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Radon Levels in Union County, Ohio

EPA puts Union County in Zone 1, its highest radon zone, and high results are common there: 60.2% of the 1,285 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

    60.2%

    7.9 pts above OH

    Union
    60.2%
    Ohio
    52.3%

    1,285 tests, 2008 to 2017

  • Median result

    4.8pCi/L

    Action level reached

    Union
    4.8 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    1,285

    24th of 79 by tests

    Union
    1,285
    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

    23 of 79

    Middle half

    Union
    60.2%
    Pickaway
    81.4%

    By share, of 79 with 30+ tests

Read a result for Union County

Starts at Union County's median, 4.8 pCi/L

4.8pCi/L

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

EPA’s reading of 4.8 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. Union County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Union County, At a glance

Union County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Union 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 Union County, 2008 to 2017, laboratory

Reported results by concentration band, Union County, 2008 to 2017, laboratory
Under 2 pCi/L: 13%2 to 4 pCi/L: 27%4 pCi/L and above: 60%

Of 1,285 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Union County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L16913.2%
2 to 4 pCi/L34326.7%
4 pCi/L and above77360.2%
What these figures count

These are the 1,285 tests reported to CDC for Union 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

Union County, Compared

Union County radon figures, compared

Calculated by Radon411

More than half: 60.2% of the 1,285 radon tests reported to CDC for Union County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 23rd of 79 Ohio counties with 30 or more tests for the same period. That is 7.9 points above the 52.3% pooled from 88 Ohio counties.

The median result, 4.8 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 45.4 pCi/L. The rest split 13.2% under 2 pCi/L and 26.7% from 2 to 4 (Ohio: 23.1% and 24.6%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 60.2% share point the same way here.

6 of its 7 bordering counties with figures ran higher, led by Logan County at 73.6%; 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 Union County went from 995 tests (2005 to 2014) to 1,285 (2008 to 2017), the share going from 58.7% to 60.2%.

Nationally, every county with a figure for the same period pools to 35.7%; Union County sits 24.5 points above it. CDC also counts 52 tests taken after mitigation in Union County for the same period: tests in homes where a fix was already in.

Union County against Ohio, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Union County
Union County1,28560.2%4.8 pCi/LThis county
Ohio, pooled88 counties, pooled98,84052.3%Not pooled−7.9 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−24.5 points
Pickaway Countyhighest in the state18381.4%9.5 pCi/L+21.2 points
Cuyahoga Countylowest in the state10,09221.7%2.0 pCi/L−38.5 points
Logan County, OHborders it75573.6%7.9 pCi/L+13.4 points
Delaware County, OHborders it3,73072.2%6.4 pCi/L+12.0 points
Franklin County, OHborders it16,14271.9%6.8 pCi/L+11.7 points
Champaign County, OHborders it59671.1%6.8 pCi/L+10.9 points
Madison County, OHborders it54967.4%5.7 pCi/L+7.2 points
Marion County, OHborders it26565.3%6.4 pCi/L+5.1 points
Hardin County, OHborders it6743.3%3.4 pCi/L−16.9 points
Where Union County's reported results fell, 2008 to 2017, laboratory, beside Ohio and the United States
ResultUnion County testsUnion County shareOhio shareU.S. shareAgainst Ohio
Under 2 pCi/L16913.2%23.1%39.6%−9.9 points
2 to 4 pCi/L34326.7%24.6%24.7%+2.1 points
4 pCi/L and above77360.2%52.3%35.7%+7.9 points
Union 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
Ashland County18 of 7934062.9%5.4 pCi/L
Preble County19 of 7921262.7%5.5 pCi/L
Richland County20 of 791,32762.2%5.2 pCi/L
Wayne County21 of 791,07461.8%4.9 pCi/L
Henry County22 of 793461.8%5.3 pCi/L
Union County23 of 791,28560.2%4.8 pCi/L
Stark County24 of 793,31760.1%4.9 pCi/L
Shelby County25 of 7924860.1%5.2 pCi/L
Putnam County26 of 799459.6%4.8 pCi/L
Crawford County27 of 7921059.5%4.7 pCi/L
Huron County28 of 7979659.4%4.9 pCi/L
Union County ten-year laboratory totals, against Ohio's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestOhio, pooledChange from the row before
2008 to 20171,28560.2%4.8 pCi/L45.4 pCi/L52.3%−0.6 points
2007 to 20161,21560.8%4.9 pCi/L44.5 pCi/L52.8%Level
2006 to 20151,14260.8%4.9 pCi/L61.8 pCi/L52.9%+2.1 points
2005 to 201499558.7%4.8 pCi/L61.8 pCi/L52.4%First on file

