Skip to content
RADON411

Locations and reports

Radon Levels in Marion County, Ohio

EPA puts Marion County in Zone 1, its highest radon zone, and high results are common there: 65.3% of the 265 radon tests reported to CDC (2008 to 2017, laboratory totals) 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

    65.3%

    13.0 pts above OH

    Marion
    65.3%
    Ohio
    52.3%

    265 tests, 2008 to 2017

  • Median result

    6.4pCi/L

    Action level reached

    Marion
    6.4 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    265

    51st of 79 by tests

    Marion
    265
    Franklin
    16,142

    Reported to CDC before any fix, 2008 to 2017

  • 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 Ohio

    13 of 79

    Top quarter

    Marion
    65.3%
    Pickaway
    81.4%

    By share, of 79 with 30+ tests

Read a result for Marion County

Starts at Marion County's median, 6.4 pCi/L

6.4pCi/L

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

EPA’s reading of 6.4 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. Marion County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Marion County, At a glance

Marion County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Marion 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 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Marion County, 2008 to 2017, laboratory

Reported results by concentration band, Marion County, 2008 to 2017, laboratory
Under 2 pCi/L: 16%2 to 4 pCi/L: 19%4 pCi/L and above: 65%

Of 265 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Marion County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L4215.8%
2 to 4 pCi/L5018.9%
4 pCi/L and above17365.3%
What these figures count

These are the 265 tests reported to CDC for Marion County over 2008 to 2017 (laboratory totals), 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: 2008 to 2017, laboratory. Measured data period: 2005 to 2017 (every CDC figure on file).

How to read these figures

Marion County, Compared

Marion County radon figures, compared

Calculated by Radon411

More than half: 65.3% of the 265 radon tests reported to CDC for Marion County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 13th of 79 Ohio counties with 30 or more tests for the same period. That is 13.0 points above the 52.3% pooled from 88 Ohio counties.

The median result, 6.4 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 87.6 pCi/L. The rest split 15.8% under 2 pCi/L and 18.9% from 2 to 4 (Ohio: 23.1% and 24.6%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 65.3% share point the same way here.

1 of its 6 bordering counties with figures ran higher, led by Delaware County at 72.2%; Hardin County reported 43.3%. Statewide for the same period, Pickaway County reported the highest share (81.4%) and Cuyahoga County the lowest (21.7%).

CDC's ten-year totals for Marion County went from 314 tests (2005 to 2014) to 265 (2008 to 2017), the share going from 60.5% to 65.3%.

Nationally, every county with a figure for the same period pools to 35.7%; Marion County sits 29.6 points above it. CDC also counts 15 tests taken after mitigation in Marion County for the same period: tests in homes where a fix was already in.

Marion County against Ohio, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Marion County
Marion County26565.3%6.4 pCi/LThis county
Ohio, pooled88 counties, pooled98,84052.3%Not pooled−13.0 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−29.6 points
Pickaway Countyhighest in the state18381.4%9.5 pCi/L+16.1 points
Cuyahoga Countylowest in the state10,09221.7%2.0 pCi/L−43.6 points
Delaware County, OHborders it3,73072.2%6.4 pCi/L+6.9 points
Morrow County, OHborders it17765.0%5.7 pCi/L−0.3 points
Union County, OHborders it1,28560.2%4.8 pCi/L−5.1 points
Crawford County, OHborders it21059.5%4.7 pCi/L−5.8 points
Wyandot County, OHborders it32046.3%3.6 pCi/L−19.0 points
Hardin County, OHborders it6743.3%3.4 pCi/L−22.0 points
Where Marion County's reported results fell, 2008 to 2017, laboratory, beside Ohio and the United States
ResultMarion County testsMarion County shareOhio shareU.S. shareAgainst Ohio
Under 2 pCi/L4215.8%23.1%39.6%−7.3 points
2 to 4 pCi/L5018.9%24.6%24.7%−5.7 points
4 pCi/L and above17365.3%52.3%35.7%+13.0 points
Marion County ranked among the 79 Ohio counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Auglaize County8 of 7913971.2%6.5 pCi/L
Champaign County9 of 7959671.1%6.8 pCi/L
Madison County10 of 7954967.4%5.7 pCi/L
Ross County11 of 7936867.1%7.0 pCi/L
Tuscarawas County12 of 7954766.4%5.6 pCi/L
Marion County13 of 7926565.3%6.4 pCi/L
Miami County14 of 7969065.2%6.1 pCi/L
Morrow County15 of 7917765.0%5.7 pCi/L
Holmes County16 of 796764.2%5.8 pCi/L
Darke County17 of 7927063.7%5.6 pCi/L
Ashland County18 of 7934062.9%5.4 pCi/L
Marion County ten-year laboratory totals, against Ohio's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestOhio, pooledChange from the row before
2008 to 201726565.3%6.4 pCi/L87.6 pCi/L52.3%+1.6 points
2007 to 201626263.7%6.4 pCi/L60.0 pCi/L52.8%Level
2006 to 201526263.7%6.4 pCi/L60.0 pCi/L52.9%+3.2 points
2005 to 201431460.5%5.8 pCi/L60.0 pCi/L52.4%First on file

