Median AQI
37
Good
Massachusetts · County air quality · 2025
Plymouth County, Massachusetts
Median AQI 37 (Good). 84.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 37
- Median AQI
- 84.4%
- Good-air days
- 0
- Unhealthy days
- 67%
- Cleaner than US counties
Plymouth County's 2025 annual median AQI is 37 (Good), with 84.4% Good days
According to the U.S. EPA Air Quality System, Plymouth County, Massachusetts posted a 2025 median AQI of 37 (Good) across 365 monitored days, with 84.4% 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
- Plymouth County's 84.4% Good-day share places it cleaner than 67% of US counties in 2025.
- Closest median-AQI peer
- Alachua County (88.7% Good, median AQI 37) is the closest nationwide median-AQI peer to Plymouth County.
- 2020–2025 trend
- Plymouth County's floor-qualifying Good-day share is improving from 2020 to 2025 (73.0% → 84.4%).
- Median AQI
- Median AQI: 37
The verdict
According to EPA Air Quality System monitoring data, Plymouth County's air is good on balance, cleaner than the median US county, a median AQI of 37 with 84.4% of 365 monitored days in the Good band, driven mainly by Ozone.
- 37
- median AQI · Good
- 84.4%
- days rated Good
- Ozone
- main pollutant
- 67%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 108.
Annual release change
What changed in the 2025 EPA annual release
- Plymouth County's Good-day share fell from 87.2% to 84.4% between the 2024 and 2025 EPA annual releases, a change of -2.8 percentage points.
- Plymouth County's peak single-day AQI rose from 97 to 108 between the 2024 and 2025 EPA annual releases, a change of +11 points.
Data updated August 2026
- Good · 308 d · 84.4%
- Moderate · 56 d · 15.3%
- USG · 1 d · 0.3%
How did Plymouth County's air break down in 2025?
Across 365 monitored days, 308 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 108 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 37 (Good).
Insights: Plymouth County, Massachusetts. EPA monitors recorded a median Air Quality Index of 37 across 365 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 308 (84%) fell in the Good band (AQI 0–50), 56 (15%) registered Moderate readings, and 1 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 54, while the worst single-day peak hit 108 (USG). Residents averaged about 25.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (251 days), PM2.5 (114 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +11.4 points and the median AQI moved -5 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 108, 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
84.4%
308 of 365 monitored days
9.4pp vs 75% target
Max AQI Recorded
108
USG
Health Grade
Good
90th-pct AQI 54
Plymouth County, Good days share 84.4%
National avg
Share of monitored days at AQI 0-50 (Good band).
Plymouth 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
308
84%
Moderate
56
15%
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 Plymouth County.
- Ozone · 251 days · 68.8%
- PM2.5 · 114 days · 31.2%
Health Impact Assessment
Good
Plymouth County's air quality is rated Good based on 84.4% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 84.4% of monitored days in 2025, with a median AQI of 37. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 251 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Plymouth County. Highest Good-day share: 2024 (87.2%, 319 of 366 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 267 | 99 | 0 | 0 | 42 | 80 |
| 2021 | 257 | 106 | 2 | 0 | 39 | 112 |
| 2022 | 235 | 125 | 0 | 0 | 45 | 93 |
| 2023 | 278 | 83 | 4 | 0 | 40 | 114 |
| 2024 | 319 | 47 | 0 | 0 | 37 | 97 |
| 2025 | 308 | 56 | 1 | 0 | 37 | 108 |
How does Plymouth County compare?
How Plymouth County compares
Good-day share better than US avg
Plymouth County 84.4%
Alachua County, Florida 88.7%
US average 76.0%
Median AQI better than US avg
Plymouth County 37
Alachua County, Florida 37
US average 39
Unhealthy days / yr better than US avg
Plymouth County 0
Alachua County, Florida 0
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Plymouth County (37), not geographic neighbors.
Alachua County, Florida
Median AQI 37 · 88.7% Good-day share
Arlington County, Virginia
Median AQI 37 · 87.4% Good-day share
Bennington County, Vermont
Median AQI 37 · 83.6% Good-day share
Boone County, Missouri
Median AQI 37 · 93.2% Good-day share
Bradford County, Pennsylvania
Median AQI 37 · 81.6% Good-day share
Brown County, Wisconsin
Median AQI 37 · 73.2% 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).
Norfolk County, Massachusetts
84.4% Good-day share · median AQI 39
Morris County, New Jersey
84.4% Good-day share · median AQI 39
Delaware County, Ohio
84.3% Good-day share · median AQI 40
Penobscot County, Maine
84.2% Good-day share · median AQI 33
Seminole County, Florida
84.6% Good-day share · median AQI 39
Miami County, Ohio
84.2% Good-day share · median AQI 40
Massachusetts counties on the map
Geographic containment only, other monitored counties in Massachusetts, independent of AQI or pollutant match.
What Plymouth 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 37 means for exposure. Health guidance
- See how Plymouth County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Plymouth 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 Plymouth County, Massachusetts? ▼
Plymouth County has a median Air Quality Index (AQI) of 37, rated "Good." 84.4% of monitored days (308 out of 365) had good air quality in 2025.
What is the main pollutant in Plymouth County? ▼
The primary pollutant in Plymouth County is Ozone (O3), which was the main AQI driver on 251 days (69% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Plymouth County have? ▼
In 2025, Plymouth County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 108.
Is the air quality in Plymouth County getting better or worse? ▼
Air quality in Plymouth County is improving 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 87.2% (319 of 366 days). Lowest qualifying Good-day share: 2022 at 65.3% (235 of 360 days).
What does AQI 37 mean for health? ▼
Plymouth County's median AQI of 37 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Plymouth County compare to other counties in Massachusetts? ▼
Plymouth County has 84.4% 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 Plymouth County? ▼
The 90th percentile AQI for Plymouth County is 54, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 108 (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 Plymouth 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.