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)

Clark County had the highest share of days with good air quality (US EPA AQI) of 23 counties in 2018, at 93.9%. Cook County had the lowest, at 23.6%. The median was 58.4%. Source: EPA.

Keep it in a collection in your library.
Highest93.9Clark County
Lowest23.6Cook County
Median58.423 counties

Map of counties, 2018

Adams County: 93.1 %Champaign County: 57.3 %Clark County: 93.9 %Cook County: 23.6 %DuPage County: 37.7 %Effingham County: 87.3 %Hamilton County: 58.6 %Jersey County: 58.4 %Jo Daviess County: 93.9 %Kane County: 75.5 %Lake County: 86.4 %McHenry County: 58.8 %McLean County: 46.3 %Macon County: 44.4 %Macoupin County: 90.1 %Madison County: 43.6 %Peoria County: 51.8 %Randolph County: 62 %Rock Island County: 58.1 %St. Clair County: 41.8 %Sangamon County: 52.1 %Will County: 53.2 %Winnebago County: 85.6 %
  • Under 45
  • 45 to 56
  • 56 to 59
  • 59 to 87
  • 87 and over
  • No data
%, 2018. 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, 2018

Clark County: 93.9 %Clark County93.9Jo Daviess County: 93.9 %Jo Daviess County93.9Adams County: 93.1 %Adams County93.1Macoupin County: 90.1 %Macoupin County90.1Effingham County: 87.3 %Effingham County87.3Lake County: 86.4 %Lake County86.4Winnebago County: 85.6 %Winnebago County85.6Kane County: 75.5 %Kane County75.5Randolph County: 62 %Randolph County62McHenry County: 58.8 %McHenry County58.8Hamilton County: 58.6 %Hamilton County58.6Jersey County: 58.4 %Jersey County58.4Rock Island County: 58.1 %Rock Island County58.1Champaign County: 57.3 %Champaign County57.3Will County: 53.2 %Will County53.2Sangamon County: 52.1 %Sangamon County52.1Peoria County: 51.8 %Peoria County51.8McLean County: 46.3 %McLean County46.3Macon County: 44.4 %Macon County44.4Madison County: 43.6 %Madison County43.6St. Clair County: 41.8 %St. Clair County41.8DuPage County: 37.7 %DuPage County37.7Cook County: 23.6 %Cook County23.6Clark County: 93.9 %Clark County93.9Jo Daviess County: 93.9 %Jo Daviess County93.9Adams County: 93.1 %Adams County93.1Macoupin County: 90.1 %Macoupin County90.1Effingham County: 87.3 %Effingham County87.3Lake County: 86.4 %Lake County86.4Winnebago County: 85.6 %Winnebago County85.6Kane County: 75.5 %Kane County75.5Randolph County: 62 %Randolph County62McHenry County: 58.8 %McHenry County58.8Hamilton County: 58.6 %Hamilton County58.6Jersey County: 58.4 %Jersey County58.4Rock Island County: 58.1 %Rock Island County58.1Champaign County: 57.3 %Champaign County57.3Will County: 53.2 %Will County53.2Sangamon County: 52.1 %Sangamon County52.1Peoria County: 51.8 %Peoria County51.8McLean County: 46.3 %McLean County46.3Macon County: 44.4 %Macon County44.4Madison County: 43.6 %Madison County43.6St. Clair County: 41.8 %St. Clair County41.8DuPage County: 37.7 %DuPage County37.7Cook County: 23.6 %Cook County23.6
%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

RankCounties2018 (%)Change on 2017
1Clark County93.9+6.3 pts
1Jo Daviess County93.9+0.4 pts
3Adams County93.1+5.8 pts
4Macoupin County90.1+0.8 pts
5Effingham County87.3+2.9 pts
6Lake County86.4-1.9 pts
7Winnebago County85.6+2 pts
8Kane County75.5-2.4 pts
9Randolph County62+17.4 pts
10McHenry County58.8-4.5 pts
11Hamilton County58.6-1.7 pts
12Jersey County58.4+3.4 pts
13Rock Island County58.1-16.3 pts
14Champaign County57.3-6.5 pts
15Will County53.2-10.1 pts
16Sangamon County52.1-19.9 pts
17Peoria County51.8+4.7 pts
18McLean County46.3-8.9 pts
19Macon County44.4-14.6 pts
20Madison County43.6+1.1 pts
21St. Clair County41.8-23.1 pts
22DuPage County37.7-29.8 pts
23Cook County23.6-7.6 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 2018 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), 2018) [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), 2018. 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=2018. 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), 2018. 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=2018.

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), 2018 [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=2018 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_illinois_by_2018,
  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), 2018}},
  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=2018. 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), 2018
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=2018
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/illinois-counties?period=2018.
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  -
Embed this chart

Paste this code into a web page or a blog post. The chart shows the latest figures as they are published, with the source and a link back to this page.

Sites that read oEmbed links, such as WordPress, can show the chart when you paste the link to this page.

Use this data in your AI agent

Invokely serves this figure to AI assistants through MCP and to code through a REST API, with the source and citation attached. Ask your assistant:

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}
GET https://invokely.ai/api/v1/rank?indicator=aqi_good_days_share&level=province&within=Illinois&limit=10

Connect Claude, ChatGPT or Cursor in one step. The trial answers 25 calls a day without an account.

Questions

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

Clark County, with 93.9% in 2018, according to EPA.

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

Cook County, with 23.6% in 2018.

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 2018 and are not ranked.

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

Counties in Illinois: more rankings