Median AQI
41
Good
Massachusetts · County air quality · 2025
Middlesex County, Massachusetts
Median AQI 41 (Good). 76.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 41
- Median AQI
- 76.5%
- Good-air days
- 0
- Unhealthy days
- 43%
- 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 41 with 76.5% of 243 monitored days in the Good band, driven mainly by Ozone.
- 41
- median AQI · Good
- 76.5%
- days rated Good
- Ozone
- main pollutant
- 43%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 105.
Data updated July 2026
- Good · 186 d · 76.5%
- Moderate · 56 d · 23.0%
- USG · 1 d · 0.4%
How did Middlesex County's air break down in 2025?
Across 243 monitored days, 186 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 105 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 243 days monitored. Median AQI: 41 (Good).
Insights: Middlesex County, Massachusetts. EPA monitors recorded a median Air Quality Index of 41 across 243 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 186 (77%) fell in the Good band (AQI 0–50), 56 (23%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 58, while the worst single-day peak hit 105 (USG). Residents averaged about 23.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (152 days), PM2.5 (90 days), NO2 (1 days). Across the 2020–2025 trend window, good days shifted by -120 and the median AQI moved +2 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 105, usg but never Unhealthy. Nearby counties show comparable readings: Barnstable (92.5% good, AQI 38); Berkshire (71.6% good, AQI 41); Bristol (77.8% good, AQI 41); Dukes (95.2% good, AQI 37); Essex (77.0% good, AQI 42); Franklin (72.8% good, AQI 40).
Air Quality Summary
Good Days
76.5%
186 of 243 monitored days
1.5pp vs 75% target
Max AQI Recorded
105
USG
Health Grade
Good
90th-pct AQI 58
Middlesex County, Good days share 76.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Middlesex County, Unhealthy-or-worse share 0.0%
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 243 monitored days across EPA AQI categories.
Good
186
77%
Moderate
56
23%
USG
1
0%
Unhealthy
0
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 · 152 days · 62.6%
- PM2.5 · 90 days · 37.0%
- NO₂ · 1 days · 0.4%
Health Impact Assessment
Good
Middlesex County's air quality is rated Good based on 76.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 76.5% of monitored days in 2025, with a median AQI of 41. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 152 of 243 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Middlesex County. Best year: 2024 (314 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 306 | 60 | 0 | 0 | 39 | 93 |
| 2021 | 299 | 64 | 1 | 1 | 37 | 153 |
| 2022 | 310 | 55 | 0 | 0 | 38 | 90 |
| 2023 | 281 | 83 | 1 | 0 | 39 | 101 |
| 2024 | 314 | 52 | 0 | 0 | 37 | 93 |
| 2025 | 186 | 56 | 1 | 0 | 41 | 105 |
How does Middlesex County compare?
How Middlesex County compares
Good-day share about average
Middlesex County 76.5%
Barnstable County, Massachusetts 92.5%
US average 76.5%
Median AQI worse than US avg
Middlesex County 41
Barnstable County, Massachusetts 38
US average 38
Unhealthy days / yr about average
Middlesex County 0
Barnstable County, Massachusetts 0
US average 0
Other Counties in Massachusetts
Air quality comparison for nearby counties in Massachusetts.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Middlesex County.
Albemarle County, Virginia
AQI 41 · 77.9% good
Anderson County, South Carolina
AQI 41 · 90.0% good
Berkshire County, Massachusetts
AQI 41 · 71.6% good
Berrien County, Michigan
AQI 41 · 80.9% good
Blair County, Pennsylvania
AQI 41 · 71.9% good
Bonner County, Idaho
AQI 41 · 71.3% good
What Middlesex County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 41 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 Massachusetts neighbours before drawing a conclusion about the region. All of Massachusetts
Based on 243 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, Massachusetts? ▼
Middlesex County has a median Air Quality Index (AQI) of 41, rated "Good." 76.5% of monitored days (186 out of 243) 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 152 days (63% 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 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 105.
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: 2024 with 314 good days. Worst year: 2025 with 186 good days.
What does AQI 41 mean for health? ▼
Middlesex County's median AQI of 41 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 Massachusetts? ▼
Middlesex County has 76.5% good air days. Nearby counties in Massachusetts: Barnstable (92.5% good), Berkshire (71.6% good), Bristol (77.8% good), Dukes (95.2% good), Essex (77.0% good), Franklin (72.8% good).
What is the 90th percentile AQI in Middlesex County? ▼
The 90th percentile AQI for Middlesex County is 58, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 105 (USG).
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.