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Radon Levels in Howard County, Maryland

EPA puts Howard County in Zone 1, its highest radon zone, and 30.8% of its 4,549 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

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  • Share at 4+ pCi/L

    30.8%

    6.9 pts above MD

    Howard
    30.8%
    Maryland
    23.9%

    4,549 tests, 2008 to 2017

  • Median result

    2.0pCi/L

    Below action level

    Howard
    2.0 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    4,549

    4th of 19 by tests

    Howard
    4,549
    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

    6 of 19

    Middle half

    Howard
    30.8%
    Washington
    56.0%

    By share, of 19 with 30+ tests

Read a result for Howard County

Starts at Howard County's median, 2.0 pCi/L

2.0pCi/L

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

EPA’s reading of 2.0 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. Howard County's median reported result, 2008 to 2017.

Interpret this readingWhat a fix costs

The data

Howard County, At a glance

Howard County at a glance

Predicted: EPA radon zone

EPA Zone 1: highest predicted potential

EPA puts all of Howard 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 91% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Howard County, 2008 to 2017, laboratory
Under 2 pCi/L: 50%2 to 4 pCi/L: 19%4 pCi/L and above: 31%

Of 4,549 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Howard County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L2,26049.7%
2 to 4 pCi/L88819.5%
4 pCi/L and above1,40130.8%
What these figures count

These are the 4,549 tests reported to CDC for Howard 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

Howard County, Compared

Howard County radon figures, compared

Calculated by Radon411

In Howard County, about 3 in 10 reported tests over the 2008 to 2017 laboratory totals (1,401 of 4,549) were at or above EPA's action level. Against the national pool of 35.7%, the gap is 4.9 points lower. Across 24 Maryland counties pooled together, 23.9% of tests reached that level, so Howard County runs 6.9 points above its state.

Of 6 neighbours with a figure, 1 reported a larger share at or above 4 pCi/L and 5 a smaller one; the spread is 17.4% (Anne Arundel County) to 41.9% (Carroll County). By share, Howard County is 6th of 19 Maryland counties with 30 or more tests for the same period.

Half the tests came in under 2.0 pCi/L and half over it; the single highest reached 172.3 pCi/L. Below the action level, 2,260 tests (49.7%) came in under 2 pCi/L, against 56.6% statewide, and 888 tests (19.5%) fell between 2 and 4, against 19.5% statewide.

Zone 1 on EPA's map (predicted average above 4 pCi/L) and a 30.8% share point the same way here. USGS geology rates 91% of the county's land high radon potential. CDC also counts 418 tests taken after mitigation in Howard County for the same period: tests in homes where a fix was already in.

CDC's ten-year totals for Howard County went from 3,185 tests (2005 to 2014) to 4,549 (2008 to 2017), the share going from 32.3% to 30.8%.

Howard County against Maryland, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Howard County
Howard County4,54930.8%2.0 pCi/LThis county
Maryland, pooled24 counties, pooled47,94123.9%Not pooled−6.9 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+4.9 points
Washington Countyhighest in the state1,25956.0%5.2 pCi/L+25.2 points
Wicomico Countylowest in the state320.0%0.9 pCi/L−30.8 points
Carroll County, MDborders it3,33441.9%2.8 pCi/L+11.1 points
Frederick County, MDborders it3,93328.1%1.5 pCi/L−2.7 points
Baltimore County, MDborders it7,70619.1%1.4 pCi/L−11.7 points
Prince George's County, MDborders it1,60319.0%1.4 pCi/L−11.8 points
Montgomery County, MDborders it10,96918.0%1.3 pCi/L−12.8 points
Anne Arundel County, MDborders it4,08817.4%1.4 pCi/L−13.4 points
Where Howard County's reported results fell, 2008 to 2017, laboratory, beside Maryland and the United States
ResultHoward County testsHoward County shareMaryland shareU.S. shareAgainst Maryland
Under 2 pCi/L2,26049.7%56.6%39.6%−6.9 points
2 to 4 pCi/L88819.5%19.5%24.7%Level
4 pCi/L and above1,40130.8%23.9%35.7%+6.9 points
Howard 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
Howard County ten-year laboratory totals, against Maryland's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestMaryland, pooledChange from the row before
2008 to 20174,54930.8%2.0 pCi/L172.3 pCi/L23.9%−0.9 points
2007 to 20164,13131.7%2.0 pCi/L172.3 pCi/L24.4%−0.5 points
2006 to 20153,56932.2%2.1 pCi/L172.3 pCi/L24.9%−0.1 points
2005 to 20143,18532.3%2.1 pCi/L172.3 pCi/L24.6%First on file

