Median AQI
39
Good
Massachusetts · County air quality · 2025
Hampden County, Massachusetts
Median AQI 39 (Good). 77.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 39
- Median AQI
- 77.0%
- Good-air days
- 0
- Unhealthy days
- 48%
- Cleaner than US counties
Hampden County's 2025 annual median AQI is 39 (Good), with 77.0% Good days
According to the U.S. EPA Air Quality System, Hampden County, Massachusetts posted a 2025 median AQI of 39 (Good) across 365 monitored days, with 77.0% 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
- Hampden County's 77.0% Good-day share places it cleaner than 48% of US counties in 2025.
- Closest median-AQI peer
- Aiken County (90.6% Good, median AQI 39) is the closest nationwide median-AQI peer to Hampden County.
- 2020–2025 trend
- Hampden County's floor-qualifying Good-day share is improving from 2020 to 2025 (68.0% → 77.0%).
- Median AQI
- Median AQI: 39
The verdict
According to EPA Air Quality System monitoring data, Hampden County's air is good on balance, more polluted than the median US county, a median AQI of 39 with 77.0% of 365 monitored days in the Good band, driven mainly by Ozone.
- 39
- median AQI · Good
- 77.0%
- days rated Good
- Ozone
- main pollutant
- 48%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 124.
Annual release change
What changed in the 2025 EPA annual release
- Hampden County's Good-day share fell from 79.2% to 77.0% between the 2024 and 2025 EPA annual releases, a change of -2.2 percentage points.
Data updated August 2026
- Good · 281 d · 77.0%
- Moderate · 79 d · 21.6%
- USG · 5 d · 1.4%
How did Hampden County's air break down in 2025?
Across 365 monitored days, 281 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 124 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 39 (Good).
Insights: Hampden County, Massachusetts. EPA monitors recorded a median Air Quality Index of 39 across 365 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 281 (77%) fell in the Good band (AQI 0–50), 79 (22%) registered Moderate readings, and 5 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 61, while the worst single-day peak hit 124 (USG). Residents averaged about 23.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (221 days), PM2.5 (136 days), NO2 (8 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +9.0 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 124, 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
77.0%
281 of 365 monitored days
2.0pp vs 75% target
Max AQI Recorded
124
USG
Health Grade
Good
90th-pct AQI 61
Hampden County, Good days share 77.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Hampden 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
281
77%
Moderate
79
22%
USG
5
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 Hampden County.
- Ozone · 221 days · 60.5%
- PM2.5 · 136 days · 37.3%
- NO₂ · 8 days · 2.2%
Health Impact Assessment
Good
Hampden County's air quality is rated Good based on 77.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 77.0% of monitored days in 2025, with a median AQI of 39. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 221 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Hampden County. Highest Good-day share: 2024 (79.2%, 290 of 366 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 249 | 117 | 0 | 0 | 44 | 99 |
| 2021 | 215 | 148 | 2 | 0 | 46 | 116 |
| 2022 | 262 | 102 | 1 | 0 | 42 | 115 |
| 2023 | 220 | 139 | 4 | 2 | 46 | 153 |
| 2024 | 290 | 74 | 2 | 0 | 37 | 119 |
| 2025 | 281 | 79 | 5 | 0 | 39 | 124 |
How does Hampden County compare?
How Hampden County compares
Good-day share better than US avg
Hampden County 77.0%
Aiken County, South Carolina 90.6%
US average 76.0%
Median AQI about average
Hampden County 39
Aiken County, South Carolina 39
US average 39
Unhealthy days / yr better than US avg
Hampden County 0
Aiken County, South Carolina 0
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Hampden County (39), not geographic neighbors.
Aiken County, South Carolina
Median AQI 39 · 90.6% Good-day share
Albany County, New York
Median AQI 39 · 68.5% Good-day share
Anderson County, Tennessee
Median AQI 39 · 94.3% Good-day share
Bay County, Michigan
Median AQI 39 · 70.4% Good-day share
Berkeley County, West Virginia
Median AQI 39 · 83.6% Good-day share
Boyd County, Kentucky
Median AQI 39 · 80.0% 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).
White Pine County, Nevada
77.0% Good-day share · median AQI 45
Franklin County, Massachusetts
76.9% Good-day share · median AQI 38
Blair County, Pennsylvania
76.9% Good-day share · median AQI 40
DeKalb County, Alabama
77.3% Good-day share · median AQI 41
La Paz County, Arizona
76.6% Good-day share · median AQI 44
Middlesex County, Connecticut
77.5% Good-day share · median AQI 42
Massachusetts counties on the map
Geographic containment only, other monitored counties in Massachusetts, independent of AQI or pollutant match.
What Hampden 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 39 means for exposure. Health guidance
- See how Hampden County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Hampden 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 Hampden County, Massachusetts? ▼
Hampden County has a median Air Quality Index (AQI) of 39, rated "Good." 77.0% of monitored days (281 out of 365) had good air quality in 2025.
What is the main pollutant in Hampden County? ▼
The primary pollutant in Hampden County is Ozone (O3), which was the main AQI driver on 221 days (61% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Hampden County have? ▼
In 2025, Hampden County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 124.
Is the air quality in Hampden County getting better or worse? ▼
Air quality in Hampden 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 79.2% (290 of 366 days). Lowest qualifying Good-day share: 2021 at 58.9% (215 of 365 days).
What does AQI 39 mean for health? ▼
Hampden County's median AQI of 39 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Hampden County compare to other counties in Massachusetts? ▼
Hampden County has 77.0% 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 Hampden County? ▼
The 90th percentile AQI for Hampden County is 61, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 124 (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 Hampden 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.