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 2014, at 91.8%. Cook County had the lowest, at 20.8%. The median was 78.2%. Source: EPA.

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Highest91.8Jo Daviess County
Lowest20.8Cook County
Median78.223 counties

Map of counties, 2014

Adams County: 90.1 %Champaign County: 75.9 %Clark County: 87.7 %Cook County: 20.8 %DuPage County: 78.2 %Effingham County: 88.2 %Hamilton County: 76.6 %Jersey County: 76.4 %Jo Daviess County: 91.8 %Kane County: 86.5 %Lake County: 89.3 %McHenry County: 68.1 %McLean County: 86 %Macon County: 58.6 %Macoupin County: 89.7 %Madison County: 45.2 %Peoria County: 55.9 %Randolph County: 69.1 %Rock Island County: 89.1 %St. Clair County: 53.2 %Sangamon County: 89.1 %Will County: 47.2 %Winnebago County: 88.9 %
  • Under 57
  • 57 to 76
  • 76 to 87
  • 87 to 89
  • 89 and over
  • No data
%, 2014. 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, 2014

Jo Daviess County: 91.8 %Jo Daviess County91.8Adams County: 90.1 %Adams County90.1Macoupin County: 89.7 %Macoupin County89.7Lake County: 89.3 %Lake County89.3Rock Island County: 89.1 %Rock Island County89.1Sangamon County: 89.1 %Sangamon County89.1Winnebago County: 88.9 %Winnebago County88.9Effingham County: 88.2 %Effingham County88.2Clark County: 87.7 %Clark County87.7Kane County: 86.5 %Kane County86.5McLean County: 86 %McLean County86DuPage County: 78.2 %DuPage County78.2Hamilton County: 76.6 %Hamilton County76.6Jersey County: 76.4 %Jersey County76.4Champaign County: 75.9 %Champaign County75.9Randolph County: 69.1 %Randolph County69.1McHenry County: 68.1 %McHenry County68.1Macon County: 58.6 %Macon County58.6Peoria County: 55.9 %Peoria County55.9St. Clair County: 53.2 %St. Clair County53.2Will County: 47.2 %Will County47.2Madison County: 45.2 %Madison County45.2Cook County: 20.8 %Cook County20.8Jo Daviess County: 91.8 %Jo Daviess County91.8Adams County: 90.1 %Adams County90.1Macoupin County: 89.7 %Macoupin County89.7Lake County: 89.3 %Lake County89.3Rock Island County: 89.1 %Rock Island County89.1Sangamon County: 89.1 %Sangamon County89.1Winnebago County: 88.9 %Winnebago County88.9Effingham County: 88.2 %Effingham County88.2Clark County: 87.7 %Clark County87.7Kane County: 86.5 %Kane County86.5McLean County: 86 %McLean County86DuPage County: 78.2 %DuPage County78.2Hamilton County: 76.6 %Hamilton County76.6Jersey County: 76.4 %Jersey County76.4Champaign County: 75.9 %Champaign County75.9Randolph County: 69.1 %Randolph County69.1McHenry County: 68.1 %McHenry County68.1Macon County: 58.6 %Macon County58.6Peoria County: 55.9 %Peoria County55.9St. Clair County: 53.2 %St. Clair County53.2Will County: 47.2 %Will County47.2Madison County: 45.2 %Madison County45.2Cook County: 20.8 %Cook County20.8
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2014 (%)Change on 2013
1Jo Daviess County91.8+2.2 pts
2Adams County90.1-3 pts
3Macoupin County89.7+1.6 pts
4Lake County89.3+2.4 pts
5Rock Island County89.1-6.4 pts
5Sangamon County89.1-0.2 pts
7Winnebago County88.9-4.8 pts
8Effingham County88.2+1.8 pts
9Clark County87.7-4.1 pts
10Kane County86.5-7.9 pts
11McLean County86+0.6 pts
12DuPage County78.2+19.4 pts
13Hamilton County76.6-6.7 pts
14Jersey County76.4-3.2 pts
15Champaign County75.9-4.6 pts
16Randolph County69.1-20.6 pts
17McHenry County68.1+2.2 pts
18Macon County58.6-3.3 pts
19Peoria County55.9+10.4 pts
20St. Clair County53.2+4.7 pts
21Will County47.2-27 pts
22Madison County45.2+4.4 pts
23Cook County20.80 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 2014 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), 2014) [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), 2014. 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=2014. 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), 2014. 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=2014.

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), 2014 [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=2014 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2014,
  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), 2014}},
  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=2014. 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), 2014
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=2014
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2014.
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.
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Questions

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

Jo Daviess County, with 91.8% in 2014, according to EPA.

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

Cook County, with 20.8% in 2014.

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 2014 and are not ranked.

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

Counties in Illinois: more rankings