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Radon Levels in St. Mary Parish, Louisiana

EPA puts St. Mary Parish in Zone 3, its lowest radon zone, and most reported tests there come back low, though 0.0% of 54 (2018) reached 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

    0.0%

    0.5 pts below LA

    St. Mary Parish
    0.0%
    Louisiana
    0.5%

    54 tests, 2018

  • Median result

    0.4pCi/L

    Below action level

    St. Mary Parish
    0.4 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2018

  • Tests reported

    54

    12th of 12 by tests

    St. Mary Parish
    54
    Jefferson Parish
    504

    Reported to CDC before any fix, 2018

  • EPA radon zone

    Zone 3

    Lower potential

    1. 1
    2. 2
    3. Zone 3

    1 highest, 3 lowest

    EPA predicts under 2 pCi/L indoors

  • Rank in Louisiana

    2 of 12

    Top quarter

    St. Mary Parish
    0.0%
    Lafayette Parish
    8.0%

    By share, of 12 with 30+ tests

Read a result for St. Mary Parish

Starts at St. Mary Parish's median, 0.4 pCi/L

0.4pCi/L

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

EPA’s reading of 0.4 pCi/L

Below 2 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 below 2 pCi/L, the level at which EPA says a fix should be considered. St. Mary Parish's median reported result, 2018.

Interpret this reading

The data

St. Mary Parish, At a glance

St. Mary Parish at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for St. Mary Parish as a whole is an average indoor of less than 2 pCi/L (less than 75 Bq/m³). A low zone clears no address in it: homes above EPA's turn up in all three zones.

USGS geologic potential: Low across 93% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for St. Mary Parish, 2018

Share of reported tests at or above 4 pCi/L, by year, St. Mary Parish
Share of reported tests at or above 4 pCi/L, by year, St. Mary Parish
YearShare of reported tests at or above 4 pCi/L
20160.0%
20180.0%

2018: 0.0%. Across these years the share ran from 0.0% to 0.0%. Each column counts radon tests reported to CDC that year, not homes.

What these figures count

These are the 54 tests reported to CDC for St. Mary Parish over 2018, many from homes being sold or already suspected of a problem: they describe the parish's reported tests, not this house.

Measured: CDC tracking data. Figure above: 2018. Measured data period: 2016 to 2018 (every CDC figure on file).

How to read these figures

St. Mary Parish, Compared

St. Mary Parish radon figures, compared

Calculated by Radon411

Only 0 of 54 radon tests reported for St. Mary Parish in 2018 reached 4 pCi/L (0.0%). The median result, 0.4 pCi/L, sits below 2, well under EPA's 4 pCi/L action level, and the top reported result was 0.6 pCi/L.

Across 28 Louisiana counties pooled together, 0.5% of tests reached that level, so St. Mary Parish runs 0.5 points below its state. Against the national pool of 29.1%, the gap is 29.1 points lower. Its place in the state: 2nd of 12 Louisiana counties with 30 or more tests for the same period.

Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 0.0% share point the same way here.

Below the action level, 54 tests (100.0%) came in under 2 pCi/L, against 97.2% statewide, and 0 tests (0.0%) fell between 2 and 4, against 2.3% statewide. USGS geology rates 93% of the county's land low radon potential.

The busiest year on file was 2018, with 54 tests reported; the quietest, 2016, had 3.

St. Mary Parish against Louisiana, the United States and nearby counties, 2018
AreaTestsAt or above 4 pCi/LMedianDifference from St. Mary Parish
St. Mary Parish540.0%0.4 pCi/LThis county
Louisiana, pooled28 counties, pooled2,3430.5%Not pooled+0.5 points
United States, pooled773 counties, pooled389,55129.1%Not pooled+29.1 points
Lafayette Parishhighest in the state758.0%1.2 pCi/L+8.0 points
Ascension Parishlowest in the state660.0%0.5 pCi/LLevel
Terrebonne Parish, LAborders it30.0%1.2 pCi/LLevel
Where St. Mary Parish's reported results fell, 2018, beside Louisiana and the United States
ResultSt. Mary Parish testsSt. Mary Parish shareLouisiana shareU.S. shareAgainst Louisiana
Under 2 pCi/L54100.0%97.2%48.3%+2.8 points
2 to 4 pCi/L00.0%2.3%22.6%−2.3 points
4 pCi/L and above00.0%0.5%29.1%−0.5 points
St. Mary Parish ranked among the 12 Louisiana counties with 30 or more tests, by share at or above 4 pCi/L, 2018
CountyRankTestsAt or above 4 pCi/LMedian
Jefferson Parish2 of 125040.0%0.4 pCi/L
East Baton Rouge Parish2 of 125010.0%0.2 pCi/L
Orleans Parish2 of 123510.0%0.6 pCi/L
Calcasieu Parish2 of 121890.0%0.6 pCi/L
Caddo Parish2 of 121110.0%0.2 pCi/L
Rapides Parish2 of 121110.0%0.2 pCi/L
Red River Parish2 of 121020.0%0.2 pCi/L
Ouachita Parish2 of 12960.0%0.6 pCi/L
De Soto Parish2 of 12840.0%0.2 pCi/L
Ascension Parish2 of 12660.0%0.5 pCi/L
St. Mary Parish2 of 12540.0%0.4 pCi/L
St. Mary Parish radon tests by year, against Louisiana's pooled share
YearTestsAt or above 4 pCi/LMedianHighestLouisiana, pooledChange from the row before
2018540.0%0.4 pCi/L0.6 pCi/L0.5%Level
201630.0%0.8 pCi/L0.8 pCi/L2.0%First on file

