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 Rock County, Wisconsin
EPA puts Rock County in Zone 1, its highest radon zone, and high results are common there: 49.9% of the 451 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
49.9%
14.2 pts above WI
- Rock
- 49.9%
- Wisconsin
- 35.7%
451 tests, 2023
Median result
3.9pCi/L
Below action level
- Rock
- 3.9 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2023
Tests reported
451
9th of 69 by tests
- Rock
- 451
- 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
12 of 69
Top quarter
- Rock
- 49.9%
- Langlade
- 65.8%
By share, of 69 with 30+ tests
Read a result for Rock County
Starts at Rock County's median, 3.9 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 3.9 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. Rock County's median reported result, 2023.
The data
Rock County, At a glance
Rock County at a glance
Predicted: EPA radon zone
EPA Zone 1: highest predicted potential
EPA puts all of Rock 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 Rock County, 2023
| Year | Share of reported tests at or above 4 pCi/L |
|---|---|
| 2014 | 60.6% |
| 2015 | 50.7% |
| 2016 | 53.3% |
| 2017 | 56.8% |
| 2019 | 53.4% |
| 2020 | 46.1% |
| 2021 | 45.7% |
| 2022 | 55.2% |
| 2023 | 49.9% |
2023: 49.9%. Across these years the share ran from 45.7% to 60.6%. Each column counts radon tests reported to CDC that year, not homes.
What these figures count
These are the 451 tests reported to CDC for Rock 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).
Rock County, Compared
Rock County radon figures, compared
Calculated by Radon411In Rock County, about 5 in 10 reported tests in 2023 (225 of 451) were at or above EPA's action level. Against the national pool of 33.4%, the gap is 16.5 points higher. Across 72 Wisconsin counties pooled together, 35.7% of tests reached that level, so Rock County runs 14.2 points above its state.
Of 4 neighbours with a figure, none reported a larger share at or above 4 pCi/L; the spread is 33.6% (Green County) to 44.3% (Jefferson County). By share, Rock County is 12th of 69 Wisconsin counties with 30 or more tests for the same period.
Year by year, 2014 opened at 60.6% and 2023 closed at 49.9% (down 10.7 points), with 2014 the high at 60.6%. Half the tests came in under 3.9 pCi/L and half over it; the single highest reached 180.2 pCi/L. Below the action level, 125 tests (27.7%) came in under 2 pCi/L, against 41.4% statewide, and 101 tests (22.4%) fell between 2 and 4, against 23.0% statewide.
Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 49.9% share point the same way here. USGS geology rates 100% of the county's land high radon potential. CDC also counts 24 tests taken after mitigation in Rock County for the same period: tests in homes where a fix was already in.
The busiest year on file was 2015, with 550 tests reported; the quietest, 2022, had 230.
Rock CountyWisconsin, pooled
49.9% % of tests35.7% % of tests
| Year | Tests | Rock County, at or above 4 pCi/L | Wisconsin, pooled |
|---|---|---|---|
| 2014 | 424 | 60.6% | 51.1% |
| 2015 | 550 | 50.7% | 51.6% |
| 2016 | 366 | 53.3% | 45.2% |
| 2017 | 361 | 56.8% | 44.8% |
| 2019 | 292 | 53.4% | 42.9% |
| 2020 | 295 | 46.1% | 27.0% |
| 2021 | 269 | 45.7% | 39.9% |
| 2022 | 230 | 55.2% | 42.9% |
| 2023 | 451 | 49.9% | 35.7% |
Pooled shares: Radon411's sum of CDC's county counts for one period.
Rock County, Geology
The ground under Rock County
PredictedUSGS's geologic map puts all of Rock County's mapped land in one class: high radon potential. 99.0% 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 | 99.0% |
| Galesburg Plain | Illinoian glacial deposits and loess over Paleozoic sedimentary rocks. | High | 13 | 1.0% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
Rock County beside the ground around it
Rock County has more high-potential ground than Wisconsin as a whole: 100% of its land against 34.8%. Of the 6 counties that border it, none has as large a share of high-potential ground; Boone County, IL comes nearest at 100%.
The map rates the ground, not the air in any home.
