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)

Jo Daviess County had the highest share of days with good air quality (US EPA AQI) of 23 counties in 2024, at 93.7%. Cook County had the lowest, at 35.8%. The median was 69.4%. Source: EPA.

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Highest93.7Jo Daviess County
Lowest35.8Cook County
Median69.423 counties

Map of counties, 2024

Adams County: 90.1 %Champaign County: 68.3 %Clark County: 91.5 %Cook County: 35.8 %DuPage County: 62.8 %Effingham County: 90.4 %Hamilton County: 64.5 %Jersey County: 74.5 %Jo Daviess County: 93.7 %Kane County: 72.3 %Lake County: 82 %McHenry County: 66.9 %McLean County: 69.4 %Macon County: 65 %Macoupin County: 93.4 %Madison County: 52.7 %Peoria County: 68.6 %Randolph County: 79 %Rock Island County: 75.7 %St. Clair County: 59.6 %Sangamon County: 71.9 %Will County: 67.1 %Winnebago County: 69 %
  • Under 65
  • 65 to 69
  • 69 to 73
  • 73 to 87
  • 87 and over
  • No data
%, 2024. 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, 2024

Jo Daviess County: 93.7 %Jo Daviess County93.7Macoupin County: 93.4 %Macoupin County93.4Clark County: 91.5 %Clark County91.5Effingham County: 90.4 %Effingham County90.4Adams County: 90.1 %Adams County90.1Lake County: 82 %Lake County82Randolph County: 79 %Randolph County79Rock Island County: 75.7 %Rock Island County75.7Jersey County: 74.5 %Jersey County74.5Kane County: 72.3 %Kane County72.3Sangamon County: 71.9 %Sangamon County71.9McLean County: 69.4 %McLean County69.4Winnebago County: 69 %Winnebago County69Peoria County: 68.6 %Peoria County68.6Champaign County: 68.3 %Champaign County68.3Will County: 67.1 %Will County67.1McHenry County: 66.9 %McHenry County66.9Macon County: 65 %Macon County65Hamilton County: 64.5 %Hamilton County64.5DuPage County: 62.8 %DuPage County62.8St. Clair County: 59.6 %St. Clair County59.6Madison County: 52.7 %Madison County52.7Cook County: 35.8 %Cook County35.8Jo Daviess County: 93.7 %Jo Daviess County93.7Macoupin County: 93.4 %Macoupin County93.4Clark County: 91.5 %Clark County91.5Effingham County: 90.4 %Effingham County90.4Adams County: 90.1 %Adams County90.1Lake County: 82 %Lake County82Randolph County: 79 %Randolph County79Rock Island County: 75.7 %Rock Island County75.7Jersey County: 74.5 %Jersey County74.5Kane County: 72.3 %Kane County72.3Sangamon County: 71.9 %Sangamon County71.9McLean County: 69.4 %McLean County69.4Winnebago County: 69 %Winnebago County69Peoria County: 68.6 %Peoria County68.6Champaign County: 68.3 %Champaign County68.3Will County: 67.1 %Will County67.1McHenry County: 66.9 %McHenry County66.9Macon County: 65 %Macon County65Hamilton County: 64.5 %Hamilton County64.5DuPage County: 62.8 %DuPage County62.8St. Clair County: 59.6 %St. Clair County59.6Madison County: 52.7 %Madison County52.7Cook County: 35.8 %Cook County35.8
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2024 (%)Change on 2023
1Jo Daviess County93.7+8.1 pts
2Macoupin County93.4+8.5 pts
3Clark County91.5+2.1 pts
4Effingham County90.4+15.2 pts
5Adams County90.1+15 pts
6Lake County82+7.1 pts
7Randolph County79+20.1 pts
8Rock Island County75.7+26.9 pts
9Jersey County74.5+20 pts
10Kane County72.3+10.5 pts
11Sangamon County71.9+22 pts
12McLean County69.4+19 pts
13Winnebago County69+23.2 pts
14Peoria County68.6+13.6 pts
15Champaign County68.3+14.6 pts
16Will County67.1+24.6 pts
17McHenry County66.9+14.6 pts
18Macon County65+18.2 pts
19Hamilton County64.5+6.1 pts
20DuPage County62.8+22.8 pts
21St. Clair County59.6+24.5 pts
22Madison County52.7+21.6 pts
23Cook County35.8+7.3 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 2024 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), 2024) [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), 2024. 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=2024. 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), 2024. 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=2024.

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), 2024 [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=2024 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2024,
  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), 2024}},
  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=2024. 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), 2024
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=2024
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2024.
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)?

Jo Daviess County, with 93.7% in 2024, according to EPA.

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

Cook County, with 35.8% in 2024.

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 2024 and are not ranked.

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

Counties in Illinois: more rankings