Median AQI
42
Good
Connecticut · County air quality · 2025
Middlesex County, Connecticut
Median AQI 42 (Good). 77.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 42
- Median AQI
- 77.5%
- Good-air days
- 1
- Unhealthy days
- 49%
- Cleaner than US counties
Middlesex County's 2025 annual median AQI is 42 (Good), with 77.5% Good days
According to the U.S. EPA Air Quality System, Middlesex County, Connecticut posted a 2025 median AQI of 42 (Good) across 213 monitored days, with 77.5% rated Good and 1 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Middlesex County's 77.5% Good-day share places it cleaner than 49% of US counties in 2025.
- Closest median-AQI peer
- Ada County (74.0% Good, median AQI 42) is the closest nationwide median-AQI peer to Middlesex County.
- 2020–2025 trend
- Middlesex County's floor-qualifying Good-day share is worsening from 2020 to 2025 (87.5% → 77.5%).
- Median AQI
- Median AQI: 42
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 77.5% of 213 monitored days in the Good band, driven mainly by Ozone.
- 42
- median AQI · Good
- 77.5%
- days rated Good
- Ozone
- main pollutant
- 49%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 151.
Annual release change
What changed in the 2025 EPA annual release
- Middlesex County's Good-day share fell from 85.2% to 77.5% between the 2024 and 2025 EPA annual releases, a change of -7.7 percentage points.
- Middlesex County's peak single-day AQI fell from 177 to 151 between the 2024 and 2025 EPA annual releases, a change of -26 points.
Data updated August 2026
- Good · 165 d · 77.5%
- Moderate · 42 d · 19.7%
- USG · 5 d · 2.3%
- Unhealthy · 1 d · 0.5%
How did Middlesex County's air break down in 2025?
Across 213 monitored days, 165 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). 213 days monitored. Median AQI: 42 (Good).
Insights: Middlesex County, Connecticut. EPA monitors recorded a median Air Quality Index of 42 across 213 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 165 (77%) fell in the Good band (AQI 0–50), 42 (20%) registered Moderate readings, and 5 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 77, while the worst single-day peak hit 151 (Unhealthy). Residents averaged about 23.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (213 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -10.0 points and the median AQI moved +3 points, placing the multi-year trajectory at worsening. The annual record contains 1 day at Unhealthy or worse. On that day, the EPA recommends that sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Fairfield (64.7% good, AQI 44); Hartford (68.0% good, AQI 42); Litchfield (81.4% good, AQI 39); New Haven (67.1% good, AQI 43); New London (81.3% good, AQI 38); Tolland (83.7% good, AQI 40).
Air Quality Summary
Good Days
77.5%
165 of 213 monitored days
2.5pp vs 75% target
Max AQI Recorded
151
Unhealthy
Health Grade
Good
90th-pct AQI 77
Middlesex County, Good days share 77.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Middlesex County, Unhealthy-or-worse share 4.7%
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 213 monitored days across EPA AQI categories.
Good
165
77%
Moderate
42
20%
USG
5
2%
Unhealthy
1
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Middlesex County.
- Ozone · 213 days · 100.0%
Health Impact Assessment
Good
Middlesex County's air quality is rated Good based on 77.5% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 5 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 213 of 213 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Middlesex County. Highest Good-day share: 2020 (87.5%, 182 of 208 monitored 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 | 165 | 42 | 5 | 1 | 42 | 151 |
How does Middlesex County compare?
How Middlesex County compares
Good-day share better than typical
Middlesex County 77.5%
Ada County, Idaho 74.0%
Typical US county 76.0%
Median AQI worse than typical
Middlesex County 42
Ada County, Idaho 42
Typical US county 39
Unhealthy days / yr about typical
Middlesex County 1
Ada County, Idaho 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Middlesex County (42), not geographic neighbors.
Ada County, Idaho
Median AQI 42 · 74.0% Good-day share
Adams County, Pennsylvania
Median AQI 42 · 74.4% Good-day share
Allen County, Ohio
Median AQI 42 · 67.0% Good-day share
Asotin County, Washington
Median AQI 42 · 61.9% Good-day share
Baldwin County, Alabama
Median AQI 42 · 75.5% Good-day share
Bell County, Kentucky
Median AQI 42 · 67.0% Good-day share
Same primary pollutant: Ozone
Ozone led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Lycoming County, Pennsylvania
77.5% Good-day share · median AQI 40
Polk County, Arkansas
77.5% Good-day share · median AQI 41
Warren County, New Jersey
77.5% Good-day share · median AQI 37
Multnomah County, Oregon
77.5% Good-day share · median AQI 37
Portage County, Ohio
77.5% Good-day share · median AQI 43
DeKalb County, Alabama
77.3% Good-day share · median AQI 41
Connecticut counties on the map
Geographic containment only, other monitored counties in Connecticut, independent of AQI or pollutant match.
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 213 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
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." 77.5% of monitored days (165 out of 213) 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 213 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 across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2020 at 87.5% (182 of 208 days). Lowest qualifying Good-day share: 2025 at 77.5% (165 of 213 days).
What does AQI 42 mean for health? ▼
Middlesex County's median AQI of 42 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Middlesex County compare to other counties in Connecticut? ▼
Middlesex County has 77.5% good air days. Nearby counties in Connecticut: Fairfield (64.7% good), Hartford (68.0% good), Litchfield (81.4% good), New Haven (67.1% good), New London (81.3% good), Tolland (83.7% 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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Middlesex County, Connecticut · 2020–2025
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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.