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Radon Levels in Humboldt County, California

EPA puts Humboldt County in Zone 3, its lowest radon zone, and only 20 radon tests from it are on file with CDC (2008 to 2017, laboratory totals). 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

  • Nearest county

    11.4%

    Butte County, 140 miles

    Butte
    11.4%
    California
    10.3%

    Share at 4+ pCi/L, 35 tests, 2008 to 2017

  • Ground above low

    None

    42.7 pts below California

    Humboldt
    0.0%
    California
    42.7%

    Share of land USGS rates moderate or high potential

  • Tests reported

    22

    Too few for a share

    Humboldt
    22
    Needed
    30

    2007 to 2016, laboratory

  • 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

Read a result for Humboldt County

Starts at EPA's action level

4.0pCi/L

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

EPA’s reading of 4.0 pCi/L

At or above 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 at or above 4 pCi/L. EPA recommends fixing a home when the radon level is 4 pCi/L or higher.

Interpret this readingWhat a fix costs

The data

Humboldt County, At a glance

Humboldt County at a glance

Predicted: EPA radon zone

EPA Zone 3: lower predicted potential

is EPA's lowest class, and its forecast for Humboldt 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 99% of the county's area

Predicted: EPA Map of Radon Zones

Measured: tests reported to CDC

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

CDC holds 20 reported tests for Humboldt County over the 2008 to 2017 laboratory totals, with a median of 1.3 pCi/L: too few to describe a county (Radon411 reads a share from 30 tests up). Humboldt County beside the counties around it is how this page reads it.

What these figures count

Only 20 tests were reported to CDC for Humboldt County over 2008 to 2017 (laboratory totals), and tests are often run on homes being sold or already suspected of a problem: so few describe neither the county nor 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

Humboldt County, Compared

Humboldt County radon figures, compared

Calculated by Radon411

CDC's largest figure for Humboldt County rests on 22 tests over the 2007 to 2016 laboratory totals, 4 of them at or above 4 pCi/L, so each test moves its share by 4.5 points and no rank or comparison is drawn from it. Results reached as high as 64.3 pCi/L, with a middle value of 1.1 pCi/L.

The nearest counties with a figure are Butte County (140 miles away) at 11.4%, Sonoma County (159 miles away) at 0.8% and Napa County (172 miles away) at 10.5%. Pooled across 56 California counties, 10.3% of reported tests reached that level; across every county in the country with a figure, 35.7%.

Statewide for the same period, El Dorado County reported the highest share (30.3%) and Sonoma County the lowest (0.8%). EPA's 1993 map puts Humboldt County in Zone 3, its lowest label: a predicted county average under 2 pCi/L. USGS geology rates 99% of the county's land low radon potential.

California, the United States and the counties around Humboldt County, 2008 to 2017, laboratory
AreaTestsAt or above 4 pCi/LMedian
California, pooled56 counties, pooled9,41510.3%Not pooled
United States, pooled2,661 counties, pooled2,013,54435.7%Not pooled
El Dorado Countyhighest in the state27430.3%1.6 pCi/L
Sonoma Countylowest in the state1310.8%0.5 pCi/L
Siskiyou County, CAborders it825.0%1.1 pCi/L
Del Norte County, CAborders it10.0%0.1 pCi/L
Mendocino County, CAborders it110.0%0.7 pCi/L
The nearest California counties to Humboldt County with 30 or more tests, 2008 to 2017, laboratory
CountyDistanceTestsAt or above 4 pCi/LMedian
Butte County140 mi3511.4%0.8 pCi/L
Sonoma County159 mi1310.8%0.5 pCi/L
Napa County172 mi5710.5%0.9 pCi/L
Yolo County174 mi482.1%0.9 pCi/L
Nevada County191 mi26025.0%2.0 pCi/L
Marin County191 mi956.3%0.8 pCi/L
Solano County197 mi3815.8%0.9 pCi/L
Placer County202 mi2837.4%1.1 pCi/L
Humboldt County ten-year laboratory totals, against California's pooled share
PeriodTestsAt or above 4 pCi/LMedianHighestCalifornia, pooledChange from the row before
2008 to 20172020.0%1.3 pCi/L64.3 pCi/L10.3%+1.8 points
2007 to 20162218.2%1.1 pCi/L64.3 pCi/L10.6%−2.9 points
2006 to 20151921.1%1.6 pCi/L64.3 pCi/L10.6%Level
2005 to 20141921.1%1.6 pCi/L64.3 pCi/L10.0%First on file

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

Humboldt County, Geology

The ground under Humboldt County

Predicted

USGS's geologic map puts all of Humboldt County's mapped land in one class: low radon potential. The map puts 77.7% of Humboldt County in the area it calls Northern Coast Ranges and 21.5% in Klamath Mountains. USGS gives both a radon index of 6, in the low band (5 to 8).

