Median AQI
48
Good
Illinois · County air quality · 2025
Madison County, Illinois
Median AQI 48 (Good). 55.3% of monitored days rated Good, direct from EPA Air Quality System stations.
- 48
- Median AQI
- 55.3%
- Good-air days
- 1
- Unhealthy days
- 10%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Madison County's air is good on balance, among the more polluted 10% of US counties, a median AQI of 48 with 55.3% of 304 monitored days in the Good band, driven mainly by PM2.5.
- 48
- median AQI · Good
- 55.3%
- days rated Good
- PM2.5
- main pollutant
- 10%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 260.
Data updated July 2026
- Good · 168 d · 55.3%
- Moderate · 132 d · 43.4%
- USG · 3 d · 1.0%
- Very Unhealthy · 1 d · 0.3%
How did Madison County's air break down in 2025?
Across 304 monitored days, 168 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). 304 days monitored. Median AQI: 48 (Good).
Insights: Madison County, Illinois. EPA monitors recorded a median Air Quality Index of 48 across 304 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 168 (55%) fell in the Good band (AQI 0–50), 132 (43%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 73, while the worst single-day peak hit 260 (Very Unhealthy). Residents averaged about 16.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (164 days), Ozone (137 days), PM10 (3 days). Across the 2020–2025 trend window, good days shifted by +39 and the median AQI moved -6 points, placing the multi-year trajectory at improving. Residents and visitors experienced 1 unhealthy-or-worse days combined, periods when the EPA recommends sensitive groups limit prolonged outdoor exertion. 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
55.3%
168 of 304 monitored days
-19.7pp vs 75% target
Max AQI Recorded
260
Very Unhealthy
Health Grade
Moderate
90th-pct AQI 73
Madison County, Good days share 55.3%
National avg
Share of monitored days at AQI 0-50 (Good band).
Madison County, Unhealthy-or-worse share 0.3%
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 304 monitored days across EPA AQI categories.
Good
168
55%
Moderate
132
43%
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 · 164 days · 53.9%
- Ozone · 137 days · 45.1%
- PM10 · 3 days · 1.0%
Health Impact Assessment
Moderate
Madison County's air quality is rated Moderate based on 55.3% 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 164 of 304 monitored days.
5-Year Air Quality Trend
Air quality is improving in Madison County. Best year: 2024 (193 good 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 | 168 | 132 | 3 | 0 | 48 | 260 |
How does Madison County compare?
How Madison County compares
Good-day share worse than US avg
Madison County 55.3%
Adams County, Illinois 94.4%
US average 76.5%
Median AQI worse than US avg
Madison County 48
Adams County, Illinois 38
US average 38
Unhealthy days / yr worse than US avg
Madison County 1
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 Madison County.
Black Hawk County, Iowa
AQI 48 · 56.6% good
Box Elder County, Utah
AQI 48 · 60.0% good
Brookings County, South Dakota
AQI 48 · 53.0% good
Clear Creek County, Colorado
AQI 48 · 64.4% good
Coconino County, Arizona
AQI 48 · 62.5% good
Columbia County, Florida
AQI 48 · 56.4% good
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 48 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 304 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 Madison County, Illinois? ▼
Madison County has a median Air Quality Index (AQI) of 48, rated "Good." 55.3% of monitored days (168 out of 304) 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 164 days (54% 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 based on 5-year trends (2020-2025). Best year: 2024 with 193 good days. Worst year: 2021 with 82 good days.
What does AQI 48 mean for health? ▼
Madison County's median AQI of 48 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 Madison County compare to other counties in Illinois? ▼
Madison County has 55.3% 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 Madison County? ▼
The 90th percentile AQI for Madison County is 73, 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.
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.