Median AQI
53
Moderate
Illinois · County air quality · 2025
Cook County, Illinois
Median AQI 53 (Moderate). 40.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 53
- Median AQI
- 40.5%
- Good-air days
- 5
- Unhealthy days
- 3%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Cook County's air is moderate on balance, among the most polluted counties nationally, a median AQI of 53 with 40.5% of 274 monitored days in the Good band, driven mainly by PM2.5.
- 53
- median AQI · Moderate
- 40.5%
- days rated Good
- PM2.5
- main pollutant
- 3%
- cleaner than US counties
5 days reached Unhealthy or worse; the worst single reading hit AQI 168.
Data updated July 2026
- Good · 111 d · 40.5%
- Moderate · 139 d · 50.7%
- USG · 19 d · 6.9%
- Unhealthy · 5 d · 1.8%
How did Cook County's air break down in 2025?
Across 274 monitored days, 111 fell in the Good band (AQI 0–50) and 5 reached Unhealthy or worse. The worst single day peaked at AQI 168 (Unhealthy).
EPA AQS →Air quality data from EPA Air Quality System (2025). 274 days monitored. Median AQI: 53 (Moderate).
Insights: Cook County, Illinois. EPA monitors recorded a median Air Quality Index of 53 across 274 measured days in 2025, earning a Moderate classification and a Poor health grade. Of those days, 111 (41%) fell in the Good band (AQI 0–50), 139 (51%) registered Moderate readings, and 19 (7%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 94, while the worst single-day peak hit 168 (Unhealthy). Residents averaged about 12.2 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (145 days), Ozone (97 days), NO2 (23 days), PM10 (8 days), CO (1 days). Across the 2020–2025 trend window, good days shifted by +25 and the median AQI moved -5 points, placing the multi-year trajectory at improving. Residents and visitors experienced 5 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); DuPage (61.2% good, AQI 44); Effingham (95.7% good, AQI 35); Hamilton (66.5% good, AQI 43).
Air Quality Summary
Good Days
40.5%
111 of 274 monitored days
-34.5pp vs 75% target
Max AQI Recorded
168
Unhealthy
Health Grade
Poor
90th-pct AQI 94
Cook County, Good days share 40.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Cook County, Unhealthy-or-worse share 1.8%
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 274 monitored days across EPA AQI categories.
Good
111
41%
Moderate
139
51%
USG
19
7%
Unhealthy
5
2%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Cook County.
- PM2.5 · 145 days · 52.9%
- Ozone · 97 days · 35.4%
- NO₂ · 23 days · 8.4%
- PM10 · 8 days · 2.9%
- CO · 1 days · 0.4%
Health Impact Assessment
Poor
Cook County's air quality is rated Poor based on 40.5% good air days. There were 5 unhealthy days in 2025 when outdoor activity should be limited.
Air quality is acceptable most days, but 19 days reached "Unhealthy for Sensitive Groups" (AQI 101-150) and peak AQI reached 168. Sensitive groups should monitor real-time conditions at AirNow.gov.
The dominant pollutant is PM2.5, driving AQI on 145 of 274 monitored days.
5-Year Air Quality Trend
Air quality is improving in Cook County. Best year: 2024 (131 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 86 | 260 | 16 | 4 | 58 | 164 |
| 2021 | 87 | 257 | 20 | 1 | 59 | 156 |
| 2022 | 117 | 240 | 7 | 1 | 54 | 151 |
| 2023 | 104 | 227 | 27 | 5 | 57 | 223 |
| 2024 | 131 | 224 | 10 | 1 | 54 | 166 |
| 2025 | 111 | 139 | 19 | 5 | 53 | 168 |
How does Cook County compare?
How Cook County compares
Good-day share worse than US avg
Cook County 40.5%
Adams County, Illinois 94.4%
US average 76.5%
Median AQI worse than US avg
Cook County 53
Adams County, Illinois 38
US average 38
Unhealthy days / yr worse than US avg
Cook County 5
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 Cook County.
Bexar County, Texas
AQI 53 · 39.8% good
Cuyahoga County, Ohio
AQI 53 · 43.6% good
Dallas County, Texas
AQI 53 · 41.0% good
Jefferson County, Alabama
AQI 53 · 36.8% good
Kings County, California
AQI 53 · 39.8% good
Lake County, Indiana
AQI 53 · 38.8% good
What Cook County's air means for you
On most days the air here is fine, but PM2.5 drives 5 unhealthy-or-worse days 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 53 means for exposure. Health guidance
- See how Cook County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Cook County with its Illinois neighbours before drawing a conclusion about the region. All of Illinois
Based on 274 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 Cook County, Illinois? ▼
Cook County has a median Air Quality Index (AQI) of 53, rated "Moderate." 40.5% of monitored days (111 out of 274) had good air quality in 2025.
What is the main pollutant in Cook County? ▼
The primary pollutant in Cook County is Fine Particles (PM2.5), which was the main AQI driver on 145 days (53% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Cook County have? ▼
In 2025, Cook County recorded 5 unhealthy air days (5 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 168.
Is the air quality in Cook County getting better or worse? ▼
Air quality in Cook County is improving based on 5-year trends (2020-2025). Best year: 2024 with 131 good days. Worst year: 2020 with 86 good days.
What does AQI 53 mean for health? ▼
Cook County's median AQI of 53 falls in the "Moderate" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Cook County compare to other counties in Illinois? ▼
Cook County has 40.5% good air days. Nearby counties in Illinois: Adams (94.4% good), Champaign (63.7% good), Clark (92.1% good), DuPage (61.2% good), Effingham (95.7% good), Hamilton (66.5% good).
What is the 90th percentile AQI in Cook County? ▼
The 90th percentile AQI for Cook County is 94, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 168 (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.