Median AQI
42
Good
Connecticut · County air quality · 2025
Middlesex County, Connecticut
Median AQI 42 (Good). 77.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 42
- Median AQI
- 77.9%
- Good-air days
- 1
- Unhealthy days
- 46%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Middlesex County's air is good on balance, more polluted than the median US county, a median AQI of 42 with 77.9% of 122 monitored days in the Good band, driven mainly by Ozone.
- 42
- median AQI · Good
- 77.9%
- days rated Good
- Ozone
- main pollutant
- 46%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 151.
Data updated July 2026
- Good · 95 d · 77.9%
- Moderate · 25 d · 20.5%
- USG · 1 d · 0.8%
- Unhealthy · 1 d · 0.8%
How did Middlesex County's air break down in 2025?
Across 122 monitored days, 95 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 151 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 122 days monitored. Median AQI: 42 (Good).
Insights: Middlesex County, Connecticut. EPA monitors recorded a median Air Quality Index of 42 across 122 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 95 (78%) fell in the Good band (AQI 0–50), 25 (20%) registered Moderate readings, and 1 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 77, while the worst single-day peak hit 151 (Unhealthy). Residents averaged about 23.4 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (122 days). Across the 2020–2025 trend window, good days shifted by -87 and the median AQI moved +3 points, placing the multi-year trajectory at worsening. 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: Fairfield (66.8% good, AQI 44); Hartford (66.8% good, AQI 44); Litchfield (81.8% good, AQI 41); New Haven (68.5% good, AQI 44); New London (79.0% good, AQI 40); Tolland (79.8% good, AQI 42).
Air Quality Summary
Good Days
77.9%
95 of 122 monitored days
2.9pp vs 75% target
Max AQI Recorded
151
Unhealthy
Health Grade
Good
90th-pct AQI 77
Middlesex County, Good days share 77.9%
National avg
Share of monitored days at AQI 0-50 (Good band).
Middlesex County, Unhealthy-or-worse share 0.8%
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 122 monitored days across EPA AQI categories.
Good
95
78%
Moderate
25
20%
USG
1
1%
Unhealthy
1
1%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Middlesex County.
- Ozone · 122 days · 100.0%
Health Impact Assessment
Good
Middlesex County's air quality is rated Good based on 77.9% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 1 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 151. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is Ozone, driving AQI on 122 of 122 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Middlesex County. Best year: 2020 (182 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 182 | 24 | 2 | 0 | 39 | 129 |
| 2021 | 171 | 25 | 10 | 1 | 39 | 151 |
| 2022 | 173 | 33 | 6 | 0 | 40 | 147 |
| 2023 | 170 | 35 | 4 | 0 | 43 | 140 |
| 2024 | 178 | 25 | 5 | 1 | 40 | 177 |
| 2025 | 95 | 25 | 1 | 1 | 42 | 151 |
How does Middlesex County compare?
How Middlesex County compares
Good-day share better than US avg
Middlesex County 77.9%
Fairfield County, Connecticut 66.8%
US average 76.5%
Median AQI worse than US avg
Middlesex County 42
Fairfield County, Connecticut 44
US average 38
Unhealthy days / yr worse than US avg
Middlesex County 1
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 Middlesex County.
Adams County, Pennsylvania
AQI 42 · 73.8% good
Anderson County, Tennessee
AQI 42 · 91.0% good
Andrew County, Missouri
AQI 42 · 84.3% good
Armstrong County, Pennsylvania
AQI 42 · 68.1% good
Baldwin County, Alabama
AQI 42 · 72.2% good
Belmont County, Ohio
AQI 42 · 63.0% good
What Middlesex 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 42 means for exposure. Health guidance
- See how Middlesex County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Middlesex County with its Connecticut neighbours before drawing a conclusion about the region. All of Connecticut
Based on 122 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 Middlesex County, Connecticut? ▼
Middlesex County has a median Air Quality Index (AQI) of 42, rated "Good." 77.9% of monitored days (95 out of 122) had good air quality in 2025.
What is the main pollutant in Middlesex County? ▼
The primary pollutant in Middlesex County is Ozone (O3), which was the main AQI driver on 122 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Middlesex County have? ▼
In 2025, Middlesex County recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 151.
Is the air quality in Middlesex County getting better or worse? ▼
Air quality in Middlesex County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 182 good days. Worst year: 2025 with 95 good days.
What does AQI 42 mean for health? ▼
Middlesex County's median AQI of 42 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 Middlesex County compare to other counties in Connecticut? ▼
Middlesex County has 77.9% good air days. Nearby counties in Connecticut: Fairfield (66.8% good), Hartford (66.8% good), Litchfield (81.8% good), New Haven (68.5% good), New London (79.0% good), Tolland (79.8% good).
What is the 90th percentile AQI in Middlesex County? ▼
The 90th percentile AQI for Middlesex County is 77, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 151 (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.