Median AQI
36
Good
Massachusetts · County air quality · 2025
Barnstable County, Massachusetts
Median AQI 36 (Good). 93.7% of monitored days rated Good, direct from EPA Air Quality System stations.
- 36
- Median AQI
- 93.7%
- Good-air days
- 0
- Unhealthy days
- 89%
- Cleaner than US counties
Barnstable County's 2025 annual median AQI is 36 (Good), with 93.7% Good days
According to the U.S. EPA Air Quality System, Barnstable County, Massachusetts posted a 2025 median AQI of 36 (Good) across 364 monitored days, with 93.7% 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
- Barnstable County's 93.7% Good-day share places it cleaner than 89% of US counties in 2025.
- Closest median-AQI peer
- Ashland County (81.6% Good, median AQI 36) is the closest nationwide median-AQI peer to Barnstable County.
- 2020–2025 trend
- Barnstable County's floor-qualifying Good-day share is stable from 2020 to 2025 (96.6% → 93.7%).
- Median AQI
- Median AQI: 36
The verdict
According to EPA Air Quality System monitoring data, Barnstable County's air is good on balance, cleaner than most US counties, a median AQI of 36 with 93.7% of 364 monitored days in the Good band, driven mainly by Ozone.
- 36
- median AQI · Good
- 93.7%
- days rated Good
- Ozone
- main pollutant
- 89%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 129.
Annual release change
What changed in the 2025 EPA annual release
- Barnstable County's peak single-day AQI rose from 101 to 129 between the 2024 and 2025 EPA annual releases, a change of +28 points.
Data updated August 2026
- Good · 341 d · 93.7%
- Moderate · 20 d · 5.5%
- USG · 3 d · 0.8%
How did Barnstable County's air break down in 2025?
Across 364 monitored days, 341 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 129 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 364 days monitored. Median AQI: 36 (Good).
Insights: Barnstable County, Massachusetts. EPA monitors recorded a median Air Quality Index of 36 across 364 measured days in 2025, earning a Good classification and a Excellent health grade. Of those days, 341 (94%) fell in the Good band (AQI 0–50), 20 (5%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 46, while the worst single-day peak hit 129 (USG). Residents averaged about 28.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (357 days), PM2.5 (7 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -2.9 points and the median AQI moved +0 points, placing the multi-year trajectory at stable. The worst single reading over the 2020–2025 window was AQI 129, usg but never Unhealthy. Nearby counties show comparable readings: 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); Hampden (77.0% good, AQI 39).
Air Quality Summary
Good Days
93.7%
341 of 364 monitored days
18.7pp vs 75% target
Max AQI Recorded
129
USG
Health Grade
Excellent
90th-pct AQI 46
Barnstable County, Good days share 93.7%
National avg
Share of monitored days at AQI 0-50 (Good band).
Barnstable 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 364 monitored days across EPA AQI categories.
Good
341
94%
Moderate
20
5%
USG
3
1%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Barnstable County.
- Ozone · 357 days · 98.1%
- PM2.5 · 7 days · 1.9%
Health Impact Assessment
Excellent
Barnstable County's air quality is rated Excellent based on 93.7% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 93.7% of monitored days in 2025, with a median AQI of 36. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 357 of 364 monitored days.
5-Year Air Quality Trend
Air quality is stable in Barnstable County. Highest Good-day share: 2020 (96.6%, 339 of 351 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 339 | 10 | 2 | 0 | 36 | 108 |
| 2021 | 331 | 25 | 3 | 0 | 36 | 126 |
| 2022 | 341 | 19 | 0 | 0 | 36 | 93 |
| 2023 | 331 | 24 | 3 | 0 | 37 | 140 |
| 2024 | 333 | 19 | 1 | 0 | 36 | 101 |
| 2025 | 341 | 20 | 3 | 0 | 36 | 129 |
How does Barnstable County compare?
How Barnstable County compares
Good-day share better than typical
Barnstable County 93.7%
Ashland County, Wisconsin 81.6%
Typical US county 76.0%
Median AQI better than typical
Barnstable County 36
Ashland County, Wisconsin 36
Typical US county 39
Unhealthy days / yr better than typical
Barnstable County 0
Ashland County, Wisconsin 8
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Barnstable County (36), not geographic neighbors.
Ashland County, Wisconsin
Median AQI 36 · 81.6% Good-day share
Atlantic County, New Jersey
Median AQI 36 · 82.2% Good-day share
Bannock County, Idaho
Median AQI 36 · 88.8% Good-day share
Beltrami County, Minnesota
Median AQI 36 · 79.1% Good-day share
Benton County, Washington
Median AQI 36 · 80.3% Good-day share
Brooke County, West Virginia
Median AQI 36 · 73.0% 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).
Pointe Coupee County, Louisiana
93.7% Good-day share · median AQI 35
Windham County, Connecticut
93.7% Good-day share · median AQI 35
Clark County, Illinois
93.8% Good-day share · median AQI 33
St. James County, Louisiana
93.9% Good-day share · median AQI 33
Okaloosa County, Florida
93.9% Good-day share · median AQI 36
Alexander County, North Carolina
93.5% Good-day share · median AQI 38
Massachusetts counties on the map
Geographic containment only, other monitored counties in Massachusetts, independent of AQI or pollutant match.
What Barnstable 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 36 means for exposure. Health guidance
- See how Barnstable County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Barnstable County with its Massachusetts neighbours before drawing a conclusion about the region. All of Massachusetts
Based on 364 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 Barnstable County, Massachusetts? ▼
Barnstable County has a median Air Quality Index (AQI) of 36, rated "Good." 93.7% of monitored days (341 out of 364) had good air quality in 2025.
What is the main pollutant in Barnstable County? ▼
The primary pollutant in Barnstable County is Ozone (O3), which was the main AQI driver on 357 days (98% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Barnstable County have? ▼
In 2025, Barnstable County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 129.
Is the air quality in Barnstable County getting better or worse? ▼
Air quality in Barnstable 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: 2020 at 96.6% (339 of 351 days). Lowest qualifying Good-day share: 2021 at 92.2% (331 of 359 days).
What does AQI 36 mean for health? ▼
Barnstable County's median AQI of 36 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Barnstable County compare to other counties in Massachusetts? ▼
Barnstable County has 93.7% good air days. Nearby counties in Massachusetts: Berkshire (78.1% good), Bristol (83.8% good), Dukes (95.9% good), Essex (81.1% good), Franklin (76.9% good), Hampden (77.0% good).
What is the 90th percentile AQI in Barnstable County? ▼
The 90th percentile AQI for Barnstable County is 46, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 129 (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 Barnstable 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.