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Radon Levels in Seneca County, Ohio

EPA puts Seneca County in Zone 1, its highest radon zone, and high results are common there: 58.9% of the 540 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

    58.9%

    6.6 pts above OH

    Seneca
    58.9%
    Ohio
    52.3%

    540 tests, 2008 to 2017

  • Median result

    4.9pCi/L

    Action level reached

    Seneca
    4.9 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    540

    39th of 79 by tests

    Seneca
    540
    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

    31 of 79

    Middle half

    Seneca
    58.9%
    Pickaway
    81.4%

    By share, of 79 with 30+ tests

Read a result for Seneca County

Starts at Seneca County's median, 4.9 pCi/L

4.9pCi/L

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

EPA’s reading of 4.9 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. Seneca County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Seneca County, At a glance

Seneca County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Seneca 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 99% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Seneca County, 2008 to 2017, laboratory
Under 2 pCi/L: 19%2 to 4 pCi/L: 22%4 pCi/L and above: 59%

Of 540 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Seneca County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L10319.1%
2 to 4 pCi/L11922.0%
4 pCi/L and above31858.9%
What these figures count

These are the 540 tests reported to CDC for Seneca 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

Seneca County, Compared

Seneca County radon figures, compared

Calculated by Radon411

More than half: 58.9% of the 540 radon tests reported to CDC for Seneca County over the 2008 to 2017 laboratory totals read at least 4 pCi/L. That places it 31st of 79 Ohio counties with 30 or more tests for the same period. That is 6.6 points above the 52.3% pooled from 88 Ohio counties.

The median result, 4.9 pCi/L, sits above EPA's 4 pCi/L action level, and the top reported result was 68.8 pCi/L. The rest split 19.1% under 2 pCi/L and 22.0% from 2 to 4 (Ohio: 23.1% and 24.6%). Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 58.9% share point the same way here.

3 of its 6 bordering counties with figures ran higher, led by Crawford County at 59.5%; Wood County reported 45.5%. 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 Seneca County went from 485 tests (2005 to 2014) to 540 (2008 to 2017), the share going from 54.2% to 58.9%.

Nationally, every county with a figure for the same period pools to 35.7%; Seneca County sits 23.2 points above it. CDC also counts 88 tests taken after mitigation in Seneca County for the same period: tests in homes where a fix was already in.

Seneca County against Ohio, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Seneca County
Seneca County54058.9%4.9 pCi/LThis county
Ohio, pooled88 counties, pooled98,84052.3%Not pooled−6.6 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−23.2 points
Pickaway Countyhighest in the state18381.4%9.5 pCi/L+22.5 points
Cuyahoga Countylowest in the state10,09221.7%2.0 pCi/L−37.2 points
Crawford County, OHborders it21059.5%4.7 pCi/L+0.6 points
Huron County, OHborders it79659.4%4.9 pCi/L+0.5 points
Sandusky County, OHborders it64959.2%5.0 pCi/L+0.3 points
Wyandot County, OHborders it32046.3%3.6 pCi/L−12.6 points
Hancock County, OHborders it1,38246.2%3.7 pCi/L−12.7 points
Wood County, OHborders it1,19245.5%3.5 pCi/L−13.4 points
Where Seneca County's reported results fell, 2008 to 2017, laboratory, beside Ohio and the United States
ResultSeneca County testsSeneca County shareOhio shareU.S. shareAgainst Ohio
Under 2 pCi/L10319.1%23.1%39.6%−4.0 points
2 to 4 pCi/L11922.0%24.6%24.7%−2.6 points
4 pCi/L and above31858.9%52.3%35.7%+6.6 points
Seneca 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
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
Sandusky County29 of 7964959.2%5.0 pCi/L
Coshocton County30 of 7911258.9%5.1 pCi/L
Seneca County31 of 7954058.9%4.9 pCi/L
Greene County32 of 792,60258.6%4.9 pCi/L
Monroe County33 of 794858.3%5.0 pCi/L
Mercer County34 of 7914957.7%5.1 pCi/L
Clark County35 of 791,44757.5%4.9 pCi/L
Carroll County36 of 7948657.4%4.8 pCi/L
Seneca County ten-year laboratory totals, against Ohio's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestOhio, pooledChange from the row before
2008 to 201754058.9%4.9 pCi/L68.8 pCi/L52.3%+0.7 points
2007 to 201652258.2%4.8 pCi/L68.8 pCi/L52.8%+1.2 points
2006 to 201550557.0%4.7 pCi/L68.8 pCi/L52.9%+2.8 points
2005 to 201448554.2%4.4 pCi/L68.8 pCi/L52.4%First on file

