Median AQI
34
Good
Maine · County air quality · 2025
Knox County, Maine
Median AQI 34 (Good). 97.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 34
- Median AQI
- 97.3%
- Good-air days
- 0
- Unhealthy days
- 94%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Knox County's air is good on balance, among the cleanest 10% of US counties, a median AQI of 34 with 97.3% of 301 monitored days in the Good band, driven mainly by Ozone.
- 34
- median AQI · Good
- 97.3%
- days rated Good
- Ozone
- main pollutant
- 94%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 77.
Data updated July 2026
- Good · 293 d · 97.3%
- Moderate · 8 d · 2.7%
How did Knox County's air break down in 2025?
Across 301 monitored days, 293 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 77 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 301 days monitored. Median AQI: 34 (Good).
Insights: Knox County, Maine. EPA monitors recorded a median Air Quality Index of 34 across 301 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 293 (97%) fell in the Good band (AQI 0–50), 8 (3%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 44, while the worst single-day peak hit 77 (Moderate). Residents averaged about 29.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (301 days). Across the 2020–2025 trend window, good days shifted by +151 and the median AQI moved +3 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 77, moderate but never Unhealthy. Nearby counties show comparable readings: Androscoggin (85.5% good, AQI 35); Aroostook (84.1% good, AQI 36); Cumberland (74.0% good, AQI 39); Hancock (89.1% good, AQI 37); Kennebec (83.5% good, AQI 36); Oxford (74.0% good, AQI 39).
Air Quality Summary
Good Days
97.3%
293 of 301 monitored days
22.3pp vs 75% target
Max AQI Recorded
77
Moderate
Health Grade
Excellent
90th-pct AQI 44
Knox County, Good days share 97.3%
National avg
Share of monitored days at AQI 0-50 (Good band).
Knox 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 301 monitored days across EPA AQI categories.
Good
293
97%
Moderate
8
3%
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 Knox County.
- Ozone · 301 days · 100.0%
Health Impact Assessment
Excellent
Knox County's air quality is rated Excellent based on 97.3% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 97.3% of monitored days in 2025, with a median AQI of 34. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 301 of 301 monitored days.
5-Year Air Quality Trend
Air quality is stable in Knox County. Best year: 2024 (343 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 142 | 6 | 0 | 0 | 31 | 71 |
| 2021 | 192 | 17 | 2 | 0 | 35 | 119 |
| 2022 | 206 | 6 | 0 | 0 | 35 | 80 |
| 2023 | 296 | 5 | 0 | 0 | 33 | 87 |
| 2024 | 343 | 12 | 0 | 0 | 33 | 84 |
| 2025 | 293 | 8 | 0 | 0 | 34 | 77 |
How does Knox County compare?
How Knox County compares
Good-day share better than US avg
Knox County 97.3%
Androscoggin County, Maine 85.5%
US average 76.5%
Median AQI better than US avg
Knox County 34
Androscoggin County, Maine 35
US average 38
Unhealthy days / yr about average
Knox County 0
Androscoggin County, Maine 0
US average 0
Other Counties in Maine
Air quality comparison for nearby counties in Maine.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Knox County.
Belknap County, New Hampshire
AQI 34 · 87.9% good
Dyer County, Tennessee
AQI 34 · 79.4% good
Flagler County, Florida
AQI 34 · 97.4% good
Grant County, Oregon
AQI 34 · 66.7% good
Highlands County, Florida
AQI 34 · 93.2% good
Johnson County, Texas
AQI 34 · 91.6% good
What Knox 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 34 means for exposure. Health guidance
- See how Knox County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Knox County with its Maine neighbours before drawing a conclusion about the region. All of Maine
Based on 301 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 Knox County, Maine? ▼
Knox County has a median Air Quality Index (AQI) of 34, rated "Good." 97.3% of monitored days (293 out of 301) had good air quality in 2025.
What is the main pollutant in Knox County? ▼
The primary pollutant in Knox County is Ozone (O3), which was the main AQI driver on 301 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Knox County have? ▼
In 2025, Knox County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 77.
Is the air quality in Knox County getting better or worse? ▼
Air quality in Knox County is stable based on 5-year trends (2020-2025). Best year: 2024 with 343 good days. Worst year: 2020 with 142 good days.
What does AQI 34 mean for health? ▼
Knox County's median AQI of 34 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 Knox County compare to other counties in Maine? ▼
Knox County has 97.3% good air days. Nearby counties in Maine: Androscoggin (85.5% good), Aroostook (84.1% good), Cumberland (74.0% good), Hancock (89.1% good), Kennebec (83.5% good), Oxford (74.0% good).
What is the 90th percentile AQI in Knox County? ▼
The 90th percentile AQI for Knox County is 44, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 77 (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.