What Is the Air Quality in Middlesex, Connecticut?
Middlesex, Connecticut has an Air Quality Grade of B (good) with a 5-year median AQI of 40. The dominant pollutant is Ground-Level Ozone, and air quality has been stable over the past decade.
Middlesex, Connecticut Air Quality Snapshot
| Air Quality Grade | B65/100 |
| 5-Year Median AQI | 40 (Good) |
| Most Recent Median AQI (2023) | 43 (Good) |
| Dominant Pollutant | Ground-Level Ozone |
| 10-Year Trend | Stable (-0.29 AQI/yr) |
| Unhealthy Days (last 5 yr) | 30 |
| National Rank (cleanest = #1) | #512 of 1,020 (50th cleanest percentile) |
| Connecticut Rank | #5 of 8 |
What Does the B Grade Mean?
Middlesex, Connecticut earns a B — air quality is reliably in the safe range for most residents most of the time, with a 5-year median AQI of 40. Sensitive groups will see occasional caution days, but the typical resident will not need to change behavior based on air quality.
Middlesex, Connecticut's 5-year median AQI of 40 is right around the national average of 41 across the 1,020 monitored U.S. cities tracked here. Within Connecticut, Middlesex, Connecticut's air quality is roughly typical for the state, where the average city posts a 5-year median AQI of 41.
For context within Connecticut: Hartford, Connecticut currently holds the state's cleanest grade (B, AQI 42), while Fairfield, Connecticut sits at the bottom (C, AQI 46).
What's in Middlesex, Connecticut's Air?
The dominant pollutant in Middlesex, Connecticut is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle exhaust and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs, triggers asthma attacks, and reduces lung function — even healthy adults can feel chest tightness and shortness of breath after exercising in elevated ozone.
Days by Dominant Pollutant (2023)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Ground-Level Ozone | 209 | 100% |
Is the Air Getting Better or Worse?
Air quality in Middlesex, Connecticut has held roughly steady over the past decade, with year-to-year shifts in median AQI of less than half a point. That stability makes the city's long-run grade a reliable signal of what residents can expect day-to-day.
In 2014, Middlesex, Connecticut posted a median AQI of 43. By 2023 that figure was 43 — a flat reading of 0 AQI points across 10 years of EPA records.
Year-by-Year AQI in Middlesex, Connecticut
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 43 | 140 | 11 | Ozone |
| 2015 | 43 | 134 | 12 | Ozone |
| 2016 | 44 | 125 | 13 | Ozone |
| 2017 | 40 | 171 | 8 | Ozone |
| 2018 | 40 | 168 | 8 | Ozone |
| 2019 | 41 | 175 | 7 | Ozone |
| 2020 | 39 | 182 | 2 | Ozone |
| 2021 | 39 | 171 | 11 | Ozone |
| 2022 | 40 | 173 | 6 | Ozone |
| 2023 | 43 | 170 | 4 | Ozone |
Health Context for Middlesex, Connecticut
Across the past five years, this area has logged 30 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 6 days per year. That is roughly typical for a U.S. metro, with most caution days clustered in summer (ozone) or wildfire season.
For most healthy adults, current air quality in this area does not require any change in behavior. People with severe asthma, COPD, or recent cardiac events should still keep an eye on daily AQI alerts, especially during wildfire season. Because ozone peaks in the afternoon on hot sunny days, plan outdoor exercise for early morning or after sunset on bad-air days.
How This Grade Is Calculated
The AirHistory Air Quality Grade combines four signals: the 5-year median AQI (40% of the score), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). All four come directly from the EPA Air Quality System (AQS), which aggregates readings from federally certified monitors. Read the full methodology.
Middlesex, Connecticut has an Air Quality Grade of B (good) with a 5-year median AQI of 40. The dominant pollutant is Ground-Level Ozone, and air quality has been stable over the past decade.
This answer pulls from the EPA Air Quality System (AQS), the authoritative federal source for U.S. air quality and pollution monitoring. The headline number above is the direct answer; what follows is the additional context most readers need to use the answer for a real decision rather than just a fact lookup.
A practical caveat: the headline answer above reflects the most recent the EPA Air Quality System (AQS) vintage; underlying data is often revised for months after first publication, and the right reference for any specific decision is whichever vintage is current at the time of the decision. The as-of date is stamped on every page.
Source: EPA Outdoor Air Quality Data, 2026.