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 32 counties in 1999, at 61. Polk County had the lowest, at 3. The median was 44. Source: the U.S. Environmental Protection Agency.

Keep it in a collection in your library.
Highest61Milwaukee County
Lowest3Polk County
Median4432 counties

Map of counties, 1999

Brown County: 43 indexColumbia County: 44 indexDane County: 44 indexDodge County: 48 indexDoor County: 41 indexDouglas County: 39 indexFlorence County: 38 indexFond du Lac County: 44 indexGrant County: 58 indexJefferson County: 53 indexKenosha County: 50 indexKewaunee County: 42 indexManitowoc County: 44 indexMarathon County: 40 indexMilwaukee County: 61 indexOneida County: 39 indexOutagamie County: 44 indexOzaukee County: 47 indexPolk County: 3 indexRacine County: 24 indexRock County: 53 indexSt. Croix County: 41 indexSauk County: 41 indexSheboygan County: 41 indexTaylor County: 37 indexVernon County: 40 indexVilas County: 37 indexWalworth County: 46 indexWashington County: 44 indexWaukesha County: 37 indexWinnebago County: 45 indexWood County: 51 index
  • Under 38
  • 38 to 41
  • 41 to 44
  • 44 to 50
  • 50 and over
  • No data
index, 1999. 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, 1999

Milwaukee County: 61 indexMilwaukee County61Grant County: 58 indexGrant County58Jefferson County: 53 indexJefferson County53Rock County: 53 indexRock County53Wood County: 51 indexWood County51Kenosha County: 50 indexKenosha County50Dodge County: 48 indexDodge County48Ozaukee County: 47 indexOzaukee County47Walworth County: 46 indexWalworth County46Winnebago County: 45 indexWinnebago County45Columbia County: 44 indexColumbia County44Dane County: 44 indexDane County44Fond du Lac County: 44 indexFond du Lac County44Manitowoc County: 44 indexManitowoc County44Outagamie County: 44 indexOutagamie County44Washington County: 44 indexWashington County44Brown County: 43 indexBrown County43Kewaunee County: 42 indexKewaunee County42Door County: 41 indexDoor County41Sauk County: 41 indexSauk County41Sheboygan County: 41 indexSheboygan County41St. Croix County: 41 indexSt. Croix County41Marathon County: 40 indexMarathon County40Vernon County: 40 indexVernon County40Douglas County: 39 indexDouglas County39Oneida County: 39 indexOneida County39Florence County: 38 indexFlorence County38Taylor County: 37 indexTaylor County37Vilas County: 37 indexVilas County37Waukesha County: 37 indexWaukesha County37Racine County: 24 indexRacine County24Polk County: 3 indexPolk County3Milwaukee County: 61 indexMilwaukee County61Grant County: 58 indexGrant County58Jefferson County: 53 indexJefferson County53Rock County: 53 indexRock County53Wood County: 51 indexWood County51Kenosha County: 50 indexKenosha County50Dodge County: 48 indexDodge County48Ozaukee County: 47 indexOzaukee County47Walworth County: 46 indexWalworth County46Winnebago County: 45 indexWinnebago County45Columbia County: 44 indexColumbia County44Dane County: 44 indexDane County44Fond du Lac County: 44 indexFond du Lac County44Manitowoc County: 44 indexManitowoc County44Outagamie County: 44 indexOutagamie County44Washington County: 44 indexWashington County44Brown County: 43 indexBrown County43Kewaunee County: 42 indexKewaunee County42Door County: 41 indexDoor County41Sauk County: 41 indexSauk County41Sheboygan County: 41 indexSheboygan County41St. Croix County: 41 indexSt. Croix County41Marathon County: 40 indexMarathon County40Vernon County: 40 indexVernon County40Douglas County: 39 indexDouglas County39Oneida County: 39 indexOneida County39Florence County: 38 indexFlorence County38Taylor County: 37 indexTaylor County37Vilas County: 37 indexVilas County37Waukesha County: 37 indexWaukesha County37Racine County: 24 indexRacine County24Polk County: 3 indexPolk County3
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12.

Full ranking

RankCounties1999
index		Change on 1998
1Milwaukee County61+19
2Grant County58n/a
3Jefferson County53+6
3Rock County53+4
5Wood County51n/a
6Kenosha County50-8
7Dodge County48+2
8Ozaukee County47-1
9Walworth County46-1
10Winnebago County450
11Columbia County44-1
11Dane County44+7
11Fond du Lac County44-3
11Manitowoc County44-2
11Outagamie County440
11Washington County44-2
17Brown County43+1
18Kewaunee County42-2
19Door County41-3
19Sauk County41+3
19Sheboygan County41-2
19St. Croix County410
23Marathon County40-4
23Vernon County40-2
25Douglas County39+20
25Oneida County390
27Florence County38-3
28Taylor County37+1
28Vilas County37+4
28Waukesha County37+6
31Racine County24-1
32Polk County3-16

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. 16 counties with data in other periods have no value for 1999 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), 1999) [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), 1999. 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=1999. 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), 1999. 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=1999.

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), 1999 [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=1999 (Accessed: 12 October 2026).

BibTeX
@misc{environmental_counties_in_wisconsin_by_1999,
  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), 1999}},
  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=1999. 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), 1999
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=1999
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/wisconsin-counties?period=1999.
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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Rank Counties in Wisconsin by median Air Quality Index (US EPA) using official data, and cite the source.
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Questions

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

Milwaukee County, with 61 in 1999, according to the U.S. Environmental Protection Agency.

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

Polk County, with 3 in 1999.

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. 16 counties have no value for 1999 and are not ranked.

Median Air Quality Index (US EPA): other rankings

Counties in Wisconsin: more rankings