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Radon Levels in Marin County, California

EPA puts Marin County in Zone 3, its lowest radon zone, and most reported tests there come back low, though 6.3% of 95 (2008 to 2017, laboratory totals) 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

    6.3%

    4.0 pts below CA

    Marin
    6.3%
    California
    10.3%

    95 tests, 2008 to 2017

  • Median result

    0.8pCi/L

    Below action level

    Marin
    0.8 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    95

    21st of 33 by tests

    Marin
    95
    Los Angeles
    2,363

    Reported to CDC before any fix, 2008 to 2017

  • 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 California

    26 of 33

    Lowest quarter

    Marin
    6.3%
    El Dorado
    30.3%

    By share, of 33 with 30+ tests

Read a result for Marin County

Starts at Marin County's median, 0.8 pCi/L

0.8pCi/L

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

EPA’s reading of 0.8 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. Marin County's median reported result, 2008 to 2017.

Interpret this reading

The data

Marin County, At a glance

Marin County at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for Marin County 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 96% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Marin County, 2008 to 2017, laboratory
Under 2 pCi/L: 77%2 to 4 pCi/L: 17%4 pCi/L and above: 6%

Of 95 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Marin County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L7376.8%
2 to 4 pCi/L1616.8%
4 pCi/L and above66.3%
What these figures count

These are the 95 tests reported to CDC for Marin 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

Marin County, Compared

Marin County radon figures, compared

Calculated by Radon411

Marin County reports fewer high tests than California overall. 6.3% of its 95 tests over the 2008 to 2017 laboratory totals were at or above 4 pCi/L. That is 4.0 points below the 10.3% pooled from 56 California counties.

Ordered from highest share down, it comes 26th of 33 California counties with 30 or more tests for the same period. Nationally, every county with a figure for the same period pools to 35.7%; Marin County sits 29.4 points below it.

Sonoma County, the one bordering county with a figure for the same period, reported 0.8%. Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 6.3% share point the same way here.

Half the tests came in under 0.8 pCi/L and half over it; the single highest reached 8.1 pCi/L. The rest split 76.8% under 2 pCi/L and 16.8% from 2 to 4 (California: 76.8% and 12.9%).

CDC's ten-year totals for Marin County went from 111 tests (2005 to 2014) to 95 (2008 to 2017), the share going from 11.7% to 6.3%.

Marin County against California, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Marin County
Marin County956.3%0.8 pCi/LThis county
California, pooled56 counties, pooled9,41510.3%Not pooled+4.0 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+29.4 points
El Dorado Countyhighest in the state27430.3%1.6 pCi/L+24.0 points
Riverside Countylowest in the state2261.3%0.6 pCi/L−5.0 points
Sonoma County, CAborders it1310.8%0.5 pCi/L−5.5 points
Where Marin County's reported results fell, 2008 to 2017, laboratory, beside California and the United States
ResultMarin County testsMarin County shareCalifornia shareU.S. shareAgainst California
Under 2 pCi/L7376.8%76.8%39.6%Level
2 to 4 pCi/L1616.8%12.9%24.7%+3.9 points
4 pCi/L and above66.3%10.3%35.7%−4.0 points
Marin County ranked among the 33 California counties with 30 or more tests, by share at or above 4 pCi/L, 2008 to 2017, laboratory
CountyRankTestsAt or above 4 pCi/LMedian
San Mateo County21 of 334689.2%0.9 pCi/L
Contra Costa County22 of 332858.8%0.9 pCi/L
Sacramento County23 of 331947.7%0.9 pCi/L
Placer County24 of 332837.4%1.1 pCi/L
Alameda County25 of 334786.7%0.7 pCi/L
Marin County26 of 33956.3%0.8 pCi/L
San Bernardino County27 of 331784.5%0.7 pCi/L
Orange County28 of 334313.9%0.7 pCi/L
San Francisco County29 of 332462.8%0.5 pCi/L
San Diego County30 of 336922.6%0.2 pCi/L
Yolo County31 of 33482.1%0.9 pCi/L
Marin County ten-year laboratory totals, against California's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestCalifornia, pooledChange from the row before
2008 to 2017956.3%0.8 pCi/L8.1 pCi/L10.3%−0.2 points
2007 to 2016936.5%0.9 pCi/L8.1 pCi/L10.6%−2.2 points
2006 to 2015928.7%1.1 pCi/L8.9 pCi/L10.6%−3.0 points
2005 to 201411111.7%1.0 pCi/L11.7 pCi/L10.0%First on file