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

Howard County, Geology

The ground under Howard County

Predicted

Most of the ground under Howard County is one class on USGS's geologic map: 91.2% of its land is high radon potential, with 8.8% moderate. The map puts 87.8% of Howard County in the area it calls Eastern Piedmont and 8.8% in Western Shore, with one smaller area beside them. USGS gives the first a radon index of 12, in the high band (12 to 17), and the second 10, in the moderate band (9 to 11).

USGS's geologic areas under Howard County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Eastern PiedmontPrecambrian igneous and metamorphic rocks (schist and gneiss).High1287.8%
Western ShoreCretaceous, Quaternary, minor Tertiary gravel, sand, silt, clay.Moderate108.8%
Western PiedmontPrecambrian phyllitic metamorphic rocks.High153.4%

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

Howard County beside the ground around it

Howard County has more high-potential ground than Maryland as a whole: 91.2% of its land against 36.7%. Of the 6 counties that border it, none has as large a share of high-potential ground; Carroll County comes nearest at 85.7%.

Share of land by USGS radon potential: Howard County, the 6 counties bordering it and Maryland
AreaEPA zoneLow potentialModerate potentialHigh potential
Howard CountyZone 1None8.8%91.2%
Montgomery CountyZone 1None19.8%80.2%
Baltimore CountyZone 1None16.1%81.1%
Carroll CountyZone 1None14.3%85.7%
Anne Arundel CountyZone 2None56.8%34.1%
Prince George's CountyZone 20.1%58.7%38.8%
Frederick CountyZone 133.5%19.1%47.4%
Maryland24 counties, pooled by land areaVaries29.9%27.6%36.7%

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

Local help

Howard County, Local resources

Radon testing and mitigation in Howard County

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

What it means

Howard County, Questions

Questions about radon in Howard County

Are radon levels high in Howard County?

Over the 2008 to 2017 laboratory totals, 30.8% of the radon tests reported to CDC for Howard County, Maryland were at or above 4 pCi/L, and the median reported pre-mitigation result was 2.0 pCi/L. Of 19 Maryland counties with 30 or more tests for the same period, 5 reported a higher share; pooled, the state comes to 23.9%.

Which counties border Howard County?

Howard County borders 6 counties: Montgomery County, Baltimore County, Carroll County, Anne Arundel County, Prince George's County and Frederick County.

Products

Howard County, Products

Testing a home in Howard County

In Howard County, 30.8% of 4,549 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 Howard 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

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

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Howard County, Bordering counties

Counties bordering Howard County

Calculated by Radon411

Howard County, Residents

Who lives in Howard County

The Census Bureau estimates 339,183 residents in Howard County for July 1, 2025, 6,374 more than in 2020 (+1.9%). Packed into its land, that is 1,352 people per square mile. Testing is reported here more often than across the state: CDC's 4,549 reported tests for 2008 to 2017 come to 13.4 for every 1,000 residents (2025 estimate), against 7.7 for Maryland.

Census lists no incorporated place in Howard County: every home in it reads the county's figures.

Residents: U.S. Census Bureau population estimates for July 1, 2025 (Vintage 2025).

Howard County, Homes

Homes in Howard County

Fewer than six in ten of Howard County's 124,489 homes are detached houses (51%, against 51% across Maryland); the rest are attached, in shared buildings or mobile. Much of Howard County is recent building: 28% of its homes have gone up since 2000 and the middle year built is 1989, against 1979 for Maryland. 72% 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 in Howard County, Maryland, 2019 to 2023
HomesHoward CountyMaryland
Housing units124,4892,545,532
Detached houses51%51%
In buildings of 5 or more25%23%
Mobile homes1%1%
Built before 19606%27%
Built since 200028%20%
Middle year built19891979
Owner-occupied72%67%

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