Median AQI
46
Good
Delaware · County air quality · 2025
New Castle County, Delaware
Median AQI 46 (Good). 60.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 46
- Median AQI
- 60.4%
- Good-air days
- 1
- Unhealthy days
- 15%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, New Castle County's air is good on balance, among the more polluted 10% of US counties, a median AQI of 46 with 60.4% of 182 monitored days in the Good band, driven mainly by Ozone.
- 46
- median AQI · Good
- 60.4%
- days rated Good
- Ozone
- main pollutant
- 15%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 161.
Data updated July 2026
- Good · 110 d · 60.4%
- Moderate · 68 d · 37.4%
- USG · 3 d · 1.6%
- Unhealthy · 1 d · 0.5%
How did New Castle County's air break down in 2025?
Across 182 monitored days, 110 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 161 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 182 days monitored. Median AQI: 46 (Good).
Insights: New Castle County, Delaware. EPA monitors recorded a median Air Quality Index of 46 across 182 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 110 (60%) fell in the Good band (AQI 0–50), 68 (37%) registered Moderate readings, and 3 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 69, while the worst single-day peak hit 161 (Unhealthy). Residents averaged about 18.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (106 days), PM2.5 (75 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -121 and the median AQI moved +0 points, placing the multi-year trajectory at stable. 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: Kent (70.2% good, AQI 44); Sussex (79.6% good, AQI 43).
Air Quality Summary
Good Days
60.4%
110 of 182 monitored days
-14.6pp vs 75% target
Max AQI Recorded
161
Unhealthy
Health Grade
Moderate
90th-pct AQI 69
New Castle County, Good days share 60.4%
National avg
Share of monitored days at AQI 0-50 (Good band).
New Castle 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 182 monitored days across EPA AQI categories.
Good
110
60%
Moderate
68
37%
USG
3
2%
Unhealthy
1
1%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in New Castle County.
- Ozone · 106 days · 58.2%
- PM2.5 · 75 days · 41.2%
- CO · 1 days · 0.5%
Health Impact Assessment
Moderate
New Castle County's air quality is rated Moderate based on 60.4% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 3 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 161. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 106 of 182 monitored days.
5-Year Air Quality Trend
Air quality is stable in New Castle County. Best year: 2024 (246 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 231 | 133 | 2 | 0 | 46 | 105 |
| 2021 | 195 | 166 | 4 | 0 | 48 | 144 |
| 2022 | 208 | 157 | 0 | 0 | 48 | 93 |
| 2023 | 173 | 184 | 5 | 1 | 51 | 282 |
| 2024 | 246 | 116 | 4 | 0 | 43 | 119 |
| 2025 | 110 | 68 | 3 | 1 | 46 | 161 |
How does New Castle County compare?
How New Castle County compares
Good-day share worse than US avg
New Castle County 60.4%
Kent County, Delaware 70.2%
US average 76.5%
Median AQI worse than US avg
New Castle County 46
Kent County, Delaware 44
US average 38
Unhealthy days / yr worse than US avg
New Castle County 1
Kent County, Delaware 0
US average 0
Other Counties in Delaware
Air quality comparison for nearby counties in Delaware.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to New Castle County.
Bay County, Florida
AQI 46 · 60.6% good
Blount County, Tennessee
AQI 46 · 69.6% good
Cache County, Utah
AQI 46 · 61.2% good
Chesterfield County, South Carolina
AQI 46 · 61.2% good
Clark County, Indiana
AQI 46 · 59.7% good
Clark County, Ohio
AQI 46 · 59.4% good
What New Castle 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 46 means for exposure. Health guidance
- See how New Castle County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare New Castle County with its Delaware neighbours before drawing a conclusion about the region. All of Delaware
Based on 182 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 New Castle County, Delaware? ▼
New Castle County has a median Air Quality Index (AQI) of 46, rated "Good." 60.4% of monitored days (110 out of 182) had good air quality in 2025.
What is the main pollutant in New Castle County? ▼
The primary pollutant in New Castle County is Ozone (O3), which was the main AQI driver on 106 days (58% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does New Castle County have? ▼
In 2025, New Castle County recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 161.
Is the air quality in New Castle County getting better or worse? ▼
Air quality in New Castle County is stable based on 5-year trends (2020-2025). Best year: 2024 with 246 good days. Worst year: 2025 with 110 good days.
What does AQI 46 mean for health? ▼
New Castle County's median AQI of 46 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 New Castle County compare to other counties in Delaware? ▼
New Castle County has 60.4% good air days. Nearby counties in Delaware: Kent (70.2% good), Sussex (79.6% good).
What is the 90th percentile AQI in New Castle County? ▼
The 90th percentile AQI for New Castle County is 69, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 161 (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.