Median AQI
34
Good
New Hampshire · County air quality · 2025
Belknap County, New Hampshire
Median AQI 34 (Good). 87.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 34
- Median AQI
- 87.9%
- Good-air days
- 0
- Unhealthy days
- 71%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Belknap County's air is good on balance, cleaner than the median US county, a median AQI of 34 with 87.9% of 273 monitored days in the Good band, driven mainly by PM2.5.
- 34
- median AQI · Good
- 87.9%
- days rated Good
- PM2.5
- main pollutant
- 71%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 92.
Data updated July 2026
- Good · 240 d · 87.9%
- Moderate · 33 d · 12.1%
How did Belknap County's air break down in 2025?
Across 273 monitored days, 240 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 92 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 273 days monitored. Median AQI: 34 (Good).
Insights: Belknap County, New Hampshire. EPA monitors recorded a median Air Quality Index of 34 across 273 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 240 (88%) fell in the Good band (AQI 0–50), 33 (12%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 52, while the worst single-day peak hit 92 (Moderate). Residents averaged about 26.4 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (159 days), Ozone (114 days). Across the 2020–2025 trend window, good days shifted by -102 and the median AQI moved +2 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 92, moderate but never Unhealthy. Nearby counties show comparable readings: Cheshire (76.9% good, AQI 38); Coos (85.4% good, AQI 40); Grafton (81.3% good, AQI 38); Hillsborough (82.5% good, AQI 38); Merrimack (95.9% good, AQI 38); Rockingham (82.5% good, AQI 39).
Air Quality Summary
Good Days
87.9%
240 of 273 monitored days
12.9pp vs 75% target
Max AQI Recorded
92
Moderate
Health Grade
Good
90th-pct AQI 52
Belknap County, Good days share 87.9%
National avg
Share of monitored days at AQI 0-50 (Good band).
Belknap 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 273 monitored days across EPA AQI categories.
Good
240
88%
Moderate
33
12%
USG
0
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 Belknap County.
- PM2.5 · 159 days · 58.2%
- Ozone · 114 days · 41.8%
Health Impact Assessment
Good
Belknap County's air quality is rated Good based on 87.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 87.9% of monitored days in 2025, with a median AQI of 34. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 159 of 273 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Belknap County. Best year: 2020 (342 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 342 | 17 | 0 | 0 | 32 | 65 |
| 2021 | 340 | 24 | 1 | 0 | 31 | 110 |
| 2022 | 339 | 19 | 0 | 0 | 32 | 68 |
| 2023 | 310 | 54 | 1 | 0 | 35 | 105 |
| 2024 | 311 | 54 | 0 | 0 | 35 | 78 |
| 2025 | 240 | 33 | 0 | 0 | 34 | 92 |
How does Belknap County compare?
How Belknap County compares
Good-day share better than US avg
Belknap County 87.9%
Cheshire County, New Hampshire 76.9%
US average 76.5%
Median AQI better than US avg
Belknap County 34
Cheshire County, New Hampshire 38
US average 38
Unhealthy days / yr about average
Belknap County 0
Cheshire County, New Hampshire 0
US average 0
Other Counties in New Hampshire
Air quality comparison for nearby counties in New Hampshire.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Belknap County.
Dyer County, Tennessee
AQI 34 · 79.4% good
Flagler County, Florida
AQI 34 · 97.4% good
Grant County, Oregon
AQI 34 · 66.7% good
Highlands County, Florida
AQI 34 · 93.2% good
Johnson County, Texas
AQI 34 · 91.6% good
Knox County, Maine
AQI 34 · 97.3% good
What Belknap County's air means for you
The air here stays in the breathable range year-round, with PM2.5 the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 34 means for exposure. Health guidance
- See how Belknap County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Belknap County with its New Hampshire neighbours before drawing a conclusion about the region. All of New Hampshire
Based on 273 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Belknap County, New Hampshire? ▼
Belknap County has a median Air Quality Index (AQI) of 34, rated "Good." 87.9% of monitored days (240 out of 273) had good air quality in 2025.
What is the main pollutant in Belknap County? ▼
The primary pollutant in Belknap County is Fine Particles (PM2.5), which was the main AQI driver on 159 days (58% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Belknap County have? ▼
In 2025, Belknap County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 92.
Is the air quality in Belknap County getting better or worse? ▼
Air quality in Belknap County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 342 good days. Worst year: 2025 with 240 good days.
What does AQI 34 mean for health? ▼
Belknap County's median AQI of 34 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Belknap County compare to other counties in New Hampshire? ▼
Belknap County has 87.9% good air days. Nearby counties in New Hampshire: Cheshire (76.9% good), Coos (85.4% good), Grafton (81.3% good), Hillsborough (82.5% good), Merrimack (95.9% good), Rockingham (82.5% good).
What is the 90th percentile AQI in Belknap County? ▼
The 90th percentile AQI for Belknap County is 52, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 92 (Moderate).
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.
Every figure on PlainAir 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.