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Radon Levels in Orange County, California

EPA puts Orange County in Zone 3, its lowest radon zone, and most reported tests there come back low, though 3.9% of 431 (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

    3.9%

    6.4 pts below CA

    Orange
    3.9%
    California
    10.3%

    431 tests, 2008 to 2017

  • Median result

    0.7pCi/L

    Below action level

    Orange
    0.7 pCi/L
    EPA
    4.0 pCi/L

    Midpoint of the tests, 2008 to 2017

  • Tests reported

    431

    6th of 33 by tests

    Orange
    431
    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

    28 of 33

    Lowest quarter

    Orange
    3.9%
    El Dorado
    30.3%

    By share, of 33 with 30+ tests

Read a result for Orange County

Starts at Orange County's median, 0.7 pCi/L

0.7pCi/L

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

EPA’s reading of 0.7 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. Orange County's median reported result, 2008 to 2017.

Interpret this reading

The data

Orange County, At a glance

Orange County at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for Orange 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 100% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

Reported results by concentration band, Orange County, 2008 to 2017, laboratory
Under 2 pCi/L: 85%2 to 4 pCi/L: 11%4 pCi/L and above: 4%

Of 431 pre-mitigation tests reported to CDC, not homes in this area.

Reported results by concentration band, Orange County, 2008 to 2017, laboratory: data table
Result bandTests MeasuredShare Measured
Under 2 pCi/L36584.7%
2 to 4 pCi/L4911.4%
4 pCi/L and above173.9%
What these figures count

These are the 431 tests reported to CDC for Orange 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

Orange County, Compared

Orange County radon figures, compared

Calculated by Radon411

Only 17 of 431 radon tests reported for Orange County over the 2008 to 2017 laboratory totals reached 4 pCi/L (3.9%). The median result, 0.7 pCi/L, sits below 2, well under EPA's 4 pCi/L action level, and the top reported result was 62.5 pCi/L.

Across 56 California counties pooled together, 10.3% of tests reached that level, so Orange County runs 6.4 points below its state. Against the national pool of 35.7%, the gap is 31.8 points lower. Its place in the state: 28th of 33 California counties with 30 or more tests for the same period.

Zone 3 on EPA's map (predicted average under 2 pCi/L) and a 3.9% share point the same way here. Of 4 neighbours with a figure, 2 reported a larger share at or above 4 pCi/L and 2 a smaller one; the spread is 1.3% (Riverside County) to 9.5% (Los Angeles County).

Below the action level, 365 tests (84.7%) came in under 2 pCi/L, against 76.8% statewide, and 49 tests (11.4%) fell between 2 and 4, against 12.9% statewide. USGS geology rates 100% of the county's land low radon potential.

CDC's ten-year totals for Orange County went from 521 tests (2005 to 2014) to 431 (2008 to 2017), the share going from 5.0% to 3.9%.

Orange County against California, the United States and nearby counties, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedianDifference from Orange County
Orange County4313.9%0.7 pCi/LThis county
California, pooled56 counties, pooled9,41510.3%Not pooled+6.4 points
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled+31.8 points
El Dorado Countyhighest in the state27430.3%1.6 pCi/L+26.4 points
Sonoma Countylowest in the state1310.8%0.5 pCi/L−3.1 points
Los Angeles County, CAborders it2,3639.5%0.7 pCi/L+5.6 points
San Bernardino County, CAborders it1784.5%0.7 pCi/L+0.6 points
San Diego County, CAborders it6922.6%0.2 pCi/L−1.3 points
Riverside County, CAborders it2261.3%0.6 pCi/L−2.6 points
Where Orange County's reported results fell, 2008 to 2017, laboratory, beside California and the United States
ResultOrange County testsOrange County shareCalifornia shareU.S. shareAgainst California
Under 2 pCi/L36584.7%76.8%39.6%+7.9 points
2 to 4 pCi/L4911.4%12.9%24.7%−1.5 points
4 pCi/L and above173.9%10.3%35.7%−6.4 points
Orange 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
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
Riverside County32 of 332261.3%0.6 pCi/L
Sonoma County33 of 331310.8%0.5 pCi/L
Orange County ten-year laboratory totals, against California's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestCalifornia, pooledChange from the row before
2008 to 20174313.9%0.7 pCi/L62.5 pCi/L10.3%−0.9 points
2007 to 20165224.8%0.8 pCi/L62.5 pCi/L10.6%+0.1 points
2006 to 20155304.7%0.8 pCi/L62.5 pCi/L10.6%−0.3 points
2005 to 20145215.0%0.8 pCi/L62.5 pCi/L10.0%First on file

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

Orange County, Geology

The ground under Orange County

Predicted

USGS's geologic map puts all of Orange County's mapped land in one class: low radon potential. All of it lies in the area the map calls Peninsular Ranges, to which USGS gives a radon index of 7, in the low band (5 to 8).

