Radon Testing & Mitigation in Castle Rock, Colorado

Castle Rock is located in Douglas County, which the U.S. Environmental Protection Agency designates as Radon Zone 1 — the highest-risk classification, indicating predicted average indoor radon levels at or above the EPA action level of 4 pCi/L.

Castle Rock — the Douglas County seat, named for the castle-shaped butte at its center — incorporated in 1881 but did most of its growing after 2000, expanding from roughly 1,500 residents in 1970 to over 73,000 by 2020. Unlike the metro's older inner-ring suburbs, much of Castle Rock's housing is new enough to include radon-resistant new construction (RRNC) features — but RRNC reduces radon, it does not eliminate it, and Douglas County is EPA Radon Zone 1. Testing remains the only way to know: 46.4% of the 16,872 CDPHE radon tests in Douglas County (2005–2025) came back at or above 4 pCi/L, with a median result of 3.5 pCi/L.

Across 16,872 Douglas County home tests submitted to CDPHE between 2005 and 2025, 46.4% came back at or above the EPA action level — against a statewide rate of 46.4% across 237,408 tests in the CDPHE 2005–2025 statewide dataset. Whether you own or rent in Castle Rock, testing is the only way to know your home's level.

Radon is the leading cause of lung cancer among people who have never smoked and the second-leading cause overall, behind smoking. The EPA estimates radon is responsible for about 21,000 lung-cancer deaths in the U.S. every year — more than drunk driving. The U.S. Surgeon General has issued a national health advisory urging Americans to test their homes.

Lung cancer isn't only a smoker's disease. The CDC estimates that 10–20% of U.S. lung cancers — 20,000 to 40,000 a year — occur in people who never smoked, and among never-smokers, radon is the single largest cause. Of radon's roughly 21,000 annual lung-cancer deaths, the National Cancer Institute attributes about 2,900 to people who never smoked. It's a risk that has nothing to do with whether anyone in the house has ever touched a cigarette.

The risk is cumulative and silent. You can't see, smell, or taste radon, and there are no short-term symptoms — the damage comes from years of breathing elevated levels, which is why a home in Castle Rock can carry real risk without anyone noticing. And for people who do smoke, radon and tobacco multiply each other's risk — the combination is far more dangerous than either exposure on its own.

The EPA sets the action level at 4 pCi/L (picocuries per liter): at or above that, it recommends installing a mitigation system. Between 2 and 4 pCi/L it suggests considering one. There is no known “safe” level — the only way to know where your home stands is to test it.

Radon is one of the most common sticking points in a Front Range home sale. A radon test is a routine part of the inspection period on most Castle Rock transactions, and a result above 4 pCi/L regularly becomes a negotiation item — buyers ask the seller to install a mitigation system or credit the cost at closing.

Colorado has specific disclosure rules. The state's Seller's Property Disclosure asks sellers to report known radon concentrations and any mitigation system on the property. Under Colorado law (HB 21-1195), sellers must also give buyers an EPA radon information brochure and disclose any known radon test results and installed mitigation systems before the sale closes.

The practical takeaway for Castle Rock owners: if you're thinking about selling, testing early — and mitigating if needed — takes radon off the table before it can stall a deal. A documented system and a passing post-install retest are a selling point, not a red flag.