Data / Median Air Quality Index (US EPA) / Williamson County

Median Air Quality Index (US EPA) in Williamson County

Median Air Quality Index (US EPA) in Williamson County was 36 in 1986, according to the U.S. Environmental Protection Agency. That is up 1 from 1985.

1986, index36
Change on 1985+1
Series high361986
Series low301981

Next release: Tuesday, 13 July 2027, estimated by Invokely from past publication dates (low confidence), not announced by the publisher. Release calendar

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Median Air Quality Index (US EPA) in Williamson County, 1980 to 1986

3032343619801981198219831984198519861980: 34 index1981: 30 index1982: 32 index1983: 34 index1984: 32 index1985: 35 index1986: 36 index303234361980198319861980: 34 index1981: 30 index1982: 32 index1983: 34 index1984: 32 index1985: 35 index1986: 36 index
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12, published 2026-07-13.
All 7 values
Periodindex
198636
198535
198432
198334
198232
198130
198034

Williamson County compared with nearby places

Adams County: 32 indexChampaign County: 32 indexCook County: 62 indexDuPage County: 25 indexEffingham County: 43 indexKane County: 28 indexKankakee County: 19 indexLake County: 31 indexLaSalle County: 33 indexMcHenry County: 30 indexMacon County: 37 indexMacoupin County: 36 indexMadison County: 51 indexMonroe County: 33 indexPeoria County: 33 indexRock Island County: 33 indexSt. Clair County: 39 indexSangamon County: 37 indexWill County: 34 indexWilliamson County: 36 indexWinnebago County: 42 index
  • Under 31
  • 31 to 33
  • 33 to 36
  • 36 to 39
  • 39 and over
  • No data
Median Air Quality Index (US EPA), 1986, index. Williamson County is outlined. Same source and period for every area shown. Source: U.S. Environmental Protection Agency. Boundaries: U.S. Census Bureau cartographic boundary files.

Rankings

Definition

Median of the daily Air Quality Index (AQI) values of a US county over the days of the year with an AQI, from the EPA AirData annual summaries.

More on how it is measured
  • The daily county AQI is the highest AQI among the monitors and pollutants reported that day (ozone, PM2.5, PM10, carbon monoxide and nitrogen dioxide).
  • Lower is cleaner: 0 to 50 is good, 51 to 100 moderate and above 100 unhealthy for sensitive groups.

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.

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How to cite

U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. 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: median daily AQI (Median Air Quality Index (US EPA), Williamson County, 1980 to 1986) [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: median daily AQI, Median Air Quality Index (US EPA), Williamson County, 1980 to 1986. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/en/median-air-quality-index/williamson-county-illinois. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Median Air Quality Index (US EPA), Williamson County, 1980 to 1986. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/en/median-air-quality-index/williamson-county-illinois.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Median Air Quality Index (US EPA), Williamson County, 1980 to 1986 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/en/median-air-quality-index/williamson-county-illinois (Accessed: 12 October 2026).

BibTeX
@misc{environmental_median_air_quality_index_1986,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: median daily AQI: Median Air Quality Index (US EPA), Williamson County, 1980 to 1986}},
  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/en/median-air-quality-index/williamson-county-illinois. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. 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: median daily AQI: Median Air Quality Index (US EPA), Williamson County, 1980 to 1986
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/en/median-air-quality-index/williamson-county-illinois
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/en/median-air-quality-index/williamson-county-illinois.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER  -
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Questions

What is the median Air Quality Index (US EPA) in Williamson County?

Median Air Quality Index (US EPA) in Williamson County was 36 in 1986, according to the U.S. Environmental Protection Agency. That is up 1 from 1985.

Where does this figure come from?

U.S. Environmental Protection Agency. EPA AirData annual AQI by county: median daily AQI. Retrieved by Invokely on 2026-10-12.

What was the highest median Air Quality Index (US EPA) in Williamson County?

36 in 1986, the highest value since 1980 in this series.

Related statistics for Williamson County

All statistics for Williamson County | Williamson County in figures