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Radon Levels in Williamson County, Tennessee

EPA puts Williamson County in Zone 1, its highest radon zone, and 16.5% of its 115 reported radon tests (2023) were at or above 4 pCi/L. Homes with elevated levels occur in all three zones, so only a test measures yours.

Data checked By Radon411Methodology

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  • Share at 4+ pCi/L

    16.5%

    8.4 pts below TN

    Williamson
    16.5%
    Tennessee
    24.9%

    115 tests, 2023

  • Median result

    1.3pCi/L

    Below action level

    Williamson
    1.3 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2023

  • Tests reported

    115

    6th of 15 by tests

    Williamson
    115
    Hamblen
    594

    Reported to CDC before any fix, 2023

  • EPA radon zone

    Zone 1

    Highest potential

    1. Zone 1
    2. 2
    3. 3

    1 highest, 3 lowest

    EPA predicts over 4 pCi/L indoors

  • Rank in Tennessee

    12 of 15

    Lowest quarter

    Williamson
    16.5%
    Blount
    50.0%

    By share, of 15 with 30+ tests

Read a result for Williamson County

Starts at Williamson County's median, 1.3 pCi/L

1.3pCi/L

EPA’s own thresholds; the figures are the area’s, never this home’s.

EPA’s reading of 1.3 pCi/L

Below 2 pCi/L

  1. Under 20 to 1.9 pCi/L
  2. 2 to 4consider a fix
  3. 4 and aboveEPA says fix it

This result is below 2 pCi/L, the level at which EPA says a fix should be considered. Williamson County's median reported result, 2023.

Interpret this reading

The data

Williamson County, At a glance

Williamson County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Williamson County in and predicts an average indoor of greater than 4 pCi/L (greater than 150 Bq/m³) across the county. That is one forecast for the whole county, not a reading of any address in it; homes above EPA's turn up in all three zones.

USGS geologic potential: High across 98% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Williamson County, 2023

Share of reported tests at or above 4 pCi/L, by year, Williamson County
Share of reported tests at or above 4 pCi/L, by year, Williamson County
YearShare of reported tests at or above 4 pCi/L
202225.2%
202316.5%

2023: 16.5%. Across these years the share ran from 16.5% to 25.2%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 115 tests reported to CDC for Williamson County over 2023, many from homes being sold or already suspected of a problem: they describe the county's reported tests, not this house.

Measured: CDC tracking data. Figure above: 2023. Measured data period: 2005 to 2023 (every CDC figure on file).

How to read these figures

Williamson County, Compared

Williamson County radon figures, compared

Calculated by Radon411

Williamson County's 115 reported tests in 2023 had a middle value of 1.3 pCi/L and a maximum of 17.2 pCi/L, and 16.5% reached 4 pCi/L. Tennessee as a whole (70 Tennessee counties, pooled) comes to 24.9%: 8.4 points higher than here.

Among its state's counties it ranks 12th of 15 Tennessee counties with 30 or more tests for the same period. Statewide for the same period, Blount County reported the highest share (50.0%) and Hamilton County the lowest (2.4%).

Below the action level, 73 tests (63.5%) came in under 2 pCi/L, against 49.4% statewide, and 23 tests (20.0%) fell between 2 and 4, against 25.7% statewide. The busiest year on file was 2022, with 393 tests reported; the quietest, 2023, had 115.

EPA's 1993 zone map puts Williamson County in Zone 1, a predicted county average above 4 pCi/L. USGS geology rates 98% of the county's land high radon potential.

Next door, the shares run from 20.0% in Rutherford County to 27.4% in Davidson County.

CDC also counts 16 tests taken after mitigation in Williamson County for the same period: tests in homes where a fix was already in.

Williamson County against Tennessee, the United States and nearby counties, 2023
AreaTestsAt or above 4 pCi/LMedianDifference from Williamson County
Williamson County11516.5%1.3 pCi/LThis county
Tennessee, pooled70 counties, pooled2,37824.9%Not pooled+8.4 points
United States, pooled642 counties, pooled128,74033.4%Not pooled+16.9 points
Blount Countyhighest in the state4650.0%3.9 pCi/L+33.5 points
Hamilton Countylowest in the state1692.4%0.7 pCi/L−14.1 points
Dickson County, TNborders it250.0%5.1 pCi/L+33.5 points
Maury County, TNborders it837.5%3.0 pCi/L+21.0 points
Hickman County, TNborders it333.3%1.1 pCi/L+16.8 points
Cheatham County, TNborders it728.6%3.3 pCi/L+12.1 points
Davidson County, TNborders it14627.4%2.2 pCi/L+10.9 points
Rutherford County, TNborders it5020.0%1.6 pCi/L+3.5 points
Marshall County, TNborders it10.0%1.8 pCi/L−16.5 points
Where Williamson County's reported results fell, 2023, beside Tennessee and the United States
ResultWilliamson County testsWilliamson County shareTennessee shareU.S. shareAgainst Tennessee
Under 2 pCi/L7363.5%49.4%41.4%+14.1 points
2 to 4 pCi/L2320.0%25.7%25.2%−5.7 points
4 pCi/L and above1916.5%24.9%33.4%−8.4 points
Williamson County ranked among the 15 Tennessee counties with 30 or more tests, by share at or above 4 pCi/L, 2023
CountyRankTestsAt or above 4 pCi/LMedian
Washington County5 of 154626.1%2.3 pCi/L
Bradley County6 of 1510525.7%2.2 pCi/L
Rutherford County7 of 155020.0%1.6 pCi/L
Sumner County7 of 153020.0%1.4 pCi/L
Cocke County9 of 1534718.4%1.4 pCi/L
Knox County10 of 1520317.7%1.8 pCi/L
Jefferson County11 of 153517.1%2.6 pCi/L
Williamson County12 of 1511516.5%1.3 pCi/L
Anderson County13 of 153716.2%1.4 pCi/L
Wilson County14 of 15822.4%0.9 pCi/L
Hamilton County15 of 151692.4%0.7 pCi/L
Williamson County radon tests by year, against Tennessee's pooled share
YearTestsAt or above 4 pCi/LMedianHighestTennessee, pooledChange from the row before
202311516.5%1.3 pCi/L17.2 pCi/L24.9%−8.7 points
202239325.2%2.1 pCi/L23.0 pCi/L23.8%First on file

Pooled shares: Radon411's sum of CDC's county counts for one period.

