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
According to the 2025 EPA AQS county release on PlainAirData, Middlesex County, Connecticut ranks #581 of 951 by median AQI (42) and #483 by good-day share (77.5%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.
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 →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Middlesex County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Middlesex County's Good-day share is 77.5% - cleaner than 49% of that comparison set.
Middlesex County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
77.5% 49th percentile higher than 49% of 951 US counties
Each bar is a band; taller bars hold more US counties. 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
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Middlesex County.
- Ozone · 213 days · 100.0%
2024 daily-monitor rhythm in Middlesex County
Month-by-month median AQI from EPA day-level county monitors (209 reported days in 2024). Jun posted the highest typical readings (median AQI 45, peak 177); Sep was cleanest at median 32. 1 day crossed Unhealthy (AQI >150) at least once that year.
Five-year seasonal record
EPA day-level county files · 2020–20241,045 daily readings show a recurring local pattern: Jul has the highest matched-month median AQI (45), while Sep is lowest (33). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
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
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
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
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.