Share of days unhealthy for sensitive groups or worse (US EPA AQI) in Delta County

Share of days unhealthy for sensitive groups or worse (US EPA AQI) in Delta County was 0.5% in 2025, according to EPA. That is the 9th highest of 30 counties in 2025.

20250.5%

Rank9th of 30Counties in Colorado ranked

Series high1.61999

Series low01988

Next release: Tuesday, 13 July 2027, estimated by Invokely from past publication dates (low confidence), not announced by the publisher. Release calendar

Share of days unhealthy for sensitive groups or worse (US EPA AQI) in Delta County, 1988 to 2025

%. Source: Invokely calculation with data from EPA. Retrieved 2026-10-12, published 2026-07-13.

All 36 values

Period	%
2025	0.5
2022	0
2021	0.5
2020	0
2019	0
2018	0
2017	0
2016	0
2015	0
2014	0
2013	0
2012	0
2011	0
2010	0
2009	0.8
2008	0
2007	0
2006	0
2005	0
2004	0
2003	1.6
2002	1.6
2001	0
2000	0
1999	1.6
1998	0
1997	0
1996	0
1995	0
1994	0
1993	0
1992	0
1991	0
1990	0
1989	0
1988	0

Rankings

Counties in Colorado by share of days unhealthy for sensitive groups or worse (US EPA AQI) 40 places

Definition

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.

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 (Share of days unhealthy for sensitive groups or worse (US EPA AQI), Delta County, 1988 to 2025) [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, Share of days unhealthy for sensitive groups or worse (US EPA AQI), Delta County, 1988 to 2025. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/en/unhealthy-air-quality-days-share/delta-county-colorado. 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. Share of days unhealthy for sensitive groups or worse (US EPA AQI), Delta County, 1988 to 2025. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/en/unhealthy-air-quality-days-share/delta-county-colorado.

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, Share of days unhealthy for sensitive groups or worse (US EPA AQI), Delta County, 1988 to 2025 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/en/unhealthy-air-quality-days-share/delta-county-colorado (Accessed: 12 October 2026).

BibTeX

@misc{environmental_share_of_days_unhealthy_2025,
  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: Share of days unhealthy for sensitive groups or worse (US EPA AQI), Delta County, 1988 to 2025}},
  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/unhealthy-air-quality-days-share/delta-county-colorado. 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: Share of days unhealthy for sensitive groups or worse (US EPA AQI), Delta County, 1988 to 2025
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/unhealthy-air-quality-days-share/delta-county-colorado
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/en/unhealthy-air-quality-days-share/delta-county-colorado.
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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What is the latest share of days unhealthy for sensitive groups or worse (US EPA AQI) in Delta County? Cite the source.

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Questions

What is the share of days unhealthy for sensitive groups or worse (US EPA AQI) in Delta County?

Share of days unhealthy for sensitive groups or worse (US EPA AQI) in Delta County was 0.5% in 2025, according to EPA. That is the 9th highest of 30 counties in 2025.

Where does this figure come from?

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 by Invokely on 2026-10-12.

What was the highest share of days unhealthy for sensitive groups or worse (US EPA AQI) in Delta County?

1.6% in 1999, the highest value since 1988 in this series.

Related statistics for Delta County

All statistics for Delta County | Delta County in figures