Median AQI
37
Good
Connecticut · County air quality · 2025
Windham County, Connecticut
Median AQI 37 (Good). 90.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 37
- Median AQI
- 90.9%
- Good-air days
- 0
- Unhealthy days
- 78%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Windham County's air is good on balance, cleaner than most US counties, a median AQI of 37 with 90.9% of 241 monitored days in the Good band, driven mainly by Ozone.
- 37
- median AQI · Good
- 90.9%
- days rated Good
- Ozone
- main pollutant
- 78%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 108.
Data updated July 2026
- Good · 219 d · 90.9%
- Moderate · 21 d · 8.7%
- USG · 1 d · 0.4%
How did Windham County's air break down in 2025?
Across 241 monitored days, 219 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 108 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 241 days monitored. Median AQI: 37 (Good).
Insights: Windham County, Connecticut. EPA monitors recorded a median Air Quality Index of 37 across 241 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 219 (91%) fell in the Good band (AQI 0–50), 21 (9%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 50, while the worst single-day peak hit 108 (USG). Residents averaged about 27.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (241 days). Across the 2020–2025 trend window, good days shifted by -126 and the median AQI moved +2 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 108, usg but never Unhealthy. Nearby counties show comparable readings: Fairfield (66.8% good, AQI 44); Hartford (66.8% good, AQI 44); Litchfield (81.8% good, AQI 41); Middlesex (77.9% good, AQI 42); New Haven (68.5% good, AQI 44); New London (79.0% good, AQI 40).
Air Quality Summary
Good Days
90.9%
219 of 241 monitored days
15.9pp vs 75% target
Max AQI Recorded
108
USG
Health Grade
Excellent
90th-pct AQI 50
Windham County, Good days share 90.9%
National avg
Share of monitored days at AQI 0-50 (Good band).
Windham 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 241 monitored days across EPA AQI categories.
Good
219
91%
Moderate
21
9%
USG
1
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Windham County.
- Ozone · 241 days · 100.0%
Health Impact Assessment
Excellent
Windham County's air quality is rated Excellent based on 90.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 90.9% of monitored days in 2025, with a median AQI of 37. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 241 of 241 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Windham County. Best year: 2020 (345 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 345 | 11 | 0 | 0 | 35 | 87 |
| 2021 | 331 | 29 | 1 | 0 | 34 | 140 |
| 2022 | 338 | 17 | 0 | 0 | 35 | 93 |
| 2023 | 340 | 17 | 2 | 0 | 34 | 112 |
| 2024 | 340 | 17 | 2 | 0 | 34 | 122 |
| 2025 | 219 | 21 | 1 | 0 | 37 | 108 |
How does Windham County compare?
How Windham County compares
Good-day share better than US avg
Windham County 90.9%
Fairfield County, Connecticut 66.8%
US average 76.5%
Median AQI better than US avg
Windham County 37
Fairfield County, Connecticut 44
US average 38
Unhealthy days / yr about average
Windham County 0
Fairfield County, Connecticut 1
US average 0
Other Counties in Connecticut
Air quality comparison for nearby counties in Connecticut.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Windham County.
Alachua County, Florida
AQI 37 · 90.1% good
Beltrami County, Minnesota
AQI 37 · 88.8% good
Benewah County, Idaho
AQI 37 · 74.6% good
Brevard County, Florida
AQI 37 · 89.7% good
Brown County, Indiana
AQI 37 · 94.5% good
Brown County, Wisconsin
AQI 37 · 79.0% good
What Windham 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 37 means for exposure. Health guidance
- See how Windham County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Windham County with its Connecticut neighbours before drawing a conclusion about the region. All of Connecticut
Based on 241 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 Windham County, Connecticut? ▼
Windham County has a median Air Quality Index (AQI) of 37, rated "Good." 90.9% of monitored days (219 out of 241) had good air quality in 2025.
What is the main pollutant in Windham County? ▼
The primary pollutant in Windham County is Ozone (O3), which was the main AQI driver on 241 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Windham County have? ▼
In 2025, Windham County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 108.
Is the air quality in Windham County getting better or worse? ▼
Air quality in Windham County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 345 good days. Worst year: 2025 with 219 good days.
What does AQI 37 mean for health? ▼
Windham County's median AQI of 37 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 Windham County compare to other counties in Connecticut? ▼
Windham County has 90.9% good air days. Nearby counties in Connecticut: Fairfield (66.8% good), Hartford (66.8% good), Litchfield (81.8% good), Middlesex (77.9% good), New Haven (68.5% good), New London (79.0% good).
What is the 90th percentile AQI in Windham County? ▼
The 90th percentile AQI for Windham County is 50, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 108 (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.