Counties in Illinois by highest daily Air Quality Index (US EPA), 2025

Madison County had the highest highest daily Air Quality Index (US EPA) of 23 counties in 2025, at 260. Adams County had the lowest, at 71. The median was 154. Source: the U.S. Environmental Protection Agency.

Highest260Madison County

Lowest71Adams County

Median15423 counties

Map of counties, 2025

Under 100
100 to 140
140 to 160
160 to 170
170 and over
No data

index, 2025. Hatched areas have no value for this period. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2025

index. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Retrieved 2026-10-12.

Full ranking

Rank	Counties	2025 index	Change on 2024
1	Madison County	260	+95
2	Winnebago County	169	+50
3	Cook County	168	+2
4	Rock Island County	167	+83
5	Kane County	165	+60
5	McHenry County	165	+57
7	Randolph County	161	+61
7	St. Clair County	161	+49
9	DuPage County	158	+61
10	Jersey County	156	+44
10	Will County	156	+72
12	McLean County	154	+42
12	Peoria County	154	+54
14	Sangamon County	142	+37
15	Macon County	141	+40
16	Champaign County	138	+38
17	Lake County	108	-32
18	Macoupin County	105	0
19	Hamilton County	98	-14
20	Clark County	87	-3
20	Jo Daviess County	87	-14
22	Effingham County	84	-17
23	Adams County	71	-26

Other periods

About this ranking

Highest daily Air Quality Index (AQI) value recorded in a US county during the year, from the EPA AirData annual summaries. Values above 500 are beyond the AQI scale and are published as EPA computes them.

A single day decides the value, so one wildfire or dust event can set it. Dust from the dry Owens Lake bed puts Inyo and Mono counties in California in the thousands. United States counties with air quality monitors only, about 1,000 of some 3,100 in recent years; a county without monitors has no value. The number of days with an AQI differs between counties because some monitors run only in the ozone season.

Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. 10 counties with data in other periods have no value for 2025 and are left out rather than filled from another source. Equal values share a rank.

How to cite

U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.

Citation styles

The publisher is the author. Invokely is named only as the place the figures were read, where a style asks for it.

APA 7

U.S. Environmental Protection Agency. (2026). EPA AirData annual AQI by county: highest daily AQI (Counties in Illinois by highest daily Air Quality Index (US EPA), 2025) [Data set]. Retrieved October 12, 2026, from https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual

MLA 9

EPA AirData annual AQI by county: highest daily AQI, Counties in Illinois by highest daily Air Quality Index (US EPA), 2025. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/illinois-counties. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: highest daily AQI. Counties in Illinois by highest daily Air Quality Index (US EPA), 2025. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/illinois-counties.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: highest daily AQI, Counties in Illinois by highest daily Air Quality Index (US EPA), 2025 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/highest-daily-air-quality-index/illinois-counties (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_illinois_by_2025,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: highest daily AQI: Counties in Illinois by highest daily Air Quality Index (US EPA), 2025}},
  year = {2026},
  howpublished = {Data set},
  url = {https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual},
  urldate = {2026-10-12},
  note = {Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/illinois-counties. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest daily AQI. https://aqs.epa.gov/aqsweb/airdata/download\_files.html\#Annual. Retrieved 2026-10-12.},
}

RIS

TY  - DATA
AU  - U.S. Environmental Protection Agency
TI  - EPA AirData annual AQI by county: highest daily AQI: Counties in Illinois by highest daily Air Quality Index (US EPA), 2025
PY  - 2026
DA  - 2026/07/13
PB  - U.S. Environmental Protection Agency
DB  - Invokely
UR  - https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
L2  - https://invokely.ai/rankings/highest-daily-air-quality-index/illinois-counties
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/illinois-counties.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -

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Rank Counties in Illinois by highest daily Air Quality Index (US EPA) using official data, and cite the source.

rank_places {"indicator":"aqi_max","level":"province","within":"Illinois","limit":10}
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Questions

Which county has the highest highest daily Air Quality Index (US EPA)?

Madison County, with 260 in 2025, according to the U.S. Environmental Protection Agency.

Which county has the lowest highest daily Air Quality Index (US EPA)?

Adams County, with 71 in 2025.

Where does this ranking come from?

Every value comes from one source and one period, so the places are comparable: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: highest daily AQI. Retrieved 2026-10-12. 10 counties have no value for 2025 and are not ranked.