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Radon Levels in Carroll County, Maryland

EPA puts Carroll County in Zone 1, its highest radon zone, and high results are common there: 41.9% of the 3,334 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

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  • Share at 4+ pCi/L

    41.9%

    18.0 pts above MD

    Carroll
    41.9%
    Maryland
    23.9%

    3,334 tests, 2008 to 2017

  • Median result

    2.8pCi/L

    Below action level

    Carroll
    2.8 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    3,334

    7th of 19 by tests

    Carroll
    3,334
    Montgomery
    10,969

    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 Maryland

    3 of 19

    Top quarter

    Carroll
    41.9%
    Washington
    56.0%

    By share, of 19 with 30+ tests

Read a result for Carroll County

Starts at Carroll County's median, 2.8 pCi/L

2.8pCi/L

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

EPA’s reading of 2.8 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. Carroll County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Carroll County, At a glance

Carroll County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Carroll 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 86% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Carroll County, 2008 to 2017, laboratory
Under 2 pCi/L: 42%2 to 4 pCi/L: 16%4 pCi/L and above: 42%

Of 3,334 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Carroll County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L1,38741.6%
2 to 4 pCi/L55116.5%
4 pCi/L and above1,39641.9%
What these figures count

These are the 3,334 tests reported to CDC for Carroll 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

Carroll County, Compared

Carroll County radon figures, compared

Calculated by Radon411

In Carroll County, about 4 in 10 reported tests over the 2008 to 2017 laboratory totals (1,396 of 3,334) were at or above EPA's action level. Against the national pool of 35.7%, the gap is 6.2 points higher. Across 24 Maryland counties pooled together, 23.9% of tests reached that level, so Carroll County runs 18.0 points above its state.

Of 5 neighbours with a figure, 2 reported a larger share at or above 4 pCi/L and 3 a smaller one; the spread is 19.1% (Baltimore County) to 59.7% (York County, PA). By share, Carroll County is 3rd of 19 Maryland counties with 30 or more tests for the same period.

Half the tests came in under 2.8 pCi/L and half over it; the single highest reached 355.4 pCi/L. Below the action level, 1,387 tests (41.6%) came in under 2 pCi/L, against 56.6% statewide, and 551 tests (16.5%) fell between 2 and 4, against 19.5% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 41.9% share point the same way here. USGS geology rates 86% of the county's land high radon potential. CDC also counts 303 tests taken after mitigation in Carroll County for the same period: tests in homes where a fix was already in.

CDC's ten-year totals for Carroll County went from 2,294 tests (2005 to 2014) to 3,334 (2008 to 2017), the share going from 47.3% to 41.9%.

Carroll County against Maryland, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Carroll County
Carroll County3,33441.9%2.8 pCi/LThis county
Maryland, pooled24 counties, pooled47,94123.9%Not pooled−18.0 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled−6.2 points
Washington Countyhighest in the state1,25956.0%5.2 pCi/L+14.1 points
Wicomico Countylowest in the state320.0%0.9 pCi/L−41.9 points
York County, PAborders it8,78559.7%5.4 pCi/L+17.8 points
Adams County, PAborders it3,25246.4%3.6 pCi/L+4.5 points
Howard County, MDborders it4,54930.8%2.0 pCi/L−11.1 points
Frederick County, MDborders it3,93328.1%1.5 pCi/L−13.8 points
Baltimore County, MDborders it7,70619.1%1.4 pCi/L−22.8 points
Where Carroll County's reported results fell, 2008 to 2017, laboratory, beside Maryland and the United States
ResultCarroll County testsCarroll County shareMaryland shareU.S. shareAgainst Maryland
Under 2 pCi/L1,38741.6%56.6%39.6%−15.0 points
2 to 4 pCi/L55116.5%19.5%24.7%−3.0 points
4 pCi/L and above1,39641.9%23.9%35.7%+18.0 points
Carroll County ranked among the 19 Maryland 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 County1 of 191,25956.0%5.2 pCi/L
Kent County2 of 195752.6%4.4 pCi/L
Carroll County3 of 193,33441.9%2.8 pCi/L
Garrett County4 of 1918737.4%3.0 pCi/L
Allegany County5 of 1928635.7%2.7 pCi/L
Howard County6 of 194,54930.8%2.0 pCi/L
Calvert County7 of 191,20530.2%2.0 pCi/L
Frederick County8 of 193,93328.1%1.5 pCi/L
Cecil County9 of 191,07124.9%1.9 pCi/L
Charles County10 of 1947724.1%1.8 pCi/L
Harford County11 of 195,28322.6%1.7 pCi/L
Carroll County ten-year laboratory totals, against Maryland's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestMaryland, pooledChange from the row before
2008 to 20173,33441.9%2.8 pCi/L355.4 pCi/L23.9%−2.0 points
2007 to 20162,98243.9%3.0 pCi/L355.4 pCi/L24.4%−2.7 points
2006 to 20152,49946.6%3.4 pCi/L355.4 pCi/L24.9%−0.7 points
2005 to 20142,29447.3%3.6 pCi/L355.4 pCi/L24.6%First on file

