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Radon Levels in Marathon County, Wisconsin

EPA puts Marathon County in Zone 1, its highest radon zone, and high results are common there: 51.9% of the 349 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

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    51.9%

    16.2 pts above WI

    Marathon
    51.9%
    Wisconsin
    35.7%

    349 tests, 2023

  • Median result

    4.3pCi/L

    Action level reached

    Marathon
    4.3 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2023

  • Tests reported

    349

    15th of 69 by tests

    Marathon
    349
    Dane
    2,887

    Reported to CDC before any fix, 2023

  • EPA radon zone

    Zone 1

    Highest potential

    1. Zone 1
    2. 2
    3. 3

    1 highest, 3 lowest

    EPA predicts over 4 pCi/L indoors

  • Rank in Wisconsin

    10 of 69

    Top quarter

    Marathon
    51.9%
    Langlade
    65.8%

    By share, of 69 with 30+ tests

Read a result for Marathon County

Starts at Marathon County's median, 4.3 pCi/L

4.3pCi/L

EPA’s own thresholds; the figures are the area’s, never this home’s.

EPA’s reading of 4.3 pCi/L

At or above 4 pCi/L

  1. Under 20 to 1.9 pCi/L
  2. 2 to 4consider a fix
  3. 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. Marathon County's median reported result, 2023.

Interpret this readingWhat a fix costs

The data

Marathon County, At a glance

Marathon County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Marathon County in and predicts an average indoor of greater than 4 pCi/L (greater than 150 Bq/m³) across the county. That is one forecast for the whole county, not a reading of any address in it; homes above EPA's turn up in all three zones.

USGS geologic potential: High across 84% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Marathon County, 2023

Share of reported tests at or above 4 pCi/L, by year, Marathon County
Share of reported tests at or above 4 pCi/L, by year, Marathon County
YearShare of reported tests at or above 4 pCi/L
201458.7%
201553.2%
201649.2%
201746.8%
201954.0%
202055.3%
202159.0%
202255.3%
202351.9%

2023: 51.9%. Across these years the share ran from 46.8% to 59.0%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 349 tests reported to CDC for Marathon 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).

How to read these figures

Marathon County, Compared

Marathon County radon figures, compared

Calculated by Radon411

More than half: 51.9% of the 349 radon tests reported to CDC for Marathon County in 2023 read at least 4 pCi/L. That places it 10th of 69 Wisconsin counties with 30 or more tests for the same period. That is 16.2 points above the 35.7% pooled from 72 Wisconsin counties.

The median result, 4.3 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 101.3 pCi/L. The rest split 31.8% under 2 pCi/L and 16.3% from 2 to 4 (Wisconsin: 41.4% and 23.0%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 51.9% share point the same way here.

3 of its 8 bordering counties with figures ran higher, led by Langlade County at 65.8%; Wood County reported 18.3%. Statewide for the same period, Langlade County reported the highest share (65.8%) and Bayfield County the lowest (5.7%).

The busiest year on file was 2014, with 736 tests reported; the quietest, 2019, had 248.

Over 9 years of annual figures the share fell 6.8 points, from 58.7% in 2014 to 51.9% in 2023; its peak was 59.0% in 2021. Nationally, every county with a figure for the same period pools to 33.4%; Marathon County sits 18.5 points above it. CDC also counts 17 tests taken after mitigation in Marathon County for the same period: tests in homes where a fix was already in.

