Data / Rankings / Median Air Quality Index (US EPA) / Counties in Wisconsin

Counties in Wisconsin by median Air Quality Index (US EPA)

Milwaukee County had the highest median Air Quality Index (US EPA) of 33 counties in 2001, at 59. Ashland County had the lowest, at 11. The median was 43. Source: the U.S. Environmental Protection Agency.

Keep it in a collection in your library.
Highest59Milwaukee County
Lowest11Ashland County
Median4333 counties

Map of counties, 2001

Ashland County: 11 indexBrown County: 44 indexColumbia County: 44 indexDane County: 43 indexDodge County: 41 indexDoor County: 42 indexDouglas County: 41 indexFlorence County: 39 indexFond du Lac County: 42 indexGrant County: 52 indexGreen County: 44 indexJefferson County: 47 indexKenosha County: 52 indexKewaunee County: 42 indexManitowoc County: 44 indexMarathon County: 39 indexMilwaukee County: 59 indexOneida County: 40 indexOutagamie County: 43 indexOzaukee County: 47 indexRacine County: 20 indexRock County: 50 indexSt. Croix County: 41 indexSauk County: 43 indexSheboygan County: 44 indexTaylor County: 35 indexVernon County: 41 indexVilas County: 36 indexWalworth County: 46 indexWashington County: 42 indexWaukesha County: 56 indexWinnebago County: 44 indexWood County: 44 index
  • Under 39
  • 39 to 42
  • 42 to 43
  • 43 to 44
  • 44 to 47
  • 47 and over
  • No data
index, 2001. Hatched areas have no value for this period. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Boundaries: U.S. Census Bureau cartographic boundary files.

All counties, 2001

Milwaukee County: 59 indexMilwaukee County59Waukesha County: 56 indexWaukesha County56Grant County: 52 indexGrant County52Kenosha County: 52 indexKenosha County52Rock County: 50 indexRock County50Jefferson County: 47 indexJefferson County47Ozaukee County: 47 indexOzaukee County47Walworth County: 46 indexWalworth County46Brown County: 44 indexBrown County44Columbia County: 44 indexColumbia County44Green County: 44 indexGreen County44Manitowoc County: 44 indexManitowoc County44Sheboygan County: 44 indexSheboygan County44Winnebago County: 44 indexWinnebago County44Wood County: 44 indexWood County44Dane County: 43 indexDane County43Outagamie County: 43 indexOutagamie County43Sauk County: 43 indexSauk County43Door County: 42 indexDoor County42Fond du Lac County: 42 indexFond du Lac County42Kewaunee County: 42 indexKewaunee County42Washington County: 42 indexWashington County42Dodge County: 41 indexDodge County41Douglas County: 41 indexDouglas County41St. Croix County: 41 indexSt. Croix County41Vernon County: 41 indexVernon County41Oneida County: 40 indexOneida County40Florence County: 39 indexFlorence County39Marathon County: 39 indexMarathon County39Vilas County: 36 indexVilas County36Taylor County: 35 indexTaylor County35Racine County: 20 indexRacine County20Ashland County: 11 indexAshland County11Milwaukee County: 59 indexMilwaukee County59Waukesha County: 56 indexWaukesha County56Grant County: 52 indexGrant County52Kenosha County: 52 indexKenosha County52Rock County: 50 indexRock County50Jefferson County: 47 indexJefferson County47Ozaukee County: 47 indexOzaukee County47Walworth County: 46 indexWalworth County46Brown County: 44 indexBrown County44Columbia County: 44 indexColumbia County44Green County: 44 indexGreen County44Manitowoc County: 44 indexManitowoc County44Sheboygan County: 44 indexSheboygan County44Winnebago County: 44 indexWinnebago County44Wood County: 44 indexWood County44Dane County: 43 indexDane County43Outagamie County: 43 indexOutagamie County43Sauk County: 43 indexSauk County43Door County: 42 indexDoor County42Fond du Lac County: 42 indexFond du Lac County42Kewaunee County: 42 indexKewaunee County42Washington County: 42 indexWashington County42Dodge County: 41 indexDodge County41Douglas County: 41 indexDouglas County41St. Croix County: 41 indexSt. Croix County41Vernon County: 41 indexVernon County41Oneida County: 40 indexOneida County40Florence County: 39 indexFlorence County39Marathon County: 39 indexMarathon County39Vilas County: 36 indexVilas County36Taylor County: 35 indexTaylor County35Racine County: 20 indexRacine County20Ashland County: 11 indexAshland County11
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties2001
index		Change on 2000
1Milwaukee County590
2Waukesha County56-1
3Grant County52+4
3Kenosha County52+5
5Rock County50-1
6Jefferson County47-4
6Ozaukee County47+3
8Walworth County46+2
9Brown County44+3
9Columbia County44+2
9Green County44+6
9Manitowoc County44+1
9Sheboygan County44+2
9Winnebago County44+3
9Wood County440
16Dane County43+8
16Outagamie County43+5
16Sauk County43+1
19Door County42+3
19Fond du Lac County42+2
19Kewaunee County42+1
19Washington County42+2
23Dodge County410
23Douglas County41+4
23St. Croix County41+1
23Vernon County41+1
27Oneida County40+4
28Florence County39+2
28Marathon County390
30Vilas County36+1
31Taylor County350
32Racine County20-2
33Ashland County11n/a

Other periods

About this ranking

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.

Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. 15 counties with data in other periods have no value for 2001 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: 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 (Counties in Wisconsin by median Air Quality Index (US EPA), 2001) [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, Counties in Wisconsin by median Air Quality Index (US EPA), 2001. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/median-air-quality-index/wisconsin-counties?period=2001. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Counties in Wisconsin by median Air Quality Index (US EPA), 2001. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/median-air-quality-index/wisconsin-counties?period=2001.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Counties in Wisconsin by median Air Quality Index (US EPA), 2001 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/median-air-quality-index/wisconsin-counties?period=2001 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_wisconsin_by_2001,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: median daily AQI: Counties in Wisconsin by median Air Quality Index (US EPA), 2001}},
  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/median-air-quality-index/wisconsin-counties?period=2001. 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: Counties in Wisconsin by median Air Quality Index (US EPA), 2001
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/median-air-quality-index/wisconsin-counties?period=2001
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/wisconsin-counties?period=2001.
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

Which county has the highest median Air Quality Index (US EPA)?

Milwaukee County, with 59 in 2001, according to the U.S. Environmental Protection Agency.

Which county has the lowest median Air Quality Index (US EPA)?

Ashland County, with 11 in 2001.

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: median daily AQI. Retrieved 2026-10-12. 15 counties have no value for 2001 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Wisconsin: more rankings