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Radon Levels in Clay County, Iowa

EPA puts Clay County in Zone 1, its highest radon zone, and high results are common there: 79.4% of the 744 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

    79.4%

    20.9 pts above IA

    Clay
    79.4%
    Iowa
    58.5%

    744 tests, 2008 to 2017

  • Median result

    9.6pCi/L

    Action level reached

    Clay
    9.6 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    744

    32nd of 99 by tests

    Clay
    744
    Polk
    10,945

    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 Iowa

    4 of 99

    Top quarter

    Clay
    79.4%
    Sioux
    84.2%

    By share, of 99 with 30+ tests

Read a result for Clay County

Starts at Clay County's median, 9.6 pCi/L

9.6pCi/L

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

EPA’s reading of 9.6 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: 13%2 to 4 pCi/L: 8%4 pCi/L and above: 79%

Of 744 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/L9512.8%
2 to 4 pCi/L587.8%
4 pCi/L and above59179.4%
What these figures count

These are the 744 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: 79.4% of the 744 radon tests reported to CDC for Clay County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 4th of 99 Iowa counties with 30 or more tests for the same period. That is 20.9 points above the 58.5% pooled from 99 Iowa counties.

The median result, 9.6 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 91.4 pCi/L. The rest split 12.8% under 2 pCi/L and 7.8% from 2 to 4 (Iowa: 21.8% and 19.7%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 79.4% share point the same way here.

1 of its 8 bordering counties with figures ran higher, led by O'Brien County at 81.7%; Osceola County reported 67.2%. Statewide for the same period, Sioux County reported the highest share (84.2%) and Decatur County the lowest (25.9%).

CDC's ten-year totals for Clay County went from 600 tests (2005 to 2014) to 744 (2008 to 2017), the share going from 80.8% to 79.4%.

Nationally, every county with a figure for the same period pools to 35.7%; Clay County sits 43.7 points above it. CDC also counts 69 tests taken after mitigation in Clay County for the same period: tests in homes where a fix was already in.

Clay County against Iowa, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Clay County
Clay County74479.4%9.6 pCi/LThis county
Iowa, pooled99 counties, pooled95,24558.5%Not pooled−20.9 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−43.7 points
Sioux Countyhighest in the state1,98284.2%10.7 pCi/L+4.8 points
Decatur Countylowest in the state8525.9%2.5 pCi/L−53.5 points
O'Brien County, IAborders it65281.7%8.9 pCi/L+2.3 points
Cherokee County, IAborders it79574.5%7.6 pCi/L−4.9 points
Buena Vista County, IAborders it68872.1%6.9 pCi/L−7.3 points
Emmet County, IAborders it42071.0%6.3 pCi/L−8.4 points
Palo Alto County, IAborders it46270.8%5.9 pCi/L−8.6 points
Dickinson County, IAborders it83170.3%7.0 pCi/L−9.1 points
Pocahontas County, IAborders it41768.8%6.4 pCi/L−10.6 points
Osceola County, IAborders it12267.2%6.8 pCi/L−12.2 points
Where Clay County's reported results fell, 2008 to 2017, laboratory, beside Iowa and the United States
ResultClay County testsClay County shareIowa shareU.S. shareAgainst Iowa
Under 2 pCi/L9512.8%21.8%39.6%−9.0 points
2 to 4 pCi/L587.8%19.7%24.7%−11.9 points
4 pCi/L and above59179.4%58.5%35.7%+20.9 points
Clay County ranked among the 99 Iowa counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Sioux County1 of 991,98284.2%10.7 pCi/L
Plymouth County2 of 991,40582.3%8.6 pCi/L
O'Brien County3 of 9965281.7%8.9 pCi/L
Clay County4 of 9974479.4%9.6 pCi/L
Shelby County5 of 9985778.4%9.0 pCi/L
Audubon County6 of 9918978.3%9.5 pCi/L
Lyon County7 of 9923777.6%8.3 pCi/L
Carroll County8 of 9959477.6%7.8 pCi/L
Cass County9 of 9932777.4%8.1 pCi/L
Sac County10 of 9949176.6%7.0 pCi/L
Crawford County11 of 9937374.8%7.0 pCi/L
Clay County ten-year laboratory totals, against Iowa's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestIowa, pooledChange from the row before
2008 to 201774479.4%9.6 pCi/L91.4 pCi/L58.5%−0.7 points
2007 to 201668980.1%9.7 pCi/L91.4 pCi/L58.6%−1.1 points
2006 to 201564281.2%9.8 pCi/L91.4 pCi/L59.4%+0.4 points
2005 to 201460080.8%9.8 pCi/L77.3 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 50.9% of Clay County in the area it calls Loess-Covered Drift Plains and 49.1% in Des Moines Lobe. USGS gives the first a radon index of 13 and the second 12, both in the high band (12 to 17).

