Median AQI
41
Good
Illinois · County air quality · 2025
Winnebago County, Illinois
Median AQI 41 (Good). 67.7% of monitored days rated Good, direct from EPA Air Quality System stations.
- 41
- Median AQI
- 67.7%
- Good-air days
- 1
- Unhealthy days
- 27%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Winnebago County's air is good on balance, more polluted than the median US county, a median AQI of 41 with 67.7% of 263 monitored days in the Good band, driven mainly by PM2.5.
- 41
- median AQI · Good
- 67.7%
- days rated Good
- PM2.5
- main pollutant
- 27%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 169.
Data updated July 2026
- Good · 178 d · 67.7%
- Moderate · 79 d · 30.0%
- USG · 5 d · 1.9%
- Unhealthy · 1 d · 0.4%
How did Winnebago County's air break down in 2025?
Across 263 monitored days, 178 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 169 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 263 days monitored. Median AQI: 41 (Good).
Insights: Winnebago County, Illinois. EPA monitors recorded a median Air Quality Index of 41 across 263 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 178 (68%) fell in the Good band (AQI 0–50), 79 (30%) registered Moderate readings, and 5 (2%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 71, while the worst single-day peak hit 169 (Unhealthy). Residents averaged about 20.3 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (140 days), Ozone (123 days). Across the 2020–2025 trend window, good days shifted by -27 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
67.7%
178 of 263 monitored days
-7.3pp vs 75% target
Max AQI Recorded
169
Unhealthy
Health Grade
Moderate
90th-pct AQI 71
Winnebago County, Good days share 67.7%
National avg
Share of monitored days at AQI 0-50 (Good band).
Winnebago County, Unhealthy-or-worse share 0.4%
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 263 monitored days across EPA AQI categories.
Good
178
68%
Moderate
79
30%
USG
5
2%
Unhealthy
1
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Winnebago County.
- PM2.5 · 140 days · 53.2%
- Ozone · 123 days · 46.8%
Health Impact Assessment
Moderate
Winnebago County's air quality is rated Moderate based on 67.7% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 5 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 169. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 140 of 263 monitored days.
5-Year Air Quality Trend
Air quality is improving in Winnebago County. Best year: 2024 (249 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 205 | 160 | 1 | 0 | 47 | 101 |
| 2021 | 190 | 170 | 0 | 0 | 49 | 100 |
| 2022 | 237 | 126 | 0 | 0 | 43 | 97 |
| 2023 | 167 | 178 | 17 | 1 | 52 | 242 |
| 2024 | 249 | 109 | 3 | 0 | 43 | 119 |
| 2025 | 178 | 79 | 5 | 1 | 41 | 169 |
How does Winnebago County compare?
How Winnebago County compares
Good-day share worse than US avg
Winnebago County 67.7%
Adams County, Illinois 94.4%
US average 76.5%
Median AQI worse than US avg
Winnebago County 41
Adams County, Illinois 38
US average 38
Unhealthy days / yr worse than US avg
Winnebago 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 Winnebago County.
Albemarle County, Virginia
AQI 41 · 77.9% good
Anderson County, South Carolina
AQI 41 · 90.0% good
Berkshire County, Massachusetts
AQI 41 · 71.6% good
Berrien County, Michigan
AQI 41 · 80.9% good
Blair County, Pennsylvania
AQI 41 · 71.9% good
Bonner County, Idaho
AQI 41 · 71.3% good
What Winnebago 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 41 means for exposure. Health guidance
- See how Winnebago County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Winnebago County with its Illinois neighbours before drawing a conclusion about the region. All of Illinois
Based on 263 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 Winnebago County, Illinois? ▼
Winnebago County has a median Air Quality Index (AQI) of 41, rated "Good." 67.7% of monitored days (178 out of 263) had good air quality in 2025.
What is the main pollutant in Winnebago County? ▼
The primary pollutant in Winnebago County is Fine Particles (PM2.5), which was the main AQI driver on 140 days (53% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Winnebago County have? ▼
In 2025, Winnebago County recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 169.
Is the air quality in Winnebago County getting better or worse? ▼
Air quality in Winnebago County is improving based on 5-year trends (2020-2025). Best year: 2024 with 249 good days. Worst year: 2023 with 167 good days.
What does AQI 41 mean for health? ▼
Winnebago County's median AQI of 41 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 Winnebago County compare to other counties in Illinois? ▼
Winnebago County has 67.7% 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 Winnebago County? ▼
The 90th percentile AQI for Winnebago County is 71, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 169 (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.