Median AQI
39
Good
California · County air quality · 2025
Colusa County, California
Median AQI 39 (Good). 84.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 39
- Median AQI
- 84.9%
- Good-air days
- 0
- Unhealthy days
- 66%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Colusa County's air is good on balance, cleaner than the median US county, a median AQI of 39 with 84.9% of 265 monitored days in the Good band, driven mainly by Ozone.
- 39
- median AQI · Good
- 84.9%
- days rated Good
- Ozone
- main pollutant
- 66%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 96.
Data updated July 2026
- Good · 225 d · 84.9%
- Moderate · 40 d · 15.1%
How did Colusa County's air break down in 2025?
Across 265 monitored days, 225 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 96 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 265 days monitored. Median AQI: 39 (Good).
Insights: Colusa County, California. EPA monitors recorded a median Air Quality Index of 39 across 265 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 225 (85%) fell in the Good band (AQI 0–50), 40 (15%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 54, while the worst single-day peak hit 96 (Moderate). Residents averaged about 25.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (149 days), PM2.5 (109 days), PM10 (7 days). Across the 2020–2025 trend window, good days shifted by +7 and the median AQI moved -3 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 96, 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); Del Norte (81.0% good, AQI 33); El Dorado (85.8% good, AQI 39); Fresno (40.1% good, AQI 54).
Air Quality Summary
Good Days
84.9%
225 of 265 monitored days
9.9pp vs 75% target
Max AQI Recorded
96
Moderate
Health Grade
Good
90th-pct AQI 54
Colusa County, Good days share 84.9%
National avg
Share of monitored days at AQI 0-50 (Good band).
Colusa 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 265 monitored days across EPA AQI categories.
Good
225
85%
Moderate
40
15%
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 Colusa County.
- Ozone · 149 days · 56.2%
- PM2.5 · 109 days · 41.1%
- PM10 · 7 days · 2.6%
Health Impact Assessment
Good
Colusa County's air quality is rated Good based on 84.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 84.9% of monitored days in 2025, with a median AQI of 39. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 149 of 265 monitored days.
5-Year Air Quality Trend
Air quality is improving in Colusa County. Best year: 2022 (240 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 218 | 116 | 9 | 22 | 42 | 221 |
| 2021 | 111 | 238 | 9 | 6 | 55 | 225 |
| 2022 | 240 | 122 | 1 | 0 | 44 | 105 |
| 2023 | 115 | 250 | 0 | 0 | 55 | 98 |
| 2024 | 68 | 298 | 0 | 0 | 54 | 90 |
| 2025 | 225 | 40 | 0 | 0 | 39 | 96 |
How does Colusa County compare?
How Colusa County compares
Good-day share better than US avg
Colusa County 84.9%
Amador County, California 84.8%
US average 76.5%
Median AQI worse than US avg
Colusa County 39
Amador County, California 40
US average 38
Unhealthy days / yr about average
Colusa 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 Colusa County.
Ada County, Idaho
AQI 39 · 80.6% good
Aiken County, South Carolina
AQI 39 · 89.1% good
Albany County, New York
AQI 39 · 67.4% good
Alexander County, North Carolina
AQI 39 · 92.4% good
Ashland County, Wisconsin
AQI 39 · 85.6% good
Bennington County, Vermont
AQI 39 · 82.9% good
What Colusa 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 39 means for exposure. Health guidance
- See how Colusa County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Colusa County with its California neighbours before drawing a conclusion about the region. All of California
Based on 265 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 Colusa County, California? ▼
Colusa County has a median Air Quality Index (AQI) of 39, rated "Good." 84.9% of monitored days (225 out of 265) had good air quality in 2025.
What is the main pollutant in Colusa County? ▼
The primary pollutant in Colusa County is Ozone (O3), which was the main AQI driver on 149 days (56% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Colusa County have? ▼
In 2025, Colusa County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 96.
Is the air quality in Colusa County getting better or worse? ▼
Air quality in Colusa County is improving based on 5-year trends (2020-2025). Best year: 2022 with 240 good days. Worst year: 2024 with 68 good days.
What does AQI 39 mean for health? ▼
Colusa County's median AQI of 39 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 Colusa County compare to other counties in California? ▼
Colusa County has 84.9% good air days. Nearby counties in California: Amador (84.8% good), Butte (66.1% good), Calaveras (73.9% good), Del Norte (81.0% good), El Dorado (85.8% good), Fresno (40.1% good).
What is the 90th percentile AQI in Colusa County? ▼
The 90th percentile AQI for Colusa County is 54, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 96 (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.