State air quality · 2025
Massachusetts Air Quality Ranking
Across 13 monitored counties, the typical county in Massachusetts posts a median AQI of 38 (Good) and 83.1% of monitored days were rated Good, from EPA AQS.
- 38
- Avg median AQI
- 83.1%
- Good-air days
- 13
- Counties monitored
- 79%
- Cleaner than US states
Massachusetts's typical county median AQI is 38 (Good) across 13 monitored counties
According to the U.S. EPA Air Quality System, Massachusetts monitors 13 counties with a day-weighted Good-day share of 83.1%. Cleanest: Dukes County (95.9% Good). Most polluted: Suffolk County (76.4%). These are annual summaries, not live AirNow readings.
Data source EPA Air Quality System (AQS)
- Cleaner than US states
- Massachusetts's day-weighted Good-day share of 83.1% places it cleaner than 79% of US states.
- In-state spread
- Within Massachusetts, Dukes County leads at 95.9% Good days while Suffolk County sits at 76.4% across 13 monitored counties.
- 2024→2025 release
- Massachusetts's Good-day share moved -2.7 points vs the 2024 EPA annual release.
- Typical median AQI
- Typical median AQI: 38
Verify with AirNow → · EPA AQS →
EPA state monitoring brief
AQS state key MASSACHUSETTS
Massachusetts · 2025 EPA AQS · 13 counties · med AQI 38 · 83.1% Good · Dukes→Suffolk spread
- GOOD 38 Median AQI · Good
- SHARE 83.1% Good-day share
- COUNT 13 Counties monitored
- UNH 1 Unhealthy+ days
- RANK 79% Cleaner than states
- O3 71.1% Main pollutant share
Good-day share peers
Massachusetts's 83.1% Good-day share is cleaner than 79% of monitored US states; Dukes and Suffolk County bracket the in-state range. Open Dukes County →
According to EPA AQS annual summaries on PlainAirData, Massachusetts ranks #11 of 52 by day-weighted good-day share (83.1%) and #18 by mean county median AQI (37.7). Hawaii is #1 of 52 by day-weighted good-day share (93.8% of monitored days) at mean county median-AQI #4 (27.3) ≠ Alaska #1 lowest mean county median AQI (20.3) at good-day share #8 (84.4%).
Data updated August 2026
What changed, what drives it, and how thick is the sample
Three derived views WPR-style word count does not substitute for: release-over-release movement from county annual rows, the EPA main-pollutant day mix, and how many counties sit below the 100-day ranking floor.
Release change
83.1%
Good-day share in 2025, down 2.7 percentage points vs 2024. County-mean median AQI 38, 0.8 points higher (worse) than 2024.
Pollutant mix
Ozone
Main driver on 71.1% of pollutant-assigned days.
- Ozone 71.1%
- PM2.5 28.1%
- NO2 0.8%
- PM10 0.1%
Monitoring coverage
363
Average monitored days per county in the latest release. Every monitored county clears the 100-day ranking floor.
Counties Monitored
13
Good Days %
83.1%
Avg Median AQI
38
Total Unhealthy Days
1
Best Air Quality
Dukes County
Worst Air Quality
Suffolk County
Where Massachusetts sits nationally
COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. Massachusetts's day-weighted Good-day share is 83.1% - cleaner than 79% of that comparison set.
Massachusetts Good-day share vs. every US state
Day-weighted share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
83.1% Cleaner than 79% higher than 79% of 52 US states
Each bar is a band; taller bars hold more US states. 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
Which counties in Massachusetts have the cleanest and most polluted air?
Cleanest counties in Massachusetts
| County | Good % | Median AQI | Max AQI | Unhealthy Days |
|---|---|---|---|---|
| Dukes | 95.9% | 35 | 151 | 1 |
| Barnstable | 93.7% | 36 | 129 | 0 |
| Hampshire | 85.5% | 36 | 117 | 0 |
| Norfolk | 84.4% | 39 | 126 | 0 |
| Plymouth | 84.4% | 37 | 108 | 0 |
| Bristol | 83.8% | 38 | 119 | 0 |
| Middlesex | 81.9% | 38 | 105 | 0 |
| Worcester | 81.6% | 39 | 100 | 0 |
| Essex | 81.1% | 40 | 136 | 0 |
| Berkshire | 78.1% | 38 | 124 | 0 |
| Hampden | 77.0% | 39 | 124 | 0 |
| Franklin | 76.9% | 38 | 114 | 0 |
| Suffolk | 76.4% | 37 | 106 | 0 |
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025. See our methodology for how AQI and median figures are computed.
Coverage: 13 counties in Massachusetts with active monitoring stations. Unmonitored counties are not represented.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files, aggregated to the state level · 2020–2025
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.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.