Share of reported tests at or above 4 pCi/L by year, Marathon County against Wisconsin2014 to 2023, years with 30 or more tests: Marathon County 58.7% to 51.9%, the state's pooled share 35.7% in 2023.
Marathon CountyWisconsin, pooled
Share of reported tests at or above 4 pCi/L by year, Marathon County against Wisconsin: data table
YearTestsMarathon County, at or above 4 pCi/LWisconsin, pooled
201473658.7%51.1%
201569953.2%51.6%
201642349.2%45.2%
201745546.8%44.8%
201924854.0%42.9%
202036055.3%27.0%
202145959.0%39.9%
202231855.3%42.9%
202334951.9%35.7%
Marathon County against Wisconsin, the United States and nearby counties, 2023
AreaTestsAt or above 4 pCi/LMedianDifference from Marathon County
Marathon County34951.9%4.3 pCi/LThis county
Wisconsin, pooled72 counties, pooled20,87935.7%Not pooled−16.2 points
United States, pooled642 counties, pooled128,74033.4%Not pooled−18.5 points
Iowa Countyhighest in the state14661.0%4.8 pCi/L+9.1 points
Bayfield Countylowest in the state355.7%1.5 pCi/L−46.2 points
Langlade County, WIborders it22565.8%6.0 pCi/L+13.9 points
Shawano County, WIborders it11457.0%5.0 pCi/L+5.1 points
Waupaca County, WIborders it12752.8%4.3 pCi/L+0.9 points
Portage County, WIborders it33435.6%3.0 pCi/L−16.3 points
Clark County, WIborders it7525.3%2.2 pCi/L−26.6 points
Lincoln County, WIborders it8123.5%2.1 pCi/L−28.4 points
Taylor County, WIborders it3718.9%2.1 pCi/L−33.0 points
Wood County, WIborders it20818.3%1.4 pCi/L−33.6 points
Where Marathon County's reported results fell, 2023, beside Wisconsin and the United States
ResultMarathon County testsMarathon County shareWisconsin shareU.S. shareAgainst Wisconsin
Under 2 pCi/L11131.8%41.4%41.4%−9.6 points
2 to 4 pCi/L5716.3%23.0%25.2%−6.7 points
4 pCi/L and above18151.9%35.7%33.4%+16.2 points
Marathon County ranked among the 69 Wisconsin counties with 30 or more tests, by share at or above 4 pCi/L, 2023
CountyRankTestsAt or above 4 pCi/LMedian
Crawford County5 of 6921753.9%4.4 pCi/L
Buffalo County6 of 695653.6%4.4 pCi/L
Waupaca County7 of 6912752.8%4.3 pCi/L
Oconto County8 of 6913052.3%4.5 pCi/L
Lafayette County9 of 694652.2%4.2 pCi/L
Marathon County10 of 6934951.9%4.3 pCi/L
Calumet County11 of 6912251.6%4.4 pCi/L
Rock County12 of 6945149.9%3.9 pCi/L
Grant County13 of 6932348.3%3.5 pCi/L
Washington County14 of 6939947.9%3.8 pCi/L
Vernon County15 of 6916247.5%3.8 pCi/L
Marathon County radon tests by year, against Wisconsin's pooled share
YearTestsAt or above 4 pCi/LMedianHighestWisconsin, pooledChange from the row before
202334951.9%4.3 pCi/L101.3 pCi/L35.7%−3.4 points
202231855.3%5.0 pCi/L164.6 pCi/L42.9%−3.7 points
202145959.0%4.9 pCi/L153.3 pCi/L39.9%+3.7 points
202036055.3%4.4 pCi/L139.0 pCi/L27.0%+1.3 points
201924854.0%4.2 pCi/L172.1 pCi/L42.9%+7.2 points
201745546.8%3.7 pCi/L131.9 pCi/L44.8%−2.4 points
201642349.2%3.8 pCi/L152.9 pCi/L45.2%−4.0 points
201569953.2%4.3 pCi/L411.0 pCi/L51.6%−5.5 points
201473658.7%5.0 pCi/L351.4 pCi/L51.1%First on file

Pooled shares: Radon411's sum of CDC's county counts for one period.

Marathon County, Geology

The ground under Marathon County

Predicted

Most of the ground under Marathon County is one class on USGS's geologic map: 84.0% of its land is high radon potential, with 16.0% moderate. The map puts 84.0% of Marathon County in the area it calls Wolf River Batholith and 16.0% in Central Plain/Northern Highland. USGS gives the first a radon index of 13, in the high band (12 to 17), and the second 10, in the moderate band (9 to 11).

USGS's geologic areas under Marathon County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Wolf River BatholithPlutonic granitic rocks including quartz monzonite, syenite, monzonite, anorthosite.High1384.0%
Central Plain/Northern HighlandPre-Late Wisconsinan glacial deposits derived from sandstone and carbonate rocks.Moderate1016.0%

Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.

