Middlesex, Massachusetts Air Quality
Middlesex County, Massachusetts (MA)
↑ Worseningover 10 years
How Middlesex, Massachusetts Air Quality Compares
Middlesex, Massachusetts's median AQI of 39is 5% better than the national average of 41. Air quality has worsened by 7 AQI points over the past decade. The area averages 1 unhealthy air days per year. The primary pollutant of concern is Ground-Level Ozone.
Middlesex, Massachusetts Air Quality: The Full Picture
Middlesex, Massachusetts (Middlesex County) holds an AirHistory Air Quality Grade of C (fair, scoring 60 out of 100), built from 10 years of EPA Air Quality System monitoring. Its median AQI rose 7 points between 2014 (32) and 2023 (39), and the five-year median now sits at 39 — inside the "Good" band on the EPA scale.
The pollutant driving most of the area's AQI readings is Ground-Level Ozone. In 2023, it set the daily AQI on 193 of 365 monitored days (53% of the year), making it the controlling pollutant for the C grade. The single worst reading logged that year hit an AQI of 101 (Unhealthy for Sensitive Groups), while the 90th-percentile day — the level the dirtiest 10% of days reach — came in at 57.
In 2023, Middlesex County recorded 281 "Good" air days (77% of the 365 days with valid AQI data), 83 "Moderate" days, and 1 days at "Unhealthy for Sensitive Groups" or above. Over the most recent five years on record, monitors logged 3 days that crossed into "Unhealthy for Sensitive Groups" or worse — about 1 per year.
The decade-long trend is worsening: median AQI has been climbing by roughly 0.84 points per year. The cleanest year in the record was 2014 (median AQI 32), and the most polluted was 2019 (median AQI 41). Because the data come straight from federally certified EPA monitors, these figures track the Massachusetts county the monitors physically sit in rather than a city boundary.
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10-Year AQI Trend
The solid line shows the median AQI each year. The dashed line shows the 90th percentile (worst 10% of days). Air quality in this area has been declining over the past decade.
Air Quality Day Breakdown
Number of days per year in each EPA AQI category. Green = Good (AQI 0-50), Yellow = Moderate (51-100), Orange = Unhealthy for Sensitive Groups (101-150), Red = Unhealthy or worse (151+).
Year-by-Year Data
| Year | Median AQI | 90th Pct | Max AQI | Good Days | Moderate | Unhealthy+ | Pollutant |
|---|---|---|---|---|---|---|---|
| 2023 | 39 | 57 | 101 | 281 | 83 | 1 | Ozone |
| 2022 | 38 | 54 | 90 | 310 | 55 | 0 | Ozone |
| 2021 | 37 | 55 | 153 | 299 | 64 | 2 | PM2.5 |
| 2020 | 39 | 54 | 93 | 306 | 60 | 0 | PM2.5 |
| 2019 | 41 | 54 | 80 | 289 | 75 | 0 | PM2.5 |
| 2018 | 38 | 54 | 97 | 308 | 57 | 0 | Ozone |
| 2017 | 33 | 44 | 105 | 341 | 14 | 1 | Ozone |
| 2016 | 33 | 46 | 100 | 341 | 21 | 0 | Ozone |
| 2015 | 33 | 46 | 119 | 345 | 16 | 1 | Ozone |
| 2014 | 32 | 46 | 97 | 338 | 19 | 0 | Ozone |
What This Means for Middlesex County Residents
Middlesex, Massachusetts has received an Air Quality Grade of C (60/100) based on a decade of monitoring data from the EPA's air quality monitoring program. The current median AQI of 39 falls in the "Good" range.
Concerning trends show air quality has been declining, with the median AQI rising by 7 points over the monitoring period. This could be driven by increased development, wildfire smoke, or industrial activity.
The primary pollutant affecting this area is Ground-Level Ozone. Over the past 5 years, this area has averaged 1 unhealthy air quality days per year, days when sensitive groups (children, elderly, those with respiratory conditions) should limit outdoor activity. The American Lung Association's State of the Air report provides additional context on long-term health risks from air pollution exposure.
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Frequently Asked Questions
Middlesex, Massachusetts has a current median AQI of 39, which falls in the "Good" range. The area has received an Air Quality Grade of C (60/100) based on 10 years of EPA monitoring data.
Air quality in Middlesex, Massachusetts is worsening over the past decade. The median AQI has changed by +7 points from 2014 to 2023.
Middlesex, Massachusetts averages 1 unhealthy air quality days per year over the past 5 years. On these days, sensitive groups including children, the elderly, and those with respiratory conditions should limit outdoor activity.
The primary pollutant affecting Middlesex, Massachusetts is Ground-Level Ozone. This is the dominant contributor to elevated AQI readings in the Middlesex County area.
Middlesex, Massachusetts averages 1 unhealthy air days per year. This is relatively low, making it a reasonable choice for asthma sufferers. The primary pollutant is Ground-Level Ozone, which is a known asthma trigger.
With a median AQI of 39 (Good), outdoor exercise in Middlesex, Massachusetts is generally safe year-round. Middlesex, Massachusetts averages 1 days per year when athletes should move workouts indoors.