Median AQI
42
Good
Illinois · County air quality · 2025
Hamilton County, Illinois
Median AQI 42 (Good). 67.9% of monitored days rated Good, direct from EPA Air Quality System stations.
- 42
- Median AQI
- 67.9%
- Good-air days
- 0
- Unhealthy days
- 28%
- Cleaner than US counties
Hamilton County's 2025 annual median AQI is 42 (Good), with 67.9% Good days
According to the U.S. EPA Air Quality System, Hamilton County, Illinois posted a 2025 median AQI of 42 (Good) across 361 monitored days, with 67.9% 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 67.9% Good-day share places it cleaner than 28% of US counties in 2025.
- Closest median-AQI peer
- Ada County (74.0% Good, median AQI 42) 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 (63.2% → 67.9%).
- Median AQI
- Median AQI: 42
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 42 with 67.9% of 361 monitored days in the Good band, driven mainly by PM2.5.
- 42
- median AQI · Good
- 67.9%
- days rated Good
- PM2.5
- main pollutant
- 28%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 98.
Annual release change
What changed in the 2025 EPA annual release
- Hamilton County's Good-day share rose from 64.5% to 67.9% between the 2024 and 2025 EPA annual releases, a change of +3.4 percentage points.
- Hamilton County's peak single-day AQI fell from 112 to 98 between the 2024 and 2025 EPA annual releases, a change of -14 points.
Data updated August 2026
- Good · 245 d · 67.9%
- Moderate · 116 d · 32.1%
How did Hamilton County's air break down in 2025?
Across 361 monitored days, 245 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). 361 days monitored. Median AQI: 42 (Good).
Insights: Hamilton County, Illinois. EPA monitors recorded a median Air Quality Index of 42 across 361 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 245 (68%) fell in the Good band (AQI 0–50), 116 (32%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 61, while the worst single-day peak hit 98 (Moderate). Residents averaged about 20.4 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (208 days), Ozone (153 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +4.7 points and the median AQI moved -4 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 (94.7% good, AQI 38); Champaign (65.8% good, AQI 43); Clark (93.8% good, AQI 33); Cook (40.8% good, AQI 53); DuPage (63.8% good, AQI 42); Effingham (95.5% good, AQI 35).
Air Quality Summary
Good Days
67.9%
245 of 361 monitored days
-7.1pp vs 75% target
Max AQI Recorded
98
Moderate
Health Grade
Moderate
90th-pct AQI 61
Hamilton County, Good days share 67.9%
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 361 monitored days across EPA AQI categories.
Good
245
68%
Moderate
116
32%
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 · 208 days · 57.6%
- Ozone · 153 days · 42.4%
Health Impact Assessment
Moderate
Hamilton County's air quality is rated Moderate based on 67.9% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 67.9% of monitored days in 2025, with a median AQI of 42. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 208 of 361 monitored days.
5-Year Air Quality Trend
Air quality is improving in Hamilton County. Highest Good-day share: 2025 (67.9%, 245 of 361 monitored 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 | 245 | 116 | 0 | 0 | 42 | 98 |
How does Hamilton County compare?
How Hamilton County compares
Good-day share worse than typical
Hamilton County 67.9%
Ada County, Idaho 74.0%
Typical US county 76.0%
Median AQI worse than typical
Hamilton County 42
Ada County, Idaho 42
Typical US county 39
Unhealthy days / yr better than typical
Hamilton County 0
Ada County, Idaho 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Hamilton County (42), not geographic neighbors.
Ada County, Idaho
Median AQI 42 · 74.0% Good-day share
Adams County, Pennsylvania
Median AQI 42 · 74.4% Good-day share
Allen County, Ohio
Median AQI 42 · 67.0% Good-day share
Asotin County, Washington
Median AQI 42 · 61.9% Good-day share
Baldwin County, Alabama
Median AQI 42 · 75.5% Good-day share
Bell County, Kentucky
Median AQI 42 · 67.0% 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).
McHenry County, Illinois
67.9% Good-day share · median AQI 42
Edgefield County, South Carolina
68.0% Good-day share · median AQI 43
Snohomish County, Washington
68.0% Good-day share · median AQI 41
Coffee County, Georgia
67.7% Good-day share · median AQI 44
Baltimore (City) County, Maryland
68.1% Good-day share · median AQI 43
Randolph County, Illinois
68.1% Good-day share · median AQI 43
Illinois counties on the map
Geographic containment only, other monitored counties in Illinois, 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 42 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 361 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, Illinois? ▼
Hamilton County has a median Air Quality Index (AQI) of 42, rated "Good." 67.9% of monitored days (245 out of 361) 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 208 days (58% 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 across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2025 at 67.9% (245 of 361 days). Lowest qualifying Good-day share: 2021 at 56.9% (207 of 364 days).
What does AQI 42 mean for health? ▼
Hamilton County's median AQI of 42 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 Illinois? ▼
Hamilton County has 67.9% good air days. Nearby counties in Illinois: Adams (94.7% good), Champaign (65.8% good), Clark (93.8% good), Cook (40.8% good), DuPage (63.8% good), Effingham (95.5% good).
What is the 90th percentile AQI in Hamilton County? ▼
The 90th percentile AQI for Hamilton County is 61, 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. 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, Illinois · 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.