Median AQI
44
Good
Pennsylvania · County air quality · 2025
Delaware County, Pennsylvania
Median AQI 44 (Good). 65.6% of monitored days rated Good, direct from EPA Air Quality System stations.
- 44
- Median AQI
- 65.6%
- Good-air days
- 1
- Unhealthy days
- 23%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Delaware County's air is good on balance, among the more polluted 10% of US counties, a median AQI of 44 with 65.6% of 212 monitored days in the Good band, driven mainly by PM2.5.
- 44
- median AQI · Good
- 65.6%
- days rated Good
- PM2.5
- main pollutant
- 23%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 153.
Data updated July 2026
- Good · 139 d · 65.6%
- Moderate · 70 d · 33.0%
- USG · 2 d · 0.9%
- Unhealthy · 1 d · 0.5%
How did Delaware County's air break down in 2025?
Across 212 monitored days, 139 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 153 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 212 days monitored. Median AQI: 44 (Good).
Insights: Delaware County, Pennsylvania. EPA monitors recorded a median Air Quality Index of 44 across 212 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 139 (66%) fell in the Good band (AQI 0–50), 70 (33%) registered Moderate readings, and 2 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 66, while the worst single-day peak hit 153 (Unhealthy). Residents averaged about 19.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (131 days), Ozone (81 days). Across the 2020–2025 trend window, good days shifted by -35 and the median AQI moved -8 points, placing the multi-year trajectory at improving. 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: Adams (73.8% good, AQI 42); Allegheny (51.6% good, AQI 49); Armstrong (68.1% good, AQI 42); Beaver (54.3% good, AQI 47); Berks (65.6% good, AQI 44); Blair (71.9% good, AQI 41).
Air Quality Summary
Good Days
65.6%
139 of 212 monitored days
-9.4pp vs 75% target
Max AQI Recorded
153
Unhealthy
Health Grade
Moderate
90th-pct AQI 66
Delaware County, Good days share 65.6%
National avg
Share of monitored days at AQI 0-50 (Good band).
Delaware 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 212 monitored days across EPA AQI categories.
Good
139
66%
Moderate
70
33%
USG
2
1%
Unhealthy
1
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Delaware County.
- PM2.5 · 131 days · 61.8%
- Ozone · 81 days · 38.2%
Health Impact Assessment
Moderate
Delaware County's air quality is rated Moderate based on 65.6% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 2 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 153. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 131 of 212 monitored days.
5-Year Air Quality Trend
Air quality is improving in Delaware County. Best year: 2024 (250 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 174 | 189 | 3 | 0 | 52 | 129 |
| 2021 | 182 | 180 | 3 | 0 | 51 | 142 |
| 2022 | 191 | 174 | 0 | 0 | 49 | 95 |
| 2023 | 187 | 169 | 5 | 2 | 50 | 271 |
| 2024 | 250 | 114 | 2 | 0 | 43 | 136 |
| 2025 | 139 | 70 | 2 | 1 | 44 | 153 |
How does Delaware County compare?
How Delaware County compares
Good-day share worse than US avg
Delaware County 65.6%
Adams County, Pennsylvania 73.8%
US average 76.5%
Median AQI worse than US avg
Delaware County 44
Adams County, Pennsylvania 42
US average 38
Unhealthy days / yr worse than US avg
Delaware County 1
Adams County, Pennsylvania 0
US average 0
Other Counties in Pennsylvania
Air quality comparison for nearby counties in Pennsylvania.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Delaware County.
Baltimore County, Maryland
AQI 44 · 70.4% good
Baltimore (City) County, Maryland
AQI 44 · 68.7% good
Bartholomew County, Indiana
AQI 44 · 69.2% good
Berks County, Pennsylvania
AQI 44 · 65.6% good
Bolivar County, Mississippi
AQI 44 · 70.3% good
Brewster County, Texas
AQI 44 · 69.0% good
What Delaware County's air means for you
On most days the air here is fine, but PM2.5 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-PM2.5 days, the guide explains what a median of 44 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 Pennsylvania neighbours before drawing a conclusion about the region. All of Pennsylvania
Based on 212 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, Pennsylvania? ▼
Delaware County has a median Air Quality Index (AQI) of 44, rated "Good." 65.6% of monitored days (139 out of 212) had good air quality in 2025.
What is the main pollutant in Delaware County? ▼
The primary pollutant in Delaware County is Fine Particles (PM2.5), which was the main AQI driver on 131 days (62% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Delaware County have? ▼
In 2025, Delaware County recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 153.
Is the air quality in Delaware County getting better or worse? ▼
Air quality in Delaware County is improving based on 5-year trends (2020-2025). Best year: 2024 with 250 good days. Worst year: 2025 with 139 good days.
What does AQI 44 mean for health? ▼
Delaware County's median AQI of 44 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 Pennsylvania? ▼
Delaware County has 65.6% good air days. Nearby counties in Pennsylvania: Adams (73.8% good), Allegheny (51.6% good), Armstrong (68.1% good), Beaver (54.3% good), Berks (65.6% good), Blair (71.9% good).
What is the 90th percentile AQI in Delaware County? ▼
The 90th percentile AQI for Delaware County is 66, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 153 (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.