Home Radon Testing: What Homeowners Need to Know
Home radon testing is the only way to know whether the soil beneath your house is delivering elevated levels of a naturally occurring radioactive gas into your living space. Radon is invisible, odorless, and the second-leading cause of lung cancer in the United States after smoking. For Colorado homeowners — particularly along the Front Range, which falls in EPA Radon Zone 1 — testing is not optional in any practical sense. About half of Colorado homes test above the EPA action level on first measurement. This guide summarizes EPA, CDPHE, CDC, and lung-health organization guidance current as of 2026 — consult your physician for radon-related health concerns.
Home Radon Testing: The Big Picture
Three things you need to know upfront. First, every house in Colorado should be tested. CDPHE data shows around half of Colorado homes exceed the EPA action level of 4 picocuries per liter. Geology — uranium-bearing soils and bedrock common throughout the Front Range — drives the risk. Second, testing is cheap and easy. A $15 charcoal canister or a $30 alpha-track detector covers the basic question. Third, mitigation is effective when needed. A typical sub-slab depressurization system runs $1,200 to $2,500 in Colorado and reduces radon by 90 percent or more.
The rest is sequence: when to test, how to test, how to interpret, and what to do with the result.
When to Test for Radon
At Purchase
Always test during the inspection period of a home purchase. Most real-estate contracts in Colorado include radon as a buyer-protection contingency. Sellers often pre-test before listing; verify the test was recent (within 2 years), conducted under proper closed-house conditions, and analyzed by a NRPP- or NRSB-certified lab. A buyer’s independent test during inspection is the gold standard.
Every 2 to 5 Years for Existing Homes
EPA recommends retesting every 2 to 5 years even after a previous low result. Geology does not change but home structure does — foundation cracks open, sump pits drain differently, HVAC patterns shift, additions change air-pressure dynamics. A previously low home can become elevated.
After Major Renovations
Foundation work, basement finishing, HVAC replacement, or any change that affects the home’s lower-level air-pressure balance warrants retesting. Sealing one entry point sometimes redirects soil gas to another. The piece on when to test for radon covers the timing decisions.
If You Have a Mitigation System
Retest 30 days after a new mitigation system is installed to verify it is working. Then retest every 2 years to confirm continued operation. Mitigation fans typically last 5 to 10 years before replacement is needed. Our piece on radon system repair covers fan failure detection.
How to Test
The Short-Term DIY Test
Buy a charcoal canister kit ($15 to $30 including lab analysis) at Home Depot, Lowe’s, Amazon, or direct from a lab. Place the canister in the lowest livable level (basement if used as living space, otherwise lowest occupied floor), 20 inches to 6 feet above the floor, 4 inches from walls. Close all windows and exterior doors at least 12 hours before opening the canister and throughout the test period.
Standard test duration: 48 to 96 hours. At the end, seal the canister immediately, fill out the chain-of-custody form, and mail to the lab on the same day. Results return by email or PDF in 3 to 7 business days. The piece on the DIY radon testing covers the at-home workflow.
The Long-Term DIY Test
Buy an alpha-track detector ($25 to $40). Place it in the same location as a short-term test. Leave it in place for 3 to 12 months. Seal and mail at the end. Results return in 2 to 4 weeks after lab receipt. Long-term tests give a more accurate annual average and smooth out weather variability.
The Continuous Monitor
Buy a consumer continuous radon monitor ($150 to $300) like the Airthings Corentium Home, Ecosense RadonEye, or Radonova RadonEye Plus. Place in the lowest livable level. The monitor reads continuously and shows daily, weekly, and long-term averages via an app or display. No mailing, no lab fees. Useful for ongoing peace of mind and for tracking mitigation system performance.
Hiring a Pro
NRPP- or NRSB-certified radon measurement professionals perform tests using continuous radon monitors with documented chain-of-custody. Cost runs $125 to $250 for a 48-hour test with same-day setup and pickup. Useful for real-estate transactions where defensibility matters. The piece on the radon testing process covers professional testing.
EPA Action Levels
Two key thresholds. Above 4 pCi/L, EPA recommends mitigation. Between 2 and 4 pCi/L, EPA suggests considering mitigation based on household risk factors (smokers, children, time spent in lower levels). Below 2 pCi/L is considered low risk.
Outdoor radon averages 0.4 pCi/L. The average indoor radon in the U.S. is 1.3 pCi/L. EPA’s 4 pCi/L action level was selected as a practical mitigation target — not because 3.9 pCi/L is safe and 4.1 pCi/L is dangerous, but because mitigation reliably reduces radon below this threshold at reasonable cost. The piece on whether you should test for radon covers the decision framework.