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

St. Mary Parish, Geology

The ground under St. Mary Parish

Predicted

USGS's geologic map puts all of St. Mary Parish's mapped land in one class: low radon potential. The map puts 87.8% of St. Mary Parish in the area it calls Delta and 4.8% in Terraces and Prairies. USGS gives the first a radon index of 6 and the second 8, both in the low band (5 to 8).

USGS's geologic areas under St. Mary Parish
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
DeltaAlluvium and marsh.Low687.8%
Terraces and PrairiesClay, sandy clay, silt, sand, and some gravel.Low84.8%

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

St. Mary Parish beside the ground around it

Neither St. Mary Parish nor Louisiana as a whole has any land USGS rates above low potential. None of the 4 parishes that border it has any moderate-potential ground either.

Share of land by USGS radon potential: St. Mary Parish, the 4 parishes bordering it and Louisiana
AreaEPA zoneLow potentialModerate potentialHigh potential
St. Mary ParishZone 392.6%NoneNone
Assumption ParishZone 3100%NoneNone
Iberia ParishZone 382.2%NoneNone
Terrebonne ParishZone 382.9%NoneNone
St. Martin ParishZone 3100%NoneNone
Louisiana64 parishes, pooled by land areaVaries98.4%NoneNone

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

Local help

St. Mary Parish, Local resources

Radon testing and mitigation in St. Mary Parish

What it means

St. Mary Parish, Questions

Questions about radon in St. Mary Parish

Are radon levels high in St. Mary Parish?

In 2018, 0.0% of the radon tests reported to CDC for St. Mary Parish, Louisiana were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.4 pCi/L. Of 12 Louisiana counties with 30 or more tests for the same period, 1 reported a higher share; pooled, the state comes to 0.5%.

Which counties border St. Mary Parish?

St. Mary Parish borders 4 parishes: Assumption Parish, Iberia Parish, Terrebonne Parish and St. Martin Parish.

Products

St. Mary Parish, Products

Testing a home in St. Mary Parish

In St. Mary Parish, 0.0% of 54 tests reported to CDC in 2018 reached 4 pCi/L, against 0.5% pooled across Louisiana. Zone 3 is the lowest label, yet homes at or above 4 pCi/L occur in every zone: a short-term kit (2 to 90 days) is the quick first check in St. Mary Parish.

Test kits and monitors3

See every kit and monitor

Best radon monitors for St. Mary Parish homes, compared

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St. Mary Parish, Bordering parishes

Parishes bordering St. Mary Parish

Calculated by Radon411

St. Mary Parish, Cities and towns

Cities and towns in St. Mary Parish

St. Mary Parish has been losing residents: from 49,149 in 2020 to 46,552 on July 1, 2025, a fall of 5.3% by the Census Bureau's estimates. That works out to 84 people per square mile of land. Morgan City, the largest place on its list (10,757 residents in all), extends beyond St. Mary Parish, so no share of the total is worked out for it. Testing is reported here more often than across the state: CDC's 54 reported tests for 2018 come to 1.2 for every 1,000 residents (2025 estimate), against 0.5 for Louisiana.

From Morgan City to Baldwin, CDC publishes no figure by town: each reads St. Mary Parish's.

The cities and towns in St. Mary Parish by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Morgan Cityextends beyond St. Mary Parish10,757−5.7%Counted whole
Franklin6,224−6.5%13.4%
Patterson5,565−5.7%12.0%
Berwick4,455−6.1%9.6%
Baldwin1,676−4.1%3.6%

5 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 St. Mary Parish is counted whole, so it has no share.

St. Mary Parish, Homes

Homes in St. Mary Parish

Of St. Mary Parish's 22,510 homes, 69% are detached houses, against 65% across Louisiana; 19% are mobile homes. Half of its homes were built before 1976, close to Louisiana's 1982: 24% date from before 1960 and 19% from 2000 or later. 70% of occupied homes are lived in by their owner (Louisiana: 67%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in St. Mary Parish, Louisiana, 2019 to 2023
HomesSt. Mary ParishLouisiana
Housing units22,5102,094,002
Detached houses69%65%
In buildings of 5 or more5%11%
Mobile homes19%13%
Built before 196024%19%
Built since 200019%27%
Middle year built19761982
Owner-occupied70%67%

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