Counties in Wisconsin by share of days unhealthy for sensitive groups or worse (US EPA AQI)

Sheboygan County had the highest share of days unhealthy for sensitive groups or worse (US EPA AQI) of 32 counties in 1999, at 15.5%. Polk County had the lowest, at 0%. The median was 7.3%. Source: EPA.

Highest15.5Sheboygan County

Lowest0Polk County

Median7.332 counties

Map of counties, 1999

Under 3.6
3.6 to 5.5
5.5 to 7.3
7.3 to 9.7
9.7 to 12
12 and over
No data

%, 1999. 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, 1999

%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12.

Full ranking

Rank	Counties	1999 (%)	Change on 1998
1	Sheboygan County	15.5	+3.5 pts
2	Kenosha County	15.4	-1.9 pts
3	Walworth County	13.3	+3.2 pts
4	Ozaukee County	13	-1.7 pts
5	Milwaukee County	12.3	+6 pts
5	Washington County	12.3	+6.4 pts
7	Kewaunee County	12	+0.6 pts
8	Jefferson County	11.8	+4.2 pts
9	Manitowoc County	11	-2.6 pts
10	Door County	9.8	-0.6 pts
10	Fond du Lac County	9.8	+5.5 pts
12	Rock County	9.5	-4.1 pts
13	Columbia County	8.7	+4.3 pts
14	Winnebago County	8.5	+5.2 pts
15	Dodge County	8.1	+2.1 pts
16	Outagamie County	7.5	+3.7 pts
17	Waukesha County	7.1	+4.6 pts
18	Racine County	6.3	+0.5 pts
19	Brown County	6.1	+2.3 pts
20	Oneida County	5.6	+3.9 pts
20	Sauk County	5.6	+2.6 pts
22	Florence County	5.5	+2.2 pts
23	Marathon County	5.3	+1.5 pts
24	Dane County	4.7	+1 pts
25	Vernon County	4.6	+1.7 pts
26	St. Croix County	3.9	-0.3 pts
27	Grant County	3.5	n/a
28	Taylor County	3	+1.6 pts
29	Vilas County	2.7	+1.9 pts
30	Wood County	0.9	n/a
31	Douglas County	0	0 pts
31	Polk County	0	-1.6 pts

Other periods

About this ranking

Days with a daily Air Quality Index above 100 (unhealthy for sensitive groups, unhealthy, very unhealthy or hazardous) 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; the counts are stored with each value.

Use the share rather than the number of days to compare counties, since the days with an AQI differ. 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: days with an AQI above 100 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts (Counties in Wisconsin by share of days unhealthy for sensitive groups or worse (US EPA AQI), 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts, Counties in Wisconsin by share of days unhealthy for sensitive groups or worse (US EPA AQI), 1999. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/wisconsin-counties?period=1999. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. Counties in Wisconsin by share of days unhealthy for sensitive groups or worse (US EPA AQI), 1999. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/wisconsin-counties?period=1999.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts, Counties in Wisconsin by share of days unhealthy for sensitive groups or worse (US EPA AQI), 1999 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/unhealthy-air-quality-days-share/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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts: Counties in Wisconsin by share of days unhealthy for sensitive groups or worse (US EPA AQI), 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/unhealthy-air-quality-days-share/wisconsin-counties?period=1999. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts: Counties in Wisconsin by share of days unhealthy for sensitive groups or worse (US EPA AQI), 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/unhealthy-air-quality-days-share/wisconsin-counties?period=1999
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/unhealthy-air-quality-days-share/wisconsin-counties?period=1999.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 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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Questions

Which county has the highest share of days unhealthy for sensitive groups or worse (US EPA AQI)?

Sheboygan County, with 15.5% in 1999, according to EPA.

Which county has the lowest share of days unhealthy for sensitive groups or worse (US EPA AQI)?

Polk County, with 0% 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: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 16 counties have no value for 1999 and are not ranked.