Metro area air quality · 2025

Decatur, IL

Median AQI 44 (Good). 63.7% of monitored days rated Good across this metro's EPA monitoring network.

44
Median AQI
63.7%
Good-air days
0
Unhealthy days
31%
Cleaner than US metros

Aggregated from EPA Air Quality System (2025), 273 monitored days. EPA AQS →

The verdict

The Decatur, IL metro runs good on the EPA scale, more polluted than the median US metro, a median AQI of 44 with 63.7% of 273 monitored days in the Good band.

44
median AQI · Good
63.7%
days rated Good
31%
cleaner than US metros
141
worst single reading

No day crossed into the Unhealthy tier across the metro's monitors in 2025.

Insights: Decatur, IL metro. Aggregated EPA AQS monitors across this Core-Based Statistical Area logged a median Air Quality Index of 44 over 273 measured days in 2025, which corresponds to a Good band and a Moderate metro-wide health grade. The monitored-day split lands at 174 Good days (64%), 96 Moderate (35%), and 3 Unhealthy-for-Sensitive-Groups (1%). The 90th-percentile AQI finished at 67 while the highest single-day reading hit 141 (USG). Metro residents averaged roughly 19.1 clean-air days per 30-day window across the reporting year.

Across the 2020–2025 trend window, good days shifted by -40 and the median AQI moved -2 points, which classifies the multi-year trajectory as improving. The metro avoided any Unhealthy-tier days entirely, an outcome that typically reflects favorable wind patterns, lower industrial density, or effective regional emission controls. Metros with comparable AQI profiles include Athens-Clarke County, GA (66.7% good, AQI 44); Bartlesville, OK (65.6% good, AQI 44); Bloomington, IL (64.1% good, AQI 44).

Air Quality Summary

Median AQI

44

Good

Good Days

63.7%

174 of 273 days

Max AQI

141

USG

Health Grade

Moderate

90th pct: 67

AQI Day Breakdown (2025)

Distribution of 273 monitored days across EPA AQI categories.

Good

174

64%

Moderate

96

35%

USG

3

1%

Unhealthy

0

0%

Very Unhealthy

0

0%

Hazardous

0

0%

Health Impact Assessment

Moderate

The Decatur, IL metro area's air quality is rated Moderate based on 63.7% good air days. No unhealthy air days were recorded in 2025.

Air quality is generally acceptable. Unusually sensitive people should monitor real-time conditions at AirNow.gov during high-pollution events.

How does the Decatur, IL metro compare?

How Decatur, IL compares

Good-day share worse than US avg
Decatur, IL 63.7%
Athens-Clarke County, GA 66.7%
US average 71.3%
Median AQI worse than US avg
Decatur, IL 44
Athens-Clarke County, GA 44
US average 41
Unhealthy days / yr better than US avg
Decatur, IL 0
Athens-Clarke County, GA 0
US average 1

5-Year Air Quality Trend

Air quality is improving in the Decatur, IL metro. Best year: 2024 (238 good days).

Year Good Moderate Unhealthy Median AQI
2020 214 148 0 46
2021 179 179 0 51
2022 209 154 0 46
2023 171 178 1 51
2024 238 127 0 43
2025 174 96 0 44

Metros with Similar Air Quality

Metro areas across the US with a similar median AQI to Decatur, IL.

Athens-Clarke County, GA

AQI 44 · 66.7% good

44

Bartlesville, OK

AQI 44 · 65.6% good

44

Bloomington, IL

AQI 44 · 64.1% good

44

Boston-Cambridge-Newton, MA-NH

AQI 44 · 68.3% good

44

Bridgeport-Stamford-Norwalk, CT

AQI 44 · 66.8% good

44

Buffalo-Cheektowaga-Niagara Falls, NY

AQI 44 · 61.3% good

44

More Air Quality Data

Explore EPA air quality rankings and guides across U.S. metros and counties.

Cleanest Metro Areas

U.S. metros ranked by their share of Good air-quality days.

Worst Air Quality

Counties and metros with the most unhealthy air days.

All Metro Areas

Annual AQI by CBSA for every monitored U.S. metro area.

Best and Worst City Air

What drives urban air quality and how metro areas compare.

Air Quality Guides

Frequently Asked Questions

What is the air quality in the Decatur, IL metro area? ▼
The Decatur, IL metro area has a median Air Quality Index (AQI) of 44, rated "Good." 63.7% of monitored days (174 out of 273) had good air quality in 2025.
How many unhealthy air days does Decatur, IL have? ▼
In 2025, the Decatur, IL metro area recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 141.
Is air quality in Decatur, IL getting better or worse? ▼
Air quality in the Decatur, IL metro is improving based on 5-year trends (2020-2025). Best year: 2024 with 238 good days. Worst year: 2023 with 171 good days.
What does AQI 44 mean for health? ▼
An AQI of 44 is "Good", air quality is satisfactory and poses little or no health risk. This is the best category on the EPA's AQI scale.
What is the 90th percentile AQI for Decatur, IL? ▼
The 90th percentile AQI for the Decatur, IL metro area is 67, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 141 (USG).
How does Decatur, IL compare to other metro areas? ▼
The Decatur, IL metro has 63.7% good air days. Similar metros: Athens-Clarke County, GA (66.7% good), Bartlesville, OK (65.6% good), Bloomington, IL (64.1% good).

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by CBSA dataset, 2020–2025. 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 files (metro-level) · 2020–2025

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.