What Happens If Radon Levels Are High: Plain-Language Guide
Radon does not produce immediate symptoms in most people. Persistent cough, shortness of breath, chest pain, or unexplained weight loss can be late-stage indicators of radon-caused lung cancer — but these appear only after years of exposure, not as a warning sign. When a test shows high radon levels — anything above the EPA action level of 4 pCi/L — the consequences are long-term cumulative cancer risk, not acute illness. The action framework that follows is direct: verify, contact a certified mitigation contractor, install sub-slab depressurization, verify with post-mitigation testing, and monitor ongoing performance. This guide summarizes EPA and CDC guidance current as of 2026 — consult a certified radon professional for testing and your physician for symptom evaluation.
What “high radon” actually does to a household
High radon levels do not produce acute symptoms regardless of concentration. EPA, CDC, and the U.S. Surgeon General all describe radon as a long-latency lung carcinogen — meaning the harm accumulates over years and decades and presents as lung cancer rather than as immediate illness. A homeowner cannot feel high radon. A radon test is the only signal.
The relevant exposure metric is cumulative dose. EPA’s risk model multiplies concentration by time spent exposed, producing an estimate of lifetime lung cancer risk. A household sustaining 20 pCi/L exposure for 20 years has roughly the same cumulative dose as a household sustaining 10 pCi/L exposure for 40 years. Higher concentrations reach the same dose faster, but both households face elevated long-term risk.
Lung cancer attributable to radon develops 5 to 25 years or more after sustained exposure. Smokers and former smokers face the largest risk multiplier — EPA estimates that smokers exposed to elevated radon face roughly 10 times the lung cancer risk of non-smokers exposed to the same levels. The risk-multiplication effect is the main reason CDC recommends radon mitigation as a high-priority public health intervention.
The five-step action framework
A homeowner who receives a high radon test result has a clear sequence of next steps. Each step has a typical timeline and cost on the Front Range.
Step 1 — Verify the result
If the high reading came from a short-term test, EPA recommends a second test to confirm. The confirmation test can be either a longer-term alpha track (90 days to 1 year) for accurate average exposure, or a second short-term test for a fast confirmation. If the first test was 2 to 4 pCi/L (borderline), repeat testing helps decide whether to mitigate. If the first test was clearly above 4 pCi/L, many homeowners proceed directly to mitigation without a second test.
Step 2 — Contact an NRPP-certified mitigation contractor
National Radon Proficiency Program (NRPP) and National Radon Safety Board (NRSB) maintain directories of certified mitigation contractors. The Colorado Department of Public Health and Environment also lists certified contractors operating along the Front Range. Get two or three quotes for sub-slab depressurization installation; pricing variation reflects fan size, slab penetration count, and exhaust routing.
Step 3 — Install the mitigation system
Standard sub-slab depressurization (SSD) installation takes one day on-site. The contractor cores a hole through the basement slab, installs a vertical pipe, mounts an in-line fan in the attic or on the exterior, and routes the exhaust above the roofline. Typical Front Range cost: $1,000 to $2,500. Complex installations — multiple slabs, crawl spaces, drainage features — run higher.
Step 4 — Post-mitigation testing
The mitigation system reaches full effectiveness 24 to 48 hours after installation. A post-mitigation short-term test — usually a 48-hour continuous radon monitor — confirms the system reduced concentrations below the EPA action level. A reputable contractor includes this verification test in the original quote and guarantees a result below 4 pCi/L.
Step 5 — Ongoing monitoring
EPA recommends re-testing every two years after mitigation to confirm the system continues to operate as designed. Re-test after any significant home modification — basement finish work, foundation repair, HVAC replacement, addition construction. The continuous-running fan should be checked annually; visual confirmation of the pressure indicator or audible fan operation works for most homeowners.
Insurance and warranty considerations
Homeowner’s insurance policies typically do not cover radon mitigation as a covered loss. Mitigation is classified as routine maintenance or property improvement — similar to roof replacement or HVAC upgrade — rather than as repair of a covered peril. Policy language varies, but the general industry position is that radon is a pre-existing environmental condition, not a damage event.
That positioning has practical implications. Homeowners pay mitigation costs out of pocket or finance them through home-improvement loans. Some Colorado counties offer mitigation cost-sharing programs through their public health departments, with eligibility typically tied to income or household composition. CDPHE maintains a current list of available programs.
Mitigation systems often carry contractor warranties — typically 2 to 5 years on labor and 5 to 10 years on the fan unit. The fan itself is the only mechanical component; warranty replacement of a failed fan typically runs $300 to $500 if outside the warranty period.
What happens if you do nothing
Doing nothing after a high radon test does not produce immediate consequences. The home remains habitable; daily life continues normally. The consequence is cumulative — over years of sustained exposure, the long-term lung cancer risk accumulates. That risk is not theoretical. EPA attributes roughly 21,000 lung cancer deaths in the United States annually to radon exposure, making it the second-leading cause of lung cancer after smoking and the leading cause among non-smokers.