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

Carroll County, Geology

The ground under Carroll County

Predicted

Most of the ground under Carroll County is one class on USGS's geologic map: 85.7% of its land is high radon potential, with 14.3% moderate. The map puts 64.4% of Carroll County in the area it calls Eastern Piedmont and 21.2% in Western Piedmont, with one smaller area beside them. USGS gives the first a radon index of 12 and the second 15, both in the high band (12 to 17).

USGS's geologic areas under Carroll County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Eastern PiedmontPrecambrian igneous and metamorphic rocks (schist and gneiss).High1264.4%
Western PiedmontPrecambrian phyllitic metamorphic rocks.High1521.2%
Mesozoic BasinsTriassic-Jurassic sedimentary and igneous rocks.Moderate1114.3%

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

Carroll County beside the ground around it

Carroll County has more high-potential ground than Maryland as a whole: 85.7% of its land against 36.7%. Of the 5 counties that border it, 2 have more high-potential ground, York County, PA the most at 99.2%.

Share of land by USGS radon potential: Carroll County, the 5 counties bordering it and Maryland
AreaEPA zoneLow potentialModerate potentialHigh potential
Carroll CountyZone 1None14.3%85.7%
Frederick CountyZone 133.5%19.1%47.4%
Baltimore CountyZone 1None16.1%81.1%
Howard CountyZone 1None8.8%91.2%
Adams County, PAZone 127.7%None72.3%
York County, PAZone 10.7%None99.2%
Maryland24 counties, pooled by land areaVaries29.9%27.6%36.7%

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

Local help

Carroll County, Local resources

Radon testing and mitigation in Carroll County

What Maryland's program says
Phone
1-866-703-3266

What it means

Carroll County, Questions

Questions about radon in Carroll County

Are radon levels high in Carroll County?

Over the 2008 to 2017 laboratory totals, 41.9% of the radon tests reported to CDC for Carroll County, Maryland were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.8 pCi/L. Of 19 Maryland counties with 30 or more tests for the same period, 2 reported a higher share; pooled, the state comes to 23.9%.

Which counties border Carroll County?

Carroll County borders 5 counties: Frederick County, MD, Baltimore County, MD, Howard County, MD, Adams County, PA and York County, PA.

Products

Carroll County, Products

Testing a home in Carroll County

In Carroll County, 41.9% of 3,334 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 23.9% pooled across Maryland. In Zone 1 the year-round level in Carroll 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

Carroll County is in EPA Zone 1, the highest predicted radon potential. Best radon monitors for Zone 1 counties like Carroll County

Explore

Carroll County, Bordering counties

Counties bordering Carroll County

Calculated by Radon411

Carroll County, Cities and towns

Cities and towns in Carroll County

The Census Bureau estimates 176,677 residents in Carroll County for July 1, 2025, 3,754 more than in 2020 (+2.2%). That works out to 395 people per square mile of land. No town dominates: Westminster, the largest, has 20,589 residents, 11.7% of the total. Testing is reported here more often than across the state: CDC's 3,334 reported tests for 2008 to 2017 come to 18.9 for every 1,000 residents (2025 estimate), against 7.7 for Maryland.

From Westminster to Union Bridge, CDC publishes no figure by town: each reads Carroll County's.

The cities and towns in Carroll County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Westminster20,589+2.2%11.7%
Mount Airyextends beyond Carroll County9,857+2.2%Counted whole
Taneytown8,259+13.6%4.7%
Hampstead6,281+0.7%3.6%
Manchester5,426+0.4%3.1%
Sykesville4,602+6.4%2.6%
New Windsor1,749+20.5%1.0%
Union Bridge941+0.3%0.5%

8 incorporated places, 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). A place that extends beyond Carroll County is counted whole, so it has no share.

Carroll County, Homes

Homes in Carroll County

Carroll County is a place of houses on their own lots: 77% of its 66,167 homes are detached single-family houses, against 51% across Maryland. Half of its homes were built before 1983, close to Maryland's 1979: 18% date from before 1960 and 20% from 2000 or later. Owners live in 84% of the occupied homes (Maryland: 67%), so for most households a test and a fix are their own to order.

Homes in Carroll County, Maryland, 2019 to 2023
HomesCarroll CountyMaryland
Housing units66,1672,545,532
Detached houses77%51%
In buildings of 5 or more7%23%
Mobile homes1%1%
Built before 196018%27%
Built since 200020%20%
Middle year built19831979
Owner-occupied84%67%

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