Local help
Rock County, Local resources
Radon testing and mitigation in Rock 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
Rock County, Questions
Questions about radon in Rock County
Are radon levels high in Rock County?
In 2023, 49.9% of the radon tests reported to CDC for Rock County, Wisconsin were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.9 pCi/L. Of 69 Wisconsin counties with 30 or more tests for the same period, 11 reported a higher share; pooled, the state comes to 35.7%.
Which counties border Rock County?
Rock County borders 6 counties: Winnebago County, IL, Boone County, IL, Green County, WI, Walworth County, WI, Dane County, WI and Jefferson County, WI.
Products
Rock County, Products
Testing a home in Rock County
In Rock County, 49.9% of 451 tests reported to CDC in 2023 reached 4 pCi/L, against 35.7% pooled across Wisconsin. In Zone 1 the year-round level in Rock 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
MonitorKidde
Radon Detector (RAB) battery digital radon detector with an alarm
- C-NRPP 2024/25: Not included
- Battery
- Alarm
- Screen
Method: Semiconductor sensor
First reading: Within 12 hours
Short-term kit
Air Chek Short-Term radon test kit you hang, lab fee included
- Lab fee included
In home: 72 to 168 hours
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Rock County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Rock County
Explore
Rock County, Bordering counties
Counties bordering Rock County
Calculated by Radon411- Radon levels in Jefferson County, WIEPA zone 1
44.3%at or above 4 pCi/L
Median 3.5 pCi/L · 296 tests · 5.6 points below Rock County
- Radon levels in Walworth County, WIEPA zone 1
37.9%at or above 4 pCi/L
Median 2.8 pCi/L · 340 tests · 12.0 points below Rock County
- Radon levels in Dane County, WIEPA zone 1
34.9%at or above 4 pCi/L
Median 2.5 pCi/L · 2,887 tests · 15.0 points below Rock County
- Radon levels in Green County, WIEPA zone 1
33.6%at or above 4 pCi/L
Median 2.1 pCi/L · 235 tests · 16.3 points below Rock County
- Radon levels in Winnebago County, ILEPA zone 1
No CDC figure for 2023; its page reads it against its own neighbours.
- Radon levels in Boone County, ILEPA zone 1
No CDC figure for 2023; its page reads it against its own neighbours.
Rock County, Cities and towns
Cities and towns in Rock County
The Census Bureau estimates 166,472 residents in Rock County for July 1, 2025, 2,709 more than in 2020 (+1.7%). That works out to 232 people per square mile of land. Its largest town, Janesville, has 66,929 residents, 40.2% of the total. CDC's 451 reported tests for 2023 work out to 2.7 per 1,000 residents (2025 estimate), close to Wisconsin's 3.5.
From Janesville to Footville, CDC publishes no figure by town: each reads Rock County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Janesville | 66,929 | +1.9% | 40.2% |
| Beloit | 36,649 | −0.2% | 22.0% |
| Edgertonextends beyond Rock County | 6,121 | +3.1% | Counted whole |
| Milton | 6,024 | +5.5% | 3.6% |
| Evansville | 5,837 | +2.1% | 3.5% |
| Brodheadextends beyond Rock County | 3,214 | −1.5% | Counted whole |
| Clinton | 2,336 | +5.5% | 1.4% |
| Orfordville | 1,618 | +10.4% | 1.0% |
| Footville | 898 | +7.0% | 0.5% |
9 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 Rock County is counted whole, so it has no share.
Rock County, Homes
Homes in Rock County
Of Rock County's 70,694 homes, 74% are detached houses, against 66% across Wisconsin; 11% are in buildings of five or more homes. Half of its homes were built before 1970, close to Wisconsin's 1975: 37% date from before 1960 and 15% from 2000 or later. 71% 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 | Rock County | Wisconsin |
|---|---|---|
| Housing units | 70,694 | 2,750,750 |
| Detached houses | 74% | 66% |
| In buildings of 5 or more | 11% | 17% |
| Mobile homes | 3% | 3% |
| Built before 1960 | 37% | 34% |
| Built since 2000 | 15% | 20% |
| Middle year built | 1970 | 1975 |
| Owner-occupied | 71% | 68% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.