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Radon Levels in Williamson County, Texas

EPA puts Williamson County in Zone 3, its lowest radon zone, and most reported tests there come back low, though 1.9% of 103 (2008 to 2017, laboratory totals) reached 4 pCi/L. Homes with elevated levels occur in all three zones, so only a test measures yours.

Data checked By Radon411Methodology

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    1.9%

    8.3 pts below TX

    Williamson
    1.9%
    Texas
    10.2%

    103 tests, 2008 to 2017

  • Median result

    0.7pCi/L

    Below action level

    Williamson
    0.7 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    103

    11th of 22 by tests

    Williamson
    103
    Harris
    733

    Reported to CDC before any fix, 2008 to 2017

  • EPA radon zone

    Zone 3

    Lower potential

    1. 1
    2. 2
    3. Zone 3

    1 highest, 3 lowest

    EPA predicts under 2 pCi/L indoors

  • Rank in Texas

    17 of 22

    Middle half

    Williamson
    1.9%
    Randall
    69.0%

    By share, of 22 with 30+ tests

Read a result for Williamson County

Starts at Williamson County's median, 0.7 pCi/L

0.7pCi/L

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

EPA’s reading of 0.7 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, 2008 to 2017.

Interpret this reading

The data

Williamson County, At a glance

Williamson County at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for Williamson County as a whole is an average indoor of less than 2 pCi/L (less than 75 Bq/m³). A low zone clears no address in it: homes above EPA's turn up in all three zones.

USGS geologic potential: Low across 85% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Williamson County, 2008 to 2017, laboratory

Reported results by concentration band, Williamson County, 2008 to 2017, laboratory
Under 2 pCi/L: 88%2 to 4 pCi/L: 10%4 pCi/L and above: 2%

Of 103 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Williamson County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L9188.3%
2 to 4 pCi/L109.7%
4 pCi/L and above21.9%
What these figures count

These are the 103 tests reported to CDC for Williamson County over 2008 to 2017 (laboratory totals), 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: 2008 to 2017, laboratory. Measured data period: 2005 to 2017 (every CDC figure on file).

How to read these figures

Williamson County, Compared

Williamson County radon figures, compared

Calculated by Radon411

Only 2 of 103 radon tests reported for Williamson County over the 2008 to 2017 laboratory totals reached 4 pCi/L (1.9%). The median result, 0.7 pCi/L, sits below 2, well under EPA's 4 pCi/L action level, and the top reported result was 6.7 pCi/L.

Across 138 Texas counties pooled together, 10.2% of tests reached that level, so Williamson County runs 8.3 points below its state. Against the national pool of 35.7%, the gap is 33.8 points lower. Its place in the state: 17th of 22 Texas counties with 30 or more tests for the same period.

Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 1.9% share point the same way here. Of 2 neighbours with a figure, all 2 reported a larger share at or above 4 pCi/L; the spread is 9.2% (Travis County) to 16.7% (Bell County).

Below the action level, 91 tests (88.3%) came in under 2 pCi/L, against 80.3% statewide, and 10 tests (9.7%) fell between 2 and 4, against 9.6% statewide. USGS geology rates 85% of the county's land low radon potential.

CDC's ten-year totals for Williamson County went from 64 tests (2005 to 2014) to 103 (2008 to 2017), the share going from 3.1% to 1.9%.

Williamson County against Texas, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Williamson County
Williamson County1031.9%0.7 pCi/LThis county
Texas, pooled138 counties, pooled4,61510.2%Not pooled+8.3 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+33.8 points
Randall Countyhighest in the state8769.0%6.1 pCi/L+67.1 points
Nueces Countylowest in the state430.0%0.6 pCi/L−1.9 points
Burnet County, TXborders it771.4%27.4 pCi/L+69.5 points
Bastrop County, TXborders it1361.5%6.5 pCi/L+59.6 points
Bell County, TXborders it3016.7%1.3 pCi/L+14.8 points
Travis County, TXborders it4269.2%0.9 pCi/L+7.3 points
Where Williamson County's reported results fell, 2008 to 2017, laboratory, beside Texas and the United States
ResultWilliamson County testsWilliamson County shareTexas shareU.S. shareAgainst Texas
Under 2 pCi/L9188.3%80.3%39.6%+8.0 points
2 to 4 pCi/L109.7%9.6%24.7%+0.1 points
4 pCi/L and above21.9%10.2%35.7%−8.3 points
Williamson County ranked among the 22 Texas counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Brazoria County12 of 22326.3%0.3 pCi/L
Smith County13 of 22525.8%0.8 pCi/L
Montgomery County14 of 221883.2%0.8 pCi/L
Harris County15 of 227332.9%0.4 pCi/L
Fort Bend County16 of 221052.9%0.2 pCi/L
Williamson County17 of 221031.9%0.7 pCi/L
Midland County18 of 221001.0%0.7 pCi/L
Galveston County19 of 22730.0%0.2 pCi/L
El Paso County19 of 22570.0%0.7 pCi/L
Brazos County19 of 22470.0%0.5 pCi/L
Nueces County19 of 22430.0%0.6 pCi/L
Williamson County ten-year laboratory totals, against Texas' pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestTexas, pooledChange from the row before
2008 to 20171031.9%0.7 pCi/L6.7 pCi/L10.2%−0.1 points
2007 to 2016982.0%0.7 pCi/L6.7 pCi/L10.4%−0.2 points
2006 to 2015902.2%0.7 pCi/L6.7 pCi/L11.0%−0.9 points
2005 to 2014643.1%0.8 pCi/L6.7 pCi/L10.6%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: 84.6% of its land is low radon potential, with 15.4% moderate. The map puts 42.3% of Williamson County in the area it calls East Texas Province and 42.3% in Central Texas/Gulf Coast Plain, with one smaller area beside them. USGS gives the first a radon index of 6 and the second 7, both in the low band (5 to 8).

