.9% of 182 monitored days in the Good band, driven mainly by Ozone.
- 40
- median AQI · Good
- 76.9%
- days rated Good
- Ozone
- main pollutant
- 43%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 62.
Data updated July 2026
- Good · 140 d · 76.9%
- Moderate · 42 d · 23.1%
How did Monterey County's air break down in 2025?
Across 182 monitored days, 140 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). 182 days monitored. Median AQI: 40 (Good).
Insights: Monterey County, California. EPA monitors recorded a median Air Quality Index of 40 across 182 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 140 (77%) fell in the Good band (AQI 0–50), 42 (23%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 55, while the worst single-day peak hit 62 (Moderate). Residents averaged about 23.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (90 days), PM2.5 (84 days), PM10 (7 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -119 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: 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
76.9%
140 of 182 monitored days
1.9pp vs 75% target
Max AQI Recorded
62
Moderate
Health Grade
Good
90th-pct AQI 55
Monterey County, Good days share 76.9%
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 182 monitored days across EPA AQI categories.
Good
140
77%
Moderate
42
23%
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 · 90 days · 49.5%
- PM2.5 · 84 days · 46.2%
- PM10 · 7 days · 3.8%
- CO · 1 days · 0.5%
Health Impact Assessment
Good
Monterey County's air quality is rated Good based on 76.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 76.9% of monitored days in 2025, with a median AQI of 40. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 90 of 182 monitored days.
5-Year Air Quality Trend
Air quality is improving in Monterey County. Best year: 2022 (299 good 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 | 140 | 42 | 0 | 0 | 40 | 62 |
How does Monterey County compare?
How Monterey County compares
Good-day share about average
Monterey County 76.9%
Amador County, California 84.8%
US average 76.5%
Median AQI worse than US avg
Monterey County 40
Amador County, California 40
US average 38
Unhealthy days / yr about average
Monterey 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 Monterey County.
Amador County, California
AQI 40 · 84.8% good
Anne Arundel County, Maryland
AQI 40 · 87.2% good
Arlington County, Virginia
AQI 40 · 84.8% good
Bergen County, New Jersey
AQI 40 · 71.1% good
Berkeley County, West Virginia
AQI 40 · 81.1% good
Billings County, North Dakota
AQI 40 · 80.3% good
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 40 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 182 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 Monterey County, California? ▼
Monterey County has a median Air Quality Index (AQI) of 40, rated "Good." 76.9% of monitored days (140 out of 182) 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 90 days (49% 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 based on 5-year trends (2020-2025). Best year: 2022 with 299 good days. Worst year: 2025 with 140 good days.
What does AQI 40 mean for health? ▼
Monterey County's median AQI of 40 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 Monterey County compare to other counties in California? ▼
Monterey County has 76.9% 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 Monterey County? ▼
The 90th percentile AQI for Monterey County is 55, 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.
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.