Ohio · County air quality · 2025
Hamilton County, Ohio
Median AQI 54 (Moderate). 34.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 54
- Median AQI
- 34.3%
- Good-air days
- 1
- Unhealthy days
- 2%
- Cleaner than US counties
Hamilton County's 2025 annual median AQI is 54 (Moderate), with 34.3% Good days
According to the U.S. EPA Air Quality System, Hamilton County, Ohio posted a 2025 median AQI of 54 (Moderate) across 365 monitored days, with 34.3% rated Good and 1 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Hamilton County's 34.3% Good-day share places it cleaner than 2% of US counties in 2025.
- Closest median-AQI peer
- Marion County (40.0% Good, median AQI 54) is the closest nationwide median-AQI peer to Hamilton County.
- 2020–2025 trend
- Hamilton County's floor-qualifying Good-day share is improving from 2020 to 2025 (28.7% → 34.2%).
- Median AQI
- Median AQI: 54
According to the 2025 EPA AQS county release on PlainAirData, Hamilton County, Ohio ranks #925 of 951 by median AQI (54) and #929 by good-day share (34.3%). Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.
Annual release change
What changed in the 2025 EPA annual release
- Hamilton County's Good-day share fell from 44.8% to 34.2% between the 2024 and 2025 EPA annual releases, a change of -10.6 percentage points.
- Hamilton County's peak single-day AQI rose from 140 to 154 between the 2024 and 2025 EPA annual releases, a change of +14 points.
- Hamilton County's Unhealthy-category days rose from 0 to 1 between the 2024 and 2025 EPA annual releases, a change of +1 days.
Data updated August 2026
- Good · 125 d · 34.2%
- Moderate · 227 d · 62.2%
- USG · 12 d · 3.3%
- Unhealthy · 1 d · 0.3%
How did Hamilton County's air break down in 2025?
Across 365 monitored days, 125 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 →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Hamilton County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Hamilton County's Good-day share is 34.3% - cleaner than 2% of that comparison set.
Hamilton County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
34.3% 2nd percentile higher than 2% of 951 US counties
Each bar is a band; taller bars hold more US counties. 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
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Hamilton County.
- PM2.5 · 263 days · 72.1%
- Ozone · 96 days · 26.3%
- NO₂ · 6 days · 1.6%
2024 daily-monitor rhythm in Hamilton County
Month-by-month median AQI from EPA day-level county monitors (366 reported days in 2024). Jul posted the highest typical readings (median AQI 61, peak 108); Dec was cleanest at median 42. Summer months (Jun–Aug) averaged median AQI 58 vs 48 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,827 daily readings show a recurring local pattern: Jun has the highest matched-month median AQI (64), while Apr is lowest (51). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
Hamilton County's air quality is rated Poor based on 34.3% 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 263 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Hamilton County. Highest Good-day share: 2024 (44.8%, 164 of 366 monitored 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 | 125 | 227 | 12 | 1 | 54 | 154 |
How does Hamilton County compare?
How Hamilton County compares
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 Hamilton County, Ohio · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
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.