Which county in Illinois has the best air quality?

Based on 2024 EPA AQS data, Macoupin County had the best air quality in Illinois with a median AQI of 35 and 342 good-air days.

2024 monitoring year Public dataset EPA AQS

Illinois Air Quality Rankings

Compiled by PlainEnviro Editorial, Environmental Data Editorial Team · July 13, 2026

County-level AQI rollup using EPA Air Quality System monitor records. Refreshed as new EPA AQS data lands.

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County-by-county air quality data for Illinois based on EPA AQS annual AQI readings. Data covers 23 monitored counties for 2024.

Illinois (2024): 23 counties monitored. State average median AQI: 43 (national average: 38.8). 72% of monitored days rated "Good." Most unhealthy air: Madison County (2 days above AQI 100).

Illinois air quality on this page is drawn from the EPA Air Quality System (AQS), which aggregates daily readings from federal, state, tribal, and local monitors into annual summaries per county. The current dataset covers 23 Illinois counties with at least ten days of valid AQI readings in 2024, out of the state's total monitored counties.

The state-average median AQI across those counties is 43, compared with a national average of 38.8 in the same reporting period. On average, 72% of monitored days fell into the "Good" AQI bucket (0–50). The heaviest burden of unhealthy days lands on Madison County (2 days above AQI 100), while Macoupin County records the cleanest air with a median AQI of 35.

AQI values translate ambient concentrations of pollutants (ozone, PM2.5, PM10, NO₂, SO₂, CO) onto a single 0–500 scale with six health-based bands. A low median AQI reflects typical daily conditions; peak values matter separately because short-term exposure drives most acute health effects. Counties without monitors are not "clean" - they are simply unmeasured by EPA AQS. For site-specific exposure guidance, check AirNow.gov alongside the county detail links below.

Counties with Data

State Avg Median AQI

National Avg Median AQI

38.8

Avg Good Days %

72%

Which Illinois counties have the worst air quality?

Ranked by median AQI (higher = worse). Click a county for full environmental data.

#	County	Median AQI	Good Days	Good %	Unhealthy Days	Peak AQI
1	Cook	54	131	36%	1	166
2	Madison	49	193	53%	2	165
3	St. Clair	46	218	60%	0	112
4	Dupage	45	230	63%	0	97
5	Will	44	245	67%	0	84
6	Rock Island	44	277	76%	0	84
7	Peoria	44	236	69%	0	100
8	Mclean	44	254	69%	0	112
9	Champaign	44	250	68%	0	100
10	Hamilton	44	236	64%	0	112
11	Mchenry	43	245	67%	0	108
12	Winnebago	43	249	69%	0	119
13	Macon	43	238	65%	0	101
14	Kane	42	237	72%	0	105
15	Jersey	42	248	74%	0	112
16	Adams	42	210	90%	0	97
17	Effingham	41	217	90%	0	101
18	Sangamon	41	263	72%	0	105
19	Lake	40	201	82%	0	140
20	Randolph	40	289	79%	0	100
21	Jo Daviess	36	327	94%	0	101
22	Clark	36	332	91%	0	90
23	Macoupin	35	342	93%	0	105

Frequently Asked Questions

What is the air quality like in Illinois?

In 2024, Illinois had an average median AQI of 43 across 23 monitored counties, with 72% of monitored days rated "Good" (AQI 0–50). The national average median AQI is 38.8.

Which county in Illinois has the worst air quality?

Based on 2024 EPA AQS data, Madison County had the most unhealthy air days in Illinois - 2 days above AQI 100, with a median AQI of 49.

Explore More

National AQI Rankings → Illinois Full Profile → Illinois Water Quality →

About This Data

Air quality data is from the EPA Air Quality System (AQS), Annual AQI by County dataset. Counties with fewer than 10 monitored days are excluded. "Unhealthy days" counts days with AQI above 100 (Unhealthy for Sensitive Groups or worse). All data is for 2024.

Source: U.S. Environmental Protection Agency, Air Quality System (AQS) U.S. Environmental Protection Agency, Air Quality System (AQS)

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

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Data sourced from the U.S. Environmental Protection Agency (EPA). See our methodology for details. Retrieved and formatted by PlainEnviro Editorial