Long-term kit
RL416 long-term radon test kit, lab fee included
- Lab fee included
Method: Alpha track
In home: 90 days to 12 months
Locations and reports
Radon Levels in Plymouth County, Iowa
EPA puts Plymouth County in Zone 1, its highest radon zone, and high results are common there: 82.3% of the 1,405 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
Share at 4+ pCi/L
82.3%
23.8 pts above IA
- Plymouth
- 82.3%
- Iowa
- 58.5%
1,405 tests, 2008 to 2017
Median result
8.6pCi/L
Action level reached
- Plymouth
- 8.6 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2008 to 2017
Tests reported
1,405
14th of 99 by tests
- Plymouth
- 1,405
- Polk
- 10,945
Reported to CDC before any fix, 2008 to 2017
EPA radon zone
Zone 1
Highest potential
- Zone 1
- 2
- 3
1 highest, 3 lowest
EPA predicts over 4 pCi/L indoors
Rank in Iowa
2 of 99
Top quarter
- Plymouth
- 82.3%
- Sioux
- 84.2%
By share, of 99 with 30+ tests
Read a result for Plymouth County
Starts at Plymouth County's median, 8.6 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 8.6 pCi/L
At or above 4 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 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. Plymouth County's median reported result, 2008 to 2017.
The data
Plymouth County, At a glance
Plymouth County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Plymouth County in
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 Plymouth County, 2008 to 2017, laboratory
Under 2 pCi/L: 8%2 to 4 pCi/L: 10%4 pCi/L and above: 82%
Of 1,405 pre-mitigation tests reported to CDC, not homes in this area.
| Result band | Tests Measured | Share Measured |
|---|---|---|
| Under 2 pCi/L | 106 | 7.5% |
| 2 to 4 pCi/L | 143 | 10.2% |
| 4 pCi/L and above | 1,156 | 82.3% |
What these figures count
These are the 1,405 tests reported to CDC for Plymouth 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).
Plymouth County, Compared
Plymouth County radon figures, compared
Calculated by Radon411More than half: 82.3% of the 1,405 radon tests reported to CDC for Plymouth County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 2nd of 99 Iowa counties with 30 or more tests for the same period. That is 23.8 points above the 58.5% pooled from 99 Iowa counties.
The median result, 8.6 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 57.9 pCi/L. The rest split 7.5% under 2 pCi/L and 10.2% from 2 to 4 (Iowa: 21.8% and 19.7%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 82.3% share point the same way here.
1 of its 5 bordering counties with figures ran higher, led by Sioux County at 84.2%; Union County, SD reported 51.1%. 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 Plymouth County went from 803 tests (2005 to 2014) to 1,405 (2008 to 2017), the share going from 84.1% to 82.3%.
Nationally, every county with a figure for the same period pools to 35.7%; Plymouth County sits 46.6 points above it. CDC also counts 118 tests taken after mitigation in Plymouth County for the same period: tests in homes where a fix was already in.
Pooled shares: Radon411's sum of CDC's county counts for one period.
Plymouth County, Geology
The ground under Plymouth County
PredictedUSGS's geologic map puts all of Plymouth County's mapped land in one class: high radon potential. All 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).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Loess-Covered Drift Plains | Pre-Illinoian and Illinoian glacial deposits, and loess. | High | 13 | 100% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Plymouth County beside the ground around it
On high-potential ground Plymouth County is close to Iowa as a whole: 100% of its land against 98.8%. Of the 5 counties that border it, none has as large a share of high-potential ground; Cherokee County comes nearest at 100%.
The map rates the ground, not the air in any home.
Local help
Plymouth County, Local resources
Radon testing and mitigation in Plymouth County
Listed radon businesses with an address in Plymouth County:
- Iowa HHS Radon ProgramIowa HHS Radon Program, run by Iowa Department of Health and Human Services, licenses or certifies radon professionals, offers free or discounted test kits and states the rule for disclosing radon when a home is sold, and answers questions about testing and mitigation in Iowa.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
You can reach the Hotline by calling 1-800-383-5992 to order low-cost radon test kits and find answers to your radon questions.
- Compare kits
- Professionals in Iowa185 professionals are listed in Iowa.
Listed radon professionals by town: Sioux City, Le Mars
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
Plymouth County, Questions
Questions about radon in Plymouth County
Are radon levels high in Plymouth County?