USGS's geologic areas under Clay County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Loess-Covered Drift PlainsPre-Illinoian and Illinoian glacial deposits, and loess.High1350.9%
Des Moines LobeCalcareous loamy till derived from shale, limestone, dolomite, sandstone.High1249.1%

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

On high-potential ground Clay County is close to Iowa as a whole: 100% of its land against 98.8%. Of the 8 counties that border it, none has as large a share of high-potential ground; Cherokee County comes nearest at 100%.

Share of land by USGS radon potential: Clay County, the 8 counties bordering it and Iowa
AreaEPA zoneLow potentialModerate potentialHigh potential
Clay CountyZone 1NoneNone100%
Dickinson CountyZone 1NoneNone100%
Palo Alto CountyZone 1NoneNone100%
Buena Vista CountyZone 1NoneNone100%
O'Brien CountyZone 1NoneNone100%
Emmet CountyZone 1NoneNone100%
Osceola CountyZone 1NoneNone100%
Pocahontas CountyZone 1NoneNone100%
Cherokee CountyZone 1NoneNone100%
Iowa99 counties, pooled by land areaVariesNone1.2%98.8%

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 Iowa's program says
Phone
(515) 281-4928
Licensing of radon professionals
Iowa’s radon program approves applications for persons who wish to become certified to perform radon testing or credentialed to perform mitigation and laboratories that provide radon testing devices and analytical services.
Disclosure when a home is sold or rented
However, per Iowa Code chapters 543B.9 and 543B.18 and Iowa Administrative Code 193E Chapter 14 administered by the Iowa Real Estate Commission, a seller must disclose any known tests for the presence of radon and if yes, provide the date of last report and the result(s).

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, 79.4% of the radon tests reported to CDC for Clay County, Iowa were at or above 4 pCi/L, and the median reported pre-mitigation result was 9.6 pCi/L. Of 99 Iowa counties with 30 or more tests for the same period, 3 reported a higher share; pooled, the state comes to 58.5%.

Which counties border Clay County?

Clay County borders 8 counties: Dickinson County, Palo Alto County, Buena Vista County, O'Brien County, Emmet County, Osceola County, Pocahontas County and Cherokee County.

Products

Clay County, Products

Testing a home in Clay County

In Clay County, 79.4% of 744 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 58.5% pooled across Iowa. 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

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Clay County, Bordering counties

Counties bordering Clay County

Calculated by Radon411

Clay County, Cities and towns

Cities and towns in Clay County

About 16,202 people live in Clay County by the Census Bureau's estimate for July 1, 2025, 139 fewer than in 2020 (−0.9%). That works out to 29 people per square mile of land. Spencer alone is home to 11,218 of them, 69.2% of the total. Together its 10 incorporated places hold 81.0% of those residents; the other 19.0% live outside any of them. Testing is reported here more often than across the state: CDC's 744 reported tests for 2008 to 2017 come to 45.9 for every 1,000 residents (2025 estimate), against 29.4 for Iowa.

From Spencer to Gillett Grove, 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
Spencer11,218−0.7%69.2%
Everly561−2.1%3.5%
Royal375−1.3%2.3%
Peterson313−1.9%1.9%
Fostoria228−2.6%1.4%
Dickens145−1.4%0.9%
Webb135−2.9%0.8%
Greenville65−1.5%0.4%
Rossie46−2.1%0.3%
Gillett Grove31−3.1%0.2%

10 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).

Clay County, Homes

Homes in Clay County

Clay County is a place of houses on their own lots: 76% of its 8,141 homes are detached single-family houses, against 72% across Iowa. Half of its homes were built before 1966, close to Iowa's 1971: 44% date from before 1960 and 15% from 2000 or later. 73% of occupied homes are lived in by their owner (Iowa: 72%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Clay County, Iowa, 2019 to 2023
HomesClay CountyIowa
Housing units8,1411,427,175
Detached houses76%72%
In buildings of 5 or more14%14%
Mobile homes2%3%
Built before 196044%39%
Built since 200015%21%
Middle year built19661971
Owner-occupied73%72%

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