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Radon Levels in Sussex County, Delaware

EPA puts Sussex County in Zone 3, its lowest radon zone, and most reported tests there come back low, though 10.4% of 451 (2008 to 2017, laboratory totals) reached 4 pCi/L. Homes with elevated levels occur in all three zones, so only a test measures yours.

Data checked By Radon411Methodology

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  • Share at 4+ pCi/L

    10.4%

    6.3 pts below DE

    Sussex
    10.4%
    Delaware
    16.7%

    451 tests, 2008 to 2017

  • Median result

    1.0pCi/L

    Below action level

    Sussex
    1.0 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    451

    3rd of 3 by tests

    Sussex
    451
    New Castle
    10,247

    Reported to CDC before any fix, 2008 to 2017

  • EPA radon zone

    Zone 3

    Lower potential

    1. 1
    2. 2
    3. Zone 3

    1 highest, 3 lowest

    EPA predicts under 2 pCi/L indoors

  • Rank in Delaware

    3 of 3

    Lowest quarter

    Sussex
    10.4%
    New Castle
    17.0%

    By share, of 3 with 30+ tests

Read a result for Sussex County

Starts at Sussex County's median, 1.0 pCi/L

1.0pCi/L

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

EPA’s reading of 1.0 pCi/L

Below 2 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 below 2 pCi/L, the level at which EPA says a fix should be considered. Sussex County's median reported result, 2008 to 2017.

Interpret this reading

The data

Sussex County, At a glance

Sussex County at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for Sussex County as a whole is an average indoor of less than 2 pCi/L (less than 75 Bq/m³). A low zone clears no address in it: homes above EPA's turn up in all three zones.

USGS geologic potential: Low across 97% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Sussex County, 2008 to 2017, laboratory
Under 2 pCi/L: 72%2 to 4 pCi/L: 18%4 pCi/L and above: 10%

Of 451 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Sussex County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L32572.1%
2 to 4 pCi/L7917.5%
4 pCi/L and above4710.4%
What these figures count

These are the 451 tests reported to CDC for Sussex 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

Sussex County, Compared

Sussex County radon figures, compared

Calculated by Radon411

Sussex County reports fewer high tests than Delaware overall. 10.4% of its 451 tests over the 2008 to 2017 laboratory totals were at or above 4 pCi/L. That is 6.3 points below the 16.7% pooled from 3 Delaware counties.

Ordered from highest share down, it comes 3rd of 3 Delaware counties with 30 or more tests for the same period. Nationally, every county with a figure for the same period pools to 35.7%; Sussex County sits 25.3 points below it.

Next door, the shares run from 0.0% in Wicomico County, MD to 16.9% in Kent County, with Wicomico County, MD below Sussex County. Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 10.4% share point the same way here.

Half the tests came in under 1.0 pCi/L and half over it; the single highest reached 19.7 pCi/L. The rest split 72.1% under 2 pCi/L and 17.5% from 2 to 4 (Delaware: 55.3% and 28.0%).

CDC's ten-year totals for Sussex County went from 323 tests (2005 to 2014) to 451 (2008 to 2017), the share going from 11.1% to 10.4%.

Sussex County against Delaware, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Sussex County
Sussex County45110.4%1.0 pCi/LThis county
Delaware, pooled3 counties, pooled12,21416.7%Not pooled+6.3 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+25.3 points
New Castle Countyhighest in the state10,24717.0%1.8 pCi/L+6.6 points
Kent County, DEborders it1,51616.9%1.6 pCi/L+6.5 points
Dorchester County, MDborders it616.7%1.6 pCi/L+6.3 points
Caroline County, MDborders it120.0%0.8 pCi/L−10.4 points
Wicomico County, MDborders it320.0%0.9 pCi/L−10.4 points
Worcester County, MDborders it70.0%0.6 pCi/L−10.4 points
Where Sussex County's reported results fell, 2008 to 2017, laboratory, beside Delaware and the United States
ResultSussex County testsSussex County shareDelaware shareU.S. shareAgainst Delaware
Under 2 pCi/L32572.1%55.3%39.6%+16.8 points
2 to 4 pCi/L7917.5%28.0%24.7%−10.5 points
4 pCi/L and above4710.4%16.7%35.7%−6.3 points
Sussex County ranked among the 3 Delaware counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
New Castle County1 of 310,24717.0%1.8 pCi/L
Kent County2 of 31,51616.9%1.6 pCi/L
Sussex County3 of 345110.4%1.0 pCi/L
Sussex County ten-year laboratory totals, against Delaware's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestDelaware, pooledChange from the row before
2008 to 201745110.4%1.0 pCi/L19.7 pCi/L16.7%−0.2 points
2007 to 201640710.6%1.1 pCi/L19.7 pCi/L17.3%−0.8 points
2006 to 201534111.4%1.1 pCi/L19.7 pCi/L17.6%+0.3 points
2005 to 201432311.1%1.1 pCi/L19.7 pCi/L17.9%First on file

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

Sussex County, Geology

The ground under Sussex County

Predicted

USGS's geologic map puts all of Sussex County's mapped land in one class: low radon potential. 96.6% of it lies in the area the map calls Coastal Plain, to which USGS gives a radon index of 7, in the low band (5 to 8).

