Radon in Colorado.
Colorado sits on some of the most radon-productive geology in the country — roughly half of the homes we test come back above the EPA action level. Here’s what radon is, what it does, what the numbers look like across the Denver metro, and what testing and fixing it actually costs.
What radon is — and why it’s a Colorado problem
Radon is a naturally occurring radioactive gas produced as uranium in soil and rock breaks down. You can’t see it, smell it, or taste it. It seeps out of the ground everywhere on earth — the question is only how much, and whether it collects inside your house.
It enters through the paths your foundation offers: slab and wall cracks, the sump pit, floor drains, utility penetrations, and bare crawlspace soil. Because warm air rising through a house pulls soil gas in behind it (the stack effect), radon concentrates in the lowest lived-in levels — and reads highest in winter, when homes are sealed tight.
Colorado’s granite-heavy, uranium-bearing geology produces more of it than almost anywhere in the country. The EPA maps every Front Range county as Zone 1, its highest-risk category, and Colorado test data backs that up: roughly half of homes test at or above the 4.0 pCi/L action level.
Average indoor radon: Denver metro vs the nation
County averages below come from CDPHE test data (2005–2024). Averages hide the spread — neighboring houses on the same street routinely test wildly differently, which is why the average matters less than your number.
pCi/L = picocuries per liter of air. The Colorado test average reflects homes that got tested — a population that skews toward known problem areas, so treat it as an upper-ish estimate.
What the risk actually looks like
Radon decays into radioactive particles that lodge in lung tissue as you breathe. The EPA attributes about 21,000 US lung-cancer deaths a year to radon — about 2,900 of them in people who never smoked. It’s the second-leading cause of lung cancer overall and the leading cause among non-smokers.
Two honest framings belong side by side. First: this is a slow, dose-over-decades risk — nobody is harmed by a week in a high-radon house. Second: the doses in Colorado homes are genuinely high by national standards, the exposure is fixable for about the price of a water heater, and smoking multiplies the risk roughly tenfold — the combination is far worse than either alone.
| Lifetime exposure level | Never-smokers | Smokers |
|---|---|---|
| 1.3 pCi/L (US average) | 2 in 1,000 get lung cancer | 20 in 1,000 get lung cancer |
| 4 pCi/L (action level) | 7 in 1,000 get lung cancer | 62 in 1,000 get lung cancer |
| 8 pCi/L | 15 in 1,000 get lung cancer | 120 in 1,000 get lung cancer |
| 20 pCi/L | 36 in 1,000 get lung cancer | 260 in 1,000 get lung cancer |
EPA lifetime-risk estimates from A Citizen’s Guide to Radon, per 1,000 people exposed at that average level over a lifetime.
Testing is the easy part
During a home purchase
We set a continuous radon monitor in the lowest livable level for ~48 hours under closed-house conditions. Hourly logging shows swings (and tampering), results are ready at pickup, and the report becomes part of your negotiation file. Colorado law licenses paid radon measurement professionals, so credentials are checkable.
Between transactions
A DIY charcoal kit ($15–$30, or free/discounted through the CDPHE program) is a legitimate screening tool. Place it low in the house, follow closed-house instructions, and mail it promptly. Confirm any high reading with a follow-up or professional test before spending mitigation money.
When to retest
Every couple of years even with past good results; after any mitigation install; after major renovations, additions, or new HVAC; and after finishing a basement — the room you just built is the one closest to the source. Winter tests read highest and are the most conservative.
What it costs to know — and to fix
Ranges reflect published Denver-metro pricing for 2025–2026. The pattern to notice: knowing costs almost nothing, and even the fix is a one-time, water-heater-sized expense — not a remodel.
How the fix works: a mitigation system (sub-slab depressurization) is a sealed pipe through the foundation floor with a continuously running inline fan that pulls soil gas from under the slab and vents it above the roof — soil gas never enters the house. Installed correctly, systems routinely bring homes under 2 pCi/L and often under 1. Cost drivers are foundation type (crawlspaces need membrane sealing), home footprint, and where the pipe can run; permits typically add $75–$200.
