EnviroNearby

Williamson County, Tennessee air quality history

Daily Air Quality Index summaries from EPA monitors in Williamson, Tennessee, 1988–2025.

89%
Good days in 2025
40
Median daily AQI, 2025
0
Days above 100 in 2025
243
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Williamson, Tennessee on 243 days. 216 of those days (89%) were in the Good range, and the median daily AQI was 40. That median is about the same as the typical monitored county in Tennessee (median 39 across 23 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 243 days. Across the years with fuller monitoring, the share of Good days rose: an average of 51% in 1988–1990 versus 85% in 2023–2025.

EPA did not report an AQI for every day of 2025 here (243 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Share of monitored days in each AQI category, by year0%25%50%75%100%1988: Good 65 of 174 days1988: Moderate 51 of 174 days1988: Unhealthy for Sensitive Groups 37 of 174 days1988: Unhealthy 20 of 174 days1988: Very Unhealthy 1 of 174 days19881989: Good 123 of 193 days1989: Moderate 55 of 193 days1989: Unhealthy for Sensitive Groups 15 of 193 days1990: Good 113 of 212 days1990: Moderate 67 of 212 days1990: Unhealthy for Sensitive Groups 30 of 212 days1990: Unhealthy 2 of 212 days1991: Good 158 of 214 days1991: Moderate 48 of 214 days1991: Unhealthy for Sensitive Groups 7 of 214 days1991: Unhealthy 1 of 214 days1992: Good 146 of 207 days1992: Moderate 55 of 207 days1992: Unhealthy for Sensitive Groups 6 of 207 days1993: Good 129 of 213 days1993: Moderate 62 of 213 days1993: Unhealthy for Sensitive Groups 20 of 213 days1993: Unhealthy 2 of 213 days19931994: Good 151 of 213 days1994: Moderate 44 of 213 days1994: Unhealthy for Sensitive Groups 18 of 213 days1995: Good 102 of 214 days1995: Moderate 71 of 214 days1995: Unhealthy for Sensitive Groups 35 of 214 days1995: Unhealthy 6 of 214 days1996: Good 103 of 213 days1996: Moderate 76 of 213 days1996: Unhealthy for Sensitive Groups 27 of 213 days1996: Unhealthy 7 of 213 days1997: Good 91 of 202 days1997: Moderate 71 of 202 days1997: Unhealthy for Sensitive Groups 30 of 202 days1997: Unhealthy 10 of 202 days1998: Good 92 of 214 days1998: Moderate 72 of 214 days1998: Unhealthy for Sensitive Groups 39 of 214 days1998: Unhealthy 9 of 214 days1998: Very Unhealthy 2 of 214 days19981999: Good 95 of 239 days1999: Moderate 84 of 239 days1999: Unhealthy for Sensitive Groups 47 of 239 days1999: Unhealthy 13 of 239 days2000: Good 127 of 267 days2000: Moderate 95 of 267 days2000: Unhealthy for Sensitive Groups 37 of 267 days2000: Unhealthy 8 of 267 days2001: Good 137 of 245 days2001: Moderate 89 of 245 days2001: Unhealthy for Sensitive Groups 19 of 245 days2002: Good 148 of 245 days2002: Moderate 60 of 245 days2002: Unhealthy for Sensitive Groups 27 of 245 days2002: Unhealthy 10 of 245 days2003: Good 145 of 244 days2003: Moderate 79 of 244 days2003: Unhealthy for Sensitive Groups 19 of 244 days2003: Unhealthy 1 of 244 days20032004: Good 174 of 233 days2004: Moderate 54 of 233 days2004: Unhealthy for Sensitive Groups 5 of 233 days2005: Good 138 of 235 days2005: Moderate 78 of 235 days2005: Unhealthy for Sensitive Groups 19 of 235 days2006: Good 145 of 233 days2006: Moderate 82 of 233 days2006: Unhealthy for Sensitive Groups 6 of 233 days2007: Good 121 of 241 days2007: Moderate 87 of 241 days2007: Unhealthy for Sensitive Groups 30 of 241 days2007: Unhealthy 3 of 241 days2008: Good 167 of 245 days2008: Moderate 75 of 245 days2008: Unhealthy for Sensitive Groups 3 of 245 days20082009: Good 208 of 241 days2009: Moderate 32 of 241 days2009: Unhealthy for Sensitive Groups 1 of 241 days2010: Good 162 of 240 days2010: Moderate 69 of 240 days2010: Unhealthy for Sensitive Groups 9 of 240 days2011: Good 154 of 243 days2011: Moderate 83 of 243 days2011: Unhealthy for Sensitive Groups 6 of 243 days2012: Good 185 of 244 days2012: Moderate 52 of 244 days2012: Unhealthy for Sensitive Groups 7 of 244 days2013: Good 227 of 244 days2013: Moderate 17 of 244 days20132014: Good 210 of 234 days2014: Moderate 23 of 234 days2014: Unhealthy for Sensitive Groups 1 of 234 days2015: Good 223 of 241 days2015: Moderate 18 of 241 days2016: Good 219 of 244 days2016: Moderate 24 of 244 days2016: Unhealthy for Sensitive Groups 1 of 244 days2017: Good 197 of 215 days2017: Moderate 18 of 215 days2018: Good 212 of 245 days2018: Moderate 32 of 245 days2018: Unhealthy for Sensitive Groups 1 of 245 days20182019: Good 215 of 245 days2019: Moderate 30 of 245 days2020: Good 231 of 241 days2020: Moderate 10 of 241 days2021: Good 225 of 243 days2021: Moderate 18 of 243 days2022: Good 209 of 244 days2022: Moderate 33 of 244 days2022: Unhealthy for Sensitive Groups 2 of 244 days2023: Good 197 of 244 days2023: Moderate 45 of 244 days2023: Unhealthy for Sensitive Groups 2 of 244 days20232024: Good 200 of 238 days2024: Moderate 38 of 238 days2025: Good 216 of 243 days2025: Moderate 27 of 243 days2025
  • Good
  • Moderate
  • Unhealthy for Sensitive Groups
  • Unhealthy
  • Very Unhealthy
  • Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median and 90th-percentile daily AQI, by year0501001502001988: 90th percentile AQI 1561989: 90th percentile AQI 931990: 90th percentile AQI 1051991: 90th percentile AQI 841992: 90th percentile AQI 871993: 90th percentile AQI 1011994: 90th percentile AQI 1001995: 90th percentile AQI 1151996: 90th percentile AQI 1121997: 90th percentile AQI 1221998: 90th percentile AQI 1331999: 90th percentile AQI 1402000: 90th percentile AQI 1152001: 90th percentile AQI 932002: 90th percentile AQI 1192003: 90th percentile AQI 972004: 90th percentile AQI 742005: 90th percentile AQI 972006: 90th percentile AQI 842007: 90th percentile AQI 1082008: 90th percentile AQI 802009: 90th percentile AQI 542010: 90th percentile AQI 842011: 90th percentile AQI 742012: 90th percentile AQI 742013: 90th percentile AQI 482014: 90th percentile AQI 512015: 90th percentile AQI 492016: 90th percentile AQI 512017: 90th percentile AQI 502018: 90th percentile AQI 582019: 90th percentile AQI 512020: 90th percentile AQI 472021: 90th percentile AQI 502022: 90th percentile AQI 542023: 90th percentile AQI 612024: 90th percentile AQI 542025: 90th percentile AQI 511988: Median AQI 641989: Median AQI 461990: Median AQI 491991: Median AQI 401992: Median AQI 421993: Median AQI 461994: Median AQI 421995: Median AQI 511996: Median AQI 511997: Median AQI 581998: Median AQI 611999: Median AQI 642000: Median AQI 542001: Median AQI 482002: Median AQI 472003: Median AQI 482004: Median AQI 422005: Median AQI 472006: Median AQI 462007: Median AQI 502008: Median AQI 452009: Median AQI 392010: Median AQI 452011: Median AQI 472012: Median AQI 442013: Median AQI 372014: Median AQI 382015: Median AQI 362016: Median AQI 402017: Median AQI 402018: Median AQI 392019: Median AQI 422020: Median AQI 372021: Median AQI 392022: Median AQI 412023: Median AQI 432024: Median AQI 432025: Median AQI 40198819931998200320082013201820232025
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2025

