Skip to content
RADON411

Locations and reports

Radon Levels in Contra Costa County, California

EPA puts Contra Costa County in Zone 2, its middle radon zone, and most reported tests there come back low, though 8.8% of 285 (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

How to test this homeFind a certified tester

  • Share at 4+ pCi/L

    8.8%

    1.5 pts below CA

    Contra Costa
    8.8%
    California
    10.3%

    285 tests, 2008 to 2017

  • Median result

    0.9pCi/L

    Below action level

    Contra Costa
    0.9 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    285

    9th of 33 by tests

    Contra Costa
    285
    Los Angeles
    2,363

    Reported to CDC before any fix, 2008 to 2017

  • EPA radon zone

    Zone 2

    Moderate potential

    1. 1
    2. Zone 2
    3. 3

    1 highest, 3 lowest

    EPA predicts 2 to 4 pCi/L indoors

  • Rank in California

    22 of 33

    Middle half

    Contra Costa
    8.8%
    El Dorado
    30.3%

    By share, of 33 with 30+ tests

Read a result for Contra Costa County

Starts at Contra Costa County's median, 0.9 pCi/L

0.9pCi/L

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

EPA’s reading of 0.9 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. Contra Costa County's median reported result, 2008 to 2017.

Interpret this reading

The data

Contra Costa County, At a glance

Contra Costa County at a glance

Predicted: EPA radon zone

EPA Zone 2: moderate predicted potential

Contra Costa County is a county: EPA's forecast for it as a whole is an average indoor of 2 to 4 pCi/L (75 to 150 Bq/m³). It describes no single address, and homes above EPA's are found in every zone, this one included.

USGS geologic potential: Moderate across 74% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

Radon tests reported for Contra Costa County, 2008 to 2017, laboratory

Reported results by concentration band, Contra Costa County, 2008 to 2017, laboratory
Under 2 pCi/L: 79%2 to 4 pCi/L: 12%4 pCi/L and above: 9%

Of 285 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Contra Costa County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L22679.3%
2 to 4 pCi/L3411.9%
4 pCi/L and above258.8%
What these figures count

These are the 285 tests reported to CDC for Contra Costa 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

Contra Costa County, Compared

Contra Costa County radon figures, compared

Calculated by Radon411

Contra Costa County reports fewer high tests than California overall. 8.8% of its 285 tests over the 2008 to 2017 laboratory totals were at or above 4 pCi/L. That is 1.5 points below the 10.3% pooled from 56 California counties.

Ordered from highest share down, it comes 22nd 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%; Contra Costa County sits 26.9 points below it.

Next door, the shares run from 6.7% in Alameda County to 15.8% in Solano County, with Sacramento County and Alameda County below Contra Costa County. The reported tests do not follow EPA's Zone 2 label for Contra Costa County (predicted average between 2 and 4 pCi/L): 8.8% of them reached 4 pCi/L, fewer than that label suggests.

Half the tests came in under 0.9 pCi/L and half over it; the single highest reached 17.6 pCi/L. The rest split 79.3% under 2 pCi/L and 11.9% from 2 to 4 (California: 76.8% and 12.9%).

CDC's ten-year totals for Contra Costa County went from 235 tests (2005 to 2014) to 285 (2008 to 2017), the share going from 5.5% to 8.8%.

Contra Costa County against California, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Contra Costa County
Contra Costa County2858.8%0.9 pCi/LThis county
California, pooled56 counties, pooled9,41510.3%Not pooled+1.5 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+26.9 points
El Dorado Countyhighest in the state27430.3%1.6 pCi/L+21.5 points
Sonoma Countylowest in the state1310.8%0.5 pCi/L−8.0 points
Solano County, CAborders it3815.8%0.9 pCi/L+7.0 points
San Joaquin County, CAborders it7414.9%1.4 pCi/L+6.1 points
Sacramento County, CAborders it1947.7%0.9 pCi/L−1.1 points
Alameda County, CAborders it4786.7%0.7 pCi/L−2.1 points
Where Contra Costa County's reported results fell, 2008 to 2017, laboratory, beside California and the United States
ResultContra Costa County testsContra Costa County shareCalifornia shareU.S. shareAgainst California
Under 2 pCi/L22679.3%76.8%39.6%+2.5 points
2 to 4 pCi/L3411.9%12.9%24.7%−1.0 points
4 pCi/L and above258.8%10.3%35.7%−1.5 points
Contra Costa 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
Fresno County17 of 3310011.0%1.5 pCi/L
Napa County18 of 335710.5%0.9 pCi/L
Los Angeles County19 of 332,3639.5%0.7 pCi/L
Santa Clara County20 of 337939.2%0.9 pCi/L
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
Contra Costa County ten-year laboratory totals, against California's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestCalifornia, pooledChange from the row before
2008 to 20172858.8%0.9 pCi/L17.6 pCi/L10.3%+1.3 points
2007 to 20162807.5%0.8 pCi/L17.6 pCi/L10.6%−0.2 points
2006 to 20152747.7%0.8 pCi/L17.6 pCi/L10.6%+2.2 points
2005 to 20142355.5%0.7 pCi/L17.6 pCi/L10.0%First on file

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

Contra Costa County, Geology

The ground under Contra Costa County

Predicted

USGS's geologic map splits Contra Costa County: 74.1% of its land is moderate radon potential and 21.8% low. The map puts 74.1% of Contra Costa County in the area it calls Southern Coast Ranges and 21.8% in Great Valley. USGS gives the first a radon index of 9, in the moderate band (9 to 11), and the second 8, in the low band (5 to 8).

