Median AQI
44
Good
California · County air quality · 2025
Shasta County, California
Median AQI 44 (Good). 84.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 44
- 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, Shasta County's air is good on balance, cleaner than the median US county, a median AQI of 44 with 84.9% of 232 monitored days in the Good band, driven mainly by Ozone.
- 44
- 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 100.
Data updated July 2026
- Good · 197 d · 84.9%
- Moderate · 35 d · 15.1%
How did Shasta County's air break down in 2025?
Across 232 monitored days, 197 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 100 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 232 days monitored. Median AQI: 44 (Good).
Insights: Shasta County, California. EPA monitors recorded a median Air Quality Index of 44 across 232 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 197 (85%) fell in the Good band (AQI 0–50), 35 (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 100 (Moderate). Residents averaged about 25.5 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (203 days), PM2.5 (21 days), PM10 (8 days). Across the 2020–2025 trend window, good days shifted by -89 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 100, 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
84.9%
197 of 232 monitored days
9.9pp vs 75% target
Max AQI Recorded
100
Moderate
Health Grade
Good
90th-pct AQI 54
Shasta County, Good days share 84.9%
National avg
Share of monitored days at AQI 0-50 (Good band).
Shasta 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 232 monitored days across EPA AQI categories.
Good
197
85%
Moderate
35
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 Shasta County.
- Ozone · 203 days · 87.5%
- PM2.5 · 21 days · 9.1%
- PM10 · 8 days · 3.4%
Health Impact Assessment
Good
Shasta 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 44. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 203 of 232 monitored days.
5-Year Air Quality Trend
Air quality is improving in Shasta County. Best year: 2020 (286 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 286 | 72 | 6 | 2 | 41 | 171 |
| 2021 | 222 | 111 | 15 | 12 | 46 | 240 |
| 2022 | 253 | 106 | 6 | 0 | 45 | 119 |
| 2023 | 247 | 112 | 5 | 0 | 45 | 1435 |
| 2024 | 268 | 94 | 3 | 0 | 44 | 232 |
| 2025 | 197 | 35 | 0 | 0 | 44 | 100 |
How does Shasta County compare?
How Shasta County compares
Good-day share better than US avg
Shasta County 84.9%
Amador County, California 84.8%
US average 76.5%
Median AQI worse than US avg
Shasta County 44
Amador County, California 40
US average 38
Unhealthy days / yr about average
Shasta 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 Shasta County.
Baltimore County, Maryland
AQI 44 · 70.4% good
Baltimore (City) County, Maryland
AQI 44 · 68.7% good
Bartholomew County, Indiana
AQI 44 · 69.2% good
Berks County, Pennsylvania
AQI 44 · 65.6% good
Bolivar County, Mississippi
AQI 44 · 70.3% good
Brewster County, Texas
AQI 44 · 69.0% good
What Shasta 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 44 means for exposure. Health guidance
- See how Shasta County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Shasta County with its California neighbours before drawing a conclusion about the region. All of California
Based on 232 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 Shasta County, California? ▼
Shasta County has a median Air Quality Index (AQI) of 44, rated "Good." 84.9% of monitored days (197 out of 232) had good air quality in 2025.
What is the main pollutant in Shasta County? ▼
The primary pollutant in Shasta County is Ozone (O3), which was the main AQI driver on 203 days (88% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Shasta County have? ▼
In 2025, Shasta County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 100.
Is the air quality in Shasta County getting better or worse? ▼
Air quality in Shasta County is improving based on 5-year trends (2020-2025). Best year: 2020 with 286 good days. Worst year: 2025 with 197 good days.
What does AQI 44 mean for health? ▼
Shasta County's median AQI of 44 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 Shasta County compare to other counties in California? ▼
Shasta County has 84.9% 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 Shasta County? ▼
The 90th percentile AQI for Shasta County is 54, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 100 (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.