USGS's geologic areas under Orange County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Peninsular RangesGranite, metasedimentary and metavolcanic rocks, marine sedimentary rocks, and coastal sediments.Low799.5%

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

Orange County beside the ground around it

Across California 0.1% of the land is rated high; none of Orange County's is. Of the 4 counties that border it, 3 have more moderate-potential ground, Los Angeles County the most at 44.5%.

Share of land by USGS radon potential: Orange County, the 4 counties bordering it and California
AreaEPA zoneLow potentialModerate potentialHigh potential
Orange CountyZone 399.5%NoneNone
Los Angeles CountyZone 254.8%44.5%None
San Diego CountyZone 399.5%NoneNone
Riverside CountyZone 295.8%4.2%None
San Bernardino CountyZone 284.3%15.7%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

Orange County, Local resources

Radon testing and mitigation in Orange 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

Orange County, Questions

Questions about radon in Orange County

Are radon levels high in Orange County?

Over the 2008 to 2017 laboratory totals, 3.9% of the radon tests reported to CDC for Orange County, California were at or above 4 pCi/L, and the median reported pre-mitigation result was 0.7 pCi/L. Of 33 California counties with 30 or more tests for the same period, 27 reported a higher share; pooled, the state comes to 10.3%.

Which counties border Orange County?

Orange County borders 4 counties: Los Angeles County, San Diego County, Riverside County and San Bernardino County.

Products

Orange County, Products

Testing a home in Orange County

In Orange County, 3.9% of 431 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 Orange County.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Orange County homes, compared

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Orange County, Bordering counties

Counties bordering Orange County

Calculated by Radon411

Orange County, Cities and towns

Cities and towns in Orange County

About 3,149,507 people live in Orange County by the Census Bureau's estimate for July 1, 2025, 36,660 fewer than in 2020 (−1.2%). Packed into its land, that is 3,972 people per square mile. No town dominates: Anaheim, the largest, has 341,008 residents, 10.8% of the total. Together its 34 incorporated places hold 95.8% of those residents; the other 4.2% live outside any of them. Reported testing is thin here: 431 tests over 2008 to 2017, or 0.1 for every 1,000 residents (2025 estimate), where California as a whole comes to 0.2.

From Anaheim to Villa Park, CDC publishes no figure by town: each reads Orange County's.

The 12 largest cities and towns in Orange County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Anaheim341,008−1.6%10.8%
Irvine318,764+3.4%10.1%
Santa Ana315,586+1.4%10.0%
Huntington Beach191,451−3.5%6.1%
Garden Grove170,455−0.8%5.4%
Fullerton138,675−3.3%4.4%
Orange138,365−0.8%4.4%
Costa Mesa108,881−2.8%3.5%
Mission Viejo90,484−3.2%2.9%
Westminster88,966−2.0%2.8%
Lake Forest86,333+0.5%2.7%
Newport Beach81,977−3.7%2.6%
  • Buena Park
  • Tustin
  • Yorba Linda
  • Laguna Niguel
  • San Clemente
  • La Habra
  • Fountain Valley
  • Placentia
  • Aliso Viejo
  • Cypress
  • Brea
  • Rancho Santa Margarita
  • Stanton
  • San Juan Capistrano
  • Dana Point
  • Laguna Hills
  • Seal Beach
  • Laguna Beach
  • Laguna Woods
  • La Palma
  • Los Alamitos
  • Villa Park

34 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).

Orange County, Homes

Homes in Orange County

Fewer than six in ten of Orange County's 1,138,473 homes are detached houses (50%, against 57% across California); the rest are attached, in shared buildings or mobile. Half of its homes were built before 1977, close to California's 1976: 17% date from before 1960 and 17% from 2000 or later. 56% 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 Orange County, California, 2019 to 2023
HomesOrange CountyCalifornia
Housing units1,138,47314,532,683
Detached houses50%57%
In buildings of 5 or more26%24%
Mobile homes3%4%
Built before 196017%27%
Built since 200017%18%
Middle year built19771976
Owner-occupied56%56%

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