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Radon Levels in Clay County, South Dakota

EPA puts Clay County in Zone 1, its highest radon zone, and high results are common there: 51.8% of the 83 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

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  • Share at 4+ pCi/L

    51.8%

    6.7 pts below SD

    Clay
    51.8%
    SD
    58.5%

    83 tests, 2008 to 2017

  • Median result

    4.2pCi/L

    Action level reached

    Clay
    4.2 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    83

    11th of 23 by tests

    Clay
    83
    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

    19 of 23

    Lowest quarter

    Clay
    51.8%
    Lake
    76.0%

    By share, of 23 with 30+ tests

Read a result for Clay County

Starts at Clay County's median, 4.2 pCi/L

4.2pCi/L

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

EPA’s reading of 4.2 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. Clay County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Clay County, At a glance

Clay County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

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

Reported results by concentration band, Clay County, 2008 to 2017, laboratory
Under 2 pCi/L: 31%2 to 4 pCi/L: 17%4 pCi/L and above: 52%

Of 83 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Clay County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L2631.3%
2 to 4 pCi/L1416.9%
4 pCi/L and above4351.8%
What these figures count

These are the 83 tests reported to CDC for Clay 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

Clay County, Compared

Clay County radon figures, compared

Calculated by Radon411

More than half: 51.8% of the 83 radon tests reported to CDC for Clay County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 19th of 23 South Dakota counties with 30 or more tests for the same period. That is 6.7 points below the 58.5% pooled from 62 South Dakota counties.

The median result, 4.2 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 23.5 pCi/L. The rest split 31.3% under 2 pCi/L and 16.9% 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 51.8% share point the same way here.

5 of its 6 bordering counties with figures ran higher, led by Dixon County, NE at 78.2%; Union County reported 51.1%. 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 Clay County went from 53 tests (2005 to 2014) to 83 (2008 to 2017), the share going from 54.7% to 51.8%.

Nationally, every county with a figure for the same period pools to 35.7%; Clay County sits 16.1 points above it.

Clay County against South Dakota, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Clay County
Clay County8351.8%4.2 pCi/LThis county
South Dakota, pooled62 counties, pooled6,27558.5%Not pooled+6.7 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−16.1 points
Lake Countyhighest in the state5076.0%8.5 pCi/L+24.2 points
Davison Countylowest in the state8448.8%3.8 pCi/L−3.0 points
Dixon County, NEborders it27578.2%7.6 pCi/L+26.4 points
Cedar County, NEborders it48776.8%8.3 pCi/L+25.0 points
Yankton County, SDborders it11872.9%6.3 pCi/L+21.1 points
Turner County, SDborders it5858.6%4.8 pCi/L+6.8 points
Lincoln County, SDborders it79556.5%4.7 pCi/L+4.7 points
Union County, SDborders it34851.1%4.1 pCi/L−0.7 points
Where Clay County's reported results fell, 2008 to 2017, laboratory, beside South Dakota and the United States
ResultClay County testsClay County shareSouth Dakota shareU.S. shareAgainst South Dakota
Under 2 pCi/L2631.3%23.6%39.6%+7.7 points
2 to 4 pCi/L1416.9%17.9%24.7%−1.0 points
4 pCi/L and above4351.8%58.5%35.7%−6.7 points
Clay 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
Clay 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 20178351.8%4.2 pCi/L23.5 pCi/L58.5%+4.6 points
2007 to 20167247.2%3.7 pCi/L16.0 pCi/L57.4%−9.5 points
2006 to 20156056.7%5.0 pCi/L16.0 pCi/L58.8%+2.0 points
2005 to 20145354.7%5.0 pCi/L16.0 pCi/L59.6%First on file

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

Clay County, Geology

The ground under Clay County

Predicted

USGS's geologic map puts all of Clay County's mapped land in one class: high radon potential. The map puts 74.1% of Clay County in the area it calls Missouri Coteau/James River Lowland/Coteau des Prairies and 25.6% in Loess-Covered Drift Plains. USGS gives the first a radon index of 14 and the second 13, both in the high band (12 to 17).

USGS's geologic areas under Clay County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Missouri Coteau/James River Lowland/Coteau des PrairiesGlacial drift derived largely from Cretaceous marine shale.High1474.1%
Loess-Covered Drift PlainsPre-Illinoian and Illinoian glacial deposits, and loess.High1325.6%

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

Clay County beside the ground around it

Clay County has more high-potential ground than South Dakota as a whole: 100% of its land against 56.7%. Of the 6 counties that border it, none has as large a share of high-potential ground; Dixon County, NE comes nearest at 100%.

Share of land by USGS radon potential: Clay County, the 6 counties bordering it and South Dakota
AreaEPA zoneLow potentialModerate potentialHigh potential
Clay CountyZone 1NoneNone100%
Union CountyZone 1NoneNone100%
Yankton CountyZone 1NoneNone100%
Lincoln CountyZone 1NoneNone100%
Cedar County, NEZone 1NoneNone100%
Turner CountyZone 1NoneNone100%
Dixon County, NEZone 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

Clay County, Local resources

Radon testing and mitigation in Clay County

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

What it means

Clay County, Questions

Questions about radon in Clay County

Are radon levels high in Clay County?

Over the 2008 to 2017 laboratory totals, 51.8% of the radon tests reported to CDC for Clay County, South Dakota were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.2 pCi/L. Of 23 South Dakota counties with 30 or more tests for the same period, 18 reported a higher share; pooled, the state comes to 58.5%.

Which counties border Clay County?

Clay County borders 6 counties: Union County, SD, Yankton County, SD, Lincoln County, SD, Cedar County, NE, Turner County, SD and Dixon County, NE.

Products

Clay County, Products

Testing a home in Clay County

In Clay County, 51.8% of 83 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 Clay 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

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

Explore

Clay County, Bordering counties

Counties bordering Clay County

Calculated by Radon411

Clay County, Cities and towns

Cities and towns in Clay County

Clay County's population has barely moved since 2020: 15,031 residents on July 1, 2025 against 15,004 then, by the Census Bureau's estimates. That works out to 36 people per square mile of land. Vermillion alone is home to 11,720 of them, 78.0% of the total. CDC's 83 reported tests for 2008 to 2017 work out to 5.5 per 1,000 residents (2025 estimate), close to South Dakota's 6.7.

From Vermillion to Wakonda, CDC publishes no figure by town: each reads Clay County's.

The cities and towns in Clay County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Vermillion11,720Level78.0%
Ireneextends beyond Clay County439+2.6%Counted whole
Wakonda338−2.6%2.2%

3 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 Clay County is counted whole, so it has no share.

Clay County, Homes

Homes in Clay County

Of Clay County's 6,243 homes, 56% are detached houses, against 67% across South Dakota; 27% are in buildings of five or more homes. Half of its homes were built before 1976, close to South Dakota's 1979: 30% date from before 1960 and 25% from 2000 or later. 52% 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 Clay County, South Dakota, 2019 to 2023
HomesClay CountySouth Dakota
Housing units6,243402,364
Detached houses56%67%
In buildings of 5 or more27%16%
Mobile homes4%8%
Built before 196030%29%
Built since 200025%28%
Middle year built19761979
Owner-occupied52%69%

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