Median AQI
40
Good
Tennessee · County air quality · 2025
Williamson County, Tennessee
Median AQI 40 (Good). 88.5% of monitored days rated Good, direct from EPA Air Quality System stations.
- 40
- Median AQI
- 88.5%
- Good-air days
- 0
- Unhealthy days
- 72%
- Cleaner than US counties
The verdict
According to EPA Air Quality System monitoring data, Williamson County's air is good on balance, cleaner than the median US county, a median AQI of 40 with 88.5% of 122 monitored days in the Good band, driven mainly by Ozone.
- 40
- median AQI · Good
- 88.5%
- days rated Good
- Ozone
- main pollutant
- 72%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 80.
Data updated July 2026
- Good · 108 d · 88.5%
- Moderate · 14 d · 11.5%
How did Williamson County's air break down in 2025?
Across 122 monitored days, 108 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 80 (Moderate).
EPA AQS →Air quality data from EPA Air Quality System (2025). 122 days monitored. Median AQI: 40 (Good).
Insights: Williamson County, Tennessee. EPA monitors recorded a median Air Quality Index of 40 across 122 measured days in 2025, earning a Good classification and a Good health grade. Of those days, 108 (89%) fell in the Good band (AQI 0–50), 14 (11%) registered Moderate readings, and 0 (0%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 51, while the worst single-day peak hit 80 (Moderate). Residents averaged about 26.6 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as Ozone (122 days). Across the 2020–2025 trend window, good days shifted by -123 and the median AQI moved +3 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 80, moderate but never Unhealthy. Nearby counties show comparable readings: Anderson (91.0% good, AQI 42); Blount (69.6% good, AQI 46); Claiborne (96.3% good, AQI 36); Davidson (51.8% good, AQI 50); DeKalb (95.4% good, AQI 35); Dyer (79.4% good, AQI 34).
Air Quality Summary
Good Days
88.5%
108 of 122 monitored days
13.5pp vs 75% target
Max AQI Recorded
80
Moderate
Health Grade
Good
90th-pct AQI 51
Williamson County, Good days share 88.5%
National avg
Share of monitored days at AQI 0-50 (Good band).
Williamson County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 122 monitored days across EPA AQI categories.
Good
108
89%
Moderate
14
11%
USG
0
0%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Williamson County.
- Ozone · 122 days · 100.0%
Health Impact Assessment
Good
Williamson County's air quality is rated Good based on 88.5% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 88.5% of monitored days in 2025, with a median AQI of 40. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is Ozone, driving AQI on 122 of 122 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Williamson County. Best year: 2020 (231 good days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 231 | 10 | 0 | 0 | 37 | 71 |
| 2021 | 225 | 18 | 0 | 0 | 39 | 67 |
| 2022 | 209 | 33 | 2 | 0 | 41 | 122 |
| 2023 | 197 | 45 | 2 | 0 | 43 | 122 |
| 2024 | 200 | 38 | 0 | 0 | 43 | 87 |
| 2025 | 108 | 14 | 0 | 0 | 40 | 80 |
How does Williamson County compare?
How Williamson County compares
Good-day share better than US avg
Williamson County 88.5%
Anderson County, Tennessee 91.0%
US average 76.5%
Median AQI worse than US avg
Williamson County 40
Anderson County, Tennessee 42
US average 38
Unhealthy days / yr about average
Williamson County 0
Anderson County, Tennessee 0
US average 0
Other Counties in Tennessee
Air quality comparison for nearby counties in Tennessee.
Counties with Similar Air Quality
Counties across the US with a similar median AQI to Williamson County.
Amador County, California
AQI 40 · 84.8% good
Anne Arundel County, Maryland
AQI 40 · 87.2% good
Arlington County, Virginia
AQI 40 · 84.8% good
Bergen County, New Jersey
AQI 40 · 71.1% good
Berkeley County, West Virginia
AQI 40 · 81.1% good
Billings County, North Dakota
AQI 40 · 80.3% good
What Williamson County's air means for you
The air here stays in the breathable range year-round, with Ozone the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-Ozone days, the guide explains what a median of 40 means for exposure. Health guidance
- See how Williamson County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Williamson County with its Tennessee neighbours before drawing a conclusion about the region. All of Tennessee
Based on 122 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
More Air Quality Data
Air Quality Guides
Frequently Asked Questions
What is the air quality in Williamson County, Tennessee? ▼
Williamson County has a median Air Quality Index (AQI) of 40, rated "Good." 88.5% of monitored days (108 out of 122) had good air quality in 2025.
What is the main pollutant in Williamson County? ▼
The primary pollutant in Williamson County is Ozone (O3), which was the main AQI driver on 122 days (100% of monitored days). Can trigger asthma, reduce lung function, and cause chest pain.
How many unhealthy air days does Williamson County have? ▼
In 2025, Williamson County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 80.
Is the air quality in Williamson County getting better or worse? ▼
Air quality in Williamson County is worsening based on 5-year trends (2020-2025). Best year: 2020 with 231 good days. Worst year: 2025 with 108 good days.
What does AQI 40 mean for health? ▼
Williamson County's median AQI of 40 falls in the "Good" category. See the <a href="/guides/understanding-aqi" class="underline">AQI category guide</a> for what each range means for health.
How does Williamson County compare to other counties in Tennessee? ▼
Williamson County has 88.5% good air days. Nearby counties in Tennessee: Anderson (91.0% good), Blount (69.6% good), Claiborne (96.3% good), Davidson (51.8% good), DeKalb (95.4% good), Dyer (79.4% good).
What is the 90th percentile AQI in Williamson County? ▼
The 90th percentile AQI for Williamson County is 51, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 80 (Moderate).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer.
Every figure on PlainAir is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.