State air quality · 2025
New Hampshire Air Quality Ranking
Across 7 monitored counties, the typical county in New Hampshire posts a median AQI of 37 (Good) and 84.9% of monitored days were rated Good, from EPA AQS.
- 37
- Avg median AQI
- 84.9%
- Good-air days
- 7
- Counties monitored
- 87%
- Cleaner than US states
New Hampshire's typical county median AQI is 37 (Good) across 7 monitored counties
According to the U.S. EPA Air Quality System, New Hampshire monitors 7 counties with a day-weighted Good-day share of 84.9%. Cleanest: Merrimack County (96.7% Good). Most polluted: Cheshire County (78.9%). These are annual summaries, not live AirNow readings.
Data source EPA Air Quality System (AQS)
- Cleaner than US states
- New Hampshire's day-weighted Good-day share of 84.9% places it cleaner than 87% of US states.
- In-state spread
- Within New Hampshire, Merrimack County leads at 96.7% Good days while Cheshire County sits at 78.9% across 7 monitored counties.
- 2024→2025 release
- New Hampshire's Good-day share moved -1.6 points vs the 2024 EPA annual release.
- Typical median AQI
- Typical median AQI: 37
Verify with AirNow → · EPA AQS →
EPA state monitoring brief
AQS state key NEW-HAMPSHIRE
New Hampshire · 2025 EPA AQS · 7 counties · med AQI 37 · 84.9% Good · Merrimack→Cheshire spread
- GOOD 37 Median AQI · Good
- SHARE 84.9% Good-day share
- COUNT 7 Counties monitored
- UNH 0 Unhealthy+ days
- RANK 87% Cleaner than states
- O3 73.3% Main pollutant share
New Hampshire's 84.9% Good-day share is cleaner than 87% of monitored US states; Merrimack and Cheshire County bracket the in-state range. Open Merrimack County →
According to EPA AQS annual summaries on PlainAirData, New Hampshire ranks #7 of 52 by day-weighted good-day share (84.9%) and #17 by mean county median AQI (37.4). 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
84.9%
Good-day share in 2025, down 1.6 percentage points vs 2024. County-mean median AQI 37, within half a point of the 2024 county-mean median.
Pollutant mix
Ozone
Main driver on 73.3% of pollutant-assigned days.
- Ozone 73.3%
- PM2.5 25.7%
- CO 1.0%
Monitoring coverage
340
Average monitored days per county in the latest release. Every monitored county clears the 100-day ranking floor.
Counties Monitored
7
Good Days %
84.9%
Avg Median AQI
37
Total Unhealthy Days
0
Best Air Quality
Merrimack County
Worst Air Quality
Cheshire County
Where New Hampshire sits nationally
COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. New Hampshire's day-weighted Good-day share is 84.9% - cleaner than 87% of that comparison set.
New Hampshire Good-day share vs. every US state
Day-weighted share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
84.9% Cleaner than 87% higher than 87% 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 New Hampshire have the cleanest and most polluted air?
Cleanest counties in New Hampshire
| County | Good % | Median AQI | Max AQI | Unhealthy Days |
|---|---|---|---|---|
| Merrimack | 96.7% | 37 | 90 | 0 |
| Belknap | 88.8% | 32 | 92 | 0 |
| Hillsborough | 84.9% | 38 | 123 | 0 |
| Rockingham | 84.7% | 38 | 150 | 0 |
| Grafton | 83.6% | 36 | 113 | 0 |
| Coos | 81.5% | 44 | 119 | 0 |
| Cheshire | 78.9% | 37 | 115 | 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: 7 counties in New Hampshire 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.