A Radon Test: A Plain Overview for First-Time Homeowners
A radon test is the only way to know whether the home you live in contains elevated concentrations of a colorless, odorless radioactive gas that is the second-leading cause of lung cancer in the United States. The premise of this overview is intentionally entry-level — written for someone who has never thought about radon, just bought their first house, or is researching what a real-estate agent or inspector mentioned in passing. This guide explains what radon is, why testing matters, how a test works, and what the results mean — all without assuming prior knowledge. Information reflects EPA, CDC, CDPHE, and lung.org guidance current as of 2026; consult your physician for any radon-exposure health concerns and a state-certified radon professional for testing decisions.
What is radon, really?
Radon is a chemical element — a noble gas like helium or neon, but radioactive. It forms naturally from the decay of uranium in soil and rock. Because uranium is widely distributed in the Earth’s crust, radon is everywhere. The element seeps up through the soil, enters buildings through foundation cracks and openings, and accumulates in indoor air. In open spaces outdoors, radon dilutes quickly into the atmosphere and poses minimal risk. In enclosed buildings — particularly basements with direct soil contact — it can accumulate to concentrations that pose meaningful health risk over years of exposure.
The radioactive part matters because radon and its decay products emit alpha particles. When alpha particles strike lung tissue from inhaled radon and its progeny, they damage cells in ways that can lead to lung cancer decades later. The American Lung Association estimates roughly 21,000 U.S. lung-cancer deaths per year are attributable to radon.
What a radon test does
A radon test measures the concentration of radon in indoor air over a defined period, typically expressed in picocuries per liter (pCi/L). One picocurie equals 0.037 disintegrations per second — a small but measurable rate of radioactive decay. EPA established 4.0 pCi/L as the action level above which homeowners are recommended to install a mitigation system.
The test does three things. First, it tells you the current concentration in your home. Second, it gives you a basis for comparison against the EPA action level. Third, it creates a record — a baseline you can re-test against in the future to detect changes.
Why testing matters everywhere — but especially in Colorado
EPA’s Radon Zone map classifies counties into three zones based on predicted average indoor radon screening levels. Zone 1 (predicted average above 4.0 pCi/L) is the highest-risk designation. The entire state of Colorado sits in Zone 1, meaning Colorado homes are expected on average to test above the action level. CDPHE estimates roughly half of Colorado homes exceed 4.0 pCi/L.
The reasons are geological. The Front Range bedrock contains granitic and metamorphic formations with elevated uranium content. The Pierre Shale of the eastern plains carries similar profiles. Fractures in foundation bedrock provide easy pathways for radon to migrate upward. Combine the geology with widespread basement construction and the result is elevated household radon across the state. The Colorado radon testing pillar walks the regional context in more depth.
How the test actually works
Three test technologies cover most of the residential market:
Charcoal canister
A small canister containing activated carbon sits in the home for 2 to 7 days, absorbing radon from the air. After the test, the homeowner seals the canister and mails it to a lab. The lab measures the absorbed radon (and its short-lived decay products) using gamma spectroscopy and calculates the original air concentration. Cost: $15 to $30 with lab analysis included. Results: one to two weeks.
Continuous radon monitor (CRM)
A professional-grade electronic device draws air through a chamber and counts alpha particles in real time. CRMs deliver hour-by-hour data across the test period, usually 48 hours. Used by state-certified professionals for real-estate transactions and post-mitigation clearance. Cost: typically $150 to $300 for a professional test.
Alpha-track detector
A long-term integrating device using a polymer film that records alpha-particle damage. The film is etched and read under a microscope after 90 days to one year. Useful for capturing year-round average exposure that accounts for seasonal variation. Cost: $20 to $50 with lab analysis.
The how is radon detected guide walks the physics behind each technology.
What the closed-house protocol means
Short-term radon tests require closed-house conditions: windows and exterior doors closed for at least 12 hours before and during the test, with HVAC operating normally. The reason is methodological — open windows dilute indoor radon with outdoor air, producing artificially low readings that do not represent normal occupied conditions. Testing during the closed-house protocol mimics conditions during heating and cooling seasons when occupants rely on mechanical ventilation rather than open windows.
Where to place the test
Lowest livable level of the home — typically the basement in Colorado, the main level in homes without basements. The placement surface should be 20 inches above the floor and at least 3 feet from exterior walls, drafts, vents, sunlight, and humidity sources. The chosen room should be one where occupants spend meaningful time, not an unused storage area.
Reading the result
The lab returns a concentration in pCi/L:
Below 2.0 pCi/L
EPA considers this the practical lower bound for routine measurement. Most homes at this level are reasonable to retest in two years and otherwise leave alone.
2.0 to 4.0 pCi/L
EPA recommends considering mitigation. WHO’s stricter 2.7 pCi/L guideline puts this range in mitigation territory. Children, current and former smokers, and individuals with respiratory conditions warrant more aggressive response.
4.0 pCi/L and above
EPA action level. Install mitigation. Mitigation costs $1,200 to $2,500 typical for a Front Range home.
10 pCi/L and above
EPA recommends expedited mitigation. Concentrations in this range produce substantial multi-year risk and justify quick action.
What happens after a high reading
A common path: order a confirmatory test (either a second short-term test or a 90-day alpha-track detector), engage a state-certified radon mitigation contractor for a quote, install a sub-slab depressurization system, and verify post-mitigation performance with a follow-up test. The whole sequence typically takes 30 to 60 days from first elevated reading to confirmed mitigation.
Mitigation systems are non-invasive — a PVC pipe runs from below the slab, up through an exterior wall, and out the roof, with an inline fan creating suction that diverts soil gas. The pipe is sometimes routed through the garage or a closet. Property values are typically unaffected; some buyers actively prefer mitigated homes because the work is documented.
What testing does not do
Three limitations to set expectations realistically. First, the test measures present concentration, not future concentration. Radon levels fluctuate based on weather, HVAC, and soil moisture. Second, the test does not identify the entry path. Mitigation contractors perform suction-point testing to map the path. Third, the test is air-based — it does not measure radon dissolved in well water, which is a separate measurement (and a relevant one for homes on private wells).
How often to test
EPA recommends testing every two years for ongoing surveillance, plus immediate testing after major renovations like basement finishes, foundation work, HVAC replacement, or significant air-sealing projects. Buyers should test before purchase. Sellers should disclose any recent results. Post-mitigation systems should be tested annually for the first two years and every two years thereafter.
When to call a professional
Real-estate testing, post-mitigation clearance, and homeowners uncertain about test placement or interpretation should engage a state-certified radon measurement professional. CDPHE and the National Radon Proficiency Program (NRPP) maintain certified-professional lookup tools. For routine surveillance, DIY charcoal canister kits are reliable when used correctly. For health concerns or symptoms, consult a physician — radon-induced lung cancer follows the same diagnostic pathway as other lung cancers and persistent respiratory symptoms warrant medical evaluation regardless of test results.
References
- EPA Radon Information — U.S. Environmental Protection Agency
- CDPHE Radon Program — Colorado Department of Public Health and Environment
- American Lung Association Radon Resource — Lung.org
- CDC Radon Health Information — Centers for Disease Control and Prevention
Front Range homeowners planning a first radon test or looking for a professional measurement before a purchase can reach out through our contact page for a connection to a state-certified Colorado radon professional.