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

Union County, Geology

The ground under Union County

Predicted

USGS's geologic map puts all of Union 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 Union 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.

Union County beside the ground around it

Union County has more high-potential ground than Ohio as a whole: 100% of its land against 62.8%. Of the 7 counties that border it, none has as large a share of high-potential ground; Champaign County comes nearest at 100%.

Share of land by USGS radon potential: Union County, the 7 counties bordering it and Ohio
AreaEPA zoneLow potentialModerate potentialHigh potential
Union CountyZone 1NoneNone100%
Marion CountyZone 1NoneNone100%
Logan CountyZone 1NoneNone100%
Delaware CountyZone 1NoneNone100%
Champaign CountyZone 1NoneNone100%
Hardin CountyZone 1NoneNone100%
Madison CountyZone 1NoneNone100%
Franklin CountyZone 1NoneNone100%
Ohio88 counties, pooled by land areaVariesNone37.2%62.8%

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

Local help

Union County, Local resources

Radon testing and mitigation in Union County

Listed radon businesses with an address in Union County:

Listed radon professionals by town: Dublin, Marysville

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

Union County, Questions

Questions about radon in Union County

Are radon levels high in Union County?

Over the 2008 to 2017 laboratory totals, 60.2% of the radon tests reported to CDC for Union County, Ohio were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.8 pCi/L. Of 79 Ohio counties with 30 or more tests for the same period, 22 reported a higher share; pooled, the state comes to 52.3%.

Which counties border Union County?

Union County borders 7 counties: Marion County, Logan County, Delaware County, Champaign County, Hardin County, Madison County and Franklin County.

Products

Union County, Products

Testing a home in Union County

In Union County, 60.2% of 1,285 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 Union 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

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

Explore

Union County, Bordering counties

Counties bordering Union County

Calculated by Radon411

Union County, Cities and towns

Cities and towns in Union County

Union County is growing: the Census Bureau puts it at 73,446 residents on July 1, 2025, up 16.5% from 63,069 in 2020. That works out to 170 people per square mile of land. Dublin, the largest place on its list (49,094 residents in all), extends beyond Union County, so no share of the total is worked out for it. Testing is reported here more often than across the state: CDC's 1,285 reported tests for 2008 to 2017 come to 17.5 for every 1,000 residents (2025 estimate), against 8.3 for Ohio.

From Dublin to Unionville Center, CDC publishes no figure by town: each reads Union County's.

The cities and towns in Union County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Dublinextends beyond Union County49,094−0.5%Counted whole
Marysville30,190+17.4%41.1%
Plain Cityextends beyond Union County4,322+4.0%Counted whole
Richwood2,640+18.3%3.6%
Milford Center959+18.4%1.3%
Magnetic Springs316+17.9%0.4%
Unionville Center235−5.2%0.3%

7 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 Union County is counted whole, so it has no share.

Union County, Homes

Homes in Union County

Union County is a place of houses on their own lots: 81% of its 24,298 homes are detached single-family houses, against 69% across Ohio. Much of Union County is recent building: 43% of its homes have gone up since 2000 and the middle year built is 1995, against 1970 for Ohio. Owners live in 79% of the occupied homes (Ohio: 67%), so for most households a test and a fix are their own to order.

Homes in Union County, Ohio, 2019 to 2023
HomesUnion CountyOhio
Housing units24,2985,271,573
Detached houses81%69%
In buildings of 5 or more6%15%
Mobile homes4%4%
Built before 196022%38%
Built since 200043%16%
Middle year built19951970
Owner-occupied79%67%

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