Long-term kit
RL116 long-term radon test kit
- Lab fee extra
Method: Alpha track
In home: 90 days to 12 months
Locations and reports
Radon Levels in Day County, South Dakota
EPA puts Day County in Zone 1, its highest radon zone, and only 11 radon tests from it are 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
65.3%
Codington County
- Codington
- 65.3%
- Brown
- 59.6%
3 of 7 neighbours have 30+ tests, 2006 to 2015
Nearest county
65.3%
Codington County, 34 miles
- Codington
- 65.3%
- SD
- 58.8%
Share at 4+ pCi/L, 72 tests, 2006 to 2015
Ground above low
100.0%
1.0 pts above SD
- Day
- 100.0%
- SD
- 99.0%
Share of land USGS rates moderate or high potential
Tests reported
11
Too few for a share
- Day
- 11
- 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 Day 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
Day County, At a glance
Day County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Day 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 Day County, 2008 to 2017, laboratory
CDC holds 11 reported tests for Day County over the 2008 to 2017 laboratory totals, with a median of 4.5 pCi/L: too few to describe a county (Radon411 reads a share from 30 tests up). Day County beside the counties around it is how this page reads it.
What these figures count
Only 11 tests were reported to CDC for Day 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).
Day County, Compared
Day County radon figures, compared
Calculated by Radon411CDC's largest figure for Day County rests on 11 tests over the 2008 to 2017 laboratory totals, 6 of them at or above 4 pCi/L, so each test moves its share by 9.1 points and no rank or comparison is drawn from it. Results reached as high as 20.5 pCi/L, with a middle value of 4.5 pCi/L.
Of the counties bordering it, 3 have a figure over the 2006 to 2015 laboratory totals: Codington County reported the largest share at or above 4 pCi/L, 65.3%, and Brown County the smallest, 59.6%, with Grant County at 64.6% between. Farther out, Kingsbury County (69 miles away) at 68.8%, Beadle County (74 miles away) at 63.0% and Edmunds County (78 miles away) at 61.8% 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 Day 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.
Day County, Geology
The ground under Day County
PredictedUSGS's geologic map puts all of Day County's mapped land in one class: high radon potential. 99.1% 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 | 99.1% |
| Glacial Lake Dakota | Silt and clay glacial lake deposits. | High | 14 | 0.9% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Day County beside the ground around it
Day 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; Clark County comes nearest at 100%.
The map rates the ground, not the air in any home.
Local help
Day County, Local resources
Radon testing and mitigation in Day 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
Day County, Questions
Questions about radon in Day County
Are radon levels high in Day County?
Over the 2008 to 2017 laboratory totals, 54.5% of the radon tests reported to CDC for Day County, South Dakota were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.5 pCi/L. Of the counties bordering it, 3 have a figure over the 2006 to 2015 laboratory totals: Codington County reported the largest share at or above 4 pCi/L, 65.3%, and Brown County the smallest, 59.6%, with Grant County at 64.6% between.
Which counties border Day County?
Day County borders 7 counties: Marshall County, Roberts County, Codington County, Brown County, Clark County, Grant County and Spink County.
Products
Day County, Products
Testing a home in Day County
No CDC figure names Day County; the nearest county with one, Codington County (34 miles away), reported 65.3% 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 Day 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
lüft plug-in radon and indoor air quality monitor
- C-NRPP 2024/25: Approved
- Plug-in
- Wi-Fi, Bluetooth
- App only
First reading: Within 90 minutes
Short-term kitempowerDX
Standard Radon Test short-term mail-in radon test kit
- Lab fee included
Method: Passive sampler
In home: 48 to 96 hours
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Day County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Day County
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Day County, Bordering counties
Counties bordering Day County
Calculated by Radon411- Radon levels in Marshall CountyEPA zone 1
83.3%at or above 4 pCi/L
Median 10.3 pCi/L · 12 tests
- Radon levels in Clark CountyEPA zone 1
66.7%at or above 4 pCi/L
Median 6.2 pCi/L · 3 tests
- Radon levels in Codington CountyEPA zone 1
65.3%at or above 4 pCi/L
Median 6.0 pCi/L · 72 tests
- Radon levels in Grant CountyEPA zone 1
64.6%at or above 4 pCi/L
Median 6.0 pCi/L · 82 tests
- Radon levels in Brown CountyEPA zone 1
59.6%at or above 4 pCi/L
Median 4.4 pCi/L · 52 tests
- Radon levels in Spink CountyEPA zone 1
58.8%at or above 4 pCi/L
Median 4.9 pCi/L · 17 tests
- Radon levels in Roberts CountyEPA zone 1
42.1%at or above 4 pCi/L
Median 3.1 pCi/L · 19 tests
Day County, Cities and towns
Cities and towns in Day County
The Census Bureau estimates 5,484 residents in Day County for July 1, 2025, 42 more than in 2020 (+0.8%). Spread over its land, that is 5.3 people to the square mile. Its largest town, Webster, has 1,681 residents, 30.7% of the total. Together its 8 incorporated places hold 52.2% of those residents; the other 47.8% live outside any of them. Reported testing is thin here: 11 tests over 2008 to 2017, or 2.0 for every 1,000 residents (2025 estimate), where South Dakota as a whole comes to 6.7.
From Webster to Butler, CDC publishes no figure by town: each reads Day County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Webster | 1,681 | −2.4% | 30.7% |
| Waubay | 474 | +0.2% | 8.6% |
| Bristol | 278 | −2.5% | 5.1% |
| Roslyn | 182 | −1.1% | 3.3% |
| Pierpont | 125 | −3.8% | 2.3% |
| Andover | 71 | +4.4% | 1.3% |
| Grenville | 48 | +2.1% | 0.9% |
| Butler | 4 | Level | Under 0.1% |
8 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).
Day County, Homes
Homes in Day County
Day County is a place of houses on their own lots: 86% of its 3,434 homes are detached single-family houses, against 67% across South Dakota. The homes are older than South Dakota's as a whole: half were built before 1964 (1979 for South Dakota), and 46% date from before 1960. 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 | Day County | South Dakota |
|---|---|---|
| Housing units | 3,434 | 402,364 |
| Detached houses | 86% | 67% |
| In buildings of 5 or more | 4% | 16% |
| Mobile homes | 5% | 8% |
| Built before 1960 | 46% | 29% |
| Built since 2000 | 18% | 28% |
| Middle year built | 1964 | 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.