Is radon high in Castle Rock, Colorado?

Low. Averaging the model's ZIP-code predictions across Castle Rock (weighted by how much of the city each ZIP covers) gives a long-term basement mean of 1.6 pCi/L - close to the US average of 1.8 and about 38% below the Colorado average of 2.6. That is below the EPA's 2 pCi/L "consider fixing" threshold, though individual homes still vary. Castle Rock ranks 20 of 21 Colorado cities on this site.

Which Castle Rock ZIP codes have the highest radon?

Predictions for the 3 modeled ZIP codes in Castle Rock range from 1.4 to 1.8 pCi/L in the basement. 0% of them are predicted at or above 4 pCi/L and 0% at or above 2. The city is fairly uniform, so the city-wide number is a reasonable guide for any neighborhood.

Should I test for radon in Castle Rock?

Yes, once. The EPA recommends testing every home regardless of area. A short-term kit costs little and settles the question for your house; Castle Rock's low predicted average describes the typical home, not yours.

What does EPA Radon Zone 1 mean for Castle Rock?

Castle Rock sits mainly in Douglas County, which the EPA classifies as Zone 1 - highest potential (predicted average > 4 pCi/L). The zone map dates from 1993 and covers whole counties. The newer model's city average of 1.6 pCi/L does not line up with Zone 1, which is common - the EPA itself says high-radon homes exist in every zone. Neither replaces a test of your own home.

Radon levels in Castle Rock, Colorado

LowEPA Zone 1

Predicted long-term basement average of 1.6 pCi/L (model estimate) across the 3 modeled ZIP codes in Castle Rock - close to the US average of 1.8, about 38% below the Colorado average of 2.6. Ranked #20 of 21 Colorado cities on this site. Most of the city lies in Douglas County.

Test Castle Rock, CO once anyway - radon varies house to house

The predicted basement average here is 1.6 pCi/L, below the EPA's 2 pCi/L "consider fixing" threshold. That's good news for the area - but the EPA still recommends every home be tested, because foundation cracks, soil pockets and ventilation make one house very different from its neighbor. One inexpensive kit settles it.

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Predicted (model estimate). The pCi/L figures on this page are long-term area averages from a peer-reviewed national model built on millions of real radon tests (2001-2021). They describe the typical home in this area - not your home, which can be several times higher or lower. The EPA radon zone is a separate, 1993 county-level classification. How the numbers work.

1.6 ± 0.07

pCi/L predicted basement mean (model SD)

1.7 ± 0.06

pCi/L predicted upstairs (above-ground) mean

1.4-1.8

range of ZIP-level basement predictions in the city

of Castle Rock ZIPs predicted ≥ 4 pCi/L (0% ≥ 2)

What this means for Castle Rock

The typical Castle Rock home is predicted below 2 pCi/L, the lowest band. That describes the average house; the ZIP-level predictions inside the city still span 1.4 to 1.8 pCi/L.

The city figure is not a separate estimate: it is the mean of the model's ZIP-code predictions, weighted by the share of the city's land in each ZIP (Census 2020 ZCTA-to-place relationship file). Note that the EPA classifies Douglas County as Zone 1 while the newer model predicts 1.6 pCi/L for Castle Rock; where they disagree, we grade on the model but show both.

Comparable Colorado cities: Highlands Ranch (1.6), Commerce City (1.7), Parker (1.9), Thornton (2.1).

ZIP codes in Castle Rock

ZIPs that overlap the city, largest share of its land first. Predicted basement and upstairs means are model estimates; ZIPs the model does not cover show a dash.

ZIP	Share of city	Basement	Upstairs	Grade
80104	40%	1.8 ± 0.07	1.9	Low
80109	38%	1.4 ± 0.07	1.6	Low
80108	23%	1.6 ± 0.07	1.6	Low

Testing help in Colorado

Colorado's radon program page is at cdphe.colorado.gov; it lists testing and certified-mitigator guidance. EPA state radon contacts page. If a test comes back at 4 pCi/L or higher, mitigation typically costs $800-$2,500 installed; use EPA find-a-professional and read the mitigation guide.

Castle Rock radon questions

Is radon high in Castle Rock, Colorado?: Low. Averaging the model's ZIP-code predictions across Castle Rock (weighted by how much of the city each ZIP covers) gives a long-term basement mean of 1.6 pCi/L - close to the US average of 1.8 and about 38% below the Colorado average of 2.6. That is below the EPA's 2 pCi/L "consider fixing" threshold, though individual homes still vary. Castle Rock ranks 20 of 21 Colorado cities on this site.
Which Castle Rock ZIP codes have the highest radon?: Predictions for the 3 modeled ZIP codes in Castle Rock range from 1.4 to 1.8 pCi/L in the basement. 0% of them are predicted at or above 4 pCi/L and 0% at or above 2. The city is fairly uniform, so the city-wide number is a reasonable guide for any neighborhood.
Should I test for radon in Castle Rock?: Yes, once. The EPA recommends testing every home regardless of area. A short-term kit costs little and settles the question for your house; Castle Rock's low predicted average describes the typical home, not yours.
What does EPA Radon Zone 1 mean for Castle Rock?: Castle Rock sits mainly in Douglas County, which the EPA classifies as Zone 1 - highest potential (predicted average > 4 pCi/L). The zone map dates from 1993 and covers whole counties. The newer model's city average of 1.6 pCi/L does not line up with Zone 1, which is common - the EPA itself says high-radon homes exist in every zone. Neither replaces a test of your own home.

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