Over the 2008 to 2017 laboratory totals, 82.3% of the radon tests reported to CDC for Plymouth County, Iowa were at or above 4 pCi/L, and the median reported pre-mitigation result was 8.6 pCi/L. Of 99 Iowa counties with 30 or more tests for the same period, 1 reported a higher share; pooled, the state comes to 58.5%.
Which counties border Plymouth County?
Plymouth County borders 5 counties: Union County, SD, Sioux County, IA, Woodbury County, IA, Cherokee County, IA and O'Brien County, IA.
Products
Plymouth County, Products
Testing a home in Plymouth County
In Plymouth County, 82.3% of 1,405 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 Plymouth 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 monitorMonitorSafetySiren
Pro4 plug-in radon detector with a display and alarm
- C-NRPP 2024/25: Not included
- Plug-in
- Alarm
- Screen
First reading: Within 48 hours
Short-term kit
Radon Short Term Test Kit short-term charcoal radon test kit, lab fee included
- Lab fee included
Method: Charcoal
In home: 2 to 7 days
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Plymouth County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Plymouth County
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Plymouth County, Bordering counties
Counties bordering Plymouth County
Calculated by Radon411- Radon levels in Sioux County, IAEPA zone 1
84.2%at or above 4 pCi/L
Median 10.7 pCi/L · 1,982 tests · 1.9 points above Plymouth County
- Radon levels in O'Brien County, IAEPA zone 1
81.7%at or above 4 pCi/L
Median 8.9 pCi/L · 652 tests · 0.6 points below Plymouth County
- Radon levels in Cherokee County, IAEPA zone 1
74.5%at or above 4 pCi/L
Median 7.6 pCi/L · 795 tests · 7.8 points below Plymouth County
- Radon levels in Woodbury County, IAEPA zone 1
69.2%at or above 4 pCi/L
Median 6.1 pCi/L · 3,541 tests · 13.1 points below Plymouth County
- Radon levels in Union County, SDEPA zone 1
51.1%at or above 4 pCi/L
Median 4.1 pCi/L · 348 tests · 31.2 points below Plymouth County
Plymouth County, Cities and towns
Cities and towns in Plymouth County
Plymouth County's population has barely moved since 2020: 25,697 residents on July 1, 2025 against 25,697 then, by the Census Bureau's estimates. That works out to 30 people per square mile of land. Sioux City, the largest place on its list (86,356 residents in all), extends beyond Plymouth County, so no share of the total is worked out for it. Testing is reported here more often than across the state: CDC's 1,405 reported tests for 2008 to 2017 come to 54.7 for every 1,000 residents (2025 estimate), against 29.4 for Iowa.
From Sioux City to Struble, CDC publishes no figure by town: each reads Plymouth County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Sioux Cityextends beyond Plymouth County | 86,356 | +0.5% | Counted whole |
| Le Mars | 10,760 | +1.5% | 41.9% |
| Remsen | 1,663 | −0.7% | 6.5% |
| Akron | 1,531 | −1.3% | 6.0% |
| Kingsley | 1,373 | −2.1% | 5.3% |
| Hinton | 948 | +1.6% | 3.7% |
| Merrill | 700 | −2.6% | 2.7% |
| Brunsville | 128 | −0.8% | 0.5% |
| Westfield | 119 | −2.5% | 0.5% |
| Oyens | 88 | −2.2% | 0.3% |
| Craig | 73 | −2.7% | 0.3% |
| Struble | 73 | Level | 0.3% |
Places in Plymouth County with a city page: 1 of 1
12 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 Plymouth County is counted whole, so it has no share.
Plymouth County, Homes
Homes in Plymouth County
Plymouth County is a place of houses on their own lots: 77% of its 10,863 homes are detached single-family houses, against 72% across Iowa. Half of its homes were built before 1972, close to Iowa's 1971: 39% date from before 1960 and 16% from 2000 or later. 74% 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 | Plymouth County | Iowa |
|---|---|---|
| Housing units | 10,863 | 1,427,175 |
| Detached houses | 77% | 72% |
| In buildings of 5 or more | 10% | 14% |
| Mobile homes | 4% | 3% |
| Built before 1960 | 39% | 39% |
| Built since 2000 | 16% | 21% |
| Middle year built | 1972 | 1971 |
| Owner-occupied | 74% | 72% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.