Median AQI
40
Good
California · County air quality · 2025
Glenn County, California
Median AQI 40 (Good). 80.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 80.0%
- Good-air days
- 0
- Unhealthy days
- 56%
- Cleaner than US counties
Glenn County's 2025 annual median AQI is 40 (Good), with 80.0% Good days
According to the U.S. EPA Air Quality System, Glenn County, California posted a 2025 median AQI of 40 (Good) across 365 monitored days, with 80.0% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Glenn County's 80.0% Good-day share places it cleaner than 56% of US counties in 2025.
- Closest median-AQI peer
- Anderson County (91.3% Good, median AQI 40) is the closest nationwide median-AQI peer to Glenn County.
- 2020–2025 trend
- Glenn County's floor-qualifying Good-day share is improving from 2020 to 2025 (55.4% → 80.0%).
- Median AQI
- Median AQI: 40
The verdict
According to EPA Air Quality System monitoring data, Glenn County's air is good on balance, cleaner than the median US county, a median AQI of 40 with 80.0% of 365 monitored days in the Good band, driven mainly by Ozone.
- 40
- median AQI · Good
- 80.0%
- days rated Good
- Ozone
- main pollutant
- 56%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 94.
Annual release change
What changed in the 2025 EPA annual release
- Glenn County's peak single-day AQI rose from 80 to 94 between the 2024 and 2025 EPA annual releases, a change of +14 points.
Data updated August 2026
- Good · 292 d · 80.0%
- Moderate · 73 d · 20.0%
How did Glenn County's air break down in 2025?
Across 365 monitored days, 292 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 94 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 40 (Good).
Insights: Glenn County, California. EPA monitors recorded a median Air Quality Index of 40 across 365 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 292 (80%) fell in the Good band (AQI 0–50), 73 (20%) 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 94 (Moderate). Residents averaged about 24.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (224 days), PM2.5 (117 days), PM10 (24 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +25.1 points and the median AQI moved -6 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 94, moderate but never Unhealthy. 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
80.0%
292 of 365 monitored days
5.0pp vs 75% target
Max AQI Recorded
94
Moderate
Health Grade
Good
90th-pct AQI 54
Glenn County, Good days share 80.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Glenn 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 365 monitored days across EPA AQI categories.
Good
292
80%
Moderate
73
20%
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 Glenn County.
- Ozone · 224 days · 61.4%
- PM2.5 · 117 days · 32.1%
- PM10 · 24 days · 6.6%
Health Impact Assessment
Good
Glenn County's air quality is rated Good based on 80.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 80.0% of monitored days in 2025, with a median AQI of 40. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 224 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Glenn County. Highest Good-day share: 2025 (80.0%, 292 of 365 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 200 | 135 | 7 | 19 | 46 | 221 |
| 2021 | 244 | 106 | 9 | 6 | 42 | 192 |
| 2022 | 264 | 95 | 1 | 0 | 42 | 104 |
| 2023 | 273 | 90 | 0 | 0 | 41 | 97 |
| 2024 | 292 | 74 | 0 | 0 | 39 | 80 |
| 2025 | 292 | 73 | 0 | 0 | 40 | 94 |
How does Glenn County compare?
How Glenn County compares
Good-day share better than US avg
Glenn County 80.0%
Anderson County, South Carolina 91.3%
US average 76.0%
Median AQI worse than US avg
Glenn County 40
Anderson County, South Carolina 40
US average 39
Unhealthy days / yr better than US avg
Glenn County 0
Anderson County, South Carolina 0
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Glenn County (40), not geographic neighbors.
Anderson County, South Carolina
Median AQI 40 · 91.3% Good-day share
Andrew County, Missouri
Median AQI 40 · 86.9% Good-day share
Anne Arundel County, Maryland
Median AQI 40 · 87.6% Good-day share
Bergen County, New Jersey
Median AQI 40 · 71.1% Good-day share
Blair County, Pennsylvania
Median AQI 40 · 76.9% Good-day share
Brazos County, Texas
Median AQI 40 · 68.5% 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).
Caddo County, Louisiana
80.0% Good-day share · median AQI 38
Rock County, Wisconsin
79.9% Good-day share · median AQI 42
Nevada County, California
79.8% Good-day share · median AQI 42
Cecil County, Maryland
79.8% Good-day share · median AQI 41
Monongalia County, West Virginia
79.8% Good-day share · median AQI 42
Grand County, Colorado
79.8% Good-day share · median AQI 44
California counties on the map
Geographic containment only, other monitored counties in California, independent of AQI or pollutant match.
What Glenn 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 40 means for exposure. Health guidance
- See how Glenn County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Glenn 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 Glenn County, California? ▼
Glenn County has a median Air Quality Index (AQI) of 40, rated "Good." 80.0% of monitored days (292 out of 365) had good air quality in 2025.
What is the main pollutant in Glenn County? ▼
The primary pollutant in Glenn County is Ozone (O3), which was the main AQI driver on 224 days (61% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Glenn County have? ▼
In 2025, Glenn County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 94.
Is the air quality in Glenn County getting better or worse? ▼
Air quality in Glenn County is improving across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2025 at 80.0% (292 of 365 days). Lowest qualifying Good-day share: 2020 at 55.4% (200 of 364 days).
What does AQI 40 mean for health? ▼
Glenn County's median AQI of 40 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Glenn County compare to other counties in California? ▼
Glenn County has 80.0% 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 Glenn County? ▼
The 90th percentile AQI for Glenn County is 54, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 94 (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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Glenn 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.