Median AQI
40
Good
Ohio · County air quality · 2025
Knox County, Ohio
Median AQI 40 (Good). 84.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 84.1%
- Good-air days
- 1
- Unhealthy days
- 63%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Knox County's air is good on balance, cleaner than the median US county, a median AQI of 40 with 84.1% of 214 monitored days in the Good band, driven mainly by Ozone.
- 40
- median AQI · Good
- 84.1%
- days rated Good
- Ozone
- main pollutant
- 63%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 159.
Data updated July 2026
- Good · 180 d · 84.1%
- Moderate · 32 d · 15.0%
- USG · 1 d · 0.5%
- Unhealthy · 1 d · 0.5%
How did Knox County's air break down in 2025?
Across 214 monitored days, 180 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 159 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 214 days monitored. Median AQI: 40 (Good).
Insights: Knox County, Ohio. EPA monitors recorded a median Air Quality Index of 40 across 214 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 180 (84%) fell in the Good band (AQI 0–50), 32 (15%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 54, while the worst single-day peak hit 159 (Unhealthy). Residents averaged about 25.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (214 days). Across the 2020–2025 trend window, good days shifted by -46 and the median AQI moved +2 points, placing the multi-year trajectory at worsening. Residents and visitors experienced 1 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Allen (65.9% good, AQI 43); Ashtabula (81.2% good, AQI 43); Athens (80.3% good, AQI 33); Belmont (63.0% good, AQI 42); Butler (33.3% good, AQI 55); Clark (59.4% good, AQI 46).
Air Quality Summary
Good Days
84.1%
180 of 214 monitored days
9.1pp vs 75% target
Max AQI Recorded
159
Unhealthy
Health Grade
Good
90th-pct AQI 54
Knox County, Good days share 84.1%
National avg
Share of monitored days at AQI 0-50 (Good band).
Knox County, Unhealthy-or-worse share 0.5%
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 214 monitored days across EPA AQI categories.
Good
180
84%
Moderate
32
15%
USG
1
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 Knox County.
- Ozone · 214 days · 100.0%
Health Impact Assessment
Good
Knox County's air quality is rated Good based on 84.1% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 1 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 159. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 214 of 214 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Knox County. Best year: 2020 (226 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 226 | 19 | 0 | 0 | 38 | 80 |
| 2021 | 225 | 20 | 0 | 0 | 38 | 97 |
| 2022 | 225 | 19 | 1 | 0 | 39 | 119 |
| 2023 | 200 | 44 | 1 | 0 | 42 | 105 |
| 2024 | 208 | 36 | 1 | 0 | 41 | 101 |
| 2025 | 180 | 32 | 1 | 1 | 40 | 159 |
How does Knox County compare?
How Knox County compares
Good-day share better than US avg
Knox County 84.1%
Allen County, Ohio 65.9%
US average 76.5%
Median AQI worse than US avg
Knox County 40
Allen County, Ohio 43
US average 38
Unhealthy days / yr worse than US avg
Knox County 1
Allen County, Ohio 0
US average 0
Other Counties in Ohio
Air quality comparison for nearby counties in Ohio.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Knox County.
Amador County, California
AQI 40 · 84.8% good
Anne Arundel County, Maryland
AQI 40 · 87.2% good
Arlington County, Virginia
AQI 40 · 84.8% good
Bergen County, New Jersey
AQI 40 · 71.1% good
Berkeley County, West Virginia
AQI 40 · 81.1% good
Billings County, North Dakota
AQI 40 · 80.3% good
What Knox 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 40 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 Ohio neighbours before drawing a conclusion about the region. All of Ohio
Based on 214 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, Ohio? ▼
Knox County has a median Air Quality Index (AQI) of 40, rated "Good." 84.1% of monitored days (180 out of 214) 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 214 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 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 159.
Is the air quality in Knox County getting better or worse? ▼
Air quality in Knox County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 226 good days. Worst year: 2025 with 180 good days.
What does AQI 40 mean for health? ▼
Knox County's median AQI of 40 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 Ohio? ▼
Knox County has 84.1% good air days. Nearby counties in Ohio: Allen (65.9% good), Ashtabula (81.2% good), Athens (80.3% good), Belmont (63.0% good), Butler (33.3% good), Clark (59.4% good).
What is the 90th percentile AQI in Knox County? ▼
The 90th percentile AQI for Knox County is 54, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 159 (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.
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.