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 2020, at 95.6%. Cook County had the lowest, at 23.5%. The median was 67.9%. Source: EPA.

Keep it in a collection in your library.
Highest95.6Clark County
Lowest23.5Cook County
Median67.923 counties

Map of counties, 2020

Adams County: 90.2 %Champaign County: 68.3 %Clark County: 95.6 %Cook County: 23.5 %DuPage County: 58.2 %Effingham County: 94 %Hamilton County: 63.2 %Jersey County: 72.1 %Jo Daviess County: 95.5 %Kane County: 73.1 %Lake County: 82.6 %McHenry County: 57.5 %McLean County: 56.9 %Macon County: 59.1 %Macoupin County: 95.1 %Madison County: 35.3 %Peoria County: 67.9 %Randolph County: 66.4 %Rock Island County: 65.9 %St. Clair County: 71.2 %Sangamon County: 69.8 %Will County: 50.4 %Winnebago County: 56 %
  • Under 57
  • 57 to 65
  • 65 to 70
  • 70 to 87
  • 87 and over
  • No data
%, 2020. 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, 2020

Clark County: 95.6 %Clark County95.6Jo Daviess County: 95.5 %Jo Daviess County95.5Macoupin County: 95.1 %Macoupin County95.1Effingham County: 94 %Effingham County94Adams County: 90.2 %Adams County90.2Lake County: 82.6 %Lake County82.6Kane County: 73.1 %Kane County73.1Jersey County: 72.1 %Jersey County72.1St. Clair County: 71.2 %St. Clair County71.2Sangamon County: 69.8 %Sangamon County69.8Champaign County: 68.3 %Champaign County68.3Peoria County: 67.9 %Peoria County67.9Randolph County: 66.4 %Randolph County66.4Rock Island County: 65.9 %Rock Island County65.9Hamilton County: 63.2 %Hamilton County63.2Macon County: 59.1 %Macon County59.1DuPage County: 58.2 %DuPage County58.2McHenry County: 57.5 %McHenry County57.5McLean County: 56.9 %McLean County56.9Winnebago County: 56 %Winnebago County56Will County: 50.4 %Will County50.4Madison County: 35.3 %Madison County35.3Cook County: 23.5 %Cook County23.5Clark County: 95.6 %Clark County95.6Jo Daviess County: 95.5 %Jo Daviess County95.5Macoupin County: 95.1 %Macoupin County95.1Effingham County: 94 %Effingham County94Adams County: 90.2 %Adams County90.2Lake County: 82.6 %Lake County82.6Kane County: 73.1 %Kane County73.1Jersey County: 72.1 %Jersey County72.1St. Clair County: 71.2 %St. Clair County71.2Sangamon County: 69.8 %Sangamon County69.8Champaign County: 68.3 %Champaign County68.3Peoria County: 67.9 %Peoria County67.9Randolph County: 66.4 %Randolph County66.4Rock Island County: 65.9 %Rock Island County65.9Hamilton County: 63.2 %Hamilton County63.2Macon County: 59.1 %Macon County59.1DuPage County: 58.2 %DuPage County58.2McHenry County: 57.5 %McHenry County57.5McLean County: 56.9 %McLean County56.9Winnebago County: 56 %Winnebago County56Will County: 50.4 %Will County50.4Madison County: 35.3 %Madison County35.3Cook County: 23.5 %Cook County23.5
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2020 (%)Change on 2019
1Clark County95.6-0.4 pts
2Jo Daviess County95.5-1.2 pts
3Macoupin County95.1-0.1 pts
4Effingham County94+2.6 pts
5Adams County90.2+3.9 pts
6Lake County82.6-3.1 pts
7Kane County73.1-0.8 pts
8Jersey County72.1+8.4 pts
9St. Clair County71.2+11.7 pts
10Sangamon County69.8+8.1 pts
11Champaign County68.3+6.7 pts
12Peoria County67.9+3.1 pts
13Randolph County66.4-5.8 pts
14Rock Island County65.9+4.7 pts
15Hamilton County63.2+3.9 pts
16Macon County59.1+10.7 pts
17DuPage County58.2+2.5 pts
18McHenry County57.5+10.1 pts
19McLean County56.9+5.5 pts
20Winnebago County56+2.6 pts
21Will County50.4+1.4 pts
22Madison County35.3-3.3 pts
23Cook County23.5+1.6 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 2020 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), 2020) [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), 2020. 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=2020. 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), 2020. 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=2020.

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), 2020 [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=2020 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2020,
  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), 2020}},
  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=2020. 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), 2020
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=2020
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2020.
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 95.6% in 2020, according to EPA.

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

Cook County, with 23.5% in 2020.

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 2020 and are not ranked.

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

Counties in Illinois: more rankings