USGS's geologic areas under Contra Costa County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Southern Coast RangesMetamorphosed marine sedimentary rocks, ultramafic rocks, Triasic metamorphic rocks, Mesozoic granitic rocks, and a series of fault-bounded linear basins in which Teriary marine and continental sedimentary rocks were deposited.Moderate974.1%
Great ValleyQuaternary alluvial gravel, sand, silt, clay.Low821.8%

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

Contra Costa County beside the ground around it

Across California 0.1% of the land is rated high; none of Contra Costa County's is. Of the 4 counties that border it, 1 has more moderate-potential ground, Alameda County the most at 98.5%.

Share of land by USGS radon potential: Contra Costa County, the 4 counties bordering it and California
AreaEPA zoneLow potentialModerate potentialHigh potential
Contra Costa CountyZone 221.8%74.1%None
Alameda CountyZone 20.8%98.5%None
Solano CountyZone 394.2%NoneNone
San Joaquin CountyZone 384.6%15.0%None
Sacramento CountyZone 378.0%20.3%None
California58 counties, pooled by land areaVaries57.2%42.6%0.1%

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

Local help

Contra Costa County, Local resources

Radon testing and mitigation in Contra Costa County

Listed radon businesses with an address in Contra Costa County:

Listed radon professionals by town: Walnut Creek

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

Contra Costa County, Questions

Questions about radon in Contra Costa County

Are radon levels high in Contra Costa County?

Over the 2008 to 2017 laboratory totals, 8.8% of the radon tests reported to CDC for Contra Costa County, California were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.9 pCi/L. Of 33 California counties with 30 or more tests for the same period, 21 reported a higher share; pooled, the state comes to 10.3%.

Which counties border Contra Costa County?

Contra Costa County borders 4 counties: Alameda County, Solano County, San Joaquin County and Sacramento County.

Products

Contra Costa County, Products

Testing a home in Contra Costa County

In Contra Costa County, 8.8% of 285 tests reported to CDC over the 2008 to 2017 laboratory totals reached 4 pCi/L, against 10.3% pooled across California. In Zone 2 a short-term kit (2 to 90 days) is the quick first check in Contra Costa County; a long-term test tells the year-round average.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Contra Costa County homes, compared

Explore

Contra Costa County, Bordering counties

Counties bordering Contra Costa County

Calculated by Radon411

Contra Costa County, Cities and towns

Cities and towns in Contra Costa County

Contra Costa County's population has barely moved since 2020: 1,170,070 residents on July 1, 2025 against 1,166,249 then, by the Census Bureau's estimates. Packed into its land, that is 1,632 people per square mile. No town dominates: Concord, the largest, has 123,261 residents, 10.5% of the total. Together its 19 incorporated places hold 84.9% of those residents; the other 15.1% live outside any of them. CDC's 285 reported tests for 2008 to 2017 work out to 0.2 per 1,000 residents (2025 estimate), close to California's 0.2.

From Concord to Clayton, CDC publishes no figure by town: each reads Contra Costa County's.

The 12 largest cities and towns in Contra Costa County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Concord123,261−1.6%10.5%
Antioch118,958+3.1%10.2%
Richmond114,861−1.1%9.8%
San Ramon86,209−0.3%7.4%
Pittsburg77,542+1.6%6.6%
Walnut Creek70,012−0.3%6.0%
Brentwood66,607+3.3%5.7%
Oakley47,583+9.4%4.1%
Danville43,302−0.6%3.7%
Martinez36,849−0.9%3.1%
Pleasant Hill34,017−1.6%2.9%
San Pablo31,678−1.4%2.7%
  • Hercules
  • El Cerrito
  • Lafayette
  • Orinda
  • Pinole
  • Moraga
  • Clayton

19 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).

Contra Costa County, Homes

Homes in Contra Costa County

Of Contra Costa County's 426,585 homes, 67% are detached houses, against 57% across California; 17% are in buildings of five or more homes. Half of its homes were built before 1978, close to California's 1976: 23% date from before 1960 and 18% from 2000 or later. 68% 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 Contra Costa County, California, 2019 to 2023
HomesContra Costa CountyCalifornia
Housing units426,58514,532,683
Detached houses67%57%
In buildings of 5 or more17%24%
Mobile homes2%4%
Built before 196023%27%
Built since 200018%18%
Middle year built19781976
Owner-occupied68%56%

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