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Radon Levels in Marion County, Indiana

EPA puts Marion County in Zone 1, its highest radon zone, and high results are common there: 40.1% of the 3,668 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

    40.1%

    1.2 pts above IN

    Marion
    40.1%
    Indiana
    38.9%

    3,668 tests, 2008 to 2017

  • Median result

    3.1pCi/L

    Below action level

    Marion
    3.1 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    3,668

    3rd of 69 by tests

    Marion
    3,668
    St. Joseph
    7,346

    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 Indiana

    42 of 69

    Middle half

    Marion
    40.1%
    Miami
    63.3%

    By share, of 69 with 30+ tests

Read a result for Marion County

Starts at Marion County's median, 3.1 pCi/L

3.1pCi/L

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

EPA’s reading of 3.1 pCi/L

Between 2 and 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 between 2 and 4 pCi/L. EPA recommends considering a fix when the level is between 2 and 4 pCi/L. 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: 35%2 to 4 pCi/L: 25%4 pCi/L and above: 40%

Of 3,668 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/L1,29835.4%
2 to 4 pCi/L90024.5%
4 pCi/L and above1,47040.1%
What these figures count

These are the 3,668 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

In Marion County, about 4 in 10 reported tests over the 2008 to 2017 laboratory totals (1,470 of 3,668) were at or above EPA's action level. Against the national pool of 35.7%, the gap is 4.4 points higher. Across 92 Indiana counties pooled together, 38.9% of tests reached that level, so Marion County runs 1.2 points above its state.

Of 7 neighbours with a figure, 3 reported a larger share at or above 4 pCi/L and 4 a smaller one; the spread is 33.9% (Hancock County) to 50.0% (Shelby County). By share, Marion County is 42nd of 69 Indiana counties with 30 or more tests for the same period.

Half the tests came in under 3.1 pCi/L and half over it; the single highest reached 60.0 pCi/L. Below the action level, 1,298 tests (35.4%) came in under 2 pCi/L, against 32.8% statewide, and 900 tests (24.5%) fell between 2 and 4, against 28.3% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 40.1% share point the same way here. USGS geology rates 100% of the county's land high radon potential. CDC also counts 406 tests taken after mitigation in Marion County for the same period: tests in homes where a fix was already in.

CDC's ten-year totals for Marion County went from 3,640 tests (2005 to 2014) to 3,668 (2008 to 2017), the share going from 40.7% to 40.1%.

Marion County against Indiana, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Marion County
Marion County3,66840.1%3.1 pCi/LThis county
Indiana, pooled92 counties, pooled43,14838.9%Not pooled−1.2 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−4.4 points
Miami Countyhighest in the state3063.3%5.3 pCi/L+23.2 points
Jasper Countylowest in the state7114.1%1.2 pCi/L−26.0 points
Shelby County, INborders it8450.0%4.0 pCi/L+9.9 points
Morgan County, INborders it22644.2%3.5 pCi/L+4.1 points
Hendricks County, INborders it1,17041.5%3.2 pCi/L+1.4 points
Boone County, INborders it61439.7%3.0 pCi/L−0.4 points
Johnson County, INborders it55639.4%2.9 pCi/L−0.7 points
Hamilton County, INborders it3,47937.6%2.7 pCi/L−2.5 points
Hancock County, INborders it27133.9%2.6 pCi/L−6.2 points
Where Marion County's reported results fell, 2008 to 2017, laboratory, beside Indiana and the United States
ResultMarion County testsMarion County shareIndiana shareU.S. shareAgainst Indiana
Under 2 pCi/L1,29835.4%32.8%39.6%+2.6 points
2 to 4 pCi/L90024.5%28.3%24.7%−3.8 points
4 pCi/L and above1,47040.1%38.9%35.7%+1.2 points
Marion County ranked among the 69 Indiana counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
Washington County37 of 693842.1%3.5 pCi/L
Allen County38 of 693,99041.8%3.3 pCi/L
LaGrange County39 of 6912041.7%3.4 pCi/L
Hendricks County40 of 691,17041.5%3.2 pCi/L
Montgomery County41 of 6915241.4%2.9 pCi/L
Marion County42 of 693,66840.1%3.1 pCi/L
Boone County43 of 6961439.7%3.0 pCi/L
Marshall County44 of 6938339.7%3.2 pCi/L
Johnson County45 of 6955639.4%2.9 pCi/L
Owen County46 of 6911039.1%2.8 pCi/L
Hamilton County47 of 693,47937.6%2.7 pCi/L
Marion County ten-year laboratory totals, against Indiana's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestIndiana, pooledChange from the row before
2008 to 20173,66840.1%3.1 pCi/L60.0 pCi/L38.9%+0.5 points
2007 to 20163,71139.6%3.1 pCi/L60.0 pCi/L39.2%−1.2 points
2006 to 20153,61440.8%3.2 pCi/L60.0 pCi/L39.6%+0.1 points
2005 to 20143,64040.7%3.2 pCi/L60.0 pCi/L39.3%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 Huron-Erie Lobe, to which USGS gives a radon index of 13, in the high band (12 to 17).