Williamson County, Geology

The ground under Williamson County

Predicted

Most of the ground under Williamson County is one class on USGS's geologic map: 98.2% of its land is high radon potential, with 1.8% moderate. The map puts 74.0% of Williamson County in the area it calls Nashville Basin and 24.2% in Cherty Highland, with one smaller area beside them. USGS gives the first a radon index of 14 and the second 12, both in the high band (12 to 17).

USGS's geologic areas under Williamson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Nashville BasinMississippian Limestone.High1474.0%
Cherty HighlandCherty dolostone.High1224.2%
Highland RimPaleozoic sedimentary rocks (limestone, shale, chert, and dolostone).Moderate111.8%

Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.

Williamson County beside the ground around it

Williamson County has more high-potential ground than Tennessee as a whole: 98.2% of its land against 47.2%. Of the 7 counties that border it, 4 have more high-potential ground, Marshall County the most at 100%.

Share of land by USGS radon potential: Williamson County, the 7 counties bordering it and Tennessee
AreaEPA zoneLow potentialModerate potentialHigh potential
Williamson CountyZone 1None1.8%98.2%
Maury CountyZone 3NoneNone100%
Davidson CountyZone 1NoneNone100%
Cheatham CountyZone 2None21.0%79.0%
Marshall CountyZone 1NoneNone100%
Rutherford CountyZone 1NoneNone100%
Dickson CountyZone 2None73.2%26.8%
Hickman CountyZone 1None21.2%78.8%
Tennessee95 counties, pooled by land areaVaries30.5%22.3%47.2%

The map rates the ground, not the air in any home.

Local help

Williamson County, Local resources

Radon testing and mitigation in Williamson County

What Tennessee's program says
Phone
1-800-232-1139
Email
TDEC.Radon@tn.gov

What it means

Williamson County, Questions

Questions about radon in Williamson County

Are radon levels high in Williamson County?

In 2023, 16.5% of the radon tests reported to CDC for Williamson County, Tennessee were at or above 4 pCi/L, and the median reported pre-mitigation result was 1.3 pCi/L. Of 15 Tennessee counties with 30 or more tests for the same period, 11 reported a higher share; pooled, the state comes to 24.9%.

Which counties border Williamson County?

Williamson County borders 7 counties: Maury County, Davidson County, Cheatham County, Marshall County, Rutherford County, Dickson County and Hickman County.

Products

Williamson County, Products

Testing a home in Williamson County

In Williamson County, 16.5% of 115 tests reported to CDC in 2023 reached 4 pCi/L, against 24.9% pooled across Tennessee. In Zone 1 the year-round level in Williamson County is the one to know: a long-term test (more than 90 days) or a monitor, with a short-term kit for a first answer.

Test kits and monitors3

See every kit and monitor

Williamson County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Williamson County

Explore

Williamson County, Bordering counties

Counties bordering Williamson County

Calculated by Radon411

Williamson County, Cities and towns

Cities and towns in Williamson County

Williamson County is growing: the Census Bureau puts it at 272,061 residents on July 1, 2025, up 9.0% from 249,614 in 2020. That works out to 467 people per square mile of land. Its largest town, Franklin, has 90,226 residents, 33.2% of the total. CDC's 115 reported tests for 2023 work out to 0.4 per 1,000 residents (2025 estimate), close to Tennessee's 0.3.

From Franklin to Eagleville, CDC publishes no figure by town: each reads Williamson County's.

The cities and towns in Williamson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Franklin90,226+7.2%33.2%
Smyrnaextends beyond Williamson County63,321+18.4%Counted whole
Spring Hillextends beyond Williamson County61,336+19.5%Counted whole
Brentwood45,313−0.3%16.7%
Nolensville15,789+12.5%5.8%
Fairview10,877+14.4%4.0%
Thompson's Station9,469+25.3%3.5%
Eaglevilleextends beyond Williamson County964+18.4%Counted whole

8 incorporated places, as listed in the U.S. Census Bureau’s 2020 place reference files. Residents: U.S. Census Bureau population estimates for July 1, 2025 (Vintage 2025). A place that extends beyond Williamson County is counted whole, so it has no share.

Williamson County, Homes

Homes in Williamson County

Williamson County is a place of houses on their own lots: 78% of its 94,418 homes are detached single-family houses, against 68% across Tennessee. Much of Williamson County is recent building: 52% of its homes have gone up since 2000 and the middle year built is 2001, against 1986 for Tennessee. Owners live in 80% of the occupied homes (Tennessee: 67%), so for most households a test and a fix are their own to order.

Homes in Williamson County, Tennessee, 2019 to 2023
HomesWilliamson CountyTennessee
Housing units94,4183,095,472
Detached houses78%68%
In buildings of 5 or more13%14%
Mobile homes2%8%
Built before 19604%18%
Built since 200052%29%
Middle year built20011986
Owner-occupied80%67%

Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.