USGS's geologic areas under Williamson County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
East Texas ProvinceCretaceous through Quaternary marine sandstone, shale, chalk, limestone, siltstone, clay, and lignite.Low642.3%
Central Texas/Gulf Coast PlainCretaceous through Quaternary marine sandstone, shale, chalk, limestone, siltstone, clay, and lignite.Low742.3%
Cretaceous Gulfian seriesCretaceous sandstone, shale, limestone, siltstone.Moderate1015.4%

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

Across Texas 0.5% of the land is rated high; none of Williamson County's is. Of the 6 counties that border it, 2 have more moderate-potential ground, Burnet County the most at 28.7%.

Share of land by USGS radon potential: Williamson County, the 6 counties bordering it and Texas
AreaEPA zoneLow potentialModerate potentialHigh potential
Williamson CountyZone 384.6%15.4%None
Bell CountyZone 382.2%17.8%None
Travis CountyZone 386.5%13.5%None
Burnet CountyZone 371.3%28.7%None
Milam CountyZone 3100%NoneNone
Bastrop CountyZone 3100%NoneNone
Lee CountyZone 3100%NoneNone
Texas254 counties, pooled by land areaVaries72.4%26.8%0.5%

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 Texas's program says
Licensing of radon professionals
There are no radon certification requirements for the State of Texas at this time, but the National Radon Proficiency Program lists radon testers and mitigators within Texas that have passed a rigorous national certification program.

What it means

Williamson County, Questions

Questions about radon in Williamson County

Are radon levels high in Williamson County?

Over the 2008 to 2017 laboratory totals, 1.9% of the radon tests reported to CDC for Williamson County, Texas were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.7 pCi/L. Of 22 Texas counties with 30 or more tests for the same period, 16 reported a higher share; pooled, the state comes to 10.2%.

Which counties border Williamson County?

Williamson County borders 6 counties: Bell County, Travis County, Burnet County, Milam County, Bastrop County and Lee County.

Products

Williamson County, Products

Testing a home in Williamson County

In Williamson County, 1.9% of 103 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 10.2% pooled across Texas. Zone 3 is the lowest label, yet homes at or above 4 pCi/L occur in every zone: a short-term kit (2 to 90 days) is the quick first check in Williamson County.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Williamson County homes, compared

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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 752,827 residents on July 1, 2025, up 22.3% from 615,611 in 2020. That works out to 675 people per square mile of land. Austin, the largest place on its list (1,002,632 residents in all), extends beyond Williamson County, so no share of the total is worked out for it. CDC's 103 reported tests for 2008 to 2017 work out to 0.1 per 1,000 residents (2025 estimate), close to Texas' 0.1.

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

The 12 largest cities and towns in Williamson County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Austinextends beyond Williamson County1,002,632+4.2%Counted whole
Round Rockextends beyond Williamson County141,282+17.2%Counted whole
Georgetown106,907+55.6%14.2%
Leanderextends beyond Williamson County91,132+49.7%Counted whole
Cedar Parkextends beyond Williamson County79,032+1.3%Counted whole
Pflugervilleextends beyond Williamson County68,316+4.3%Counted whole
Hutto46,048+63.9%6.1%
Taylor18,402+11.9%2.4%
Liberty Hill13,317+261.9%1.8%
Jarrell5,870+228.9%0.8%
Bartlettextends beyond Williamson County1,678+2.9%Counted whole
Thorndaleextends beyond Williamson County1,456+15.6%Counted whole
  • Florence
  • Granger
  • Thrall
  • Weir
  • Coupland

17 incorporated places, as listed in the U.S. Census Bureau’s 2020 place reference files. “(part)” marks a place that extends into another county. 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

Of Williamson County's 251,600 homes, 73% are detached houses, against 65% across Texas; 18% are in buildings of five or more homes. Much of Williamson County is recent building: 62% of its homes have gone up since 2000 and the middle year built is 2005, against 1990 for Texas. 67% of occupied homes are lived in by their owner (Texas: 63%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Williamson County, Texas, 2019 to 2023
HomesWilliamson CountyTexas
Housing units251,60011,890,808
Detached houses73%65%
In buildings of 5 or more18%21%
Mobile homes2%7%
Built before 19603%13%
Built since 200062%37%
Middle year built20051990
Owner-occupied67%63%

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