Short-term kit
Air Chek Short-Term radon test kit you hang, lab fee included
- Lab fee included
In home: 72 to 168 hours
Locations and reports
Radon Levels in St. Mary's County, Maryland
EPA puts St. Mary's County in Zone 2, its middle radon zone, and 20.9% of its 636 reported radon tests (2008 to 2017, laboratory totals) were 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
Share at 4+ pCi/L
20.9%
3.0 pts below MD
- St. Mary's
- 20.9%
- Maryland
- 23.9%
636 tests, 2008 to 2017
Median result
1.8pCi/L
Below action level
- St. Mary's
- 1.8 pCi/L
- EPA
- 4.0 pCi/L
Midpoint of the tests, 2008 to 2017
Tests reported
636
13th of 19 by tests
- St. Mary's
- 636
- Montgomery
- 10,969
Reported to CDC before any fix, 2008 to 2017
EPA radon zone
Zone 2
Moderate potential
- 1
- Zone 2
- 3
1 highest, 3 lowest
EPA predicts 2 to 4 pCi/L indoors
Rank in Maryland
12 of 19
Middle half
- St. Mary's
- 20.9%
- Washington
- 56.0%
By share, of 19 with 30+ tests
Read a result for St. Mary's County
Starts at St. Mary's County's median, 1.8 pCi/L
EPA’s own thresholds; the figures are the area’s, never this home’s.
EPA’s reading of 1.8 pCi/L
Below 2 pCi/L
- Under 20 to 1.9 pCi/L
- 2 to 4consider a fix
- 4 and aboveEPA says fix it
This result is below 2 pCi/L, the level at which EPA says a fix should be considered. St. Mary's County's median reported result, 2008 to 2017.
The data
St. Mary's County, At a glance
St. Mary's County at a glance
Predicted: EPA radon zone
EPA Zone 2: moderate predicted potential
St. Mary's County is a
USGS geologic potential: Moderate across 86% of the county's area
Predicted: EPA Map of Radon Zones
Measured: tests reported to CDC
Radon tests reported for St. Mary's County, 2008 to 2017, laboratory
Under 2 pCi/L: 55%2 to 4 pCi/L: 24%4 pCi/L and above: 21%
Of 636 pre-mitigation tests reported to CDC, not homes in this area.
| Result band | Tests Measured | Share Measured |
|---|---|---|
| Under 2 pCi/L | 349 | 54.9% |
| 2 to 4 pCi/L | 154 | 24.2% |
| 4 pCi/L and above | 133 | 20.9% |
What these figures count
These are the 636 tests reported to CDC for St. Mary's 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).
St. Mary's County, Compared
St. Mary's County radon figures, compared
Calculated by Radon411St. Mary's County's 636 reported tests over the 2008 to 2017 laboratory totals had a middle value of 1.8 pCi/L and a maximum of 68.8 pCi/L, and 20.9% reached 4 pCi/L. Maryland as a whole (24 Maryland counties, pooled) comes to 23.9%: 3.0 points higher than here.
Among its state's counties it ranks 12th of 19 Maryland counties with 30 or more tests for the same period. Statewide for the same period, Washington County reported the highest share (56.0%) and Wicomico County the lowest (0.0%).
Below the action level, 349 tests (54.9%) came in under 2 pCi/L, against 56.6% statewide, and 154 tests (24.2%) fell between 2 and 4, against 19.5% statewide. CDC's ten-year totals for St. Mary's County went from 438 tests (2005 to 2014) to 636 (2008 to 2017), the share going from 21.7% to 20.9%.
EPA's 1993 zone map puts St. Mary's County in Zone 2, a predicted county average between 2 and 4 pCi/L. USGS geology rates 86% of the county's land moderate radon potential.
Next door, the shares run from 24.1% in Charles County to 30.2% in Calvert County.
CDC also counts 35 tests taken after mitigation in St. Mary's County for the same period: tests in homes where a fix was already in.
Pooled shares: Radon411's sum of CDC's county counts for one period.
St. Mary's County, Geology
The ground under St. Mary's County
PredictedUSGS's geologic map puts all of St. Mary's County's mapped land in one class: moderate radon potential. 86.5% of it lies in the area the map calls Western Shore, to which USGS gives a radon index of 10, in the moderate band (9 to 11).
| Area | The rock, in USGS's words | Potential | Radon index | Share of land |
|---|---|---|---|---|
| Western Shore | Cretaceous, Quaternary, minor Tertiary gravel, sand, silt, clay. | Moderate | 10 | 86.5% |
Names, descriptions and indexes: U.S. Geological Survey, Geologic Radon Potential Map (2025). Shares: Radon411, from the map cut to the boundary.