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

Marion County, Geology

The ground under Marion County

Predicted

USGS's geologic map puts all of Marion County's mapped land in one class: high radon potential. All of it lies in the area the map calls Till Plains, to which USGS gives a radon index of 14, in the high band (12 to 17).

USGS's geologic areas under Marion County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Till PlainsGlacial deposits underlain by Ordovician through Devonian limestone and shale.High14100%

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

Marion County beside the ground around it

Marion County has more high-potential ground than Ohio as a whole: 100% of its land against 62.8%. Of the 6 counties that border it, none has as large a share of high-potential ground; Crawford County comes nearest at 100%.

Share of land by USGS radon potential: Marion County, the 6 counties bordering it and Ohio
AreaEPA zoneLow potentialModerate potentialHigh potential
Marion CountyZone 1NoneNone100%
Union CountyZone 1NoneNone100%
Delaware CountyZone 1NoneNone100%
Morrow CountyZone 1NoneNone100%
Wyandot CountyZone 1NoneNone100%
Crawford CountyZone 1NoneNone100%
Hardin CountyZone 1NoneNone100%
Ohio88 counties, pooled by land areaVariesNone37.2%62.8%

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

Local help

Marion County, Local resources

Radon testing and mitigation in Marion County

What Ohio's program says
Phone
614-728-0272
Email
indoor.radon@odh.ohio.gov
Licensing of radon professionals
This program licenses companies and persons directly involved with the radon testing and mitigation industry.

What it means

Marion County, Questions

Questions about radon in Marion County

Are radon levels high in Marion County?

Over the 2008 to 2017 laboratory totals, 65.3% of the radon tests reported to CDC for Marion County, Ohio were at or above 4 pCi/L, and the median reported pre-mitigation result was 6.4 pCi/L. Of 79 Ohio counties with 30 or more tests for the same period, 12 reported a higher share; pooled, the state comes to 52.3%.

Which counties border Marion County?

Marion County borders 6 counties: Union County, Delaware County, Morrow County, Wyandot County, Crawford County and Hardin County.

Products

Marion County, Products

Testing a home in Marion County

In Marion County, 65.3% of 265 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 52.3% pooled across Ohio. In Zone 1 the year-round level in Marion 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

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

Explore

Marion County, Bordering counties

Counties bordering Marion County

Calculated by Radon411

Marion County, Cities and towns

Cities and towns in Marion County

Marion County's population has barely moved since 2020: 65,115 residents on July 1, 2025 against 65,385 then, by the Census Bureau's estimates. That works out to 161 people per square mile of land. Marion alone is home to 35,587 of them, 54.7% of the total. Together its 8 incorporated places hold 60.3% of those residents; the other 39.7% live outside any of them. Reported testing is thin here: 265 tests over 2008 to 2017, or 4.1 for every 1,000 residents (2025 estimate), where Ohio as a whole comes to 8.3.

From Marion to Green Camp, CDC publishes no figure by town: each reads Marion County's.

The cities and towns in Marion County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Marion35,587−1.3%54.7%
Prospect1,056−1.5%1.6%
La Rue674−1.2%1.0%
Caledonia559−0.5%0.9%
New Bloomington418+1.0%0.6%
Morral367−1.9%0.6%
Waldo320−0.6%0.5%
Green Camp305−1.3%0.5%

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).

Marion County, Homes

Homes in Marion County

Marion County is a place of houses on their own lots: 77% of its 27,381 homes are detached single-family houses, against 69% across Ohio. Half of its homes were built before 1963, close to Ohio's 1970: 46% date from before 1960 and 11% from 2000 or later. 68% of occupied homes are lived in by their owner (Ohio: 67%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Marion County, Ohio, 2019 to 2023
HomesMarion CountyOhio
Housing units27,3815,271,573
Detached houses77%69%
In buildings of 5 or more9%15%
Mobile homes5%4%
Built before 196046%38%
Built since 200011%16%
Middle year built19631970
Owner-occupied68%67%

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