Radon in Delaware: radon map by county
EPA's Map of Radon Zones puts 0 of Delaware's 3 counties in Zone 1 (predicted average indoor radon above 4 pCi/L), 1 in Zone 2 and 2 in Zone 3. Of 10,735 building tests that radon laboratories reported for 2008 to 2017, 16.8% were at or above EPA's 4 pCi/L action level, against 36.5% nationally (number 39 of 42 states with 1,000+ tests). The highest county share is New Castle County (17% of 9,184 tests).
Radon by county in Delaware
| County | EPA zone | Lab tests | Median | 4+ pCi/L |
|---|---|---|---|---|
| Kent County | Zone 3 | 1,222 | 1.6 | 16.9% |
| New Castle County | Zone 2 | 9,184 | 1.8 | 17% |
| Sussex County | Zone 3 | 329 | 1.0 | 10.4% |
Lab tests: buildings tested by radon laboratories, 2008 to 2017; median in pCi/L; “4+ pCi/L” is the share of tests at or above EPA's action level. n/a: no value on file (no tests, or too few to report).
What the zones mean
Zone 1 counties have a predicted average indoor radon screening level above 4 pCi/L, Zone 2 from 2 to 4 pCi/L and Zone 3 below 2 pCi/L. The prediction is from 1993 indoor measurements, geology, aerial radioactivity, soils and foundation types. EPA: “This map should not be used to determine if individual homes need to be tested.” EPA recommends fixing a home at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, because there is no known safe level of exposure.
Radon in Delaware: questions
What radon zone is Delaware?
EPA's 1993 map does not give states one zone; it zones each county. In Delaware, 0 counties are Zone 1, 1 Zone 2 and 2 Zone 3. EPA says the map "should not be used to determine if individual homes need to be tested".
Which Delaware counties have the highest radon levels?
By the share of laboratory tests at or above 4 pCi/L for 2008 to 2017 (counties with 20 or more tests): New Castle County 17% (median 1.8 pCi/L, 9,184 tests); Kent County 16.9% (median 1.6 pCi/L, 1,222 tests); Sussex County 10.4% (median 1.0 pCi/L, 329 tests).
How common is high radon in Delaware?
16.8% of the 10,735 tests in the laboratory data for Delaware (2008 to 2017) were at or above 4 pCi/L; the national share was 36.5%. Buildings are tested by choice, often because of a sale or a concern, so the share describes the tests on file, not every home.
Should I test for radon in Delaware?
Yes. EPA recommends testing every home no matter where it is, fixing it at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L, because there is no known safe level of radon exposure.
States bordering Delaware
Other free hazard lookups
- Radon by county and ZIP codeEPA radon zone and the radon test results on file for any county, ZIP code or address.
- Fault lines near an addressUSGS-mapped earthquake faults within 5 miles, their age and slip rate, and the USGS seismic hazard value.
- Power lines near an addressHigh-voltage transmission lines within 1 mile from federal maps, with voltage and distance.
- Flood zone lookupThe FEMA flood zone mapped at any US address. Not an official flood zone determination.
- Superfund sites near an addressThe nearest EPA Superfund (NPL) sites and their status.
All of these, plus wetlands, FEMA hazard ratings and the federal contamination lists, are in the Environmental Records Report for any US address ($29).
Superfund sites in Delaware · Radon map of every state
Data compiled 2026-10-20. County-level screening information, not a radon measurement of any building.