Data / Rankings / Share of days with good air quality (US EPA AQI) / Counties in Illinois

Counties in Illinois by share of days with good air quality (US EPA AQI)

Clark County had the highest share of days with good air quality (US EPA AQI) of 23 counties in 2023, at 89.4%. Cook County had the lowest, at 28.5%. The median was 53.7%. Source: EPA.

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Highest89.4Clark County
Lowest28.5Cook County
Median53.723 counties

Map of counties, 2023

Adams County: 75.1 %Champaign County: 53.7 %Clark County: 89.4 %Cook County: 28.5 %DuPage County: 40 %Effingham County: 75.2 %Hamilton County: 58.4 %Jersey County: 54.5 %Jo Daviess County: 85.6 %Kane County: 61.8 %Lake County: 74.9 %McHenry County: 52.3 %McLean County: 50.4 %Macon County: 46.8 %Macoupin County: 84.9 %Madison County: 31.1 %Peoria County: 55 %Randolph County: 58.9 %Rock Island County: 48.8 %St. Clair County: 35.1 %Sangamon County: 49.9 %Will County: 42.5 %Winnebago County: 45.8 %
  • Under 44
  • 44 to 50
  • 50 to 56
  • 56 to 75
  • 75 and over
  • No data
%, 2023. Hatched areas have no value for this period. Source: Invokely calculation with data from EPA. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2023

Clark County: 89.4 %Clark County89.4Jo Daviess County: 85.6 %Jo Daviess County85.6Macoupin County: 84.9 %Macoupin County84.9Effingham County: 75.2 %Effingham County75.2Adams County: 75.1 %Adams County75.1Lake County: 74.9 %Lake County74.9Kane County: 61.8 %Kane County61.8Randolph County: 58.9 %Randolph County58.9Hamilton County: 58.4 %Hamilton County58.4Peoria County: 55 %Peoria County55Jersey County: 54.5 %Jersey County54.5Champaign County: 53.7 %Champaign County53.7McHenry County: 52.3 %McHenry County52.3McLean County: 50.4 %McLean County50.4Sangamon County: 49.9 %Sangamon County49.9Rock Island County: 48.8 %Rock Island County48.8Macon County: 46.8 %Macon County46.8Winnebago County: 45.8 %Winnebago County45.8Will County: 42.5 %Will County42.5DuPage County: 40 %DuPage County40St. Clair County: 35.1 %St. Clair County35.1Madison County: 31.1 %Madison County31.1Cook County: 28.5 %Cook County28.5Clark County: 89.4 %Clark County89.4Jo Daviess County: 85.6 %Jo Daviess County85.6Macoupin County: 84.9 %Macoupin County84.9Effingham County: 75.2 %Effingham County75.2Adams County: 75.1 %Adams County75.1Lake County: 74.9 %Lake County74.9Kane County: 61.8 %Kane County61.8Randolph County: 58.9 %Randolph County58.9Hamilton County: 58.4 %Hamilton County58.4Peoria County: 55 %Peoria County55Jersey County: 54.5 %Jersey County54.5Champaign County: 53.7 %Champaign County53.7McHenry County: 52.3 %McHenry County52.3McLean County: 50.4 %McLean County50.4Sangamon County: 49.9 %Sangamon County49.9Rock Island County: 48.8 %Rock Island County48.8Macon County: 46.8 %Macon County46.8Winnebago County: 45.8 %Winnebago County45.8Will County: 42.5 %Will County42.5DuPage County: 40 %DuPage County40St. Clair County: 35.1 %St. Clair County35.1Madison County: 31.1 %Madison County31.1Cook County: 28.5 %Cook County28.5
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2023 (%)Change on 2022
1Clark County89.4-5.5 pts
2Jo Daviess County85.6-10 pts
3Macoupin County84.9-12.1 pts
4Effingham County75.2-14.8 pts
5Adams County75.1-21.9 pts
6Lake County74.9-7 pts
7Kane County61.8-4.5 pts
8Randolph County58.9-7.9 pts
9Hamilton County58.4-3.8 pts
10Peoria County55-1.7 pts
11Jersey County54.5-11.3 pts
12Champaign County53.7-7.4 pts
13McHenry County52.3-2.2 pts
14McLean County50.4-6.3 pts
15Sangamon County49.9-10.9 pts
16Rock Island County48.8-10.6 pts
17Macon County46.8-10.5 pts
18Winnebago County45.8-19.5 pts
19Will County42.5-10.4 pts
20DuPage County40-14.5 pts
21St. Clair County35.1-10.1 pts
22Madison County31.1-5.8 pts
23Cook County28.5-3.6 pts

Other periods

About this ranking

Days with a daily Air Quality Index of 0 to 50 (good) 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; both counts are stored with each value.

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 2023 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: good days 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

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: good days as a share of days with an AQI, calculated with stored counts (Counties in Illinois by share of days with good air quality (US EPA AQI), 2023) [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: good days as a share of days with an AQI, calculated with stored counts, Counties in Illinois by share of days with good air quality (US EPA AQI), 2023. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2023. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. Counties in Illinois by share of days with good air quality (US EPA AQI), 2023. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2023.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts, Counties in Illinois by share of days with good air quality (US EPA AQI), 2023 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2023 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2023,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts: Counties in Illinois by share of days with good air quality (US EPA AQI), 2023}},
  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/good-air-quality-days-share/illinois-counties?period=2023. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days 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: good days as a share of days with an AQI, calculated with stored counts: Counties in Illinois by share of days with good air quality (US EPA AQI), 2023
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/good-air-quality-days-share/illinois-counties?period=2023
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2023.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days 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  -
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Rank Counties in Illinois by share of days with good air quality (US EPA AQI) using official data, and cite the source.
rank_places {"indicator":"aqi_good_days_share","level":"province","within":"Illinois","limit":10}
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Questions

Which county has the highest share of days with good air quality (US EPA AQI)?

Clark County, with 89.4% in 2023, according to EPA.

Which county has the lowest share of days with good air quality (US EPA AQI)?

Cook County, with 28.5% in 2023.

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: good days as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 10 counties have no value for 2023 and are not ranked.

Share of days with good air quality (US EPA AQI): other rankings

Counties in Illinois: more rankings