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 Jefferson County, Wisconsin
EPA puts Jefferson County in Zone 1, its highest radon zone, and high results are common there: 44.3% of the 296 radon tests reported to CDC (2023) 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
44.3%
8.6 pts above WI
- Jefferson
- 44.3%
- Wisconsin
- 35.7%
296 tests, 2023
Median result
3.5pCi/L
Below action level
- Jefferson
- 3.5 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2023
Tests reported
296
22nd of 69 by tests
- Jefferson
- 296
- Dane
- 2,887
Reported to CDC before any fix, 2023
EPA radon zone
Zone 1
Highest potential
- Zone 1
- 2
- 3
1 highest, 3 lowest
EPA predicts over 4 pCi/L indoors
Rank in Wisconsin
20 of 69
Middle half
- Jefferson
- 44.3%
- Langlade
- 65.8%
By share, of 69 with 30+ tests
Read a result for Jefferson County
Starts at Jefferson County's median, 3.5 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 3.5 pCi/L
Between 2 and 4 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 4 and aboveEPA says fix it
This result is between 2 and 4 pCi/L. EPA recommends considering a fix when the level is between 2 and 4 pCi/L. Jefferson County's median reported result, 2023.
The data
Jefferson County, At a glance
Jefferson County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Jefferson 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 Jefferson County, 2023
| Year | Share of reported tests at or above 4 pCi/L |
|---|---|
| 2014 | 43.3% |
| 2015 | 54.5% |
| 2016 | 36.6% |
| 2017 | 50.3% |
| 2019 | 46.0% |
| 2020 | 52.7% |
| 2021 | 26.9% |
| 2022 | 46.7% |
| 2023 | 44.3% |
2023: 44.3%. Across these years the share ran from 26.9% to 54.5%. Each column counts radon tests reported to CDC that year, not homes.
What these figures count
These are the 296 tests reported to CDC for Jefferson County over 2023, 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: 2023. Measured data period: 2005 to 2023 (every CDC figure on file).
Jefferson County, Compared
Jefferson County radon figures, compared
Calculated by Radon411In Jefferson County, about 4 in 10 reported tests in 2023 (131 of 296) were at or above EPA's action level. Against the national pool of 33.4%, the gap is 10.9 points higher. Across 72 Wisconsin counties pooled together, 35.7% of tests reached that level, so Jefferson County runs 8.6 points above its state.
Of 5 neighbours with a figure, 2 reported a larger share at or above 4 pCi/L and 3 a smaller one; the spread is 34.9% (Dane County) to 54.9% (Dodge County). By share, Jefferson County is 20th of 69 Wisconsin counties with 30 or more tests for the same period.
Year by year, 2014 opened at 43.3% and 2023 closed at 44.3% (about level), with 2015 the high at 54.5%. Half the tests came in under 3.5 pCi/L and half over it; the single highest reached 36.0 pCi/L. Below the action level, 88 tests (29.7%) came in under 2 pCi/L, against 41.4% statewide, and 77 tests (26.0%) fell between 2 and 4, against 23.0% statewide.
Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 44.3% share point the same way here. USGS geology rates 100% of the county's land high radon potential. CDC also counts 16 tests taken after mitigation in Jefferson County for the same period: tests in homes where a fix was already in.
The busiest year on file was 2023, with 296 tests reported; the quietest, 2022, had 150.
Jefferson CountyWisconsin, pooled
44.3% % of tests35.7% % of tests
| Year | Tests | Jefferson County, at or above 4 pCi/L | Wisconsin, pooled |
|---|---|---|---|
| 2014 | 238 | 43.3% | 51.1% |
| 2015 | 264 | 54.5% | 51.6% |
| 2016 | 276 | 36.6% | 45.2% |
| 2017 | 185 | 50.3% | 44.8% |
| 2019 | 189 | 46.0% | 42.9% |
| 2020 | 182 | 52.7% | 27.0% |
| 2021 | 160 | 26.9% | 39.9% |
| 2022 | 150 | 46.7% | 42.9% |
| 2023 | 296 | 44.3% | 35.7% |
Pooled shares: Radon411's sum of CDC's county counts for one period.
Jefferson County, Geology
The ground under Jefferson County
PredictedUSGS's geologic map puts all of Jefferson County's mapped land in one class: high radon potential. All of it lies in the area the map calls Eastern Ridges and Lowlands, to which USGS gives a radon index of 12, in the high band (12 to 17).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Eastern Ridges and Lowlands | Glacial deposits derived from Ordovician carbonate rocks. | High | 12 | 100% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Jefferson County beside the ground around it
Jefferson County has more high-potential ground than Wisconsin as a whole: 100% of its land against 34.8%. Of the 5 counties that border it, none has as large a share of high-potential ground; Dodge County comes nearest at 100%.
The map rates the ground, not the air in any home.