USGS's geologic areas under Sussex County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Coastal PlainCretaceous, Tertiary and Quaternary quartzitic fluvial and marine sediments.Low796.6%

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

Sussex County beside the ground around it

Across Delaware 14.0% of the land is rated moderate; none of Sussex County's is. None of the 5 counties that border it has any moderate-potential ground either.

Share of land by USGS radon potential: Sussex County, the 5 counties bordering it and Delaware
AreaEPA zoneLow potentialModerate potentialHigh potential
Sussex CountyZone 396.6%NoneNone
Wicomico County, MDZone 398.3%NoneNone
Caroline County, MDZone 398.7%NoneNone
Kent CountyZone 399.8%NoneNone
Worcester County, MDZone 378.1%NoneNone
Dorchester County, MDZone 391.0%NoneNone
Delaware3 counties, pooled by land areaVaries84.2%14.0%None

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

Local help

Sussex County, Local resources

Radon testing and mitigation in Sussex County

What Delaware's program says
Phone
1-800-464-HELP

What it means

Sussex County, Questions

Questions about radon in Sussex County

Are radon levels high in Sussex County?

Over the 2008 to 2017 laboratory totals, 10.4% of the radon tests reported to CDC for Sussex County, Delaware were at or above 4 pCi/L, and the median reported pre-mitigation result was 1.0 pCi/L. Of 3 Delaware counties with 30 or more tests for the same period, 2 reported a higher share; pooled, the state comes to 16.7%.

Which counties border Sussex County?

Sussex County borders 5 counties: Wicomico County, MD, Caroline County, MD, Kent County, DE, Worcester County, MD and Dorchester County, MD.

Products

Sussex County, Products

Testing a home in Sussex County

In Sussex County, 10.4% of 451 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 16.7% pooled across Delaware. Zone 3 is the lowest label, yet homes at or above 4 pCi/L occur in every zone: a short-term kit (2 to 90 days) is the quick first check in Sussex County.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Sussex County homes, compared

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Sussex County, Bordering counties

Counties bordering Sussex County

Calculated by Radon411

Sussex County, Cities and towns

Cities and towns in Sussex County

Sussex County is growing: the Census Bureau puts it at 277,140 residents on July 1, 2025, up 16.1% from 238,627 in 2020. That works out to 296 people per square mile of land. Milford, the largest place on its list (14,723 residents in all), extends beyond Sussex County, so no share of the total is worked out for it. Reported testing is thin here: 451 tests over 2008 to 2017, or 1.6 for every 1,000 residents (2025 estimate), where Delaware as a whole comes to 11.5.

From Milford to Henlopen Acres, CDC publishes no figure by town: each reads Sussex County's.

The 12 largest cities and towns in Sussex County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Milfordextends beyond Sussex County14,723+30.3%Counted whole
Seaford9,165+14.8%3.3%
Georgetown8,291+14.8%3.0%
Millsboro7,890+15.7%2.8%
Laurel4,557+15.2%1.6%
Milton3,860+15.6%1.4%
Lewes3,826+14.0%1.4%
Selbyville3,381+15.8%1.2%
Ocean View3,107+15.8%1.1%
Bridgeville2,997+15.7%1.1%
Delmar2,366+14.2%0.9%
Millville2,145+16.1%0.8%
  • Blades
  • Rehoboth Beach
  • Greenwood
  • Bethany Beach
  • Dagsboro
  • Frankford
  • Ellendale
  • South Bethany
  • Dewey Beach
  • Fenwick Island
  • Bethel
  • Slaughter Beach
  • Henlopen Acres

25 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 Sussex County is counted whole, so it has no share.

Sussex County, Homes

Homes in Sussex County

Of Sussex County's 147,904 homes, 65% are detached houses, against 59% across Delaware; 13% are mobile homes. Much of Sussex County is recent building: 48% of its homes have gone up since 2000 and the middle year built is 1999, against 1986 for Delaware. Owners live in 81% of the occupied homes (Delaware: 72%), so for most households a test and a fix are their own to order.

Homes in Sussex County, Delaware, 2019 to 2023
HomesSussex CountyDelaware
Housing units147,904457,958
Detached houses65%59%
In buildings of 5 or more9%14%
Mobile homes13%7%
Built before 196010%21%
Built since 200048%32%
Middle year built19991986
Owner-occupied81%72%

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