Counties in North Dakota by share of days with good air quality (US EPA AQI)

Billings County had the highest share of days with good air quality (US EPA AQI) of 13 counties in 1987, at 100%. Oliver County had the lowest, at 85.1%. The median was 97.5%. Source: EPA.

Highest100Billings County

Lowest85.1Oliver County

Median97.513 counties

Map of counties, 1987

Under 92
92 to 96
96 to 98
98 to 100
100 and over
No data

%, 1987. 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, 1987

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

Full ranking

Rank	Counties	1987 (%)	Change on 1986
1	Billings County	100	0 pts
1	Burke County	100	n/a
1	Grand Forks County	100	+2.9 pts
1	Williams County	100	+1.7 pts
5	Cass County	98.4	+0.1 pts
6	McKenzie County	98.3	-0.2 pts
7	Dunn County	97.5	-0.6 pts
8	Ward County	96.4	-3.6 pts
9	Stutsman County	96.2	-3.8 pts
10	Burleigh County	93.3	-3.2 pts
11	Stark County	91.4	-3 pts
12	Mercer County	86.8	-6.1 pts
13	Oliver County	85.1	-10.5 pts

Other periods

2001
2000
1998
1987
1986

About this ranking

Days with a daily Air Quality Index of 0 to 50 (good) 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; both counts are stored with each value.

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. 2 counties with data in other periods have no value for 1987 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: good days 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: good days as a share of days with an AQI, calculated with stored counts (Counties in North Dakota by share of days with good air quality (US EPA AQI), 1987) [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: good days as a share of days with an AQI, calculated with stored counts, Counties in North Dakota by share of days with good air quality (US EPA AQI), 1987. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/good-air-quality-days-share/north-dakota-counties?period=1987. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts. Counties in North Dakota by share of days with good air quality (US EPA AQI), 1987. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/good-air-quality-days-share/north-dakota-counties?period=1987.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts, Counties in North Dakota by share of days with good air quality (US EPA AQI), 1987 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/good-air-quality-days-share/north-dakota-counties?period=1987 (Accessed: 12 October 2026).

BibTeX

@misc{environmental_counties_in_north_dakota_1987,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: good days as a share of days with an AQI, calculated with stored counts: Counties in North Dakota by share of days with good air quality (US EPA AQI), 1987}},
  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/good-air-quality-days-share/north-dakota-counties?period=1987. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days 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: good days as a share of days with an AQI, calculated with stored counts: Counties in North Dakota by share of days with good air quality (US EPA AQI), 1987
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/good-air-quality-days-share/north-dakota-counties?period=1987
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/rankings/good-air-quality-days-share/north-dakota-counties?period=1987.
N1  - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: good days 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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Rank Counties in North Dakota by share of days with good air quality (US EPA AQI) using official data, and cite the source.

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Questions

Which county has the highest share of days with good air quality (US EPA AQI)?

Billings County, with 100% in 1987, according to EPA.

Which county has the lowest share of days with good air quality (US EPA AQI)?

Oliver County, with 85.1% in 1987.

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: good days as a share of days with an AQI, calculated with stored counts. Retrieved 2026-10-12. 2 counties have no value for 1987 and are not ranked.