USGS's geologic areas under Marion County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Huron-Erie LobeCalcareous clayey and loamy glacial deposits derived from marine carbonate and clastic sedimentary rocks.High13100%

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 Indiana as a whole: 100% of its land against 56.6%. Of the 7 counties that border it, none has as large a share of high-potential ground; Boone County comes nearest at 100%.

Share of land by USGS radon potential: Marion County, the 7 counties bordering it and Indiana
AreaEPA zoneLow potentialModerate potentialHigh potential
Marion CountyZone 1NoneNone100%
Johnson CountyZone 1None0.9%99.1%
Hamilton CountyZone 1NoneNone100%
Hendricks CountyZone 1NoneNone100%
Hancock CountyZone 1NoneNone100%
Shelby CountyZone 1NoneNone100%
Boone CountyZone 1NoneNone100%
Morgan CountyZone 2None44.4%55.6%
Indiana92 counties, pooled by land areaVariesNone43.4%56.6%

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

Local help

Marion County, Local resources

Radon testing and mitigation in Marion County

Listed radon businesses with an address in Marion County:

Listed radon professionals by town: Indianapolis

What Indiana's program says
Phone
1-800-272-9723
Licensing of radon professionals
According to the Indiana Radon Rule, 410 IAC 5.1, everyone who is serving Indiana by doing radon testing, analysis and mitigation (radon reduction) in residential structures must be certified by the Indiana State Department of Health.

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, 40.1% of the radon tests reported to CDC for Marion County, Indiana were at or above 4 pCi/L, and the median reported pre-mitigation result was 3.1 pCi/L. Of 69 Indiana counties with 30 or more tests for the same period, 41 reported a higher share; pooled, the state comes to 38.9%.

Which counties border Marion County?

Marion County borders 7 counties: Johnson County, Hamilton County, Hendricks County, Hancock County, Shelby County, Boone County and Morgan County.

Products

Marion County, Products

Testing a home in Marion County

In Marion County, 40.1% of 3,668 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 38.9% pooled across Indiana. 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

The Census Bureau estimates 992,196 residents in Marion County for July 1, 2025, 15,488 more than in 2020 (+1.6%). Packed into its land, that is 2,502 people per square mile. Indianapolis alone is home to 901,116 of them, 90.8% of the total. Reported testing is thin here: 3,668 tests over 2008 to 2017, or 3.7 for every 1,000 residents (2025 estimate), where Indiana as a whole comes to 6.2.

From Indianapolis to North Crows Nest, CDC publishes no figure by town: each reads Marion County's.

The 12 largest cities and towns in Marion County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Indianapolis901,116+1.6%90.8%
Lawrence50,420+2.2%5.1%
Beech Grove14,671−0.1%1.5%
Speedway14,373+3.0%1.4%
Cumberlandextends beyond Marion County8,066+24.6%Counted whole
Southport2,094−1.1%0.2%
Meridian Hills1,800+1.4%0.2%
Warren Park1,509+1.5%0.2%
Clermont1,403+1.7%0.1%
Homecroft760+1.3%Under 0.1%
Rocky Ripple658+1.1%Under 0.1%
Williams Creek435+1.6%Under 0.1%
  • Wynnedale
  • Spring Hill
  • Crows Nest
  • North Crows Nest

16 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 Marion County is counted whole, so it has no share.

Marion County, Homes

Homes in Marion County

Of Marion County's 439,034 homes, 60% are detached houses, against 73% across Indiana; 22% are in buildings of five or more homes. Half of its homes were built before 1972, close to Indiana's 1976: 34% date from before 1960 and 17% from 2000 or later. 56% of occupied homes are lived in by their owner (Indiana: 70%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Marion County, Indiana, 2019 to 2023
HomesMarion CountyIndiana
Housing units439,0342,953,344
Detached houses60%73%
In buildings of 5 or more22%13%
Mobile homes2%4%
Built before 196034%32%
Built since 200017%21%
Middle year built19721976
Owner-occupied56%70%

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