Radon in Maryland: radon map by county
EPA's Map of Radon Zones puts 8 of Maryland's 24 counties in Zone 1 (predicted average indoor radon above 4 pCi/L), 9 in Zone 2 and 7 in Zone 3. Of 35,201 building tests that radon laboratories reported for 2008 to 2017, 24.2% were at or above EPA's 4 pCi/L action level, against 36.5% nationally (number 28 of 42 states with 1,000+ tests). The highest county share is Washington County (56% of 988 tests).
Radon by county in Maryland
| County | EPA zone | Lab tests | Median | 4+ pCi/L |
|---|---|---|---|---|
| Allegany County | Zone 2 | 202 | 2.7 | 35.7% |
| Anne Arundel County | Zone 2 | 2,882 | 1.4 | 17.4% |
| Baltimore city | Zone 2 | 707 | 0.8 | 7.9% |
| Baltimore County | Zone 1 | 6,118 | 1.4 | 19.1% |
| Calvert County | Zone 1 | 806 | 2.0 | 30.2% |
| Caroline County | Zone 3 | 10 | 0.8 | 0% |
| Carroll County | Zone 1 | 2,602 | 2.8 | 41.9% |
| Cecil County | Zone 2 | 831 | 1.9 | 24.9% |
| Charles County | Zone 2 | 340 | 1.8 | 24.1% |
| Dorchester County | Zone 3 | 5 | 1.6 | 16.7% |
| Frederick County | Zone 1 | 2,947 | 1.5 | 28.1% |
| Garrett County | Zone 2 | 127 | 3.0 | 37.4% |
| Harford County | Zone 1 | 4,534 | 1.7 | 22.6% |
| Howard County | Zone 1 | 3,429 | 2.0 | 30.8% |
| Kent County | Zone 3 | 37 | 4.4 | 52.6% |
| Montgomery County | Zone 1 | 6,980 | 1.3 | 18% |
| Prince George's County | Zone 2 | 1,108 | 1.4 | 19% |
| Queen Anne's County | Zone 3 | 53 | 1.3 | 16.9% |
| Somerset County | Zone 2 | 2 | 0.3 | 0% |
| St. Mary's County | Zone 2 | 445 | 1.8 | 20.9% |
| Talbot County | Zone 3 | 23 | 1.1 | 7.7% |
| Washington County | Zone 1 | 988 | 5.2 | 56% |
| Wicomico County | Zone 3 | 18 | 0.9 | 0% |
| Worcester County | Zone 3 | 7 | 0.6 | 0% |
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 Maryland: questions
What radon zone is Maryland?
EPA's 1993 map does not give states one zone; it zones each county. In Maryland, 8 counties are Zone 1, 9 Zone 2 and 7 Zone 3. EPA says the map "should not be used to determine if individual homes need to be tested".
Which Maryland 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): Washington County 56% (median 5.2 pCi/L, 988 tests); Kent County 52.6% (median 4.4 pCi/L, 37 tests); Carroll County 41.9% (median 2.8 pCi/L, 2,602 tests); Garrett County 37.4% (median 3.0 pCi/L, 127 tests); Allegany County 35.7% (median 2.7 pCi/L, 202 tests).
How common is high radon in Maryland?
24.2% of the 35,201 tests in the laboratory data for Maryland (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 Maryland?
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 Maryland
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 Maryland · Radon map of every state
Data compiled 2026-10-20. County-level screening information, not a radon measurement of any building.