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

PollutantDays as main pollutantShare of monitored days
Ozone243100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
20252432162700004051100Ozone
2024238200380000435487Ozone
20232441974520004361122Ozone
20222442093320004154122Ozone
2021243225180000395067Ozone
2020241231100000374771Ozone
2019245215300000425184Ozone
20182452123210003958108Ozone
2017215197180000405080Ozone
20162442192410004051108Ozone
2015241223180000364984Ozone
20142342102310003851105Ozone
2013244227170000374887Ozone
20122441855270004474136Ozone
20112431548360004774129Ozone
20102401626990004584126Ozone
20092412083210003954140Ozone
20082451677530004580126Ozone
2007241121873030050108159Ozone
20062331458260004684150Ozone
200523513878190004797129Ozone
20042331745450004274112Ozone
200324414579191004897156Ozone
20022451486027100047119177Ozone
200124513789190004893143Ozone
2000267127953780054115195Ozone
1999239958447130064140195Ozone
199821492723992061133205Ozone
1997202917130100058122174Ozone
1996213103762770051112172Ozone
1995214102713560051115200Ozone
1994213151441800042100147Ozone
1993213129622020046101179Ozone
19922071465560004287143Ozone
19912141584871004084159Ozone
1990212113673020049105159Ozone
198919312355150004693150Ozone
1988174655137201064156209Ozone

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

Counties in Tennessee with a similar median AQI in 2025

All monitored counties in Tennessee

Source: U.S. EPA AirData, “Annual AQI by County” files for 1988–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem