Median AQI
40
Good
Illinois · County air quality · 2025
Winnebago County, Illinois
Median AQI 40 (Good). 68.7% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 68.7%
- Good-air days
- 1
- Unhealthy days
- 30%
- Cleaner than US counties
Winnebago County's 2025 annual median AQI is 40 (Good), with 68.7% Good days
According to the U.S. EPA Air Quality System, Winnebago County, Illinois posted a 2025 median AQI of 40 (Good) across 355 monitored days, with 68.7% 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
- Winnebago County's 68.7% Good-day share places it cleaner than 30% of US counties in 2025.
- Closest median-AQI peer
- Anderson County (91.3% Good, median AQI 40) is the closest nationwide median-AQI peer to Winnebago County.
- 2020–2025 trend
- Winnebago County's floor-qualifying Good-day share is improving from 2020 to 2025 (56.0% → 68.7%).
- Median AQI
- Median AQI: 40
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 40 with 68.7% of 355 monitored days in the Good band, driven mainly by PM2.5.
- 40
- median AQI · Good
- 68.7%
- days rated Good
- PM2.5
- main pollutant
- 30%
- cleaner than US counties
1 day reached Unhealthy or worse; the worst single reading hit AQI 169.
Annual release change
What changed in the 2025 EPA annual release
- Winnebago County's peak single-day AQI rose from 119 to 169 between the 2024 and 2025 EPA annual releases, a change of +50 points.
- Winnebago County's median AQI fell from 43 to 40 between the 2024 and 2025 EPA annual releases, a change of -3 points.
- Winnebago County's Unhealthy-category days rose from 0 to 1 between the 2024 and 2025 EPA annual releases, a change of +1 days.
Data updated August 2026
- Good · 244 d · 68.7%
- Moderate · 105 d · 29.6%
- USG · 5 d · 1.4%
- Unhealthy · 1 d · 0.3%
How did Winnebago County's air break down in 2025?
Across 355 monitored days, 244 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). 355 days monitored. Median AQI: 40 (Good).
Insights: Winnebago County, Illinois. EPA monitors recorded a median Air Quality Index of 40 across 355 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 244 (69%) fell in the Good band (AQI 0–50), 105 (30%) registered Moderate readings, and 5 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 67, while the worst single-day peak hit 169 (Unhealthy). Residents averaged about 20.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (215 days), Ozone (140 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by +12.7 points and the median AQI moved -7 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
68.7%
244 of 355 monitored days
-6.3pp vs 75% target
Max AQI Recorded
169
Unhealthy
Health Grade
Moderate
90th-pct AQI 67
Winnebago County, Good days share 68.7%
National avg
Share of monitored days at AQI 0-50 (Good band).
Winnebago County, Unhealthy-or-worse share 2.8%
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 355 monitored days across EPA AQI categories.
Good
244
69%
Moderate
105
30%
USG
5
1%
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 · 215 days · 60.6%
- Ozone · 140 days · 39.4%
Health Impact Assessment
Moderate
Winnebago County's air quality is rated Moderate based on 68.7% 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 215 of 355 monitored days.
5-Year Air Quality Trend
Air quality is improving in Winnebago County. Highest Good-day share: 2024 (69.0%, 249 of 361 monitored 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 | 244 | 105 | 5 | 1 | 40 | 169 |
How does Winnebago County compare?
How Winnebago County compares
Good-day share worse than US avg
Winnebago County 68.7%
Anderson County, South Carolina 91.3%
US average 76.0%
Median AQI worse than US avg
Winnebago County 40
Anderson County, South Carolina 40
US average 39
Unhealthy days / yr about average
Winnebago County 1
Anderson County, South Carolina 0
US average 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Winnebago County (40), not geographic neighbors.
Anderson County, South Carolina
Median AQI 40 · 91.3% Good-day share
Andrew County, Missouri
Median AQI 40 · 86.9% Good-day share
Anne Arundel County, Maryland
Median AQI 40 · 87.6% Good-day share
Bergen County, New Jersey
Median AQI 40 · 71.1% Good-day share
Blair County, Pennsylvania
Median AQI 40 · 76.9% Good-day share
Brazos County, Texas
Median AQI 40 · 68.5% 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).
Grant County, Wisconsin
68.7% Good-day share · median AQI 37
Cumberland County, North Carolina
68.8% Good-day share · median AQI 44
Catawba County, North Carolina
68.9% Good-day share · median AQI 42
Albany County, New York
68.5% Good-day share · median AQI 39
Johnson County, Kansas
68.5% Good-day share · median AQI 44
Wake County, North Carolina
68.5% 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 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 40 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 355 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 Winnebago County, Illinois? ▼
Winnebago County has a median Air Quality Index (AQI) of 40, rated "Good." 68.7% of monitored days (244 out of 355) 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 215 days (61% 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 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 69.0% (249 of 361 days). Lowest qualifying Good-day share: 2023 at 46.0% (167 of 365 days).
What does AQI 40 mean for health? ▼
Winnebago County's median AQI of 40 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Winnebago County compare to other counties in Illinois? ▼
Winnebago County has 68.7% 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 Winnebago County? ▼
The 90th percentile AQI for Winnebago County is 67, 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. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Winnebago 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.