Colorado-Specific Context
The Front Range is EPA Zone 1, the highest predicted-radon classification. CDPHE data shows that approximately 50 percent of Colorado homes test above 4 pCi/L on first measurement. Particular county hotspots include Boulder, Larimer, Jefferson, and El Paso, where bedrock geology produces higher soil-gas radon.
Colorado’s housing stock includes a high proportion of homes with finished basements used as living space — bedrooms, family rooms, home offices. This puts occupants closer to the soil-gas source than ground-floor-only homes. Pre-1990 construction tends to higher radon entry points (less sealed foundations, fewer vapor barriers under slabs).
The Colorado real-estate market treats radon as a routine inspection item. Sellers commonly disclose past test results and any mitigation systems. Buyers commonly require pre-close mitigation for elevated results. The parent guide on radon testing in Colorado covers the Front Range market context.
What to Do With Results
Below 2 pCi/L
Retest every 2 to 5 years. No immediate action needed. Continue closed-house ventilation patterns as normal.
Between 2 and 4 pCi/L
Consider mitigation based on household risk factors. Smokers in the home increase radon risk significantly. Young children spending time at floor level near the soil-gas source warrant precaution. Long basement-occupancy patterns (basement bedroom, home office) tilt toward mitigation.
Between 4 and 10 pCi/L
EPA recommends mitigation. Get a follow-up test (short-term or long-term) to confirm the initial reading before contracting a mitigation system. For real-estate transactions, a single short-term test above 4 pCi/L is usually sufficient to require mitigation.
Above 10 pCi/L
EPA recommends mitigation. Confirmation testing is less critical at this level because the magnitude of the reading is unambiguous. Mitigation systems typically reduce these readings to below 2 pCi/L. The piece on home radon mitigation systems covers mitigation design.
Mitigation Costs
A standard sub-slab depressurization system (the most common Colorado approach) runs $1,200 to $2,500 installed by a NRPP- or NRSB-certified mitigation contractor. The system consists of a fan, PVC piping from below the slab to outside the roofline, and a manometer to verify continuous operation. Annual electricity cost is $50 to $100 for the fan.
Crawlspace homes require different mitigation approaches — sealed vapor barriers plus passive or active depressurization. Pricing varies more widely at $1,500 to $4,000. Slab-on-grade homes with multiple suspect entry points may require multi-suction-point systems at higher cost. The piece on radon basement systems covers system design.
Common Testing Mistakes
Open Windows During Test
Closed-house conditions are required for valid short-term tests. Even brief ventilation during a 48-hour test can drop readings significantly. Plan around weather and family schedules — do not test during a window-open weekend.
Wrong Placement
Placing the kit on a basement floor against a wall (wrong) versus 20 inches to 6 feet above floor and 4 inches from walls (correct) changes readings. Avoid placement near windows, exterior doors, HVAC vents, or sources of disturbance.
Wrong Season
Summer tests with windows open during daytime under-report radon. Winter tests under closed-house conditions over-report relative to the annual average but match the highest-exposure season. EPA’s 4 pCi/L threshold is calibrated to closed-house conditions.
Delayed Lab Submission
Charcoal canisters lose accuracy if held more than 24 hours after the test ends. Mail same-day. Failure to mail promptly is the most common DIY testing error.
Testing for Real-Estate Transactions
Real-estate radon tests have additional protocols beyond standard homeowner testing. The test should be performed by a certified professional with documented chain-of-custody. A continuous radon monitor with tamper detection is preferred over a passive kit. The 48-hour test window is standard. Results above 4 pCi/L commonly trigger mitigation negotiation or seller-paid mitigation prior to closing.
Testing for New Construction
Colorado building codes in many Front Range counties require passive radon-resistant features in new construction — a sealed slab, a vapor barrier under the slab, a vertical PVC pipe stub from below the slab to attic space, and an attic-mounted electrical connection for a future fan. After construction, a radon test determines whether to activate the passive system with a fan (converting it to active mitigation). The piece on whether you need a radon detector covers ongoing monitoring.
References
- EPA Radon Program — U.S. Environmental Protection Agency
- Colorado Radon Program — Colorado Department of Public Health and Environment
- CDC Radon Information — Centers for Disease Control and Prevention
- Radon and Indoor Air Quality — American Lung Association
Front Range homeowners ready to schedule a certified professional radon test before a purchase or after a high DIY reading can connect with a vetted local pro through our contact page.