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

Marin County, Geology

The ground under Marin County

Predicted

USGS's geologic map puts all of Marin County's mapped land in one class: low radon potential. 96.2% of it lies in the area the map calls Northern Coast Ranges, to which USGS gives a radon index of 6, in the low band (5 to 8).

USGS's geologic areas under Marin County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Northern Coast RangesMetamorphosed marine sedimentary rocks and ultramafic rocks.Low696.2%

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

Marin County beside the ground around it

Across California 0.1% of the land is rated high; none of Marin County's is. None of the one county that borders it has any moderate-potential ground either.

Share of land by USGS radon potential: Marin County, the county bordering it and California
AreaEPA zoneLow potentialModerate potentialHigh potential
Marin CountyZone 396.2%NoneNone
Sonoma CountyZone 399.9%NoneNone
California58 counties, pooled by land areaVaries57.2%42.6%0.1%

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

Local help

Marin County, Local resources

Radon testing and mitigation in Marin County

What California's program says
Phone
1-800-745-7236
Email
Radonprogram@cdph.ca.gov
Licensing of radon professionals
The Radon Program certifies all radon service providers operating in California.

What it means

Marin County, Questions

Questions about radon in Marin County

Are radon levels high in Marin County?

Over the 2008 to 2017 laboratory totals, 6.3% of the radon tests reported to CDC for Marin County, California were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.8 pCi/L. Of 33 California counties with 30 or more tests for the same period, 25 reported a higher share; pooled, the state comes to 10.3%.

Which counties border Marin County?

Marin County borders 1 county: Sonoma County.

Products

Marin County, Products

Testing a home in Marin County

In Marin County, 6.3% of 95 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 10.3% pooled across California. 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 Marin County.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Marin County homes, compared

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Marin County, Bordering counties

Counties bordering Marin County

Calculated by Radon411

Marin County, Cities and towns

Cities and towns in Marin County

About 253,694 people live in Marin County by the Census Bureau's estimate for July 1, 2025, 7,643 fewer than in 2020 (−2.9%). That works out to 487 people per square mile of land. Its largest town, San Rafael, has 59,410 residents, 23.4% of the total. Together its 11 incorporated places hold 74.1% of those residents; the other 25.9% live outside any of them. Testing is reported here more often than across the state: CDC's 95 reported tests for 2008 to 2017 come to 0.4 for every 1,000 residents (2025 estimate), against 0.2 for California.

From San Rafael to Belvedere, CDC publishes no figure by town: each reads Marin County's.

The cities and towns in Marin County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
San Rafael59,410−2.8%23.4%
Novato51,947−2.3%20.5%
Mill Valley13,799−2.9%5.4%
Larkspur12,620−3.2%5.0%
San Anselmo12,555−2.0%4.9%
Corte Madera9,929−2.7%3.9%
Tiburon8,881−2.9%3.5%
Fairfax7,417−2.3%2.9%
Sausalito7,019−3.3%2.8%
Ross2,304−1.3%0.9%
Belvedere2,066−2.9%0.8%

11 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).

Marin County, Homes

Homes in Marin County

Of Marin County's 111,860 homes, 63% are detached houses, against 57% across California; 19% are in buildings of five or more homes. The homes are older than California's as a whole: half were built before 1966 (1976 for California), and 38% date from before 1960. 64% of occupied homes are lived in by their owner (California: 56%); in the rest a tenant can run a test, but a fix is the landlord's to make.

Homes in Marin County, California, 2019 to 2023
HomesMarin CountyCalifornia
Housing units111,86014,532,683
Detached houses63%57%
In buildings of 5 or more19%24%
Mobile homes2%4%
Built before 196038%27%
Built since 20007%18%
Middle year built19661976
Owner-occupied64%56%

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