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)

Effingham County had the highest share of days with good air quality (US EPA AQI) of 23 counties in 2021, at 95.4%. Madison County had the lowest, at 22.5%. The median was 56.9%. Source: EPA.

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Highest95.4Effingham County
Lowest22.5Madison County
Median56.923 counties

Map of counties, 2021

Adams County: 90.5 %Champaign County: 64.7 %Clark County: 94.8 %Cook County: 23.8 %DuPage County: 49 %Effingham County: 95.4 %Hamilton County: 56.9 %Jersey County: 62.5 %Jo Daviess County: 92.8 %Kane County: 66 %Lake County: 73.8 %McHenry County: 51.5 %McLean County: 52.3 %Macon County: 49.6 %Macoupin County: 94.7 %Madison County: 22.5 %Peoria County: 50.1 %Randolph County: 62.8 %Rock Island County: 52.9 %St. Clair County: 38.6 %Sangamon County: 64.4 %Will County: 43 %Winnebago County: 52.8 %
  • Under 49
  • 49 to 53
  • 53 to 63
  • 63 to 84
  • 84 and over
  • No data
%, 2021. 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, 2021

Effingham County: 95.4 %Effingham County95.4Clark County: 94.8 %Clark County94.8Macoupin County: 94.7 %Macoupin County94.7Jo Daviess County: 92.8 %Jo Daviess County92.8Adams County: 90.5 %Adams County90.5Lake County: 73.8 %Lake County73.8Kane County: 66 %Kane County66Champaign County: 64.7 %Champaign County64.7Sangamon County: 64.4 %Sangamon County64.4Randolph County: 62.8 %Randolph County62.8Jersey County: 62.5 %Jersey County62.5Hamilton County: 56.9 %Hamilton County56.9Rock Island County: 52.9 %Rock Island County52.9Winnebago County: 52.8 %Winnebago County52.8McLean County: 52.3 %McLean County52.3McHenry County: 51.5 %McHenry County51.5Peoria County: 50.1 %Peoria County50.1Macon County: 49.6 %Macon County49.6DuPage County: 49 %DuPage County49Will County: 43 %Will County43St. Clair County: 38.6 %St. Clair County38.6Cook County: 23.8 %Cook County23.8Madison County: 22.5 %Madison County22.5Effingham County: 95.4 %Effingham County95.4Clark County: 94.8 %Clark County94.8Macoupin County: 94.7 %Macoupin County94.7Jo Daviess County: 92.8 %Jo Daviess County92.8Adams County: 90.5 %Adams County90.5Lake County: 73.8 %Lake County73.8Kane County: 66 %Kane County66Champaign County: 64.7 %Champaign County64.7Sangamon County: 64.4 %Sangamon County64.4Randolph County: 62.8 %Randolph County62.8Jersey County: 62.5 %Jersey County62.5Hamilton County: 56.9 %Hamilton County56.9Rock Island County: 52.9 %Rock Island County52.9Winnebago County: 52.8 %Winnebago County52.8McLean County: 52.3 %McLean County52.3McHenry County: 51.5 %McHenry County51.5Peoria County: 50.1 %Peoria County50.1Macon County: 49.6 %Macon County49.6DuPage County: 49 %DuPage County49Will County: 43 %Will County43St. Clair County: 38.6 %St. Clair County38.6Cook County: 23.8 %Cook County23.8Madison County: 22.5 %Madison County22.5
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2021 (%)Change on 2020
1Effingham County95.4+1.4 pts
2Clark County94.8-0.8 pts
3Macoupin County94.7-0.4 pts
4Jo Daviess County92.8-2.7 pts
5Adams County90.5+0.3 pts
6Lake County73.8-8.8 pts
7Kane County66-7.1 pts
8Champaign County64.7-3.6 pts
9Sangamon County64.4-5.4 pts
10Randolph County62.8-3.6 pts
11Jersey County62.5-9.6 pts
12Hamilton County56.9-6.3 pts
13Rock Island County52.9-13 pts
14Winnebago County52.8-3.2 pts
15McLean County52.3-4.6 pts
16McHenry County51.5-6 pts
17Peoria County50.1-17.8 pts
18Macon County49.6-9.5 pts
19DuPage County49-9.2 pts
20Will County43-7.4 pts
21St. Clair County38.6-32.6 pts
22Cook County23.8+0.3 pts
23Madison County22.5-12.8 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 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: 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), 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: 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), 2021. 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=2021. 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), 2021. 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=2021.

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), 2021 [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=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: 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), 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/good-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: 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), 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/good-air-quality-days-share/illinois-counties?period=2021
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-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: 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)?

Effingham County, with 95.4% in 2021, according to EPA.

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

Madison County, with 22.5% 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: good days 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.

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

Counties in Illinois: more rankings