Marathon County beside the ground around it

Marathon County has more high-potential ground than Wisconsin as a whole: 84.0% of its land against 34.8%. Of the 8 counties that border it, none has as large a share of high-potential ground; Portage County comes nearest at 76.0%.

Share of land by USGS radon potential: Marathon County, the 8 counties bordering it and Wisconsin
AreaEPA zoneLow potentialModerate potentialHigh potential
Marathon CountyZone 1None16.0%84.0%
Lincoln CountyZone 2None100%None
Wood CountyZone 1None46.3%53.7%
Portage CountyZone 1None24.0%76.0%
Clark CountyZone 2None97.5%2.5%
Taylor CountyZone 2None100%None
Langlade CountyZone 1None44.9%55.1%
Waupaca CountyZone 1None36.6%63.4%
Shawano CountyZone 1None36.7%63.3%
Wisconsin72 counties, pooled by land areaVaries1.4%63.5%34.8%

The map rates the ground, not the air in any home.

Local help

Marathon County, Local resources

Radon testing and mitigation in Marathon County

What Wisconsin's program says
Phone
1-888-569-7236
Email
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

Marathon County, Questions

Questions about radon in Marathon County

Are radon levels high in Marathon County?

In 2023, 51.9% of the radon tests reported to CDC for Marathon County, Wisconsin were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.3 pCi/L. Of 69 Wisconsin counties with 30 or more tests for the same period, 9 reported a higher share; pooled, the state comes to 35.7%.

Which counties border Marathon County?

Marathon County borders 8 counties: Lincoln County, Wood County, Portage County, Clark County, Taylor County, Langlade County, Waupaca County and Shawano County.

Products

Marathon County, Products

Testing a home in Marathon County

In Marathon County, 51.9% of 349 tests reported to CDC in 2023 reached 4 pCi/L, against 35.7% pooled across Wisconsin. In Zone 1 the year-round level in Marathon 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

Marathon County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Marathon County

Explore

Marathon County, Bordering counties

Counties bordering Marathon County

Calculated by Radon411

Marathon County, Cities and towns

Cities and towns in Marathon County

The Census Bureau estimates 139,432 residents in Marathon County for July 1, 2025, 1,313 more than in 2020 (+1.0%). That works out to 90 people per square mile of land. Its largest town, Wausau, has 40,188 residents, 28.8% of the total. CDC's 349 reported tests for 2023 work out to 2.5 per 1,000 residents (2025 estimate), close to Wisconsin's 3.5.

From Wausau to Fenwood, CDC publishes no figure by town: each reads Marathon County's.

The 12 largest cities and towns in Marathon County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Wausau40,188+0.4%28.8%
Marshfieldextends beyond Marathon County18,980+0.2%Counted whole
Weston15,931+1.3%11.4%
Kronenwetter8,413+0.6%6.0%
Rothschild5,627+1.3%4.0%
Mosinee4,529+1.7%3.2%
Maine2,621+0.6%1.9%
Abbotsfordextends beyond Marathon County2,413+5.4%Counted whole
Schofield2,311+6.2%1.7%
Colbyextends beyond Marathon County1,957−1.8%Counted whole
Spencer1,809−0.4%1.3%
Stratford1,619+1.6%1.2%
  • Marathon City
  • Edgar
  • Athens
  • Dorchester (part)
  • Birnamwood (part)
  • Hatley
  • Unity (part)
  • Elderon
  • Fenwood

21 incorporated places, as listed in the U.S. Census Bureau’s 2020 place reference files. “(part)” marks a place that extends into another county. Residents: U.S. Census Bureau population estimates for July 1, 2025 (Vintage 2025). A place that extends beyond Marathon County is counted whole, so it has no share.

Marathon County, Homes

Homes in Marathon County

Marathon County is a place of houses on their own lots: 75% of its 60,296 homes are detached single-family houses, against 66% across Wisconsin. Half of its homes were built before 1977, close to Wisconsin's 1975: 32% date from before 1960 and 23% 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 in Marathon County, Wisconsin, 2019 to 2023
HomesMarathon CountyWisconsin
Housing units60,2962,750,750
Detached houses75%66%
In buildings of 5 or more13%17%
Mobile homes3%3%
Built before 196032%34%
Built since 200023%20%
Middle year built19771975
Owner-occupied73%68%

Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.