Metro area air quality · 2025

Decatur, IL

Median AQI 44 (Good). 64.9% of monitored days rated Good across this metro's EPA monitoring network.

Compiled by PlainAirData · August 23, 2026

44
Median AQI: 64.9%
Good-air days: 0
Unhealthy days: 34%
Cleaner than US metros

Decatur, IL posted a 2025 median AQI of 44 (Good) with 64.9% Good days

According to the U.S. EPA Air Quality System, the Decatur, IL CBSA recorded a median AQI of 44 across 365 monitored days in 2025, with 64.9% rated Good and 0 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 Decatur, IL CBSA's 64.9% Good-day share places it cleaner than 34% of US metros in 2025.
Closest median-AQI peer: Bartlesville, OK (68.0% Good, median AQI 44) is the closest nationwide median-AQI peer to Decatur, IL.
2020–2025 Good-day shift: Decatur, IL's Good-day share moved +6.4 points from 2020 to 2025 (median AQI -2).
Median AQI: Median AQI: 44

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

The verdict

Decatur, IL's peak reading 141 is in the upper max-AQI tercile among CBSA metros here. On the EPA scale the metro runs good, more polluted than the median US metro, with a median AQI of 44 and 64.9% of 365 monitored days in the Good band.

44
median AQI · Good: 64.9%
days rated Good: 34%
cleaner than US metros: 141
worst single reading

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

Decatur, IL metro's peak single-day AQI rose from 101 to 141 between the 2024 and 2025 EPA annual releases, a change of +40 points.

Decatur, IL's peak reading 141 is in the upper max-AQI tercile among CBSA metros here. Signature read: peak AQI 141 (high tercile vs CBSA corpus). Aggregated EPA AQS monitors across this Core-Based Statistical Area logged a median Air Quality Index of 44 over 365 measured days in 2025 (Good / Moderate). The monitored-day split lands at 237 Good (65%), 125 Moderate (34%), and 3 Unhealthy-for-Sensitive-Groups (1%). The 90th-percentile AQI finished at 64; the highest single-day reading hit 141 (USG).

Across 2020–2025, Good-day share shifted +6.4 points and median AQI moved -2 (improving). No Unhealthy-tier days appear in this annual dataset. Metros with comparable AQI profiles include Bartlesville, OK (68.0% good, AQI 44); Berlin, NH-VT (81.5% good, AQI 44); Bloomington, IL (66.0% good, AQI 44).

Health effects of air pollution →

Air Quality Summary

Median AQI

Good

Good Days

64.9%

237 of 365 days

Max AQI

141

USG

Health Grade

Moderate

90th pct: 64

AQI Day Breakdown (2025)

Distribution of 365 monitored days across EPA AQI categories.

Good

237

65%

Moderate

125

34%

USG

Unhealthy

Very Unhealthy

Hazardous

Health Impact Assessment

Moderate

The Decatur, IL metro area's air quality is rated Moderate based on 64.9% 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

Compare against

Good-day share worse than typical

Decatur, IL 64.9%

Bartlesville, OK 68.0%

Typical US metro 70.6%

Median AQI worse than typical

Decatur, IL 44

Bartlesville, OK 44

Typical US metro 41

Unhealthy days / yr better than typical

Decatur, IL 0

Bartlesville, OK 1

Typical US metro 1

5-Year Air Quality Trend

Air quality is improving in the Decatur, IL metro. Highest Good-day share: 2025 (65.5%, 237 of 362 monitored days).

Year	Good	Moderate	Unhealthy	Median AQI	Max AQI
2020	214	148	0	46	90
2021	179	179	0	51	112
2022	209	154	0	46	115
2023	171	178	1	51	287
2024	238	127	0	43	101
2025	237	125	0	44	141

Metros with a similar median AQI

CBSA peers nationwide whose annual median AQI sits near Decatur, IL (44).

Bartlesville, OK

Median AQI 44 · 68.0% Good-day share

Berlin, NH-VT

Median AQI 44 · 81.5% Good-day share

Bloomington, IL

Median AQI 44 · 66.0% Good-day share

Boston-Cambridge-Newton, MA-NH

Median AQI 44 · 71.0% Good-day share

Bridgeport-Stamford-Norwalk, CT

Median AQI 44 · 64.7% Good-day share

Buffalo-Cheektowaga-Niagara Falls, NY

Median AQI 44 · 63.8% Good-day share

Metros with similar unhealthy-day exposure

A second relationship type: metros whose unhealthy-or-worse day count is closest to Decatur, IL's 0 (independent of median AQI).

Aberdeen, SD

0 unhealthy-or-worse days · median AQI 27

Aberdeen, WA

0 unhealthy-or-worse days · median AQI 22

Adrian, MI

0 unhealthy-or-worse days · median AQI 41

Akron, OH

0 unhealthy-or-worse days · median AQI 45

Albany, GA

0 unhealthy-or-worse days · median AQI 46

Albany, OR

0 unhealthy-or-worse days · median AQI 27

Frequently Asked Questions

What is the 90th percentile AQI for Decatur, IL? ▼

The 90th percentile AQI for the Decatur, IL metro area is 64, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 141 (USG).

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.

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." 64.9% of monitored days (237 out of 365) had good air quality in 2025.

Is air quality in Decatur, IL getting better or worse? ▼

Air quality in the Decatur, IL 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 65.5% (237 of 362 days). Lowest qualifying Good-day share: 2023 at 48.9% (171 of 350 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.

How does Decatur, IL compare to other metro areas? ▼

The Decatur, IL metro has 64.9% good air days. Similar metros: Bartlesville, OK (68.0% good), Berlin, NH-VT (81.5% good), Bloomington, IL (66.0% good).

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by CBSA dataset for Decatur, IL, 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, Decatur, IL · 2020–2025 Accessed 2026-07-06