Median AQI
38
Good
New Hampshire · County air quality · 2025
Hillsborough County, New Hampshire
Median AQI 38 (Good). 82.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 38
- Median AQI
- 82.5%
- Good-air days
- 0
- Unhealthy days
- 58%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Hillsborough County's air is good on balance, cleaner than the median US county, a median AQI of 38 with 82.5% of 274 monitored days in the Good band, driven mainly by Ozone.
- 38
- median AQI · Good
- 82.5%
- days rated Good
- Ozone
- main pollutant
- 58%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 123.
Data updated July 2026
- Good · 226 d · 82.5%
- Moderate · 44 d · 16.1%
- USG · 4 d · 1.5%
How did Hillsborough County's air break down in 2025?
Across 274 monitored days, 226 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 123 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 274 days monitored. Median AQI: 38 (Good).
Insights: Hillsborough County, New Hampshire. EPA monitors recorded a median Air Quality Index of 38 across 274 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 226 (82%) fell in the Good band (AQI 0–50), 44 (16%) registered Moderate readings, and 4 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 56, while the worst single-day peak hit 123 (USG). Residents averaged about 24.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (152 days), PM2.5 (121 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -125 and the median AQI moved +1 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 123, usg but never Unhealthy. Nearby counties show comparable readings: Belknap (87.9% good, AQI 34); Cheshire (76.9% good, AQI 38); Coos (85.4% good, AQI 40); Grafton (81.3% good, AQI 38); Merrimack (95.9% good, AQI 38); Rockingham (82.5% good, AQI 39).
Air Quality Summary
Good Days
82.5%
226 of 274 monitored days
7.5pp vs 75% target
Max AQI Recorded
123
USG
Health Grade
Good
90th-pct AQI 56
Hillsborough County, Good days share 82.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Hillsborough 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 274 monitored days across EPA AQI categories.
Good
226
82%
Moderate
44
16%
USG
4
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 Hillsborough County.
- Ozone · 152 days · 55.5%
- PM2.5 · 121 days · 44.2%
- CO · 1 days · 0.4%
Health Impact Assessment
Good
Hillsborough County's air quality is rated Good based on 82.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 82.5% of monitored days in 2025, with a median AQI of 38. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 152 of 274 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Hillsborough County. Best year: 2020 (351 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 351 | 15 | 0 | 0 | 37 | 90 |
| 2021 | 328 | 35 | 2 | 0 | 37 | 144 |
| 2022 | 329 | 36 | 0 | 0 | 39 | 100 |
| 2023 | 305 | 55 | 5 | 0 | 39 | 119 |
| 2024 | 328 | 38 | 0 | 0 | 37 | 97 |
| 2025 | 226 | 44 | 4 | 0 | 38 | 123 |
How does Hillsborough County compare?
How Hillsborough County compares
Good-day share better than US avg
Hillsborough County 82.5%
Belknap County, New Hampshire 87.9%
US average 76.5%
Median AQI about average
Hillsborough County 38
Belknap County, New Hampshire 34
US average 38
Unhealthy days / yr about average
Hillsborough County 0
Belknap 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 Hillsborough County.
Adams County, Illinois
AQI 38 · 94.4% good
Anoka County, Minnesota
AQI 38 · 82.1% good
Atlantic County, New Jersey
AQI 38 · 79.6% good
Avery County, North Carolina
AQI 38 · 93.9% good
Barnstable County, Massachusetts
AQI 38 · 92.5% good
Bay County, Michigan
AQI 38 · 71.8% good
What Hillsborough 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 38 means for exposure. Health guidance
- See how Hillsborough County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Hillsborough County with its New Hampshire neighbours before drawing a conclusion about the region. All of New Hampshire
Based on 274 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 Hillsborough County, New Hampshire? ▼
Hillsborough County has a median Air Quality Index (AQI) of 38, rated "Good." 82.5% of monitored days (226 out of 274) had good air quality in 2025.
What is the main pollutant in Hillsborough County? ▼
The primary pollutant in Hillsborough County is Ozone (O3), which was the main AQI driver on 152 days (55% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Hillsborough County have? ▼
In 2025, Hillsborough County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 123.
Is the air quality in Hillsborough County getting better or worse? ▼
Air quality in Hillsborough County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 351 good days. Worst year: 2025 with 226 good days.
What does AQI 38 mean for health? ▼
Hillsborough County's median AQI of 38 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 Hillsborough County compare to other counties in New Hampshire? ▼
Hillsborough County has 82.5% good air days. Nearby counties in New Hampshire: Belknap (87.9% good), Cheshire (76.9% good), Coos (85.4% good), Grafton (81.3% good), Merrimack (95.9% good), Rockingham (82.5% good).
What is the 90th percentile AQI in Hillsborough County? ▼
The 90th percentile AQI for Hillsborough County is 56, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 123 (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.
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.