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Radon Levels in Williams County, North Dakota

EPA puts Williams County in Zone 1, its highest radon zone, and 33.8% of its 204 reported radon tests (2008 to 2017, laboratory totals) 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

    33.8%

    25.5 pts below ND

    Williams
    33.8%
    ND
    59.3%

    204 tests, 2008 to 2017

  • Median result

    2.9pCi/L

    Below action level

    Williams
    2.9 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    204

    8th of 29 by tests

    Williams
    204
    Burleigh
    1,558

    Reported to CDC before any fix, 2008 to 2017

  • 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 ND

    29 of 29

    Lowest quarter

    Williams
    33.8%
    Pembina
    82.5%

    By share, of 29 with 30+ tests

Read a result for Williams County

Starts at Williams County's median, 2.9 pCi/L

2.9pCi/L

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

EPA’s reading of 2.9 pCi/L

Between 2 and 4 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 between 2 and 4 pCi/L. EPA recommends considering a fix when the level is between 2 and 4 pCi/L. Williams County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Williams County, At a glance

Williams County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Williams 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 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Williams County, 2008 to 2017, laboratory
Under 2 pCi/L: 34%2 to 4 pCi/L: 32%4 pCi/L and above: 34%

Of 204 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Williams County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L7034.3%
2 to 4 pCi/L6531.9%
4 pCi/L and above6933.8%
What these figures count

These are the 204 tests reported to CDC for Williams 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

Williams County, Compared

Williams County radon figures, compared

Calculated by Radon411

In Williams County, about 3 in 10 reported tests over the 2008 to 2017 laboratory totals (69 of 204) were at or above EPA's action level. Against the national pool of 35.7%, the gap is 1.9 points lower. Across 53 North Dakota counties pooled together, 59.3% of tests reached that level, so Williams County runs 25.5 points below its state.

Of 4 neighbours with a figure, all 4 reported a larger share at or above 4 pCi/L; the spread is 56.1% (McKenzie County) to 78.6% (Richland County, MT). By share, Williams County is 29th of 29 North Dakota counties with 30 or more tests for the same period.

Half the tests came in under 2.9 pCi/L and half over it; the single highest reached 53.3 pCi/L. Below the action level, 70 tests (34.3%) came in under 2 pCi/L, against 18.3% statewide, and 65 tests (31.9%) fell between 2 and 4, against 22.4% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 33.8% share point the same way here. USGS geology rates 100% of the county's land high radon potential. CDC also counts 5 tests taken after mitigation in Williams County for the same period: tests in homes where a fix was already in.

CDC's ten-year totals for Williams County went from 305 tests (2005 to 2014) to 204 (2008 to 2017), the share going from 35.1% to 33.8%.

Williams County against North Dakota, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Williams County
Williams County20433.8%2.9 pCi/LThis county
North Dakota, pooled53 counties, pooled6,60759.3%Not pooled+25.5 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+1.9 points
Pembina Countyhighest in the state5782.5%9.3 pCi/L+48.7 points
Richland Countylowest in the state9343.0%3.6 pCi/L+9.2 points
Roosevelt County, MTborders it1693.8%8.7 pCi/L+60.0 points
Richland County, MTborders it4278.6%5.9 pCi/L+44.8 points
Sheridan County, MTborders it7075.7%6.5 pCi/L+41.9 points
Mountrail County, NDborders it4866.7%7.3 pCi/L+32.9 points
Burke County, NDborders it1163.6%7.8 pCi/L+29.8 points
McKenzie County, NDborders it4156.1%4.8 pCi/L+22.3 points
Divide County, NDborders it1450.0%4.6 pCi/L+16.2 points
Where Williams County's reported results fell, 2008 to 2017, laboratory, beside North Dakota and the United States
ResultWilliams County testsWilliams County shareNorth Dakota shareU.S. shareAgainst North Dakota
Under 2 pCi/L7034.3%18.3%39.6%+16.0 points
2 to 4 pCi/L6531.9%22.4%24.7%+9.5 points
4 pCi/L and above6933.8%59.3%35.7%−25.5 points
Williams County ranked among the 29 North Dakota counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Foster County19 of 293161.3%4.5 pCi/L
Cass County20 of 2986257.5%4.7 pCi/L
McKenzie County21 of 294156.1%4.8 pCi/L
Morton County22 of 2926853.4%4.2 pCi/L
Traill County23 of 293751.4%4.1 pCi/L
Burleigh County24 of 291,55851.1%4.0 pCi/L
Pierce County25 of 293948.7%3.8 pCi/L
Wells County26 of 298148.1%3.8 pCi/L
Ward County27 of 2944643.0%3.3 pCi/L
Richland County28 of 299343.0%3.6 pCi/L
Williams County29 of 2920433.8%2.9 pCi/L
Williams County ten-year laboratory totals, against North Dakota's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestNorth Dakota, pooledChange from the row before
2008 to 201720433.8%2.9 pCi/L53.3 pCi/L59.3%−3.4 points
2007 to 201626137.2%3.0 pCi/L53.3 pCi/L58.7%+1.8 points
2006 to 201528035.4%3.0 pCi/L27.3 pCi/L56.3%+0.3 points
2005 to 201430535.1%3.0 pCi/L27.3 pCi/L54.0%First on file

