Median AQI
44
Good
California · County air quality · 2025
Shasta County, California
Median AQI 44 (Good). 75.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 44
- Median AQI
- 75.6%
- Good-air days
- 1
- Unhealthy days
- 45%
- Cleaner than US counties
Shasta County's 2025 annual median AQI is 44 (Good), with 75.6% Good days
According to the U.S. EPA Air Quality System, Shasta County, California posted a 2025 median AQI of 44 (Good) across 365 monitored days, with 75.6% rated Good and 1 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Shasta County's 75.6% Good-day share places it cleaner than 45% of US counties in 2025.
- Closest median-AQI peer
- Allegan County (63.9% Good, median AQI 44) is the closest nationwide median-AQI peer to Shasta County.
- 2020–2025 trend
- Shasta County's floor-qualifying Good-day share is stable from 2020 to 2025 (78.1% → 75.6%).
- Median AQI
- Median AQI: 44
The verdict
According to EPA Air Quality System monitoring data, Shasta County's air is good on balance, more polluted than the median US county, a median AQI of 44 with 75.6% of 365 monitored days in the Good band, driven mainly by Ozone.
- 44
- median AQI · Good
- 75.6%
- days rated Good
- Ozone
- main pollutant
- 45%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 154.
Annual release change
What changed in the 2025 EPA annual release
- Shasta County's peak single-day AQI fell from 232 to 154 between the 2024 and 2025 EPA annual releases, a change of -78 points.
- Shasta County's Good-day share rose from 73.2% to 75.6% between the 2024 and 2025 EPA annual releases, a change of +2.4 percentage points.
- Shasta County's Unhealthy-category days rose from 0 to 1 between the 2024 and 2025 EPA annual releases, a change of +1 days.
Data updated August 2026
- Good · 276 d · 75.6%
- Moderate · 88 d · 24.1%
- Unhealthy · 1 d · 0.3%
How did Shasta County's air break down in 2025?
Across 365 monitored days, 276 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 154 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 44 (Good).
Insights: Shasta County, California. EPA monitors recorded a median Air Quality Index of 44 across 365 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 276 (76%) fell in the Good band (AQI 0–50), 88 (24%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 61, while the worst single-day peak hit 154 (Unhealthy). Residents averaged about 22.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (285 days), PM2.5 (80 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -2.5 points and the median AQI moved +3 points, placing the multi-year trajectory at stable. The annual record contains 1 day at Unhealthy or worse. On that day, the EPA recommends that sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Alameda (53.0% good, AQI 49); Amador (87.4% good, AQI 38); Butte (65.2% good, AQI 46); Calaveras (82.2% good, AQI 40); Colusa (79.2% good, AQI 40); Contra Costa (53.5% good, AQI 48).
Air Quality Summary
Good Days
75.6%
276 of 365 monitored days
0.6pp vs 75% target
Max AQI Recorded
154
Unhealthy
Health Grade
Good
90th-pct AQI 61
Shasta County, Good days share 75.6%
National avg
Share of monitored days at AQI 0-50 (Good band).
Shasta County, Unhealthy-or-worse share 2.7%
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 365 monitored days across EPA AQI categories.
Good
276
76%
Moderate
88
24%
USG
0
0%
Unhealthy
1
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 · 285 days · 78.1%
- PM2.5 · 80 days · 21.9%
Health Impact Assessment
Good
Shasta County's air quality is rated Good based on 75.6% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 0 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 154. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 285 of 365 monitored days.
5-Year Air Quality Trend
Air quality is stable in Shasta County. Highest Good-day share: 2020 (78.1%, 286 of 366 monitored 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 | 276 | 88 | 0 | 1 | 44 | 154 |
How does Shasta County compare?
How Shasta County compares
Good-day share about average
Shasta County 75.6%
Allegan County, Michigan 63.9%
US average 76.0%
Median AQI worse than US avg
Shasta County 44
Allegan County, Michigan 44
US average 39
Unhealthy days / yr about average
Shasta County 1
Allegan County, Michigan 2
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Shasta County (44), not geographic neighbors.
Allegan County, Michigan
Median AQI 44 · 63.9% Good-day share
Allen County, Indiana
Median AQI 44 · 60.0% Good-day share
Baker County, Florida
Median AQI 44 · 69.5% Good-day share
Bay County, Florida
Median AQI 44 · 66.4% Good-day share
Blount County, Tennessee
Median AQI 44 · 71.5% Good-day share
Cache County, Utah
Median AQI 44 · 66.0% Good-day share
Same primary pollutant: Ozone
Ozone led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Kent County, Delaware
75.6% Good-day share · median AQI 41
Trego County, Kansas
75.5% Good-day share · median AQI 41
Harford County, Maryland
75.5% Good-day share · median AQI 42
Collin County, Texas
75.8% Good-day share · median AQI 40
Henrico County, Virginia
75.8% Good-day share · median AQI 40
Gilpin County, Colorado
75.8% Good-day share · median AQI 46
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
What Shasta County's air means for you
On most days the air here is fine, but Ozone drives 1 unhealthy-or-worse day a year that sensitive groups should plan around.
- 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 365 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
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." 75.6% of monitored days (276 out of 365) 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 285 days (78% 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 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 154.
Is the air quality in Shasta County getting better or worse? ▼
Air quality in Shasta County is stable across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2020 at 78.1% (286 of 366 days). Lowest qualifying Good-day share: 2021 at 61.7% (222 of 365 days).
What does AQI 44 mean for health? ▼
Shasta County's median AQI of 44 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Shasta County compare to other counties in California? ▼
Shasta County has 75.6% good air days. Nearby counties in California: Alameda (53.0% good), Amador (87.4% good), Butte (65.2% good), Calaveras (82.2% good), Colusa (79.2% good), Contra Costa (53.5% good).
What is the 90th percentile AQI in Shasta County? ▼
The 90th percentile AQI for Shasta County is 61, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 154 (Unhealthy).
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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Shasta County, California · 2020–2025
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Every figure on PlainAirData 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.