Median AQI
54
Moderate
Ohio · County air quality · 2025
Hamilton County, Ohio
Median AQI 54 (Moderate). 32.1% of monitored days rated Good, direct from EPA Air Quality System stations.
- 54
- Median AQI
- 32.1%
- Good-air days
- 1
- Unhealthy days
- 2%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Hamilton County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 54 with 32.1% of 274 monitored days in the Good band, driven mainly by PM2.5.
- 54
- median AQI · Moderate
- 32.1%
- days rated Good
- PM2.5
- main pollutant
- 2%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 154.
Data updated July 2026
- Good · 88 d · 32.1%
- Moderate · 173 d · 63.1%
- USG · 12 d · 4.4%
- Unhealthy · 1 d · 0.4%
How did Hamilton County's air break down in 2025?
Across 274 monitored days, 88 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 154 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 274 days monitored. Median AQI: 54 (Moderate).
Insights: Hamilton County, Ohio. EPA monitors recorded a median Air Quality Index of 54 across 274 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 88 (32%) fell in the Good band (AQI 0–50), 173 (63%) registered Moderate readings, and 12 (4%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 74, while the worst single-day peak hit 154 (Unhealthy). Residents averaged about 9.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (182 days), Ozone (88 days), NO2 (3 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by -17 and the median AQI moved -1 points, placing the multi-year trajectory at improving. Residents and visitors experienced 1 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. Nearby counties show comparable readings: Allen (65.9% good, AQI 43); Ashtabula (81.2% good, AQI 43); Athens (80.3% good, AQI 33); Belmont (63.0% good, AQI 42); Butler (33.3% good, AQI 55); Clark (59.4% good, AQI 46).
Air Quality Summary
Good Days
32.1%
88 of 274 monitored days
-42.9pp vs 75% target
Max AQI Recorded
154
Unhealthy
Health Grade
Poor
90th-pct AQI 74
Hamilton County, Good days share 32.1%
National avg
Share of monitored days at AQI 0-50 (Good band).
Hamilton County, Unhealthy-or-worse share 0.4%
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
88
32%
Moderate
173
63%
USG
12
4%
Unhealthy
1
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Hamilton County.
- PM2.5 · 182 days · 66.4%
- Ozone · 88 days · 32.1%
- NO₂ · 3 days · 1.1%
- CO · 1 days · 0.4%
Health Impact Assessment
Poor
Hamilton County's air quality is rated Poor based on 32.1% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 12 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 154. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 182 of 274 monitored days.
5-Year Air Quality Trend
Air quality is improving in Hamilton County. Best year: 2024 (164 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 105 | 254 | 7 | 0 | 55 | 126 |
| 2021 | 109 | 251 | 5 | 0 | 56 | 140 |
| 2022 | 125 | 236 | 3 | 1 | 54 | 151 |
| 2023 | 129 | 217 | 17 | 2 | 54 | 193 |
| 2024 | 164 | 191 | 11 | 0 | 52 | 140 |
| 2025 | 88 | 173 | 12 | 1 | 54 | 154 |
How does Hamilton County compare?
How Hamilton County compares
Good-day share worse than US avg
Hamilton County 32.1%
Allen County, Ohio 65.9%
US average 76.5%
Median AQI worse than US avg
Hamilton County 54
Allen County, Ohio 43
US average 38
Unhealthy days / yr worse than US avg
Hamilton County 1
Allen County, Ohio 0
US average 0
Other Counties in Ohio
Air quality comparison for nearby counties in Ohio.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Hamilton County.
Denver County, Colorado
AQI 54 · 44.2% good
Franklin County, Ohio
AQI 54 · 39.8% good
Fresno County, California
AQI 54 · 40.1% good
Hidalgo County, Texas
AQI 54 · 36.2% good
Inyo County, California
AQI 54 · 40.7% good
Larimer County, Colorado
AQI 54 · 40.1% good
What Hamilton County's air means for you
On most days the air here is fine, but PM2.5 drives 1 unhealthy-or-worse day a year that sensitive groups should plan around.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 54 means for exposure. Health guidance
- See how Hamilton County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Hamilton County with its Ohio neighbours before drawing a conclusion about the region. All of Ohio
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 Hamilton County, Ohio? ▼
Hamilton County has a median Air Quality Index (AQI) of 54, rated "Moderate." 32.1% of monitored days (88 out of 274) had good air quality in 2025.
What is the main pollutant in Hamilton County? ▼
The primary pollutant in Hamilton County is Fine Particles (PM2.5), which was the main AQI driver on 182 days (66% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Hamilton County have? ▼
In 2025, Hamilton County recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 154.
Is the air quality in Hamilton County getting better or worse? ▼
Air quality in Hamilton County is improving based on 5-year trends (2020-2025). Best year: 2024 with 164 good days. Worst year: 2025 with 88 good days.
What does AQI 54 mean for health? ▼
Hamilton County's median AQI of 54 falls in the "Moderate" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Hamilton County compare to other counties in Ohio? ▼
Hamilton County has 32.1% good air days. Nearby counties in Ohio: Allen (65.9% good), Ashtabula (81.2% good), Athens (80.3% good), Belmont (63.0% good), Butler (33.3% good), Clark (59.4% good).
What is the 90th percentile AQI in Hamilton County? ▼
The 90th percentile AQI for Hamilton County is 74, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 154 (Unhealthy).
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.