State air quality · 2025

Connecticut Air Quality Ranking

Across 8 monitored counties, the typical county in Connecticut posts a median AQI of 40 (Good) and 76.7% of monitored days were rated Good, from EPA AQS.

40
Avg median AQI
76.7%
Good-air days
8
Counties monitored
50%
Cleaner than US states

Connecticut's typical county median AQI is 40 (Good) across 8 monitored counties

According to the U.S. EPA Air Quality System, Connecticut monitors 8 counties with a day-weighted Good-day share of 76.7%. Cleanest: Windham County (93.7% Good). Most polluted: Fairfield County (64.7%). These are annual summaries, not live AirNow readings.

Data source EPA Air Quality System (AQS)

Cleaner than US states
Connecticut's day-weighted Good-day share of 76.7% places it cleaner than 50% of US states.
In-state spread
Within Connecticut, Windham County leads at 93.7% Good days while Fairfield County sits at 64.7% across 8 monitored counties.
2024→2025 release
Connecticut's Good-day share moved -4.3 points vs the 2024 EPA annual release.
Typical median AQI
Typical median AQI: 40

Verify with AirNow → · EPA AQS →

EPA state monitoring brief

AQS state key CONNECTICUT

Connecticut · 2025 EPA AQS · 8 counties · med AQI 40 · 76.7% Good · Windham→Fairfield spread

  • GOOD 40 Median AQI · Good
  • SHARE 76.7% Good-day share
  • COUNT 8 Counties monitored
  • UNH 8 Unhealthy+ days
  • RANK 50% Cleaner than states
  • O3 71.5% Main pollutant share

Good-day share peers

Oregon76.9%Minnesota76.8%Connecticut (this)76.7%Nevada76.5%
Good-day share neighbourhood · nearest ABS peers vs Connecticut

Connecticut's 76.7% Good-day share is cleaner than 50% of monitored US states; Windham and Fairfield County bracket the in-state range. Open Windham County →

According to EPA AQS annual summaries on PlainAirData, Connecticut ranks #26 of 52 by day-weighted good-day share (76.7%) and #34 by mean county median AQI (40.4). 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

76.7%

Good-day share in 2025, down 4.3 percentage points vs 2024. County-mean median AQI 40, 1.9 points higher (worse) than 2024.

Pollutant mix

Ozone

Main driver on 71.5% of pollutant-assigned days.

  • Ozone 71.5%
  • PM2.5 26.6%
  • NO2 1.9%

Monitoring coverage

326

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

Counties Monitored

8

Good Days %

76.7%

Avg Median AQI

40

Total Unhealthy Days

8

Best Air Quality

Windham County

Worst Air Quality

Fairfield County

Where Connecticut sits nationally

COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. Connecticut's day-weighted Good-day share is 76.7% - cleaner than 50% of that comparison set.

Connecticut Good-day share vs. every US state

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

76.7% Cleaner than 50% higher than 50% 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%). Below this entry. 75.0%–80.0%: 11 US states (21%). This entry sits in this band. 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 Connecticut have the cleanest and most polluted air?

Cleanest counties in Connecticut

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

  1. 1 Windham County 93.7%
  2. 2 Tolland County 83.7%
  3. 3 Litchfield County 81.4%
  4. 4 New London County 81.3%
  5. 5 Middlesex County 77.5%
  6. 6 Hartford County 68%
  7. 7 New Haven County 67.1%
  8. 8 Fairfield County 64.7%

Good-day share · EPA Air Quality System

County Good % Median AQI
Windham 93.7% 35
Tolland 83.7% 40
Litchfield 81.4% 39
New London 81.3% 38
Middlesex 77.5% 42
Hartford 68.0% 42
New Haven 67.1% 43
Fairfield 64.7% 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: 8 counties in Connecticut 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.