Data / Highest daily Air Quality Index (US EPA) / Williams County

Highest daily Air Quality Index (US EPA) in Williams County

Highest daily Air Quality Index (US EPA) in Williams County was 77 in 2019, according to the U.S. Environmental Protection Agency. That is down 23 from 2018.

2019, index77
Change on 2018-23
Series high1822015
Series low201996

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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Highest daily Air Quality Index (US EPA) in Williams County, 1985 to 2019

05010015020019851988199119941997201420191985: 55 index1986: 91 index1987: 40 index1988: 52 index1989: 58 index1990: 143 index1991: 36 index1992: 24 index1993: 25 index1994: 56 index1995: 39 index1996: 20 index1997: 32 index1998: 44 index2013: 102 index2014: 114 index2015: 182 index2016: 94 index2017: 131 index2018: 100 index2019: 77 index05010015020019851992201320191985: 55 index1986: 91 index1987: 40 index1988: 52 index1989: 58 index1990: 143 index1991: 36 index1992: 24 index1993: 25 index1994: 56 index1995: 39 index1996: 20 index1997: 32 index1998: 44 index2013: 102 index2014: 114 index2015: 182 index2016: 94 index2017: 131 index2018: 100 index2019: 77 index
index. Source: EPA, EPA AirData annual AQI by county: highest daily AQI. Retrieved 2026-10-12, published 2026-07-13.
All 21 values
Periodindex
201977
2018100
2017131
201694
2015182
2014114
2013102
199844
199732
199620
199539
199456
199325
199224
199136
1990143
198958
198852
198740
198691
198555

Williams County compared with nearby places

Billings County: 84 indexBurke County: 67 indexBurleigh County: 90 indexCass County: 157 indexDunn County: 87 indexMcKenzie County: 77 indexMercer County: 72 indexOliver County: 84 indexWard County: 87 indexWilliams County: 77 index
  • Under 76
  • 76 to 81
  • 81 to 85
  • 85 to 88
  • 88 and over
  • No data
Highest daily Air Quality Index (US EPA), 2019, index. Williams 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

Highest daily Air Quality Index (AQI) value recorded in a US county during the year, from the EPA AirData annual summaries. Values above 500 are beyond the AQI scale and are published as EPA computes them.

A single day decides the value, so one wildfire or dust event can set it. Dust from the dry Owens Lake bed puts Inyo and Mono counties in California in the thousands. 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: highest 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: highest daily AQI (Highest daily Air Quality Index (US EPA), Williams County, 1985 to 2019) [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: highest daily AQI, Highest daily Air Quality Index (US EPA), Williams County, 1985 to 2019. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/en/highest-daily-air-quality-index/williams-county-north-dakota. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: highest daily AQI. Highest daily Air Quality Index (US EPA), Williams County, 1985 to 2019. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/en/highest-daily-air-quality-index/williams-county-north-dakota.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: highest daily AQI, Highest daily Air Quality Index (US EPA), Williams County, 1985 to 2019 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/en/highest-daily-air-quality-index/williams-county-north-dakota (Accessed: 12 October 2026).

BibTeX
@misc{environmental_highest_daily_air_quality_2019,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: highest daily AQI: Highest daily Air Quality Index (US EPA), Williams County, 1985 to 2019}},
  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/highest-daily-air-quality-index/williams-county-north-dakota. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest 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: highest daily AQI: Highest daily Air Quality Index (US EPA), Williams County, 1985 to 2019
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/highest-daily-air-quality-index/williams-county-north-dakota
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/en/highest-daily-air-quality-index/williams-county-north-dakota.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: highest 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 highest daily Air Quality Index (US EPA) in Williams County?

Highest daily Air Quality Index (US EPA) in Williams County was 77 in 2019, according to the U.S. Environmental Protection Agency. That is down 23 from 2018.

Where does this figure come from?

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

What was the highest highest daily Air Quality Index (US EPA) in Williams County?

182 in 2015, the highest value since 1985 in this series.

Related statistics for Williams County

All statistics for Williams County | Williams County in figures