Median AQI
49
Good
Illinois · County air quality · 2025
Madison County, Illinois
Median AQI 49 (Good). 52.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 49
- Median AQI
- 52.3%
- Good-air days
- 1
- Unhealthy days
- 8%
- Cleaner than US counties
Madison County's 2025 annual median AQI is 49 (Good), with 52.3% Good days
According to the U.S. EPA Air Quality System, Madison County, Illinois posted a 2025 median AQI of 49 (Good) across 365 monitored days, with 52.3% 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
- Madison County's 52.3% Good-day share places it cleaner than 8% of US counties in 2025.
- Closest median-AQI peer
- Adams County (55.1% Good, median AQI 49) is the closest nationwide median-AQI peer to Madison County.
- 2020–2025 trend
- Madison County's floor-qualifying Good-day share is improving from 2020 to 2025 (35.3% → 52.5%).
- Median AQI
- Median AQI: 49
The verdict
According to EPA Air Quality System monitoring data, Madison County's air is good on balance, among the most polluted counties nationally, a median AQI of 49 with 52.3% of 365 monitored days in the Good band, driven mainly by PM2.5.
- 49
- median AQI · Good
- 52.3%
- days rated Good
- PM2.5
- main pollutant
- 8%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 260.
Annual release change
What changed in the 2025 EPA annual release
- Madison County's peak single-day AQI rose from 165 to 260 between the 2024 and 2025 EPA annual releases, a change of +95 points.
- Madison County's Unhealthy-category days fell from 2 to 0 between the 2024 and 2025 EPA annual releases, a change of -2 days.
Data updated August 2026
- Good · 191 d · 52.3%
- Moderate · 170 d · 46.6%
- USG · 3 d · 0.8%
- Very Unhealthy · 1 d · 0.3%
How did Madison County's air break down in 2025?
Across 365 monitored days, 191 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 260 (Very Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 365 days monitored. Median AQI: 49 (Good).
Insights: Madison County, Illinois. EPA monitors recorded a median Air Quality Index of 49 across 365 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 191 (52%) fell in the Good band (AQI 0–50), 170 (47%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 70, while the worst single-day peak hit 260 (Very Unhealthy). Residents averaged about 15.7 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (240 days), Ozone (122 days), PM10 (3 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +17.0 points and the median AQI moved -5 points, placing the multi-year trajectory at improving. The annual record contains 1 day at Unhealthy or worse. On that day, the EPA recommends that sensitive groups limit prolonged outdoor exertion. 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
52.3%
191 of 365 monitored days
-22.7pp vs 75% target
Max AQI Recorded
260
Very Unhealthy
Health Grade
Moderate
90th-pct AQI 70
Madison County, Good days share 52.3%
National avg
Share of monitored days at AQI 0-50 (Good band).
Madison County, Unhealthy-or-worse share 2.7%
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 365 monitored days across EPA AQI categories.
Good
191
52%
Moderate
170
47%
USG
3
1%
Unhealthy
0
0%
Very Unhealthy
1
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Madison County.
- PM2.5 · 240 days · 65.8%
- Ozone · 122 days · 33.4%
- PM10 · 3 days · 0.8%
Health Impact Assessment
Moderate
Madison County's air quality is rated Moderate based on 52.3% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality reached unhealthy levels on 1 days in 2025 (0 Unhealthy, 1 Very Unhealthy). Sensitive groups, including people with asthma, COPD, cardiovascular conditions, children, and adults over 65, should review the daily breakdown below and follow local air-quality advisories on high-AQI days.
Sensitive group advisory
People with asthma, COPD, heart disease, children, pregnant women, and adults 65+ should follow EPA AQI alerts daily and limit outdoor exertion when AQI exceeds 100. Real-time conditions: airnow.gov.
The dominant pollutant is PM2.5, driving AQI on 240 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Madison County. Highest Good-day share: 2024 (52.7%, 193 of 366 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 129 | 233 | 3 | 0 | 54 | 112 |
| 2021 | 82 | 277 | 6 | 0 | 57 | 143 |
| 2022 | 134 | 220 | 9 | 0 | 54 | 147 |
| 2023 | 113 | 234 | 14 | 2 | 55 | 155 |
| 2024 | 193 | 167 | 4 | 2 | 49 | 165 |
| 2025 | 191 | 170 | 3 | 0 | 49 | 260 |
How does Madison County compare?
How Madison County compares
Good-day share worse than typical
Madison County 52.3%
Adams County, Colorado 55.1%
Typical US county 76.0%
Median AQI worse than typical
Madison County 49
Adams County, Colorado 49
Typical US county 39
Unhealthy days / yr about typical
Madison County 1
Adams County, Colorado 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Madison County (49), not geographic neighbors.
Adams County, Colorado
Median AQI 49 · 55.1% Good-day share
Albany County, Wyoming
Median AQI 49 · 55.6% Good-day share
Black Hawk County, Iowa
Median AQI 49 · 53.4% Good-day share
Davidson County, Tennessee
Median AQI 49 · 52.9% Good-day share
Eddy County, New Mexico
Median AQI 49 · 53.7% Good-day share
Fulton County, Georgia
Median AQI 49 · 54.3% 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).
Santa Cruz County, Arizona
52.2% Good-day share · median AQI 49
Plumas County, California
52.9% Good-day share · median AQI 45
Arkansas County, Arkansas
52.9% Good-day share · median AQI 48
Kleberg County, Texas
51.7% Good-day share · median AQI 49
Hillsborough County, Florida
53.1% Good-day share · median AQI 49
Jefferson County, Kentucky
51.5% Good-day share · median AQI 49
Illinois counties on the map
Geographic containment only, other monitored counties in Illinois, independent of AQI or pollutant match.
What Madison 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 49 means for exposure. Health guidance
- See how Madison County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Madison County with its Illinois neighbours before drawing a conclusion about the region. All of Illinois
Based on 365 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 Madison County, Illinois? ▼
Madison County has a median Air Quality Index (AQI) of 49, rated "Good." 52.3% of monitored days (191 out of 365) had good air quality in 2025.
What is the main pollutant in Madison County? ▼
The primary pollutant in Madison County is Fine Particles (PM2.5), which was the main AQI driver on 240 days (66% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Madison County have? ▼
In 2025, Madison County recorded 1 unhealthy air days (0 Unhealthy, 1 Very Unhealthy, 0 Hazardous). The maximum AQI reached 260.
Is the air quality in Madison County getting better or worse? ▼
Air quality in Madison 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: 2024 at 52.7% (193 of 366 days). Lowest qualifying Good-day share: 2021 at 22.5% (82 of 365 days).
What does AQI 49 mean for health? ▼
Madison County's median AQI of 49 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Madison County compare to other counties in Illinois? ▼
Madison County has 52.3% 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 Madison County? ▼
The 90th percentile AQI for Madison County is 70, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 260 (Very 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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Madison 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.