In a real-estate deal, a failed radon test is one of the most routine, fixable negotiation items on the Front Range — mitigation is commonly seller-funded or split. It is almost never a reason to walk away from a house.
What the state requires
Sellers must disclose (SB23-206)
Since August 7, 2023, sellers must disclose known radon test results, any mitigation or remediation performed, whether a system is installed, and provide the CDPHE radon brochure. Testing itself is not required — “never tested” is a legal answer, which is why buyers test.
Landlords too
The same law covers leases: landlords must disclose radon knowledge to tenants, and a tenant may be able to void the lease if a landlord fails to mitigate a confirmed 4.0+ pCi/L problem within 180 days of notice.
Pros must be licensed (HB21-1195)
Anyone measuring or mitigating radon for a fee in Colorado must hold a state license. Ask for the license and the post-install test result — a real mitigation job ends with a number, not just a fan.
Common radon questions
What level of radon is dangerous?
There is no known fully safe level — risk scales with concentration and time. The EPA action level is 4.0 pCi/L: fix the home at or above it. The EPA also recommends considering mitigation between 2 and 4 pCi/L, and the World Health Organization uses a stricter 2.7 pCi/L reference level. For context, the average American home sits around 1.3 pCi/L — and the average Colorado test result is several times that.
Why is radon so bad in Colorado?
Geology. Colorado’s granite and uranium-bearing soils generate more radon than most of the country, which is why the EPA maps every Front Range county as Zone 1 — its highest-risk designation. Statewide test data shows roughly half of Colorado homes at or above the 4.0 pCi/L action level. Altitude doesn’t cause radon, but tight, well-sealed Colorado homes hold what the soil delivers, especially in winter.
How much does radon mitigation cost in Denver?
Published Denver-metro pricing in 2025–2026 puts a standard sub-slab system at roughly $1,200–$2,500 installed, with many jobs landing around $1,500–$2,000. Activating a builder-installed passive system runs about $550–$850; crawlspace and multi-foundation homes trend higher ($1,900–$2,800+). Ongoing costs are modest: the fan draws $50–$70 of electricity a year and is typically replaced every 7–10 years for $180–$250.
How does a radon test work during a home inspection?
We place a continuous radon monitor in the lowest livable level for about 48 hours under closed-house conditions (windows shut, normal door use). The monitor logs hourly readings, which also reveals tampering or unusual swings, and you get the numeric result right after pickup — our add-on runs $125, or $150 where a return trip is needed. DIY charcoal kits are a fine screening tool between transactions; the CDPHE offers them free or discounted to Colorado residents.
Does a radon mitigation system actually work?
Yes — sub-slab depressurization is one of the most reliable fixes in home construction. A sealed pipe and a continuously running fan pull soil gas from beneath the foundation and vent it above the roof, typically bringing homes under 2 pCi/L and often under 1. The catches: the fan must actually run (check the system’s manometer occasionally), and the home should be retested after installation and every couple of years.
Do sellers in Colorado have to disclose radon?
Yes. Senate Bill 23-206 (effective August 7, 2023) requires sellers to disclose known radon test results, mitigation history, and whether a system is installed, along with the state’s radon brochure — and landlords must make similar disclosures to tenants. It does not require testing, so “no known tests” is a common and legal answer. That gap is exactly why buyers test during inspection.
- EPA — Health Risk of Radon (21,000 deaths/yr; risk tables; action level)
- CDPHE — “Half of Colorado homes have elevated radon levels”
- CDPHE — Understanding radon (state guidance + test kits)
- CDPHE — Testing your home for radon (free/discounted kit program)
- EPA — Map of Radon Zones: Colorado (Front Range counties = Zone 1)
- Colorado General Assembly — SB23-206, radon disclosure (effective Aug 7, 2023)
- American Cancer Society — Radon and Cancer Risk
- Colorado Radon Check — CDPHE test data by county/zip (2005–2024) & 2026 cost survey
- Martens Radon — Denver-metro mitigation cost guide (2025–2026 pricing)
Figures reflect the cited sources as of mid-2026. County averages are averages of tests submitted to CDPHE, not random samples. This page is education, not medical advice — for health questions, talk to your doctor; for your house, test it.
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