Skip to content
RADON411

Locations and reports

Radon Levels in Union County, South Dakota

EPA puts Union County in Zone 1, its highest radon zone, and high results are common there: 51.1% of the 348 radon tests reported to CDC (2008 to 2017, laboratory totals) came back 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

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    51.1%

    7.4 pts below SD

    Union
    51.1%
    SD
    58.5%

    348 tests, 2008 to 2017

  • Median result

    4.1pCi/L

    Action level reached

    Union
    4.1 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    348

    5th of 23 by tests

    Union
    348
    Minnehaha
    1,920

    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 SD

    21 of 23

    Lowest quarter

    Union
    51.1%
    Lake
    76.0%

    By share, of 23 with 30+ tests

Read a result for Union County

Starts at Union County's median, 4.1 pCi/L

4.1pCi/L

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

EPA’s reading of 4.1 pCi/L

At or above 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 at or above 4 pCi/L. EPA recommends fixing a home when the radon level is 4 pCi/L or higher. Union County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Union County, At a glance

Union County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Union 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 Union County, 2008 to 2017, laboratory

Reported results by concentration band, Union County, 2008 to 2017, laboratory
Under 2 pCi/L: 23%2 to 4 pCi/L: 26%4 pCi/L and above: 51%

Of 348 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Union County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L7922.7%
2 to 4 pCi/L9126.1%
4 pCi/L and above17851.1%
What these figures count

These are the 348 tests reported to CDC for Union 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

Union County, Compared

Union County radon figures, compared

Calculated by Radon411

More than half: 51.1% of the 348 radon tests reported to CDC for Union County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 21st of 23 South Dakota counties with 30 or more tests for the same period. That is 7.4 points below the 58.5% pooled from 62 South Dakota counties.

The median result, 4.1 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 144.7 pCi/L. The rest split 22.7% under 2 pCi/L and 26.1% from 2 to 4 (South Dakota: 23.6% and 17.9%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 51.1% share point the same way here.

All 7 of its bordering counties with figures ran higher, led by Sioux County, IA at 84.2%; the lowest of them, Clay County, reported 51.8%. Statewide for the same period, Lake County reported the highest share (76.0%) and Davison County the lowest (48.8%).

CDC's ten-year totals for Union County went from 152 tests (2005 to 2014) to 348 (2008 to 2017), the share going from 51.3% to 51.1%.

Nationally, every county with a figure for the same period pools to 35.7%; Union County sits 15.4 points above it. CDC also counts 16 tests taken after mitigation in Union County for the same period: tests in homes where a fix was already in.

Union County against South Dakota, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Union County
Union County34851.1%4.1 pCi/LThis county
South Dakota, pooled62 counties, pooled6,27558.5%Not pooled+7.4 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−15.4 points
Lake Countyhighest in the state5076.0%8.5 pCi/L+24.9 points
Davison Countylowest in the state8448.8%3.8 pCi/L−2.3 points
Sioux County, IAborders it1,98284.2%10.7 pCi/L+33.1 points
Plymouth County, IAborders it1,40582.3%8.6 pCi/L+31.2 points
Dixon County, NEborders it27578.2%7.6 pCi/L+27.1 points
Woodbury County, IAborders it3,54169.2%6.1 pCi/L+18.1 points
Dakota County, NEborders it32261.5%5.0 pCi/L+10.4 points
Lincoln County, SDborders it79556.5%4.7 pCi/L+5.4 points
Clay County, SDborders it8351.8%4.2 pCi/L+0.7 points
Where Union County's reported results fell, 2008 to 2017, laboratory, beside South Dakota and the United States
ResultUnion County testsUnion County shareSouth Dakota shareU.S. shareAgainst South Dakota
Under 2 pCi/L7922.7%23.6%39.6%−0.9 points
2 to 4 pCi/L9126.1%17.9%24.7%+8.2 points
4 pCi/L and above17851.1%58.5%35.7%−7.4 points
Union County ranked among the 23 South 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
Turner County13 of 235858.6%4.8 pCi/L
Minnehaha County14 of 231,92058.5%4.8 pCi/L
Lawrence County15 of 2316056.9%5.3 pCi/L
Lincoln County16 of 2379556.5%4.7 pCi/L
Grant County17 of 237855.1%5.0 pCi/L
Pennington County18 of 231,02953.6%4.3 pCi/L
Clay County19 of 238351.8%4.2 pCi/L
Meade County20 of 2319751.8%4.1 pCi/L
Union County21 of 2334851.1%4.1 pCi/L
McCook County22 of 233050.0%3.9 pCi/L
Davison County23 of 238448.8%3.8 pCi/L
Union County ten-year laboratory totals, against South Dakota's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestSouth Dakota, pooledChange from the row before
2008 to 201734851.1%4.1 pCi/L144.7 pCi/L58.5%+1.5 points
2007 to 201628049.6%3.9 pCi/L144.7 pCi/L57.4%−3.3 points
2006 to 201518752.9%4.2 pCi/L144.7 pCi/L58.8%+1.6 points
2005 to 201415251.3%4.1 pCi/L144.7 pCi/L59.6%First on file

