Median AQI
43
Good
Illinois · County air quality · 2025
Hamilton County, Illinois
Median AQI 43 (Good). 66.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 43
- Median AQI
- 66.5%
- Good-air days
- 0
- Unhealthy days
- 25%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Hamilton County's air is good on balance, more polluted than the median US county, a median AQI of 43 with 66.5% of 269 monitored days in the Good band, driven mainly by PM2.5.
- 43
- median AQI · Good
- 66.5%
- days rated Good
- PM2.5
- main pollutant
- 25%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 98.
Data updated July 2026
- Good · 179 d · 66.5%
- Moderate · 90 d · 33.5%
How did Hamilton County's air break down in 2025?
Across 269 monitored days, 179 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 98 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 269 days monitored. Median AQI: 43 (Good).
Insights: Hamilton County, Illinois. EPA monitors recorded a median Air Quality Index of 43 across 269 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 179 (67%) fell in the Good band (AQI 0–50), 90 (33%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 62, while the worst single-day peak hit 98 (Moderate). Residents averaged about 20.0 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (142 days), Ozone (127 days). Across the 2020–2025 trend window, good days shifted by -44 and the median AQI moved -3 points, placing the multi-year trajectory at improving. The worst single reading over the 2020–2025 window was AQI 98, moderate but never Unhealthy. Nearby counties show comparable readings: Adams (94.4% good, AQI 38); Champaign (63.7% good, AQI 44); Clark (92.1% good, AQI 36); Cook (40.5% good, AQI 53); DuPage (61.2% good, AQI 44); Effingham (95.7% good, AQI 35).
Air Quality Summary
Good Days
66.5%
179 of 269 monitored days
-8.5pp vs 75% target
Max AQI Recorded
98
Moderate
Health Grade
Moderate
90th-pct AQI 62
Hamilton County, Good days share 66.5%
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 269 monitored days across EPA AQI categories.
Good
179
67%
Moderate
90
33%
USG
0
0%
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 · 142 days · 52.8%
- Ozone · 127 days · 47.2%
Health Impact Assessment
Moderate
Hamilton County's air quality is rated Moderate based on 66.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 66.5% of monitored days in 2025, with a median AQI of 43. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 142 of 269 monitored days.
5-Year Air Quality Trend
Air quality is improving in Hamilton County. Best year: 2024 (236 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 223 | 129 | 1 | 0 | 46 | 118 |
| 2021 | 207 | 155 | 2 | 0 | 48 | 115 |
| 2022 | 227 | 138 | 0 | 0 | 45 | 88 |
| 2023 | 213 | 141 | 10 | 1 | 46 | 157 |
| 2024 | 236 | 129 | 1 | 0 | 44 | 112 |
| 2025 | 179 | 90 | 0 | 0 | 43 | 98 |
How does Hamilton County compare?
How Hamilton County compares
Good-day share worse than US avg
Hamilton County 66.5%
Adams County, Illinois 94.4%
US average 76.5%
Median AQI worse than US avg
Hamilton County 43
Adams County, Illinois 38
US average 38
Unhealthy days / yr about average
Hamilton County 0
Adams County, Illinois 0
US average 0
Other Counties in Illinois
Air quality comparison for nearby counties in Illinois.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Hamilton County.
Allegan County, Michigan
AQI 43 · 71.2% good
Allen County, Ohio
AQI 43 · 65.9% good
Ashtabula County, Ohio
AQI 43 · 81.2% good
Bell County, Kentucky
AQI 43 · 63.5% good
Boise County, Idaho
AQI 43 · 72.9% good
Brazos County, Texas
AQI 43 · 63.4% good
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 43 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 Illinois neighbours before drawing a conclusion about the region. All of Illinois
Based on 269 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, Illinois? ▼
Hamilton County has a median Air Quality Index (AQI) of 43, rated "Good." 66.5% of monitored days (179 out of 269) 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 142 days (53% 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 98.
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 236 good days. Worst year: 2025 with 179 good days.
What does AQI 43 mean for health? ▼
Hamilton County's median AQI of 43 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 Hamilton County compare to other counties in Illinois? ▼
Hamilton County has 66.5% good air days. Nearby counties in Illinois: Adams (94.4% good), Champaign (63.7% good), Clark (92.1% good), Cook (40.5% good), DuPage (61.2% good), Effingham (95.7% 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 98 (Moderate).
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.