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

Williams County, Geology

The ground under Williams County

Predicted

USGS's geologic map puts all of Williams County's mapped land in one class: high radon potential. All of it lies in the area the map calls Great Plains (Glaciated), to which USGS gives a radon index of 13, in the high band (12 to 17).

USGS's geologic areas under Williams County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Great Plains (Glaciated)Glacial deposits derived from Cretaceous shales containing higher than average amounts of uranium.High13100%

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

Williams County beside the ground around it

On high-potential ground Williams County is close to North Dakota as a whole: 100% of its land against 98.9%. Of the 7 counties that border it, none has as large a share of high-potential ground; Burke County comes nearest at 100%.

Share of land by USGS radon potential: Williams County, the 7 counties bordering it and North Dakota
AreaEPA zoneLow potentialModerate potentialHigh potential
Williams CountyZone 1NoneNone100%
McKenzie CountyZone 1NoneNone100%
Divide CountyZone 1NoneNone100%
Mountrail CountyZone 1NoneNone100%
Burke CountyZone 1NoneNone100%
Sheridan County, MTZone 1NoneNone100%
Richland County, MTZone 1NoneNone100%
Roosevelt County, MTZone 1NoneNone100%
North Dakota53 counties, pooled by land areaVariesNone1.1%98.9%

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

Local help

Williams County, Local resources

Radon testing and mitigation in Williams County

What North Dakota's program says
Phone
701-328-5166
Email
jOtto@nd.gov

What it means

Williams County, Questions

Questions about radon in Williams County

Are radon levels high in Williams County?

Over the 2008 to 2017 laboratory totals, 33.8% of the radon tests reported to CDC for Williams County, North Dakota were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.9 pCi/L. Of 29 North Dakota counties with 30 or more tests for the same period, 28 reported a higher share; pooled, the state comes to 59.3%.

Which counties border Williams County?

Williams County borders 7 counties: McKenzie County, ND, Divide County, ND, Mountrail County, ND, Burke County, ND, Sheridan County, MT, Richland County, MT and Roosevelt County, MT.

Products

Williams County, Products

Testing a home in Williams County

In Williams County, 33.8% of 204 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 59.3% pooled across North Dakota. In Zone 1 the year-round level in Williams 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

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

Explore

Williams County, Bordering counties

Counties bordering Williams County

Calculated by Radon411

Williams County, Cities and towns

Cities and towns in Williams County

The Census Bureau estimates 41,767 residents in Williams County for July 1, 2025, 577 more than in 2020 (+1.4%). That works out to 20 people per square mile of land. Williston alone is home to 29,522 of them, 70.7% of the total. Together its 8 incorporated places hold 79.1% of those residents; the other 20.9% live outside any of them. Reported testing is thin here: 204 tests over 2008 to 2017, or 4.9 for every 1,000 residents (2025 estimate), where North Dakota as a whole comes to 8.3.

From Williston to Springbrook, CDC publishes no figure by town: each reads Williams County's.

The cities and towns in Williams County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Williston29,522+0.7%70.7%
Tioga2,221+0.4%5.3%
Ray768+4.1%1.8%
Grenora230+2.7%0.6%
Wildrose119+4.4%0.3%
Epping89+3.5%0.2%
Alamo56+3.7%0.1%
Springbrook35+6.1%Under 0.1%

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).

Williams County, Homes

Homes in Williams County

Williams County has 20,291 homes, and many share a building: 36% are in buildings of five or more, 46% are detached houses, against 57% across North Dakota. Much of Williams County is recent building: 47% of its homes have gone up since 2000 and the middle year built is 1993, against 1981 for North Dakota. 57% of occupied homes are lived in by their owner (North Dakota: 63%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Williams County, North Dakota, 2019 to 2023
HomesWilliams CountyNorth Dakota
Housing units20,291374,866
Detached houses46%57%
In buildings of 5 or more36%25%
Mobile homes8%7%
Built before 196023%24%
Built since 200047%31%
Middle year built19931981
Owner-occupied57%63%

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