Data / Median Air Quality Index (US EPA) / Williamson County

Median Air Quality Index (US EPA) in Williamson County

Median Air Quality Index (US EPA) in Williamson County was 40 in 2025, according to the U.S. Environmental Protection Agency. That is down 3 from 2024.

2025, index40
Change on 2024-3
Series high641988
Series low362015

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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Median Air Quality Index (US EPA) in Williamson County, 1988 to 2025

304050607019881994200020062012201820251988: 64 index1989: 46 index1990: 49 index1991: 40 index1992: 42 index1993: 46 index1994: 42 index1995: 51 index1996: 51 index1997: 58 index1998: 61 index1999: 64 index2000: 54 index2001: 48 index2002: 47 index2003: 48 index2004: 42 index2005: 47 index2006: 46 index2007: 50 index2008: 45 index2009: 39 index2010: 45 index2011: 47 index2012: 44 index2013: 37 index2014: 38 index2015: 36 index2016: 40 index2017: 40 index2018: 39 index2019: 42 index2020: 37 index2021: 39 index2022: 41 index2023: 43 index2024: 43 index2025: 40 index304050607019882001201420251988: 64 index1989: 46 index1990: 49 index1991: 40 index1992: 42 index1993: 46 index1994: 42 index1995: 51 index1996: 51 index1997: 58 index1998: 61 index1999: 64 index2000: 54 index2001: 48 index2002: 47 index2003: 48 index2004: 42 index2005: 47 index2006: 46 index2007: 50 index2008: 45 index2009: 39 index2010: 45 index2011: 47 index2012: 44 index2013: 37 index2014: 38 index2015: 36 index2016: 40 index2017: 40 index2018: 39 index2019: 42 index2020: 37 index2021: 39 index2022: 41 index2023: 43 index2024: 43 index2025: 40 index
index. Source: EPA, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12, published 2026-07-13.
All 38 values
Periodindex
202540
202443
202343
202241
202139
202037
201942
201839
201740
201640
201536
201438
201337
201244
201147
201045
200939
200845
200750
200646
200547
200442
200348
200247
200148
200054
199964
199861
199758
199651
199551
199442
199346
199242
199140
199049
198946
198864

Williamson County compared with nearby places

Anderson County: 39 indexBlount County: 44 indexClaiborne County: 35 indexDavidson County: 49 indexDeKalb County: 34 indexDyer County: 34 indexHamilton County: 44 indexJefferson County: 41 indexKnox County: 46 indexLawrence County: 33 indexLoudon County: 39 indexMcMinn County: 37 indexMadison County: 36 indexMaury County: 33 indexMontgomery County: 32 indexPutnam County: 30 indexRoane County: 29 indexSevier County: 40 indexShelby County: 47 indexSullivan County: 40 indexSumner County: 44 indexWilliamson County: 40 indexWilson County: 38 index
  • Under 33
  • 33 to 37
  • 37 to 40
  • 40 to 44
  • 44 and over
  • No data
Median Air Quality Index (US EPA), 2025, index. Williamson 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

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.

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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 (Median Air Quality Index (US EPA), Williamson 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: median daily AQI, Median Air Quality Index (US EPA), Williamson 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/median-air-quality-index/williamson-county-tennessee. Accessed 12 Oct. 2026.

Chicago (author-date)

U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Median Air Quality Index (US EPA), Williamson 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/median-air-quality-index/williamson-county-tennessee.

Harvard

U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Median Air Quality Index (US EPA), Williamson County, 1988 to 2025 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/en/median-air-quality-index/williamson-county-tennessee (Accessed: 12 October 2026).

BibTeX
@misc{environmental_median_air_quality_index_2025,
  author = {{U.S. Environmental Protection Agency}},
  title = {{EPA AirData annual AQI by county: median daily AQI: Median Air Quality Index (US EPA), Williamson 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/median-air-quality-index/williamson-county-tennessee. 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: Median Air Quality Index (US EPA), Williamson 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/median-air-quality-index/williamson-county-tennessee
Y2  - 2026/10/12
N1  - Accessed via Invokely, https://invokely.ai/en/median-air-quality-index/williamson-county-tennessee.
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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Questions

What is the median Air Quality Index (US EPA) in Williamson County?

Median Air Quality Index (US EPA) in Williamson County was 40 in 2025, according to the U.S. Environmental Protection Agency. That is down 3 from 2024.

Where does this figure come from?

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

What was the highest median Air Quality Index (US EPA) in Williamson County?

64 in 1988, the highest value since 1988 in this series.

Related statistics for Williamson County

All statistics for Williamson County | Williamson County in figures