Median AQI
33
Good
California · County air quality · 2025
Trinity County, California
Median AQI 33 (Good). 68.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 33
- Median AQI
- 68.4%
- Good-air days
- 0
- Unhealthy days
- 28%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Trinity County's air is good on balance, more polluted than the median US county, a median AQI of 33 with 68.4% of 152 monitored days in the Good band, driven mainly by PM2.5.
- 33
- median AQI · Good
- 68.4%
- days rated Good
- PM2.5
- main pollutant
- 28%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 83.
Data updated July 2026
- Good · 104 d · 68.4%
- Moderate · 48 d · 31.6%
How did Trinity County's air break down in 2025?
Across 152 monitored days, 104 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 83 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 152 days monitored. Median AQI: 33 (Good).
Insights: Trinity County, California. EPA monitors recorded a median Air Quality Index of 33 across 152 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 104 (68%) fell in the Good band (AQI 0–50), 48 (32%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 66, while the worst single-day peak hit 83 (Moderate). Residents averaged about 20.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (152 days). Across the 2020–2025 trend window, good days shifted by -111 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 83, 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
68.4%
104 of 152 monitored days
-6.6pp vs 75% target
Max AQI Recorded
83
Moderate
Health Grade
Moderate
90th-pct AQI 66
Trinity County, Good days share 68.4%
National avg
Share of monitored days at AQI 0-50 (Good band).
Trinity 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 152 monitored days across EPA AQI categories.
Good
104
68%
Moderate
48
32%
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 Trinity County.
- PM2.5 · 152 days · 100.0%
Health Impact Assessment
Moderate
Trinity County's air quality is rated Moderate based on 68.4% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 68.4% of monitored days in 2025, with a median AQI of 33. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 152 of 152 monitored days.
5-Year Air Quality Trend
Air quality is improving in Trinity County. Best year: 2024 (258 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 215 | 100 | 12 | 13 | 36 | 211 |
| 2021 | 201 | 81 | 4 | 19 | 38 | 1217 |
| 2022 | 227 | 111 | 5 | 5 | 34 | 342 |
| 2023 | 209 | 93 | 6 | 3 | 31 | 174 |
| 2024 | 258 | 98 | 1 | 0 | 31 | 113 |
| 2025 | 104 | 48 | 0 | 0 | 33 | 83 |
How does Trinity County compare?
How Trinity County compares
Good-day share worse than US avg
Trinity County 68.4%
Amador County, California 84.8%
US average 76.5%
Median AQI better than US avg
Trinity County 33
Amador County, California 40
US average 38
Unhealthy days / yr about average
Trinity 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 Trinity County.
Ascension County, Louisiana
AQI 33 · 94.1% good
Athens County, Ohio
AQI 33 · 80.3% good
Brazoria County, Texas
AQI 33 · 84.8% good
Bristol City County, Virginia
AQI 33 · 78.3% good
Carter County, Oklahoma
AQI 33 · 80.9% good
Codington County, South Dakota
AQI 33 · 82.1% good
What Trinity 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 33 means for exposure. Health guidance
- See how Trinity County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Trinity County with its California neighbours before drawing a conclusion about the region. All of California
Based on 152 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 Trinity County, California? ▼
Trinity County has a median Air Quality Index (AQI) of 33, rated "Good." 68.4% of monitored days (104 out of 152) had good air quality in 2025.
What is the main pollutant in Trinity County? ▼
The primary pollutant in Trinity County is Fine Particles (PM2.5), which was the main AQI driver on 152 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Trinity County have? ▼
In 2025, Trinity County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 83.
Is the air quality in Trinity County getting better or worse? ▼
Air quality in Trinity County is improving based on 5-year trends (2020-2025). Best year: 2024 with 258 good days. Worst year: 2025 with 104 good days.
What does AQI 33 mean for health? ▼
Trinity County's median AQI of 33 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 Trinity County compare to other counties in California? ▼
Trinity County has 68.4% 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 Trinity County? ▼
The 90th percentile AQI for Trinity County is 66, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 83 (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.