Median AQI
44
Good
Indiana · County air quality · 2025
Hamilton County, Indiana
Median AQI 44 (Good). 64.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 44
- Median AQI
- 64.0%
- Good-air days
- 0
- Unhealthy days
- 21%
- Cleaner than US counties
Hamilton County's 2025 annual median AQI is 44 (Good), with 64.0% Good days
According to the U.S. EPA Air Quality System, Hamilton County, Indiana posted a 2025 median AQI of 44 (Good) across 364 monitored days, with 64.0% rated Good and 0 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 64.0% Good-day share places it cleaner than 21% of US counties in 2025.
- Closest median-AQI peer
- Allegan County (63.9% Good, median AQI 44) is the closest nationwide median-AQI peer to Hamilton County.
- 2020–2025 trend
- Hamilton County's floor-qualifying Good-day share is worsening from 2020 to 2025 (67.5% → 64.0%).
- Median AQI
- Median AQI: 44
The verdict
According to EPA Air Quality System monitoring data, Hamilton County's air is good on balance, among the more polluted 10% of US counties, a median AQI of 44 with 64.0% of 364 monitored days in the Good band, driven mainly by PM2.5.
- 44
- median AQI · Good
- 64.0%
- days rated Good
- PM2.5
- main pollutant
- 21%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 118.
Annual release change
What changed in the 2025 EPA annual release
- Hamilton County's Good-day share fell from 66.7% to 64.0% between the 2024 and 2025 EPA annual releases, a change of -2.7 percentage points.
- Hamilton County's peak single-day AQI rose from 108 to 118 between the 2024 and 2025 EPA annual releases, a change of +10 points.
Data updated August 2026
- Good · 233 d · 64.0%
- Moderate · 126 d · 34.6%
- USG · 5 d · 1.4%
How did Hamilton County's air break down in 2025?
Across 364 monitored days, 233 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 118 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 364 days monitored. Median AQI: 44 (Good).
Insights: Hamilton County, Indiana. EPA monitors recorded a median Air Quality Index of 44 across 364 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 233 (64%) fell in the Good band (AQI 0–50), 126 (35%) registered Moderate readings, and 5 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 62, while the worst single-day peak hit 118 (USG). Residents averaged about 19.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (201 days), Ozone (163 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -3.5 points and the median AQI moved +0 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 118, usg but never Unhealthy. Nearby counties show comparable readings: Allen (60.0% good, AQI 44); Bartholomew (72.6% good, AQI 43); Boone (89.7% good, AQI 35); Brown (95.9% good, AQI 34); Carroll (90.8% good, AQI 35); Clark (58.1% good, AQI 46).
Air Quality Summary
Good Days
64.0%
233 of 364 monitored days
-11.0pp vs 75% target
Max AQI Recorded
118
USG
Health Grade
Moderate
90th-pct AQI 62
Hamilton County, Good days share 64.0%
National avg
Share of monitored days at AQI 0-50 (Good band).
Hamilton 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 364 monitored days across EPA AQI categories.
Good
233
64%
Moderate
126
35%
USG
5
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 Hamilton County.
- PM2.5 · 201 days · 55.2%
- Ozone · 163 days · 44.8%
Health Impact Assessment
Moderate
Hamilton County's air quality is rated Moderate based on 64.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 64.0% of monitored days in 2025, with a median AQI of 44. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 201 of 364 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Hamilton County. Highest Good-day share: 2020 (67.5%, 247 of 366 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 247 | 119 | 0 | 0 | 44 | 97 |
| 2021 | 232 | 133 | 0 | 0 | 44 | 100 |
| 2022 | 241 | 124 | 0 | 0 | 43 | 100 |
| 2023 | 220 | 134 | 8 | 2 | 46 | 190 |
| 2024 | 244 | 121 | 1 | 0 | 44 | 108 |
| 2025 | 233 | 126 | 5 | 0 | 44 | 118 |
How does Hamilton County compare?
How Hamilton County compares
Good-day share worse than US avg
Hamilton County 64.0%
Allegan County, Michigan 63.9%
US average 76.0%
Median AQI worse than US avg
Hamilton County 44
Allegan County, Michigan 44
US average 39
Unhealthy days / yr better than US avg
Hamilton County 0
Allegan County, Michigan 2
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Hamilton County (44), not geographic neighbors.
Allegan County, Michigan
Median AQI 44 · 63.9% Good-day share
Allen County, Indiana
Median AQI 44 · 60.0% Good-day share
Baker County, Florida
Median AQI 44 · 69.5% Good-day share
Bay County, Florida
Median AQI 44 · 66.4% Good-day share
Blount County, Tennessee
Median AQI 44 · 71.5% Good-day share
Cache County, Utah
Median AQI 44 · 66.0% Good-day share
Same primary pollutant: PM2.5
PM2.5 led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Chesterfield County, South Carolina
64.1% Good-day share · median AQI 44
Neosho County, Kansas
64.1% Good-day share · median AQI 44
Shawnee County, Kansas
64.1% Good-day share · median AQI 44
DuPage County, Illinois
63.8% Good-day share · median AQI 42
Erie County, New York
63.8% Good-day share · median AQI 44
Kings County, New York
63.8% Good-day share · median AQI 39
Indiana counties on the map
Geographic containment only, other monitored counties in Indiana, independent of AQI or pollutant match.
What Hamilton 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 44 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 Indiana neighbours before drawing a conclusion about the region. All of Indiana
Based on 364 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in Hamilton County, Indiana? ▼
Hamilton County has a median Air Quality Index (AQI) of 44, rated "Good." 64.0% of monitored days (233 out of 364) 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 201 days (55% 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 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 118.
Is the air quality in Hamilton County getting better or worse? ▼
Air quality in Hamilton County is worsening across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2020 at 67.5% (247 of 366 days). Lowest qualifying Good-day share: 2023 at 60.4% (220 of 364 days).
What does AQI 44 mean for health? ▼
Hamilton County's median AQI of 44 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Hamilton County compare to other counties in Indiana? ▼
Hamilton County has 64.0% good air days. Nearby counties in Indiana: Allen (60.0% good), Bartholomew (72.6% good), Boone (89.7% good), Brown (95.9% good), Carroll (90.8% good), Clark (58.1% good).
What is the 90th percentile AQI in Hamilton County? ▼
The 90th percentile AQI for Hamilton County is 62, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 118 (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. 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, Indiana · 2020–2025
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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.