Counties in Illinois by highest daily Air Quality Index (US EPA)
St. Clair County had the highest highest daily Air Quality Index (US EPA) of 23 counties in 2021, at 185. Effingham County had the lowest, at 77. The median was 105. Source: the U.S. Environmental Protection Agency.
Map of counties, 2021
- Under 97
- 97 to 100
- 100 to 110
- 110 to 120
- 120 and over
- No data
All counties, 2021
Full ranking
| Rank | Counties | 2021 | Change on 2020 |
|---|---|---|---|
| 1 | St. Clair County | 185 | +85 |
| 2 | Cook County | 156 | -8 |
| 3 | Madison County | 143 | +31 |
| 4 | Lake County | 136 | -49 |
| 5 | Randolph County | 122 | +19 |
| 6 | Hamilton County | 115 | -3 |
| 7 | Jersey County | 112 | 0 |
| 7 | Kane County | 112 | -49 |
| 7 | Macon County | 112 | +22 |
| 7 | Rock Island County | 112 | +10 |
| 11 | McLean County | 107 | -29 |
| 12 | DuPage County | 105 | -49 |
| 12 | McHenry County | 105 | -42 |
| 12 | Will County | 105 | -51 |
| 15 | Peoria County | 101 | -14 |
| 16 | Winnebago County | 100 | -1 |
| 17 | Champaign County | 99 | -27 |
| 18 | Sangamon County | 98 | +1 |
| 19 | Jo Daviess County | 97 | +10 |
| 20 | Adams County | 93 | +9 |
| 20 | Macoupin County | 93 | 0 |
| 22 | Clark County | 84 | -13 |
| 23 | Effingham County | 77 | -7 |
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 2021 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
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), 2021) [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), 2021. 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?period=2021. 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), 2021. 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?period=2021.
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), 2021 [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?period=2021 (Accessed: 12 October 2026).
BibTeX
@misc{environmental_counties_in_illinois_by_2021,
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), 2021}},
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?period=2021. 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), 2021
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?period=2021
Y2 - 2026/10/12
N1 - Accessed via Invokely, https://invokely.ai/rankings/highest-daily-air-quality-index/illinois-counties?period=2021.
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 -
Use this data in your AI agent
Invokely serves this figure to AI assistants through MCP and to code through a REST API, with the source and citation attached. Ask your assistant:
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}
GET https://invokely.ai/api/v1/rank?indicator=aqi_max&level=province&within=Illinois&limit=10Questions
Which county has the highest highest daily Air Quality Index (US EPA)?
St. Clair County, with 185 in 2021, according to the U.S. Environmental Protection Agency.
Which county has the lowest highest daily Air Quality Index (US EPA)?
Effingham County, with 77 in 2021.
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 2021 and are not ranked.