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

Seneca County, Geology

The ground under Seneca County

Predicted

Most of the ground under Seneca County is one class on USGS's geologic map: 99.4% of its land is high radon potential, with 0.6% moderate. 99.4% 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 Seneca County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Till PlainsGlacial deposits underlain by Ordovician through Devonian limestone and shale.High1499.4%
Lake PlainsGlacial lake deposits, Devonian shale and limestone.Moderate100.6%

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

Seneca County beside the ground around it

Seneca County has more high-potential ground than Ohio as a whole: 99.4% of its land against 62.8%. Of the 6 counties that border it, 3 have more high-potential ground, Crawford County the most at 100%.

Share of land by USGS radon potential: Seneca County, the 6 counties bordering it and Ohio
AreaEPA zoneLow potentialModerate potentialHigh potential
Seneca CountyZone 1None0.6%99.4%
Wyandot CountyZone 1NoneNone100%
Crawford CountyZone 1NoneNone100%
Sandusky CountyZone 2None53.0%46.5%
Hancock CountyZone 1NoneNone100%
Huron CountyZone 1None32.6%67.4%
Wood CountyZone 2None8.3%91.7%
Ohio88 counties, pooled by land areaVariesNone37.2%62.8%

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

Local help

Seneca County, Local resources

Radon testing and mitigation in Seneca County

Listed radon businesses with an address in Seneca County:

Listed radon professionals by town: Tiffin, Fostoria, Republic

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

Seneca County, Questions

Questions about radon in Seneca County

Are radon levels high in Seneca County?

Over the 2008 to 2017 laboratory totals, 58.9% of the radon tests reported to CDC for Seneca County, Ohio were at or above 4 pCi/L, and the median reported pre-mitigation result was 4.9 pCi/L. Of 79 Ohio counties with 30 or more tests for the same period, 30 reported a higher share; pooled, the state comes to 52.3%.

Which counties border Seneca County?

Seneca County borders 6 counties: Wyandot County, Crawford County, Sandusky County, Hancock County, Huron County and Wood County.

Products

Seneca County, Products

Testing a home in Seneca County

In Seneca County, 58.9% of 540 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 Seneca 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

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

Explore

Seneca County, Bordering counties

Counties bordering Seneca County

Calculated by Radon411

Seneca County, Cities and towns

Cities and towns in Seneca County

About 54,522 people live in Seneca County by the Census Bureau's estimate for July 1, 2025, 539 fewer than in 2020 (−1.0%). That works out to 99 people per square mile of land. Its largest town, Tiffin, has 17,803 residents, 32.7% of the total. CDC's 540 reported tests for 2008 to 2017 work out to 9.9 per 1,000 residents (2025 estimate), close to Ohio's 8.3.

From Tiffin to New Riegel, CDC publishes no figure by town: each reads Seneca County's.

The cities and towns in Seneca County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Tiffin17,803−1.2%32.7%
Fostoriaextends beyond Seneca County12,920−1.0%Counted whole
Green Springsextends beyond Seneca County1,316+1.3%Counted whole
Attica863−0.8%1.6%
Bloomville845−2.6%1.5%
Bettsville584−1.2%1.1%
Republic538−2.4%1.0%
New Riegel279−2.4%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). A place that extends beyond Seneca County is counted whole, so it has no share.

Seneca County, Homes

Homes in Seneca County

Seneca County is a place of houses on their own lots: 79% of its 23,968 homes are detached single-family houses, against 69% across Ohio. The homes are older than Ohio's as a whole: half were built before 1958 (1970 for Ohio), and 53% date from before 1960. 73% 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 Seneca County, Ohio, 2019 to 2023
HomesSeneca CountyOhio
Housing units23,9685,271,573
Detached houses79%69%
In buildings of 5 or more7%15%
Mobile homes4%4%
Built before 196053%38%
Built since 20008%16%
Middle year built19581970
Owner-occupied73%67%

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