St. Mary's County beside the ground around it
Across Maryland 36.7% of the land is rated high; none of St. Mary's County's is. Of the 2 counties that border it, 1 has more high-potential ground, Calvert County the most at 88.3%.
The map rates the ground, not the air in any home.
Local help
St. Mary's County, Local resources
Radon testing and mitigation in St. Mary's County
- Maryland's radon programMaryland Department of HealthMaryland's radon program offers free or discounted test kits, and answers questions about testing and mitigation in Maryland.Visit the program’s page(opens in a new tab)
- Free and low-cost kitsThe program says:
Maryland residents may be eligible for a free radon test kit.
- Compare kits
- Professionals in Maryland5 professionals are listed in Maryland.
What Maryland's program says
- Phone
- 1-866-703-3266
What it means
St. Mary's County, Questions
Questions about radon in St. Mary's County
Are radon levels high in St. Mary's County?
Over the 2008 to 2017 laboratory totals, 20.9% of the radon tests reported to CDC for St. Mary's County, Maryland were at or above 4 pCi/L, and the median reported pre-mitigation result was 1.8 pCi/L. Of 19 Maryland counties with 30 or more tests for the same period, 11 reported a higher share; pooled, the state comes to 23.9%.
Which counties border St. Mary's County?
St. Mary's County borders 2 counties: Calvert County and Charles County.
Products
St. Mary's County, Products
Testing a home in St. Mary's County
In St. Mary's County, 20.9% of 636 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 23.9% pooled across Maryland. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in St. Mary's County; a long-term test tells the year-round average.
Test kits and monitors3
See every kit and monitor
Long-term kit
RL116 long-term radon test kit
- Lab fee extra
Method: Alpha track
In home: 90 days to 12 months
Monitor
RadonScan USB-powered radon monitor with a color screen
- C-NRPP 2024/25: Not included
- Plug-in
- Alarm
- Screen
First reading: Within 5 minutes
- See every kit and monitor Test kits and monitors
Test kits and monitors: 1 of 3
Explore
St. Mary's County, Bordering counties
Counties bordering St. Mary's County
Calculated by Radon411- Radon levels in Calvert CountyEPA zone 1
30.2%at or above 4 pCi/L
Median 2.0 pCi/L · 1,205 tests · 9.3 points above St. Mary's County
- Radon levels in Charles CountyEPA zone 2
24.1%at or above 4 pCi/L
Median 1.8 pCi/L · 477 tests · 3.2 points above St. Mary's County
St. Mary's County, Cities and towns
Cities and towns in St. Mary's County
The Census Bureau estimates 116,692 residents in St. Mary's County for July 1, 2025, 2,700 more than in 2020 (+2.4%). That works out to 325 people per square mile of land. Leonardtown, its one incorporated place, has 5,033 residents, 4.3% of the total. CDC's 636 reported tests for 2008 to 2017 work out to 5.5 per 1,000 residents (2025 estimate), close to Maryland's 7.7.
Leonardtown is the one incorporated place here; CDC publishes no figure by town, so it reads St. Mary's County's.
| Place | Residents | Since 2020 | Share of residents |
|---|---|---|---|
| Leonardtown | 5,033 | +10.8% | 4.3% |
One incorporated place, 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).
St. Mary's County, Homes
Homes in St. Mary's County
Of St. Mary's County's 46,078 homes, 73% are detached houses, against 51% across Maryland; 11% are in buildings of five or more homes. Much of St. Mary's County is recent building: 36% of its homes have gone up since 2000 and the middle year built is 1992, against 1979 for Maryland. 73% of occupied homes are lived in by their owner (Maryland: 67%); in the rest a tenant can run a test, but a fix is the landlord's to make.
| Homes | St. Mary's County | Maryland |
|---|---|---|
| Housing units | 46,078 | 2,545,532 |
| Detached houses | 73% | 51% |
| In buildings of 5 or more | 11% | 23% |
| Mobile homes | 3% | 1% |
| Built before 1960 | 11% | 27% |
| Built since 2000 | 36% | 20% |
| Middle year built | 1992 | 1979 |
| Owner-occupied | 73% | 67% |
Census Bureau, American Community Survey, 2019 to 2023: estimates from a sample of homes, looser for a small place. Shares by Radon411.