For sellers, doing nothing after a buyer’s inspection reveals high radon typically means accepting a price reduction or losing the deal. Most Front Range purchase contracts include an inspection contingency that allows buyers to negotiate mitigation, accept a price adjustment reflecting mitigation cost, or withdraw without penalty. Sellers who refuse to address documented high radon usually face contract termination.
Real-estate transaction implications
High radon discovered during a pre-purchase inspection becomes part of the negotiation. The standard outcomes break down predictably.
Seller-funded mitigation
The most common outcome on the Front Range. The seller agrees to install a mitigation system before closing, with the post-mitigation test result included in the closing disclosure. Buyers receive a home with a verified-functional mitigation system; sellers absorb the typical $1,000 to $2,500 cost.
Price reduction in lieu of mitigation
The seller reduces the purchase price by the estimated mitigation cost — often slightly higher than the actual cost to account for the buyer’s coordination effort. The buyer installs the system post-closing.
Closing credit
The seller credits the buyer at closing for the mitigation cost. The buyer installs the system after closing using the credit. Functionally similar to a price reduction; the closing-credit structure is sometimes preferred for tax or loan-underwriting reasons.
Buyer accepts as-is
Less common but possible. Some buyers accept high radon and the associated mitigation responsibility without seller adjustment — typically because they value other deal terms more than the mitigation cost.
Contract withdrawal
If buyer and seller cannot reach agreement, the inspection contingency allows the buyer to withdraw without penalty during the contingency period.
What if mitigation doesn’t reduce levels enough
A properly designed and installed SSD system typically reduces indoor radon below the EPA action level. Occasionally a system underperforms, with post-mitigation levels still above 4 pCi/L. This outcome usually traces to one of three causes.
Undersized fan or insufficient suction
The original design may have used a fan rated for typical homes when the specific house needed higher draw. Upgrading to a larger fan often solves the problem at modest additional cost.
Additional entry pathways not addressed
SSD covers the slab. If radon is entering through additional pathways — an unsealed sump pit, a crawl space connected to the basement, an old French drain system — those need separate treatment. Sub-membrane depressurization for crawl spaces and sealed sump-pit covers are typical add-ons.
Slab seal failure
The seal around the SSD pipe where it penetrates the slab must remain airtight for the system to work. Settlement, vibration, or poor original installation can break that seal. Reapplying urethane or polysulfide sealant restores function.
Reputable NRPP- or NRSB-certified contractors warrant their work and revisit underperforming systems at no charge during the warranty period. This is one reason credential verification is the basic step in contractor selection.
How the mitigation system actually works
Sub-slab depressurization reduces indoor radon by creating negative pressure in the soil gas beneath the basement slab, diverting radon away from foundation entry pathways and venting it to outdoor air above the roofline. The continuously running fan maintains the pressure differential.
A properly designed system typically reduces indoor radon by 80 to 99 percent. A 40 pCi/L home commonly drops to under 1 pCi/L after mitigation. Operating cost runs roughly $3 to $10 per month in electricity for the fan, plus modest heating-cost increase from the small volume of conditioned air pulled into the system.
For context on what drives high radon in the first place, see the radon testing hub for Front Range homeowners. For deeper detail on the cumulative-dose framework that explains why high levels matter even without acute symptoms, the how long radon exposure takes to cause cancer guide covers the long-latency dose response.
When to call a professional
Any homeowner with a verified radon reading above 4 pCi/L should contact an NRPP- or NRSB-certified mitigation contractor. Front Range homeowners can locate certified contractors through the CDPHE radon program directory or the NRPP national search. Get two or three quotes; confirm the quote includes post-mitigation testing.
For homeowners with current or former smoking history, basement-living-space occupancy, or children spending significant time in finished basements, the mitigation priority is higher than for households without those risk factors. The combined risk multiplier from smoking plus radon exposure produces dramatically elevated lung cancer rates that mitigation directly addresses.
Symptoms that homeowners might attribute to radon — headaches, fatigue, respiratory irritation — warrant physician evaluation for other causes, including carbon monoxide, mold, dampness, and volatile organic compounds. Radon does not produce those symptoms; the symptom set associated with radon is late-stage lung cancer, not acute exposure.
Households that have lived with high radon for years before mitigation should not panic about the prior exposure. The risk model is probabilistic — sustained high exposure increases the lifetime probability of lung cancer, but the absolute risk for any individual depends on smoking status, total exposure duration, and other factors. Mitigation stops the additional accumulation going forward and shifts the household onto a lower long-term risk trajectory. For households with current or former smokers, regular discussions with a primary care physician about lung-cancer screening eligibility — particularly under USPSTF guidelines for adults with smoking history — convert the radon exposure history into actionable medical surveillance.
References
- EPA Health Risk of Radon — U.S. Environmental Protection Agency
- CDC Radon Information — Centers for Disease Control and Prevention
- Colorado Department of Public Health and Environment Radon Program — State of Colorado
- American Lung Association: Radon Lung Cancer Risk — American Lung Association
Front Range homeowners with verified high radon readings — and buyers whose inspection turned up elevated levels — can connect with a certified local mitigation professional through our contact page for quotes and scheduling. Mitigation is straightforward; the action framework is well-established.