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 Sully County, South Dakota
EPA puts Sully County in Zone 1, its highest radon zone, and only 1 radon test from it is on file with CDC (2008 to 2017, laboratory totals). Homes with elevated levels occur in all three zones, so only a test measures yours.
Data checked By Radon411Methodology
Highest bordering
66.7%
Hughes County
- Hughes
- 66.7%
1 of 5 neighbours have 30+ tests, 2006 to 2015
Nearest county
66.7%
Hughes County, 24 miles
- Hughes
- 66.7%
- SD
- 58.8%
Share at 4+ pCi/L, 30 tests, 2006 to 2015
Ground above low
100.0%
1.0 pts above SD
- Sully
- 100.0%
- SD
- 99.0%
Share of land USGS rates moderate or high potential
Tests reported
1
Too few for a share
- Sully
- 1
- Needed
- 30
2008 to 2017, laboratory
EPA radon zone
Zone 1
Highest potential
- Zone 1
- 2
- 3
1 highest, 3 lowest
EPA predicts over 4 pCi/L indoors
Read a result for Sully County
Starts at EPA's action level
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 4.0 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.
The data
Sully County, At a glance
Sully County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Sully 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 Sully County, 2008 to 2017, laboratory
CDC holds 1 reported test for Sully County over the 2008 to 2017 laboratory totals, a result of 4.1 pCi/L: too few to describe a county (Radon411 reads a share from 30 tests up). Sully County beside the counties around it is how this page reads it.
What these figures count
Only 1 test was reported to CDC for Sully County over 2008 to 2017 (laboratory totals), and tests are often run on homes being sold or already suspected of a problem: so few describe neither the county nor this house.
Measured: CDC tracking data. Figure above: 2008 to 2017, laboratory. Measured data period: 2005 to 2017 (every CDC figure on file).
Sully County, Compared
Sully County radon figures, compared
Calculated by Radon411CDC's largest figure for Sully County rests on 1 test over the 2008 to 2017 laboratory totals, 1 of them at or above 4 pCi/L, so each test moves its share by 100.0 points and no rank or comparison is drawn from it. Results reached as high as 4.1 pCi/L, with a middle value of 4.1 pCi/L.
Hughes County, the one bordering county with a figure over the 2006 to 2015 laboratory totals, reported 66.7% of its tests at or above 4 pCi/L. Farther out, Edmunds County (66 miles away) at 61.8%, Brule County (86 miles away) at 62.5% and Beadle County (94 miles away) at 63.0% have figures too. Pooled across 63 South Dakota counties, 58.8% of reported tests reached that level; across every county in the country with a figure, 36.3%.
Statewide for the same period, Bon Homme County reported the highest share (79.5%) and Davison County the lowest (47.9%). EPA's 1993 map puts Sully County in Zone 1, the map's highest label: a predicted county average above 4 pCi/L. USGS geology rates 100% of the county's land high radon potential.
Pooled shares: Radon411's sum of CDC's county counts for one period.
Sully County, Geology
The ground under Sully County
PredictedUSGS's geologic map puts all of Sully County's mapped land in one class: high radon potential. All of it lies in the area the map calls Missouri Coteau/James River Lowland/Coteau des Prairies, to which USGS gives a radon index of 14, in the high band (12 to 17).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Missouri Coteau/James River Lowland/Coteau des Prairies | Glacial drift derived largely from Cretaceous marine shale. | High | 14 | 100% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Sully County beside the ground around it
Sully County has more high-potential ground than South Dakota as a whole: 100% of its land against 56.7%. Of the 5 counties that border it, none has as large a share of high-potential ground; Hughes County comes nearest at 100%.
The map rates the ground, not the air in any home.
Local help
Sully County, Local resources
Radon testing and mitigation in Sully County
- South Dakota's radon programDepartment of Agriculture and Natural ResourcesSouth Dakota's radon program offers free or discounted test kits, and answers questions about testing and mitigation in South Dakota.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
The Department of Agriculture and Natural Resources (DANR) will provide 500 free Radon test kits to South Dakota Residents.
