State air quality · 2025

Delaware Air Quality Ranking

Across 3 monitored counties, the typical county in Delaware posts a median AQI of 41 (Good) and 74.1% of monitored days were rated Good, from EPA AQS.

41
Avg median AQI
74.1%
Good-air days
3
Counties monitored
35%
Cleaner than US states

Delaware's typical county median AQI is 41 (Good) across 3 monitored counties

According to the U.S. EPA Air Quality System, Delaware monitors 3 counties with a day-weighted Good-day share of 74.1%. Cleanest: Sussex County (82.2% Good). Most polluted: New Castle County (64.4%). These are annual summaries, not live AirNow readings.

Data source EPA Air Quality System (AQS)

Cleaner than US states
Delaware's day-weighted Good-day share of 74.1% places it cleaner than 35% of US states.
In-state spread
Within Delaware, Sussex County leads at 82.2% Good days while New Castle County sits at 64.4% across 3 monitored counties.
2024→2025 release
Delaware's Good-day share moved -0.4 points vs the 2024 EPA annual release.
Typical median AQI
Typical median AQI: 41

Verify with AirNow → · EPA AQS →

EPA state monitoring brief

AQS state key DELAWARE

Delaware · 2025 EPA AQS · 3 counties · med AQI 41 · 74.1% Good · Sussex→New Castle spread

  • GOOD 41 Median AQI · Good
  • SHARE 74.1% Good-day share
  • COUNT 3 Counties monitored
  • UNH 1 Unhealthy+ days
  • RANK 35% Cleaner than states
  • O3 64.0% Main pollutant share

Good-day share peers

Louisiana74.6%Delaware (this)74.1%Alabama73.9%Pennsylvania73.8%
Good-day share neighbourhood · nearest ABS peers vs Delaware

Delaware's 74.1% Good-day share is cleaner than 35% of monitored US states; Sussex and New Castle County bracket the in-state range. Open Sussex County →

According to EPA AQS annual summaries on PlainAirData, Delaware ranks #34 of 52 by day-weighted good-day share (74.1%) and #38 by mean county median AQI (41.3). Hawaii is #1 of 52 by day-weighted good-day share (93.8% of monitored days) at mean county median-AQI #4 (27.3) ≠ Alaska #1 lowest mean county median AQI (20.3) at good-day share #8 (84.4%).

Data updated August 2026

What changed, what drives it, and how thick is the sample

Three derived views WPR-style word count does not substitute for: release-over-release movement from county annual rows, the EPA main-pollutant day mix, and how many counties sit below the 100-day ranking floor.

Release change

74.1%

Good-day share in 2025, down 0.4 percentage points vs 2024. County-mean median AQI 41, within half a point of the 2024 county-mean median.

Pollutant mix

Ozone

Main driver on 64.0% of pollutant-assigned days.

  • Ozone 64.0%
  • PM2.5 36.0%

Monitoring coverage

365

Average monitored days per county in the latest release. Every monitored county clears the 100-day ranking floor.

Counties Monitored

3

Good Days %

74.1%

Avg Median AQI

41

Total Unhealthy Days

1

Best Air Quality

Sussex County

Worst Air Quality

New Castle County

Where Delaware sits nationally

COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. Delaware's day-weighted Good-day share is 74.1% - cleaner than 35% of that comparison set.

Delaware Good-day share vs. every US state

Day-weighted share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release

74.1% 35th percentile higher than 35% of 52 US states

0.0%–5.0%: 0 US states (0%). Below this entry. 5.0%–10.0%: 0 US states (0%). Below this entry. 10.0%–15.0%: 0 US states (0%). Below this entry. 15.0%–20.0%: 0 US states (0%). Below this entry. 20.0%–25.0%: 0 US states (0%). Below this entry. 25.0%–30.0%: 0 US states (0%). Below this entry. 30.0%–35.0%: 0 US states (0%). Below this entry. 35.0%–40.0%: 0 US states (0%). Below this entry. 40.0%–45.0%: 0 US states (0%). Below this entry. 45.0%–50.0%: 0 US states (0%). Below this entry. 50.0%–55.0%: 0 US states (0%). Below this entry. 55.0%–60.0%: 0 US states (0%). Below this entry. 60.0%–65.0%: 2 US states (4%). Below this entry. 65.0%–70.0%: 9 US states (17%). Below this entry. 70.0%–75.0%: 11 US states (21%). This entry sits in this band. 75.0%–80.0%: 11 US states (21%). Above this entry. 80.0%–85.0%: 13 US states (25%). Above this entry. 85.0%–90.0%: 5 US states (10%). Above this entry. 90.0%–95.0%: 1 US states (2%). Above this entry. 95.0%–100.0%: 0 US states (0%). Above this entry. 100.0%–105.0%: 0 US states (0%). Above this entry. This state 0.0% 100% every floor-qualified US state, bucketed by value

Each bar is a band; taller bars hold more US states. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.

Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025

Which counties in Delaware have the cleanest and most polluted air?

Cleanest counties in Delaware

Share of monitored days rated Good (AQI 0–50)

  1. 1 Sussex County 82.2%
  2. 2 Kent County 75.6%
  3. 3 New Castle County 64.4%

Good-day share · EPA Air Quality System

County Good % Median AQI
Sussex 82.2% 39
Kent 75.6% 41
New Castle 64.4% 44

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025. See our methodology for how AQI and median figures are computed.

Coverage: 3 counties in Delaware with active monitoring stations. Unmonitored counties are not represented.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files, aggregated to the state level · 2020–2025

Every figure on PlainAirData 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.

Informational only, not professional advice. Data from EPA AQS. See our disclaimer.