Median AQI
6
Good
California · County air quality · 2025
Siskiyou County, California
Median AQI 6 (Good). 100.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 6
- Median AQI
- 100.0%
- Good-air days
- 0
- Unhealthy days
- 96%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Siskiyou County's air is good on balance, among the cleanest 5% of US counties, a median AQI of 6 with 100.0% of 15 monitored days in the Good band, driven mainly by PM2.5.
- 6
- median AQI · Good
- 100.0%
- days rated Good
- PM2.5
- main pollutant
- 96%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 10.
Data updated July 2026
- Good · 15 d · 100.0%
How did Siskiyou County's air break down in 2025?
Across 15 monitored days, 15 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 10 (Good).
EPA AQS →Air quality data from EPA Air Quality System (2025). 15 days monitored. Median AQI: 6 (Good).
Insights: Siskiyou County, California. EPA monitors recorded a median Air Quality Index of 6 across 15 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 15 (100%) fell in the Good band (AQI 0–50), 0 (0%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 9, while the worst single-day peak hit 10 (Good). Residents averaged about 30.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (15 days). Across the 2020–2025 trend window, good days shifted by -257 and the median AQI moved -33 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 10, good 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
100.0%
15 of 15 monitored days
25.0pp vs 75% target
Max AQI Recorded
10
Good
Health Grade
Excellent
90th-pct AQI 9
Siskiyou County, Good days share 100.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Siskiyou 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 15 monitored days across EPA AQI categories.
Good
15
100%
Moderate
0
0%
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 Siskiyou County.
- PM2.5 · 15 days · 100.0%
Health Impact Assessment
Excellent
Siskiyou County's air quality is rated Excellent based on 100.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 100.0% of monitored days in 2025, with a median AQI of 6. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 15 of 15 monitored days.
5-Year Air Quality Trend
Air quality is improving in Siskiyou County. Best year: 2024 (284 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 272 | 71 | 10 | 9 | 39 | 468 |
| 2021 | 237 | 96 | 7 | 21 | 43 | 210 |
| 2022 | 269 | 86 | 2 | 4 | 41 | 454 |
| 2023 | 270 | 78 | 7 | 6 | 43 | 320 |
| 2024 | 284 | 82 | 0 | 0 | 41 | 94 |
| 2025 | 15 | 0 | 0 | 0 | 6 | 10 |
How does Siskiyou County compare?
How Siskiyou County compares
Good-day share better than US avg
Siskiyou County 100.0%
Amador County, California 84.8%
US average 76.5%
Median AQI better than US avg
Siskiyou County 6
Amador County, California 40
US average 38
Unhealthy days / yr about average
Siskiyou 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 Siskiyou County.
Hopewell City County, Virginia
AQI 6 · 100.0% good
Powell County, Montana
AQI 6 · 100.0% good
Carbon County, Wyoming
AQI 7 · 100.0% good
Keweenaw County, Michigan
AQI 7 · 100.0% good
Matanuska-Susitna County, Alaska
AQI 7 · 90.3% good
North Slope County, Alaska
AQI 7 · 100.0% good
What Siskiyou 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 6 means for exposure. Health guidance
- See how Siskiyou County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Siskiyou County with its California neighbours before drawing a conclusion about the region. All of California
Based on 15 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 Siskiyou County, California? ▼
Siskiyou County has a median Air Quality Index (AQI) of 6, rated "Good." 100.0% of monitored days (15 out of 15) had good air quality in 2025.
What is the main pollutant in Siskiyou County? ▼
The primary pollutant in Siskiyou County is Fine Particles (PM2.5), which was the main AQI driver on 15 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Siskiyou County have? ▼
In 2025, Siskiyou County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 10.
Is the air quality in Siskiyou County getting better or worse? ▼
Air quality in Siskiyou County is improving based on 5-year trends (2020-2025). Best year: 2024 with 284 good days. Worst year: 2025 with 15 good days.
What does AQI 6 mean for health? ▼
Siskiyou County's median AQI of 6 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 Siskiyou County compare to other counties in California? ▼
Siskiyou County has 100.0% 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 Siskiyou County? ▼
The 90th percentile AQI for Siskiyou County is 9, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 10 (Good).
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.