Counties in Illinois by share of days unhealthy for sensitive groups or worse (US EPA AQI)
Cook County had the highest share of days unhealthy for sensitive groups or worse (US EPA AQI) of 23 counties in 2021, at 5.8%. Winnebago County had the lowest, at 0%. The median was 0.4%. Source: EPA.
Map of counties, 2021
- Under 0.3
- 0.3 to 0.5
- 0.5 to 0.8
- 0.8 and over
- No data
All counties, 2021
Full ranking
| Rank | Counties | 2021 (%) | Change on 2020 |
|---|---|---|---|
| 1 | Cook County | 5.8 | +0.3 pts |
| 2 | Lake County | 3.7 | -0.9 pts |
| 3 | Madison County | 1.6 | +0.8 pts |
| 4 | Macon County | 0.8 | +0.8 pts |
| 4 | Rock Island County | 0.8 | +0.5 pts |
| 4 | St. Clair County | 0.8 | +0.8 pts |
| 7 | Jersey County | 0.6 | +0.3 pts |
| 7 | McHenry County | 0.6 | -2.1 pts |
| 9 | DuPage County | 0.5 | -0.6 pts |
| 9 | Hamilton County | 0.5 | +0.2 pts |
| 9 | Will County | 0.5 | -0.6 pts |
| 12 | Kane County | 0.4 | -2.1 pts |
| 13 | McLean County | 0.3 | -0.5 pts |
| 13 | Peoria County | 0.3 | 0 pts |
| 13 | Randolph County | 0.3 | -0.2 pts |
| 16 | Adams County | 0 | 0 pts |
| 16 | Champaign County | 0 | -0.5 pts |
| 16 | Clark County | 0 | 0 pts |
| 16 | Effingham County | 0 | 0 pts |
| 16 | Jo Daviess County | 0 | 0 pts |
| 16 | Macoupin County | 0 | 0 pts |
| 16 | Sangamon County | 0 | 0 pts |
| 16 | Winnebago County | 0 | -0.3 pts |
Other periods
About this ranking
Days with a daily Air Quality Index above 100 (unhealthy for sensitive groups, unhealthy, very unhealthy or hazardous) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; the counts are stored with each value.
Use the share rather than the number of days to compare counties, since the days with an AQI differ. 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts (Counties in Illinois by share of days unhealthy for sensitive groups or worse (US EPA AQI), 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts, Counties in Illinois by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/illinois-counties?period=2021. Accessed 12 Oct. 2026.
Chicago (author-date)
U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. Counties in Illinois by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/illinois-counties?period=2021.
Harvard
U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts, Counties in Illinois by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2021 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/unhealthy-air-quality-days-share/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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts: Counties in Illinois by share of days unhealthy for sensitive groups or worse (US EPA AQI), 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/unhealthy-air-quality-days-share/illinois-counties?period=2021. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts: Counties in Illinois by share of days unhealthy for sensitive groups or worse (US EPA AQI), 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/unhealthy-air-quality-days-share/illinois-counties?period=2021
Y2 - 2026/10/12
N1 - Accessed via Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/illinois-counties?period=2021.
N1 - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. 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 share of days unhealthy for sensitive groups or worse (US EPA AQI) using official data, and cite the source.
rank_places {"indicator":"aqi_unhealthy_sensitive_or_worse_days_share","level":"province","within":"Illinois","limit":10}
GET https://invokely.ai/api/v1/rank?indicator=aqi_unhealthy_sensitive_or_worse_days_share&level=province&within=Illinois&limit=10Questions
Which county has the highest share of days unhealthy for sensitive groups or worse (US EPA AQI)?
Cook County, with 5.8% in 2021, according to EPA.
Which county has the lowest share of days unhealthy for sensitive groups or worse (US EPA AQI)?
Winnebago County, with 0% 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 10 counties have no value for 2021 and are not ranked.