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 2019, at 96.7%. Cook County had the lowest, at 21.9%. The median was 61.6%. Source: EPA.

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Highest96.7Jo Daviess County
Lowest21.9Cook County
Median61.623 counties

Map of counties, 2019

Adams County: 86.3 %Champaign County: 61.6 %Clark County: 96 %Cook County: 21.9 %DuPage County: 55.7 %Effingham County: 91.4 %Hamilton County: 59.3 %Jersey County: 63.7 %Jo Daviess County: 96.7 %Kane County: 73.9 %Lake County: 85.7 %McHenry County: 47.4 %McLean County: 51.4 %Macon County: 48.4 %Macoupin County: 95.2 %Madison County: 38.6 %Peoria County: 64.8 %Randolph County: 72.2 %Rock Island County: 61.2 %St. Clair County: 59.5 %Sangamon County: 61.7 %Will County: 49 %Winnebago County: 53.4 %
  • Under 50
  • 50 to 59
  • 59 to 64
  • 64 to 86
  • 86 and over
  • No data
%, 2019. 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, 2019

Jo Daviess County: 96.7 %Jo Daviess County96.7Clark County: 96 %Clark County96Macoupin County: 95.2 %Macoupin County95.2Effingham County: 91.4 %Effingham County91.4Adams County: 86.3 %Adams County86.3Lake County: 85.7 %Lake County85.7Kane County: 73.9 %Kane County73.9Randolph County: 72.2 %Randolph County72.2Peoria County: 64.8 %Peoria County64.8Jersey County: 63.7 %Jersey County63.7Sangamon County: 61.7 %Sangamon County61.7Champaign County: 61.6 %Champaign County61.6Rock Island County: 61.2 %Rock Island County61.2St. Clair County: 59.5 %St. Clair County59.5Hamilton County: 59.3 %Hamilton County59.3DuPage County: 55.7 %DuPage County55.7Winnebago County: 53.4 %Winnebago County53.4McLean County: 51.4 %McLean County51.4Will County: 49 %Will County49Macon County: 48.4 %Macon County48.4McHenry County: 47.4 %McHenry County47.4Madison County: 38.6 %Madison County38.6Cook County: 21.9 %Cook County21.9Jo Daviess County: 96.7 %Jo Daviess County96.7Clark County: 96 %Clark County96Macoupin County: 95.2 %Macoupin County95.2Effingham County: 91.4 %Effingham County91.4Adams County: 86.3 %Adams County86.3Lake County: 85.7 %Lake County85.7Kane County: 73.9 %Kane County73.9Randolph County: 72.2 %Randolph County72.2Peoria County: 64.8 %Peoria County64.8Jersey County: 63.7 %Jersey County63.7Sangamon County: 61.7 %Sangamon County61.7Champaign County: 61.6 %Champaign County61.6Rock Island County: 61.2 %Rock Island County61.2St. Clair County: 59.5 %St. Clair County59.5Hamilton County: 59.3 %Hamilton County59.3DuPage County: 55.7 %DuPage County55.7Winnebago County: 53.4 %Winnebago County53.4McLean County: 51.4 %McLean County51.4Will County: 49 %Will County49Macon County: 48.4 %Macon County48.4McHenry County: 47.4 %McHenry County47.4Madison County: 38.6 %Madison County38.6Cook County: 21.9 %Cook County21.9
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2019 (%)Change on 2018
1Jo Daviess County96.7+2.8 pts
2Clark County96+2.1 pts
3Macoupin County95.2+5.1 pts
4Effingham County91.4+4.1 pts
5Adams County86.3-6.8 pts
6Lake County85.7-0.7 pts
7Kane County73.9-1.6 pts
8Randolph County72.2+10.2 pts
9Peoria County64.8+13 pts
10Jersey County63.7+5.3 pts
11Sangamon County61.7+9.6 pts
12Champaign County61.6+4.3 pts
13Rock Island County61.2+3.1 pts
14St. Clair County59.5+17.7 pts
15Hamilton County59.3+0.7 pts
16DuPage County55.7+18 pts
17Winnebago County53.4-32.2 pts
18McLean County51.4+5.1 pts
19Will County49-4.2 pts
20Macon County48.4+4 pts
21McHenry County47.4-11.4 pts
22Madison County38.6-5 pts
23Cook County21.9-1.7 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 2019 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), 2019) [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), 2019. 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=2019. 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), 2019. 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=2019.

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), 2019 [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=2019 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2019,
  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), 2019}},
  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=2019. 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), 2019
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=2019
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2019.
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 96.7% in 2019, according to EPA.

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

Cook County, with 21.9% in 2019.

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 2019 and are not ranked.

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

Counties in Illinois: more rankings