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 2017, at 93.5%. Cook County had the lowest, at 31.2%. The median was 64.9%. Source: EPA.

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Highest93.5Jo Daviess County
Lowest31.2Cook County
Median64.923 counties

Map of counties, 2017

Adams County: 87.3 %Champaign County: 63.8 %Clark County: 87.6 %Cook County: 31.2 %DuPage County: 67.5 %Effingham County: 84.4 %Hamilton County: 60.3 %Jersey County: 55 %Jo Daviess County: 93.5 %Kane County: 77.9 %Lake County: 88.3 %McHenry County: 63.3 %McLean County: 55.2 %Macon County: 59 %Macoupin County: 89.3 %Madison County: 42.5 %Peoria County: 47.1 %Randolph County: 44.6 %Rock Island County: 74.4 %St. Clair County: 64.9 %Sangamon County: 72 %Will County: 63.3 %Winnebago County: 83.6 %
  • Under 55
  • 55 to 63
  • 63 to 72
  • 72 to 86
  • 86 and over
  • No data
%, 2017. 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, 2017

Jo Daviess County: 93.5 %Jo Daviess County93.5Macoupin County: 89.3 %Macoupin County89.3Lake County: 88.3 %Lake County88.3Clark County: 87.6 %Clark County87.6Adams County: 87.3 %Adams County87.3Effingham County: 84.4 %Effingham County84.4Winnebago County: 83.6 %Winnebago County83.6Kane County: 77.9 %Kane County77.9Rock Island County: 74.4 %Rock Island County74.4Sangamon County: 72 %Sangamon County72DuPage County: 67.5 %DuPage County67.5St. Clair County: 64.9 %St. Clair County64.9Champaign County: 63.8 %Champaign County63.8McHenry County: 63.3 %McHenry County63.3Will County: 63.3 %Will County63.3Hamilton County: 60.3 %Hamilton County60.3Macon County: 59 %Macon County59McLean County: 55.2 %McLean County55.2Jersey County: 55 %Jersey County55Peoria County: 47.1 %Peoria County47.1Randolph County: 44.6 %Randolph County44.6Madison County: 42.5 %Madison County42.5Cook County: 31.2 %Cook County31.2Jo Daviess County: 93.5 %Jo Daviess County93.5Macoupin County: 89.3 %Macoupin County89.3Lake County: 88.3 %Lake County88.3Clark County: 87.6 %Clark County87.6Adams County: 87.3 %Adams County87.3Effingham County: 84.4 %Effingham County84.4Winnebago County: 83.6 %Winnebago County83.6Kane County: 77.9 %Kane County77.9Rock Island County: 74.4 %Rock Island County74.4Sangamon County: 72 %Sangamon County72DuPage County: 67.5 %DuPage County67.5St. Clair County: 64.9 %St. Clair County64.9Champaign County: 63.8 %Champaign County63.8McHenry County: 63.3 %McHenry County63.3Will County: 63.3 %Will County63.3Hamilton County: 60.3 %Hamilton County60.3Macon County: 59 %Macon County59McLean County: 55.2 %McLean County55.2Jersey County: 55 %Jersey County55Peoria County: 47.1 %Peoria County47.1Randolph County: 44.6 %Randolph County44.6Madison County: 42.5 %Madison County42.5Cook County: 31.2 %Cook County31.2
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2017 (%)Change on 2016
1Jo Daviess County93.5-1.1 pts
2Macoupin County89.3-0.4 pts
3Lake County88.3-0.1 pts
4Clark County87.6+1.4 pts
5Adams County87.3-5.7 pts
6Effingham County84.4-0.2 pts
7Winnebago County83.6-1.1 pts
8Kane County77.9-2 pts
9Rock Island County74.4-12.5 pts
10Sangamon County72-11.5 pts
11DuPage County67.5-5.5 pts
12St. Clair County64.9+3.2 pts
13Champaign County63.8-5.6 pts
14McHenry County63.3-3.3 pts
14Will County63.3+8.2 pts
16Hamilton County60.3-8.8 pts
17Macon County59+3.9 pts
18McLean County55.2-22.1 pts
19Jersey County55-13.5 pts
20Peoria County47.1-9.2 pts
21Randolph County44.6-26.2 pts
22Madison County42.5-5.6 pts
23Cook County31.2-0.2 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 2017 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), 2017) [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), 2017. 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=2017. 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), 2017. 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=2017.

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), 2017 [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=2017 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2017,
  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), 2017}},
  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=2017. 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), 2017
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=2017
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2017.
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 93.5% in 2017, according to EPA.

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

Cook County, with 31.2% in 2017.

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 2017 and are not ranked.

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

Counties in Illinois: more rankings