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 2015, at 94.5%. Cook County had the lowest, at 20%. The median was 79.2%. Source: EPA.

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Highest94.5Jo Daviess County
Lowest20Cook County
Median79.223 counties

Map of counties, 2015

Adams County: 90.2 %Champaign County: 59.5 %Clark County: 86.7 %Cook County: 20 %DuPage County: 75.7 %Effingham County: 90.6 %Hamilton County: 77.8 %Jersey County: 79.2 %Jo Daviess County: 94.5 %Kane County: 79.6 %Lake County: 91.7 %McHenry County: 47.5 %McLean County: 83.7 %Macon County: 62.5 %Macoupin County: 91.5 %Madison County: 53.2 %Peoria County: 55.1 %Randolph County: 77 %Rock Island County: 84.4 %St. Clair County: 56.2 %Sangamon County: 83.7 %Will County: 42 %Winnebago County: 82.9 %
  • Under 56
  • 56 to 77
  • 77 to 83
  • 83 to 89
  • 89 and over
  • No data
%, 2015. 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, 2015

Jo Daviess County: 94.5 %Jo Daviess County94.5Lake County: 91.7 %Lake County91.7Macoupin County: 91.5 %Macoupin County91.5Effingham County: 90.6 %Effingham County90.6Adams County: 90.2 %Adams County90.2Clark County: 86.7 %Clark County86.7Rock Island County: 84.4 %Rock Island County84.4McLean County: 83.7 %McLean County83.7Sangamon County: 83.7 %Sangamon County83.7Winnebago County: 82.9 %Winnebago County82.9Kane County: 79.6 %Kane County79.6Jersey County: 79.2 %Jersey County79.2Hamilton County: 77.8 %Hamilton County77.8Randolph County: 77 %Randolph County77DuPage County: 75.7 %DuPage County75.7Macon County: 62.5 %Macon County62.5Champaign County: 59.5 %Champaign County59.5St. Clair County: 56.2 %St. Clair County56.2Peoria County: 55.1 %Peoria County55.1Madison County: 53.2 %Madison County53.2McHenry County: 47.5 %McHenry County47.5Will County: 42 %Will County42Cook County: 20 %Cook County20Jo Daviess County: 94.5 %Jo Daviess County94.5Lake County: 91.7 %Lake County91.7Macoupin County: 91.5 %Macoupin County91.5Effingham County: 90.6 %Effingham County90.6Adams County: 90.2 %Adams County90.2Clark County: 86.7 %Clark County86.7Rock Island County: 84.4 %Rock Island County84.4McLean County: 83.7 %McLean County83.7Sangamon County: 83.7 %Sangamon County83.7Winnebago County: 82.9 %Winnebago County82.9Kane County: 79.6 %Kane County79.6Jersey County: 79.2 %Jersey County79.2Hamilton County: 77.8 %Hamilton County77.8Randolph County: 77 %Randolph County77DuPage County: 75.7 %DuPage County75.7Macon County: 62.5 %Macon County62.5Champaign County: 59.5 %Champaign County59.5St. Clair County: 56.2 %St. Clair County56.2Peoria County: 55.1 %Peoria County55.1Madison County: 53.2 %Madison County53.2McHenry County: 47.5 %McHenry County47.5Will County: 42 %Will County42Cook County: 20 %Cook County20
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2015 (%)Change on 2014
1Jo Daviess County94.5+2.7 pts
2Lake County91.7+2.4 pts
3Macoupin County91.5+1.8 pts
4Effingham County90.6+2.4 pts
5Adams County90.2+0.1 pts
6Clark County86.7-1 pts
7Rock Island County84.4-4.7 pts
8McLean County83.7-2.3 pts
8Sangamon County83.7-5.4 pts
10Winnebago County82.9-6 pts
11Kane County79.6-6.9 pts
12Jersey County79.2+2.8 pts
13Hamilton County77.8+1.2 pts
14Randolph County77+7.9 pts
15DuPage County75.7-2.5 pts
16Macon County62.5+3.9 pts
17Champaign County59.5-16.4 pts
18St. Clair County56.2+3 pts
19Peoria County55.1-0.8 pts
20Madison County53.2+8 pts
21McHenry County47.5-20.6 pts
22Will County42-5.2 pts
23Cook County20-0.8 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 2015 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), 2015) [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), 2015. 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=2015. 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), 2015. 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=2015.

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), 2015 [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=2015 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2015,
  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), 2015}},
  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=2015. 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), 2015
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=2015
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2015.
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 94.5% in 2015, according to EPA.

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

Cook County, with 20% in 2015.

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 2015 and are not ranked.

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

Counties in Illinois: more rankings