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

Union County, Geology

The ground under Union County

Predicted

USGS's geologic map puts all of Union County's mapped land in one class: high radon potential. 95.6% of it lies in the area the map calls Loess-Covered Drift Plains, to which USGS gives a radon index of 13, in the high band (12 to 17).

USGS's geologic areas under Union County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Loess-Covered Drift PlainsPre-Illinoian and Illinoian glacial deposits, and loess.High1395.6%
Missouri Coteau/James River Lowland/Coteau des PrairiesGlacial drift derived largely from Cretaceous marine shale.High143.4%
Pre-Wisconsin glacial depositsLoess-mantled glacial drift.High131.0%

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

Union County beside the ground around it

Union County has more high-potential ground than South Dakota as a whole: 100% of its land against 56.7%. Of the 7 counties that border it, none has as large a share of high-potential ground; Clay County comes nearest at 100%.

Share of land by USGS radon potential: Union County, the 7 counties bordering it and South Dakota
AreaEPA zoneLow potentialModerate potentialHigh potential
Union CountyZone 1NoneNone100%
Clay CountyZone 1NoneNone100%
Plymouth County, IAZone 1NoneNone100%
Dixon County, NEZone 1NoneNone100%
Dakota County, NEZone 1NoneNone100%
Lincoln CountyZone 1NoneNone100%
Sioux County, IAZone 1NoneNone100%
Woodbury County, IAZone 1NoneNone100%
South Dakota66 counties, pooled by land areaVaries1.0%42.3%56.7%

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

Local help

Union County, Local resources

Radon testing and mitigation in Union County

What South Dakota's program says
Phone
1-800-438-3367

What it means

Union County, Questions

Questions about radon in Union County

Are radon levels high in Union County?

Over the 2008 to 2017 laboratory totals, 51.1% of the radon tests reported to CDC for Union County, South Dakota were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.1 pCi/L. Of 23 South Dakota counties with 30 or more tests for the same period, 20 reported a higher share; pooled, the state comes to 58.5%.

Which counties border Union County?

Union County borders 7 counties: Clay County, SD, Plymouth County, IA, Dixon County, NE, Dakota County, NE, Lincoln County, SD, Sioux County, IA and Woodbury County, IA.

Products

Union County, Products

Testing a home in Union County

In Union County, 51.1% of 348 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 58.5% pooled across South Dakota. In Zone 1 the year-round level in Union 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

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

Explore

Union County, Bordering counties

Counties bordering Union County

Calculated by Radon411

Union County, Cities and towns

Cities and towns in Union County

The Census Bureau estimates 17,402 residents in Union County for July 1, 2025, 547 more than in 2020 (+3.2%). That works out to 38 people per square mile of land. No town dominates: North Sioux City, the largest, has 3,036 residents, 17.4% of the total. Testing is reported here more often than across the state: CDC's 348 reported tests for 2008 to 2017 come to 20.0 for every 1,000 residents (2025 estimate), against 6.7 for South Dakota.

From North Sioux City to Jefferson, CDC publishes no figure by town: each reads Union County's.

The cities and towns in Union County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
North Sioux City3,036−0.5%17.4%
Beresfordextends beyond Union County2,105−3.4%Counted whole
Elk Point2,080−3.0%12.0%
Alcester776−5.7%4.5%
Jefferson448−5.1%2.6%

5 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 Union County is counted whole, so it has no share.

Union County, Homes

Homes in Union County

Of Union County's 7,438 homes, 71% are detached houses, against 67% across South Dakota; 16% are in buildings of five or more homes. Much of Union County is recent building: 35% of its homes have gone up since 2000 and the middle year built is 1992, against 1979 for South Dakota. 71% of occupied homes are lived in by their owner (South Dakota: 69%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Union County, South Dakota, 2019 to 2023
HomesUnion CountySouth Dakota
Housing units7,438402,364
Detached houses71%67%
In buildings of 5 or more16%16%
Mobile homes4%8%
Built before 196023%29%
Built since 200035%28%
Middle year built19921979
Owner-occupied71%69%

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