- Compare kits
- Professionals in South Dakota5 professionals are listed in South Dakota.
What South Dakota's program says
- Phone
- 1-800-438-3367
What it means
Sully County, Questions
Questions about radon in Sully County
Are radon levels high in Sully County?
Over the 2008 to 2017 laboratory totals, 100.0% of the radon tests reported to CDC for Sully County, South Dakota were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.1 pCi/L. Hughes County, the one bordering county with a figure over the 2006 to 2015 laboratory totals, reported 66.7% of its tests at or above 4 pCi/L.
Which counties border Sully County?
Sully County borders 5 counties: Potter County, Hughes County, Stanley County, Hyde County and Dewey County.
Products
Sully County, Products
Testing a home in Sully County
No CDC figure names Sully County; the nearest county with one, Hughes County (24 miles away), reported 66.7% of its tests at or above 4 pCi/L over the 2006 to 2015 laboratory totals, the only reading nearby until a test of this home. In Zone 1 the year-round level in Sully 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
Monitor
RM-65 rechargeable radon detector with a color screen and alarm
- C-NRPP 2024/25: Not included
- Battery
- Alarm
- Screen
Method: Semiconductor sensor
First reading: Within 6 hours
Short-term kit
RA100 short-term liquid scintillation radon test kit
- Lab fee extra
Method: Liquid scintillation
In home: 48 to 96 hours
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Sully County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Sully County
Explore
Sully County, Bordering counties
Counties bordering Sully County
Calculated by Radon411- Radon levels in Stanley CountyEPA zone 1
100.0%at or above 4 pCi/L
Median 5.2 pCi/L · 2 tests
- Radon levels in Hyde CountyEPA zone 1
80.0%at or above 4 pCi/L
Median 4.9 pCi/L · 5 tests
- Radon levels in Hughes CountyEPA zone 1
66.7%at or above 4 pCi/L
Median 5.5 pCi/L · 30 tests
- Radon levels in Potter CountyEPA zone 1
0.0%at or above 4 pCi/L
Median 1.2 pCi/L · 1 test
- Radon levels in Dewey CountyEPA zone 2
0.0%at or above 4 pCi/L
Median 0.6 pCi/L · 1 test
Sully County, Cities and towns
Cities and towns in Sully County
The Census Bureau estimates 1,479 residents in Sully County for July 1, 2025, 28 more than in 2020 (+1.9%). Spread over its land, that is 1.5 people to the square mile. Its largest town, Onida, has 640 residents, 43.3% of the total. Together its 2 incorporated places hold 48.1% of those residents; the other 51.9% live outside any of them. Reported testing is thin here: 1 test over 2008 to 2017, or 0.7 for every 1,000 residents (2025 estimate), where South Dakota as a whole comes to 6.7.
From Onida to Agar, CDC publishes no figure by town: each reads Sully County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Onida | 640 | −3.8% | 43.3% |
| Agar | 71 | +6.0% | 4.8% |
2 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).
Sully County, Homes
Homes in Sully County
Sully County is a place of houses on their own lots: 75% of its 908 homes are detached single-family houses, against 67% across South Dakota. Half of its homes were built before 1975, close to South Dakota's 1979: 36% date from before 1960 and 25% from 2000 or later. Owners live in 76% of the occupied homes (South Dakota: 69%), so for most households a test and a fix are their own to order.
| Homes | Sully County | South Dakota |
|---|---|---|
| Housing units | 908 | 402,364 |
| Detached houses | 75% | 67% |
| In buildings of 5 or more | 4% | 16% |
| Mobile homes | 19% | 8% |
| Built before 1960 | 36% | 29% |
| Built since 2000 | 25% | 28% |
| Middle year built | 1975 | 1979 |
| Owner-occupied | 76% | 69% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.