Median AQI
36
Good
California · County air quality · 2025
Humboldt County, California
Median AQI 36 (Good). 83.2% of monitored days rated Good, direct from EPA Air Quality System stations.
- 36
- Median AQI
- 83.2%
- Good-air days
- 0
- Unhealthy days
- 61%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Humboldt County's air is good on balance, cleaner than the median US county, a median AQI of 36 with 83.2% of 303 monitored days in the Good band, driven mainly by PM2.5.
- 36
- median AQI · Good
- 83.2%
- days rated Good
- PM2.5
- main pollutant
- 61%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 75.
Data updated July 2026
- Good · 252 d · 83.2%
- Moderate · 51 d · 16.8%
How did Humboldt County's air break down in 2025?
Across 303 monitored days, 252 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 75 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 303 days monitored. Median AQI: 36 (Good).
Insights: Humboldt County, California. EPA monitors recorded a median Air Quality Index of 36 across 303 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 252 (83%) fell in the Good band (AQI 0–50), 51 (17%) 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 75 (Moderate). Residents averaged about 25.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (206 days), Ozone (87 days), PM10 (10 days). Across the 2020–2025 trend window, good days shifted by -66 and the median AQI moved +7 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 75, 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
83.2%
252 of 303 monitored days
8.2pp vs 75% target
Max AQI Recorded
75
Moderate
Health Grade
Good
90th-pct AQI 54
Humboldt County, Good days share 83.2%
National avg
Share of monitored days at AQI 0-50 (Good band).
Humboldt 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 303 monitored days across EPA AQI categories.
Good
252
83%
Moderate
51
17%
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 Humboldt County.
- PM2.5 · 206 days · 68.0%
- Ozone · 87 days · 28.7%
- PM10 · 10 days · 3.3%
Health Impact Assessment
Good
Humboldt County's air quality is rated Good based on 83.2% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 83.2% of monitored days in 2025, with a median AQI of 36. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 206 of 303 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Humboldt County. Best year: 2022 (340 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 318 | 44 | 3 | 1 | 29 | 180 |
| 2021 | 333 | 32 | 0 | 0 | 29 | 64 |
| 2022 | 340 | 25 | 0 | 0 | 24 | 74 |
| 2023 | 284 | 70 | 6 | 1 | 32 | 299 |
| 2024 | 266 | 100 | 0 | 0 | 38 | 86 |
| 2025 | 252 | 51 | 0 | 0 | 36 | 75 |
How does Humboldt County compare?
How Humboldt County compares
Good-day share better than US avg
Humboldt County 83.2%
Amador County, California 84.8%
US average 76.5%
Median AQI better than US avg
Humboldt County 36
Amador County, California 40
US average 38
Unhealthy days / yr about average
Humboldt 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 Humboldt County.
Aroostook County, Maine
AQI 36 · 84.1% good
Berkeley County, South Carolina
AQI 36 · 92.2% good
Carter County, Kentucky
AQI 36 · 89.8% good
Claiborne County, Tennessee
AQI 36 · 96.3% good
Clark County, Illinois
AQI 36 · 92.1% good
Cobb County, Georgia
AQI 36 · 92.5% good
What Humboldt 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 36 means for exposure. Health guidance
- See how Humboldt County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Humboldt County with its California neighbours before drawing a conclusion about the region. All of California
Based on 303 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 Humboldt County, California? ▼
Humboldt County has a median Air Quality Index (AQI) of 36, rated "Good." 83.2% of monitored days (252 out of 303) had good air quality in 2025.
What is the main pollutant in Humboldt County? ▼
The primary pollutant in Humboldt County is Fine Particles (PM2.5), which was the main AQI driver on 206 days (68% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Humboldt County have? ▼
In 2025, Humboldt County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 75.
Is the air quality in Humboldt County getting better or worse? ▼
Air quality in Humboldt County is worsening based on 5-year trends (2020-2025). Best year: 2022 with 340 good days. Worst year: 2025 with 252 good days.
What does AQI 36 mean for health? ▼
Humboldt County's median AQI of 36 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 Humboldt County compare to other counties in California? ▼
Humboldt County has 83.2% 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 Humboldt County? ▼
The 90th percentile AQI for Humboldt County is 54, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 75 (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.