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 2016, at 94.6%. Cook County had the lowest, at 31.4%. The median was 73%. Source: EPA.

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Highest94.6Jo Daviess County
Lowest31.4Cook County
Median7323 counties

Map of counties, 2016

Adams County: 93 %Champaign County: 69.4 %Clark County: 86.2 %Cook County: 31.4 %DuPage County: 73 %Effingham County: 84.6 %Hamilton County: 69.1 %Jersey County: 68.5 %Jo Daviess County: 94.6 %Kane County: 79.9 %Lake County: 88.4 %McHenry County: 66.6 %McLean County: 77.3 %Macon County: 55.1 %Macoupin County: 89.7 %Madison County: 48.1 %Peoria County: 56.3 %Randolph County: 70.8 %Rock Island County: 86.9 %St. Clair County: 61.7 %Sangamon County: 83.5 %Will County: 55.1 %Winnebago County: 84.7 %
  • Under 58
  • 58 to 69
  • 69 to 81
  • 81 to 87
  • 87 and over
  • No data
%, 2016. 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, 2016

Jo Daviess County: 94.6 %Jo Daviess County94.6Adams County: 93 %Adams County93Macoupin County: 89.7 %Macoupin County89.7Lake County: 88.4 %Lake County88.4Rock Island County: 86.9 %Rock Island County86.9Clark County: 86.2 %Clark County86.2Winnebago County: 84.7 %Winnebago County84.7Effingham County: 84.6 %Effingham County84.6Sangamon County: 83.5 %Sangamon County83.5Kane County: 79.9 %Kane County79.9McLean County: 77.3 %McLean County77.3DuPage County: 73 %DuPage County73Randolph County: 70.8 %Randolph County70.8Champaign County: 69.4 %Champaign County69.4Hamilton County: 69.1 %Hamilton County69.1Jersey County: 68.5 %Jersey County68.5McHenry County: 66.6 %McHenry County66.6St. Clair County: 61.7 %St. Clair County61.7Peoria County: 56.3 %Peoria County56.3Macon County: 55.1 %Macon County55.1Will County: 55.1 %Will County55.1Madison County: 48.1 %Madison County48.1Cook County: 31.4 %Cook County31.4Jo Daviess County: 94.6 %Jo Daviess County94.6Adams County: 93 %Adams County93Macoupin County: 89.7 %Macoupin County89.7Lake County: 88.4 %Lake County88.4Rock Island County: 86.9 %Rock Island County86.9Clark County: 86.2 %Clark County86.2Winnebago County: 84.7 %Winnebago County84.7Effingham County: 84.6 %Effingham County84.6Sangamon County: 83.5 %Sangamon County83.5Kane County: 79.9 %Kane County79.9McLean County: 77.3 %McLean County77.3DuPage County: 73 %DuPage County73Randolph County: 70.8 %Randolph County70.8Champaign County: 69.4 %Champaign County69.4Hamilton County: 69.1 %Hamilton County69.1Jersey County: 68.5 %Jersey County68.5McHenry County: 66.6 %McHenry County66.6St. Clair County: 61.7 %St. Clair County61.7Peoria County: 56.3 %Peoria County56.3Macon County: 55.1 %Macon County55.1Will County: 55.1 %Will County55.1Madison County: 48.1 %Madison County48.1Cook County: 31.4 %Cook County31.4
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2016 (%)Change on 2015
1Jo Daviess County94.6+0.1 pts
2Adams County93+2.8 pts
3Macoupin County89.7-1.8 pts
4Lake County88.4-3.3 pts
5Rock Island County86.9+2.5 pts
6Clark County86.2-0.5 pts
7Winnebago County84.7+1.8 pts
8Effingham County84.6-6 pts
9Sangamon County83.5-0.2 pts
10Kane County79.9+0.3 pts
11McLean County77.3-6.4 pts
12DuPage County73-2.7 pts
13Randolph County70.8-6.2 pts
14Champaign County69.4+9.9 pts
15Hamilton County69.1-8.7 pts
16Jersey County68.5-10.7 pts
17McHenry County66.6+19.1 pts
18St. Clair County61.7+5.5 pts
19Peoria County56.3+1.2 pts
20Macon County55.1-7.4 pts
20Will County55.1+13.1 pts
22Madison County48.1-5.1 pts
23Cook County31.4+11.4 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 2016 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), 2016) [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), 2016. 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=2016. 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), 2016. 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=2016.

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), 2016 [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=2016 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2016,
  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), 2016}},
  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=2016. 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), 2016
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=2016
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2016.
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 94.6% in 2016, according to EPA.

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

Cook County, with 31.4% in 2016.

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 2016 and are not ranked.

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

Counties in Illinois: more rankings