Median AQI
41
Good
Ohio · County air quality · 2025
Delaware County, Ohio
Median AQI 41 (Good). 82.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 41
- Median AQI
- 82.9%
- Good-air days
- 0
- Unhealthy days
- 60%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Delaware County's air is good on balance, cleaner than the median US county, a median AQI of 41 with 82.9% of 211 monitored days in the Good band, driven mainly by Ozone.
- 41
- median AQI · Good
- 82.9%
- days rated Good
- Ozone
- main pollutant
- 60%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 119.
Data updated July 2026
- Good · 175 d · 82.9%
- Moderate · 33 d · 15.6%
- USG · 3 d · 1.4%
How did Delaware County's air break down in 2025?
Across 211 monitored days, 175 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 119 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 211 days monitored. Median AQI: 41 (Good).
Insights: Delaware County, Ohio. EPA monitors recorded a median Air Quality Index of 41 across 211 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 175 (83%) fell in the Good band (AQI 0–50), 33 (16%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 61, while the worst single-day peak hit 119 (USG). Residents averaged about 24.9 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (211 days). Across the 2020–2025 trend window, good days shifted by -47 and the median AQI moved +3 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 119, usg but never Unhealthy. 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
82.9%
175 of 211 monitored days
7.9pp vs 75% target
Max AQI Recorded
119
USG
Health Grade
Good
90th-pct AQI 61
Delaware County, Good days share 82.9%
National avg
Share of monitored days at AQI 0-50 (Good band).
Delaware 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 211 monitored days across EPA AQI categories.
Good
175
83%
Moderate
33
16%
USG
3
1%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Delaware County.
- Ozone · 211 days · 100.0%
Health Impact Assessment
Good
Delaware County's air quality is rated Good based on 82.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 82.9% of monitored days in 2025, with a median AQI of 41. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 211 of 211 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Delaware County. Best year: 2022 (230 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 222 | 21 | 1 | 0 | 38 | 101 |
| 2021 | 222 | 18 | 0 | 0 | 38 | 77 |
| 2022 | 230 | 14 | 0 | 0 | 37 | 100 |
| 2023 | 211 | 33 | 1 | 0 | 41 | 108 |
| 2024 | 200 | 41 | 1 | 0 | 42 | 105 |
| 2025 | 175 | 33 | 3 | 0 | 41 | 119 |
How does Delaware County compare?
How Delaware County compares
Good-day share better than US avg
Delaware County 82.9%
Allen County, Ohio 65.9%
US average 76.5%
Median AQI worse than US avg
Delaware County 41
Allen County, Ohio 43
US average 38
Unhealthy days / yr about average
Delaware County 0
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 Delaware County.
Albemarle County, Virginia
AQI 41 · 77.9% good
Anderson County, South Carolina
AQI 41 · 90.0% good
Berkshire County, Massachusetts
AQI 41 · 71.6% good
Berrien County, Michigan
AQI 41 · 80.9% good
Blair County, Pennsylvania
AQI 41 · 71.9% good
Bonner County, Idaho
AQI 41 · 71.3% good
What Delaware 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 41 means for exposure. Health guidance
- See how Delaware County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Delaware County with its Ohio neighbours before drawing a conclusion about the region. All of Ohio
Based on 211 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 Delaware County, Ohio? ▼
Delaware County has a median Air Quality Index (AQI) of 41, rated "Good." 82.9% of monitored days (175 out of 211) had good air quality in 2025.
What is the main pollutant in Delaware County? ▼
The primary pollutant in Delaware County is Ozone (O3), which was the main AQI driver on 211 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Delaware County have? ▼
In 2025, Delaware County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 119.
Is the air quality in Delaware County getting better or worse? ▼
Air quality in Delaware County is worsening based on 5-year trends (2020-2025). Best year: 2022 with 230 good days. Worst year: 2025 with 175 good days.
What does AQI 41 mean for health? ▼
Delaware County's median AQI of 41 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 Delaware County compare to other counties in Ohio? ▼
Delaware County has 82.9% 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 Delaware County? ▼
The 90th percentile AQI for Delaware County is 61, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 119 (USG).
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.