Local help
Jefferson County, Local resources
Radon testing and mitigation in Jefferson County
- Wisconsin's radon programWisconsin Department of Health ServicesWisconsin's radon program does not license radon professionals and offers free or discounted test kits, and answers questions about testing and mitigation in Wisconsin.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
We recommend starting with your regional Radon Information Center since many centers provide test kits for free or reduced cost, and often with laboratory fees covered.
- Compare kits
- Professionals in Wisconsin19 professionals are listed in Wisconsin.
What Wisconsin's program says
- Phone
- 1-888-569-7236
- DHSRadon@dhs.wisconsin.gov
- Licensing of radon professionals
The State of Wisconsin does not require contractors performing radon measurement or mitigation work to be state licensed or nationally certified.
What it means
Jefferson County, Questions
Questions about radon in Jefferson County
Are radon levels high in Jefferson County?
In 2023, 44.3% of the radon tests reported to CDC for Jefferson County, Wisconsin were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.5 pCi/L. Of 69 Wisconsin counties with 30 or more tests for the same period, 19 reported a higher share; pooled, the state comes to 35.7%.
Which counties border Jefferson County?
Jefferson County borders 5 counties: Waukesha County, Dodge County, Walworth County, Dane County and Rock County.
Products
Jefferson County, Products
Testing a home in Jefferson County
In Jefferson County, 44.3% of 296 tests reported to CDC in 2023 reached 4 pCi/L, against 35.7% pooled across Wisconsin. In Zone 1 the year-round level in Jefferson 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
MonitorAEGTEST
HOUND-3699 radon monitor with a 2.1-inch touchscreen
- C-NRPP 2024/25: Not included
- Battery
- Alarm
- Screen
First reading: Within 1 hour
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
Jefferson County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Jefferson County
Explore
Jefferson County, Bordering counties
Counties bordering Jefferson County
Calculated by Radon411- Radon levels in Dodge CountyEPA zone 1
54.9%at or above 4 pCi/L
Median 4.8 pCi/L · 257 tests · 10.6 points above Jefferson County
- Radon levels in Rock CountyEPA zone 1
49.9%at or above 4 pCi/L
Median 3.9 pCi/L · 451 tests · 5.6 points above Jefferson County
- Radon levels in Waukesha CountyEPA zone 1
43.2%at or above 4 pCi/L
Median 3.2 pCi/L · 1,633 tests · 1.1 points below Jefferson County
- Radon levels in Walworth CountyEPA zone 1
37.9%at or above 4 pCi/L
Median 2.8 pCi/L · 340 tests · 6.4 points below Jefferson County
- Radon levels in Dane CountyEPA zone 1
34.9%at or above 4 pCi/L
Median 2.5 pCi/L · 2,887 tests · 9.4 points below Jefferson County
Jefferson County, Cities and towns
Cities and towns in Jefferson County
Jefferson County's population has barely moved since 2020: 86,505 residents on July 1, 2025 against 86,219 then, by the Census Bureau's estimates. That works out to 155 people per square mile of land. Watertown, the largest place on its list (22,704 residents in all), extends beyond Jefferson County, so no share of the total is worked out for it. CDC's 296 reported tests for 2023 work out to 3.4 per 1,000 residents (2025 estimate), close to Wisconsin's 3.5.
From Watertown to Lac La Belle, CDC publishes no figure by town: each reads Jefferson County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Watertownextends beyond Jefferson County | 22,704 | −1.2% | Counted whole |
| Whitewaterextends beyond Jefferson County | 16,038 | −0.6% | Counted whole |
| Fort Atkinson | 12,388 | −1.6% | 14.3% |
| Jefferson | 7,814 | +0.4% | 9.0% |
| Lake Mills | 6,655 | +6.1% | 7.7% |
| Waterloo | 3,600 | +3.2% | 4.2% |
| Johnson Creek | 3,435 | +2.9% | 4.0% |
| Palmyra | 1,720 | −0.5% | 2.0% |
| Cambridgeextends beyond Jefferson County | 1,679 | +1.9% | Counted whole |
| Sullivan | 663 | +1.7% | 0.8% |
| Lac La Belleextends beyond Jefferson County | 274 | −1.8% | Counted whole |
11 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 Jefferson County is counted whole, so it has no share.
Jefferson County, Homes
Homes in Jefferson County
Of Jefferson County's 36,649 homes, 71% are detached houses, against 66% across Wisconsin; 14% are in buildings of five or more homes. Half of its homes were built before 1974, close to Wisconsin's 1975: 37% date from before 1960 and 20% from 2000 or later. 73% of occupied homes are lived in by their owner (Wisconsin: 68%); in the rest a tenant can run a test, but a fix is the landlord's to make.
| Homes | Jefferson County | Wisconsin |
|---|---|---|
| Housing units | 36,649 | 2,750,750 |
| Detached houses | 71% | 66% |
| In buildings of 5 or more | 14% | 17% |
| Mobile homes | 4% | 3% |
| Built before 1960 | 37% | 34% |
| Built since 2000 | 20% | 20% |
| Middle year built | 1974 | 1975 |
| Owner-occupied | 73% | 68% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.