Metro area air quality · 2025

Chicago-Naperville-Elgin, IL-IN-WI

Median AQI 56 (Moderate). 29.9% of monitored days rated Good across this metro's EPA monitoring network.

Compiled by PlainAirData · August 23, 2026

56
Median AQI: 29.9%
Good-air days: 6
Unhealthy days: 3%
Cleaner than US metros

Chicago-Naperville-Elgin, IL-IN-WI posted a 2025 median AQI of 56 (Moderate) with 29.9% Good days

According to the U.S. EPA Air Quality System, the Chicago-Naperville-Elgin, IL-IN-WI CBSA recorded a median AQI of 56 across 365 monitored days in 2025, with 29.9% rated Good and 6 unhealthy or worse. This is an annual metro comparison, not a live advisory.

Data source EPA Air Quality System (AQS)

Cleaner than US metros: The Chicago-Naperville-Elgin, IL-IN-WI CBSA's 29.9% Good-day share places it cleaner than 3% of US metros in 2025.
Closest median-AQI peer: Cincinnati, OH-KY-IN (26.0% Good, median AQI 56) is the closest nationwide median-AQI peer to Chicago-Naperville-Elgin, IL-IN-WI.
2020–2025 Good-day shift: Chicago-Naperville-Elgin, IL-IN-WI's Good-day share moved +14.4 points from 2020 to 2025 (median AQI -4).
Median AQI: Median AQI: 56

Aggregated from EPA Air Quality System (2025), 365 monitored days. EPA AQS →

The verdict

Chicago-Naperville-Elgin, IL-IN-WI lands in the lower Good-day tercile (29.9% Good), a thinner clean-air share than most CBSA peers here. On the EPA scale the metro runs moderate, among the more polluted US metros, with a median AQI of 56 and 29.9% of 365 monitored days in the Good band.

56
median AQI · Moderate: 29.9%
days rated Good: 3%
cleaner than US metros: 219
worst single reading

6 days reached Unhealthy or worse across the metro's monitors in 2025.

Chicago-Naperville-Elgin, IL-IN-WI metro's peak single-day AQI rose from 166 to 219 between the 2024 and 2025 EPA annual releases, a change of +53 points.
Chicago-Naperville-Elgin, IL-IN-WI metro's Good-day share rose from 27.1% to 32.2% between the 2024 and 2025 EPA annual releases, a change of +5.2 percentage points.
Chicago-Naperville-Elgin, IL-IN-WI metro's Unhealthy-category days rose from 2 to 5 between the 2024 and 2025 EPA annual releases, a change of +3 days.

Chicago-Naperville-Elgin, IL-IN-WI lands in the lower Good-day tercile (29.9% Good), a thinner clean-air share than most CBSA peers here. Signature read: 29.9% Good-day share (low tercile vs CBSA corpus). Aggregated EPA AQS monitors across this Core-Based Statistical Area logged a median Air Quality Index of 56 over 365 measured days in 2025 (Moderate / Poor). The monitored-day split lands at 109 Good (30%), 224 Moderate (61%), and 26 Unhealthy-for-Sensitive-Groups (7%). The 90th-percentile AQI finished at 97; the highest single-day reading hit 219 (Very Unhealthy).

Across 2020–2025, Good-day share shifted +14.4 points and median AQI moved -4 (improving). The annual record contains 6 days at Unhealthy or worse. Metros with comparable AQI profiles include Cincinnati, OH-KY-IN (26.0% good, AQI 56); St. Louis, MO-IL (31.2% good, AQI 56); Bishop, CA (37.3% good, AQI 55).

Health effects of air pollution →

Air Quality Summary

Median AQI

Moderate

Good Days

29.9%

109 of 365 days

Max AQI

219

Very Unhealthy

Health Grade

Poor

90th pct: 97

AQI Day Breakdown (2025)

Distribution of 365 monitored days across EPA AQI categories.

Good

109

30%

Moderate

224

61%

USG

Unhealthy

Very Unhealthy

Hazardous

Health Impact Assessment

Poor

The Chicago-Naperville-Elgin, IL-IN-WI metro area's air quality is rated Poor based on 29.9% good air days. There were 6 unhealthy days in 2025 when outdoor activity should be limited.

Air quality reached unhealthy levels on 6 days in 2025 (5 Unhealthy, 1 Very Unhealthy). Sensitive groups, including people with asthma, COPD, cardiovascular conditions, children, and adults over 65, should review the daily breakdown and follow local advisories.

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.

How does the Chicago-Naperville-Elgin, IL-IN-WI metro compare?

