Median AQI
38
Good
Massachusetts · County air quality · 2025
Middlesex County, Massachusetts
Median AQI 38 (Good). 81.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 38
- Median AQI
- 81.9%
- Good-air days
- 0
- Unhealthy days
- 61%
- Cleaner than US counties
Middlesex County's 2025 annual median AQI is 38 (Good), with 81.9% Good days
According to the U.S. EPA Air Quality System, Middlesex County, Massachusetts posted a 2025 median AQI of 38 (Good) across 365 monitored days, with 81.9% rated Good and 0 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 81.9% Good-day share places it cleaner than 61% of US counties in 2025.
- Closest median-AQI peer
- Adams County (94.7% Good, median AQI 38) is the closest nationwide median-AQI peer to Middlesex County.
- 2020–2025 trend
- Middlesex County's floor-qualifying Good-day share is stable from 2020 to 2025 (83.6% → 81.9%).
- Median AQI
- Median AQI: 38
The verdict
According to EPA Air Quality System monitoring data, Middlesex County's air is good on balance, cleaner than the median US county, a median AQI of 38 with 81.9% of 365 monitored days in the Good band, driven mainly by Ozone.
- 38
- median AQI · Good
- 81.9%
- days rated Good
- Ozone
- main pollutant
- 61%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 105.
Annual release change
What changed in the 2025 EPA annual release
- Middlesex County's Good-day share fell from 85.8% to 81.9% between the 2024 and 2025 EPA annual releases, a change of -3.9 percentage points.
- Middlesex County's peak single-day AQI rose from 93 to 105 between the 2024 and 2025 EPA annual releases, a change of +12 points.
Data updated August 2026
- Good · 299 d · 81.9%
- Moderate · 65 d · 17.8%
- USG · 1 d · 0.3%
How did Middlesex County's air break down in 2025?
Across 365 monitored days, 299 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). 365 days monitored. Median AQI: 38 (Good).
Insights: Middlesex County, Massachusetts. EPA monitors recorded a median Air Quality Index of 38 across 365 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 299 (82%) fell in the Good band (AQI 0–50), 65 (18%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 56, while the worst single-day peak hit 105 (USG). Residents averaged about 24.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (230 days), PM2.5 (134 days), NO2 (1 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -1.7 points and the median AQI moved -1 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 105, usg but never Unhealthy. Nearby counties show comparable readings: Barnstable (93.7% good, AQI 36); Berkshire (78.1% good, AQI 38); Bristol (83.8% good, AQI 38); Dukes (95.9% good, AQI 35); Essex (81.1% good, AQI 40); Franklin (76.9% good, AQI 38).
Air Quality Summary
Good Days
81.9%
299 of 365 monitored days
6.9pp vs 75% target
Max AQI Recorded
105
USG
Health Grade
Good
90th-pct AQI 56
Middlesex County, Good days share 81.9%
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 365 monitored days across EPA AQI categories.
Good
299
82%
Moderate
65
18%
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 · 230 days · 63.0%
- PM2.5 · 134 days · 36.7%
- NO₂ · 1 days · 0.3%
Health Impact Assessment
Good
Middlesex County's air quality is rated Good based on 81.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 81.9% of monitored days in 2025, with a median AQI of 38. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 230 of 365 monitored days.
5-Year Air Quality Trend
Air quality is stable in Middlesex County. Highest Good-day share: 2024 (85.8%, 314 of 366 monitored 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 | 299 | 65 | 1 | 0 | 38 | 105 |
How does Middlesex County compare?
How Middlesex County compares
Good-day share better than typical
Middlesex County 81.9%
Adams County, Illinois 94.7%
Typical US county 76.0%
Median AQI better than typical
Middlesex County 38
Adams County, Illinois 38
Typical US county 39
Unhealthy days / yr better than typical
Middlesex County 0
Adams County, Illinois 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Middlesex County (38), not geographic neighbors.
Adams County, Illinois
Median AQI 38 · 94.7% Good-day share
Albemarle County, Virginia
Median AQI 38 · 81.0% Good-day share
Alexander County, North Carolina
Median AQI 38 · 93.5% Good-day share
Amador County, California
Median AQI 38 · 87.4% Good-day share
Anoka County, Minnesota
Median AQI 38 · 74.1% Good-day share
Avery County, North Carolina
Median AQI 38 · 92.3% 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).
Montgomery County, North Carolina
81.8% Good-day share · median AQI 39
Parker County, Texas
81.8% Good-day share · median AQI 38
Tioga County, Pennsylvania
82.1% Good-day share · median AQI 39
Osage County, Oklahoma
81.7% Good-day share · median AQI 42
Calaveras County, California
82.2% Good-day share · median AQI 40
Atlantic County, New Jersey
82.2% Good-day share · median AQI 36
Massachusetts counties on the map
Geographic containment only, other monitored counties in Massachusetts, independent of AQI or pollutant match.
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 38 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 365 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, Massachusetts? ▼
Middlesex County has a median Air Quality Index (AQI) of 38, rated "Good." 81.9% of monitored days (299 out of 365) 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 230 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 stable across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2024 at 85.8% (314 of 366 days). Lowest qualifying Good-day share: 2023 at 77.0% (281 of 365 days).
What does AQI 38 mean for health? ▼
Middlesex County's median AQI of 38 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 Massachusetts? ▼
Middlesex County has 81.9% good air days. Nearby counties in Massachusetts: Barnstable (93.7% good), Berkshire (78.1% good), Bristol (83.8% good), Dukes (95.9% good), Essex (81.1% good), Franklin (76.9% good).
What is the 90th percentile AQI in Middlesex County? ▼
The 90th percentile AQI for Middlesex County is 56, 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. 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, Massachusetts · 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.