USGS's geologic areas under Humboldt County
AreaThe rock, in USGS's wordsPotentialRadon indexShare of land
Northern Coast RangesMetamorphosed marine sedimentary rocks and ultramafic rocks.Low677.7%
Klamath MountainsMetasedimentary and ultramafic metavolcanic rocks, and basalt.Low621.5%

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

Humboldt County beside the ground around it

Across California 0.1% of the land is rated high; none of Humboldt County's is. Of the 4 counties that border it, 1 has more moderate-potential ground, Siskiyou County the most at 5.6%.

Share of land by USGS radon potential: Humboldt County, the 4 counties bordering it and California
AreaEPA zoneLow potentialModerate potentialHigh potential
Humboldt CountyZone 399.3%NoneNone
Trinity CountyZone 3100%NoneNone
Del Norte CountyZone 399.8%NoneNone
Siskiyou CountyZone 394.4%5.6%None
Mendocino CountyZone 3100%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

Humboldt County, Local resources

Radon testing and mitigation in Humboldt 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

Humboldt County, Questions

Questions about radon in Humboldt County

Are radon levels high in Humboldt County?

Over the 2008 to 2017 laboratory totals, 20.0% of the radon tests reported to CDC for Humboldt County, California were at or above 4 pCi/L, and the median reported pre-mitigation result was 1.3 pCi/L. Of the 8 nearest counties with a figure, within 202 miles, the share of tests at or above 4 pCi/L runs from 0.8% to 25.0%.

Which counties border Humboldt County?

Humboldt County borders 4 counties: Trinity County, Del Norte County, Siskiyou County and Mendocino County.

Products

Humboldt County, Products

Testing a home in Humboldt County

No CDC figure names Humboldt County; the nearest county with one, Butte County (140 miles away), reported 11.4% of its tests at or above 4 pCi/L over the 2008 to 2017 laboratory totals, the only reading nearby until a test of this home. 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 Humboldt County.

Test kits and monitors3

See every kit and monitor

Best radon monitors for Humboldt County homes, compared

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Humboldt County, Bordering counties

Counties bordering Humboldt County

Calculated by Radon411

Humboldt County, Cities and towns

Cities and towns in Humboldt County

About 131,647 people live in Humboldt County by the Census Bureau's estimate for July 1, 2025, 4,651 fewer than in 2020 (−3.4%). That works out to 37 people per square mile of land. No town dominates: Eureka, the largest, has 25,304 residents, 19.2% of the total. Together its 7 incorporated places hold 47.4% of those residents; the other 52.6% live outside any of them. CDC's 22 reported tests for 2007 to 2016 work out to 0.2 per 1,000 residents (2025 estimate), close to California's 0.3.

From Eureka to Trinidad, CDC publishes no figure by town: each reads Humboldt County's.

The cities and towns in Humboldt County by residents, July 1, 2025
PlaceResidentsSince 2020Share of residents
Eureka25,304−4.4%19.2%
Arcata19,047+1.1%14.5%
Fortuna12,003−3.9%9.1%
Rio Dell3,211−5.1%2.4%
Ferndale1,348−3.6%1.0%
Blue Lake1,144−5.6%0.9%
Trinidad317−5.9%0.2%

7 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).

Humboldt County, Homes

Homes in Humboldt County

Of Humboldt County's 62,625 homes, 70% are detached houses, against 57% across California; 9% are in buildings of five or more homes. Half of its homes were built before 1973, close to California's 1976: 35% date from before 1960 and 14% 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 Humboldt County, California, 2019 to 2023
HomesHumboldt CountyCalifornia
Housing units62,62514,532,683
Detached houses70%57%
In buildings of 5 or more9%24%
Mobile homes8%4%
Built before 196035%27%
Built since 200014%18%
Middle year built19731976
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.