How Chicago-Naperville-Elgin, IL-IN-WI compares

Compare against

Good-day share worse than typical

Chicago-Naperville-Elgin, IL-IN-WI 29.9%

Cincinnati, OH-KY-IN 26.0%

Typical US metro 70.6%

Median AQI worse than typical

Chicago-Naperville-Elgin, IL-IN-WI 56

Cincinnati, OH-KY-IN 56

Typical US metro 41

Unhealthy days / yr worse than typical

Chicago-Naperville-Elgin, IL-IN-WI 6

Cincinnati, OH-KY-IN 2

Typical US metro 1

5-Year Air Quality Trend

Air quality is improving in the Chicago-Naperville-Elgin, IL-IN-WI metro. Highest Good-day share: 2025 (32.2%, 109 of 338 monitored days).

Year	Good	Moderate	Unhealthy	Median AQI	Max AQI
2020	62	280	5	60	197
2021	67	270	2	61	175
2022	93	257	2	56	202
2023	76	246	8	60	246
2024	95	254	2	56	166
2025	109	224	5	56	219

Metros with a similar median AQI

CBSA peers nationwide whose annual median AQI sits near Chicago-Naperville-Elgin, IL-IN-WI (56).

Cincinnati, OH-KY-IN

Median AQI 56 · 26.0% Good-day share

St. Louis, MO-IL

Median AQI 56 · 31.2% Good-day share

Bishop, CA

Median AQI 55 · 37.3% Good-day share

Brownsville-Harlingen, TX

Median AQI 57 · 29.9% Good-day share

Hanford-Corcoran, CA

Median AQI 57 · 34.5% Good-day share

Little Rock-North Little Rock-Conway, AR

Median AQI 55 · 31.2% Good-day share

Metros with similar unhealthy-day exposure

A second relationship type: metros whose unhealthy-or-worse day count is closest to Chicago-Naperville-Elgin, IL-IN-WI's 6 (independent of median AQI).

Appleton, WI

6 unhealthy-or-worse days · median AQI 38

Marquette, MI

6 unhealthy-or-worse days · median AQI 19

Minneapolis-St. Paul-Bloomington, MN-WI

6 unhealthy-or-worse days · median AQI 47

Spokane-Spokane Valley, WA

6 unhealthy-or-worse days · median AQI 47

Dallas-Fort Worth-Arlington, TX

5 unhealthy-or-worse days · median AQI 59

Eau Claire, WI

5 unhealthy-or-worse days · median AQI 38

Frequently Asked Questions

How many unhealthy air days does Chicago-Naperville-Elgin, IL-IN-WI have? ▼

In 2025, the Chicago-Naperville-Elgin, IL-IN-WI metro area recorded 6 unhealthy air days (5 Unhealthy, 1 Very Unhealthy, 0 Hazardous). The maximum AQI reached 219.

What is the air quality in the Chicago-Naperville-Elgin, IL-IN-WI metro area? ▼

The Chicago-Naperville-Elgin, IL-IN-WI metro area has a median Air Quality Index (AQI) of 56, rated "Moderate." 29.9% of monitored days (109 out of 365) had good air quality in 2025.

Is air quality in Chicago-Naperville-Elgin, IL-IN-WI getting better or worse? ▼

Air quality in the Chicago-Naperville-Elgin, IL-IN-WI metro 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: 2025 at 32.2% (109 of 338 days). Lowest qualifying Good-day share: 2020 at 17.9% (62 of 347 days).

What does AQI 56 mean for health? ▼

An AQI of 56 is "Moderate", air quality is acceptable, but some pollutants may pose a moderate health concern for a small number of sensitive individuals.

How does Chicago-Naperville-Elgin, IL-IN-WI compare to other metro areas? ▼

The Chicago-Naperville-Elgin, IL-IN-WI metro has 29.9% good air days. Similar metros: Cincinnati, OH-KY-IN (26.0% good), St. Louis, MO-IL (31.2% good), Bishop, CA (37.3% good).

What is the 90th percentile AQI for Chicago-Naperville-Elgin, IL-IN-WI? ▼

The 90th percentile AQI for the Chicago-Naperville-Elgin, IL-IN-WI metro area is 97, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 219 (Very Unhealthy).

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by CBSA dataset for Chicago-Naperville-Elgin, IL-IN-WI, 2025. See our methodology for how AQI, median, and coverage figures are computed.

Read our methodology , how this data is sourced, computed, and verified.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by CBSA summary, Chicago-Naperville-Elgin, IL-IN-WI · 2020–2025 Accessed 2026-07-06