How to Test for Radon Gas in Your Home: A Plain-Language Guide
The question of how to test for radon gas in your home deserves an answer that explains what the gas actually is and why standard methods detect it. Radon is a noble gas, chemically inert, radioactive, colorless, odorless, and tasteless. It cannot be detected by smell, by carbon monoxide alarms, or by any standard combustion-gas detector. The only way to know radon is present is to test for it with devices designed to detect the alpha particles emitted as the gas decays. This guide summarizes EPA, CDC, and NIEHS guidance current as of 2026 — consult a certified radon professional for transactional or post-mitigation testing.
What radon gas actually is
Radon-222 is the most relevant isotope for indoor air. It is a noble gas, chemically unreactive, produced by the radioactive decay of radium-226 in soil and rock. The half-life is 3.82 days, which is short enough that radon does not persist in well-ventilated outdoor air but long enough that it accumulates in enclosed indoor spaces with limited ventilation. Radon decays by emitting alpha particles, which are helium nuclei. Detection methods count these alpha particles directly or count the radon progeny (polonium-218, polonium-214) that share the alpha-emission pathway.
Why radon gas is dangerous
The gas itself is not the primary biological concern. Radon decays into solid progeny (polonium-218, lead-214, bismuth-214, polonium-214) that attach to dust particles. Inhaled progeny lodge in lung tissue, where their continued alpha decay damages cell DNA over years of exposure. EPA estimates radon causes approximately 21,000 lung cancer deaths annually in the US, making it the second-leading cause of lung cancer after tobacco. CDC and WHO concur on the estimate. The health risk is dose-dependent: longer exposure at higher concentrations means higher risk.
How radon gas enters homes
Radon gas in soil migrates through cracks in the slab, gaps around plumbing penetrations, sump pump openings, crawl space dirt floors, and even diffuse pressure-driven flow through concrete. The driving force is the stack effect: warm air rising in the upper levels of the home creates negative pressure at ground level, drawing soil gas inward. Heating season amplifies the stack effect because temperature differential is larger. Granite, shale, and uranium-rich bedrock under the home are the highest-emission soil types. The entire Colorado Front Range sits on radon-producing geology.
Why standard gas detectors don’t detect radon
Carbon monoxide detectors, natural gas detectors, propane detectors, and household smoke alarms cannot detect radon. Each is designed for a specific chemical species: CO detectors use electrochemical sensors that respond to carbon monoxide; natural gas detectors use semiconductor sensors that respond to methane and propane combustion byproducts. Radon is chemically inert and does not trigger any standard combustion-gas sensor. The only detection methods are radiation-based: counting alpha particles from radon decay or sensing decay products on a sample medium.
The alpha particle detection principle
Radon-222 decays to polonium-218 by emitting an alpha particle of 5.49 MeV. Each detection method exploits this alpha emission. Charcoal canisters absorb radon onto activated charcoal during the exposure window; the lab measures gamma rays from the trapped radon and its progeny using gamma spectroscopy. Alpha-track detectors use plastic film that records the etched tracks of alpha particles directly. Continuous radon monitors use scintillation cells or solid-state detectors that produce electronic pulses for each alpha event. The physics is consistent; the detection medium varies.
Where to test for radon gas
Place the test in the lowest livable level of the home, where radon concentrations are typically highest because of proximity to the soil source and reduced ventilation. Specific room requirements: 20 to 70 inches above the floor; at least 4 inches from exterior walls; at least 3 feet from windows, doors, and HVAC vents; away from drafts and heat sources; in a room normally occupied. Basements register the highest readings in nearly every home because of slab proximity and reduced ventilation. See our basement radon testing guide for why.
Closed-house conditions and the gas dynamic
Closed-house conditions matter because radon gas concentration is a balance between soil-gas entry rate and indoor ventilation rate. Open windows dramatically increase ventilation, dropping indoor concentration even though the entry rate is unchanged. A short-term test with open windows underreports the baseline concentration that the home actually maintains during normal heating-season operation. EPA requires closed-house conditions (windows and exterior doors closed except for normal entry, for 12 hours before and during the test) for short-term tests. Long-term tests don’t require closed-house because the long exposure window averages out ventilation effects.
Gas concentration units explained
US testing reports concentration in picocuries per liter (pCi/L), a unit of radioactivity per volume. One picocurie equals 0.037 becquerel; the international standard reports in becquerel per cubic meter (Bq/m³). The conversion: 1 pCi/L = 37 Bq/m³. EPA’s action level of 4 pCi/L equals 148 Bq/m³. WHO’s recommendation of 100 Bq/m³ equals 2.7 pCi/L. Norway’s action level of 100 Bq/m³ matches WHO. Reports from accredited labs always state the units; converting between systems requires only the multiplication factor. See the how to check radon levels guide for the unit detail.
Short-term test approaches for radon gas
Short-term gas testing uses charcoal canisters, electret ion chambers, or continuous radon monitors over 2 to 7 days. Charcoal canisters absorb the gas during exposure and are analyzed by gamma spectroscopy. Cost: $15 to $30 including lab fee. Electret ion chambers use a charged Teflon disk that loses charge as alpha particles from decaying gas neutralize the surface. Cost: $30 to $80, often reusable. Continuous monitors count alpha events from the gas in real time. Consumer CRM cost: $200 to $400; professional CRM rental as part of a paid test: $150 to $250.
Long-term test approaches for radon gas
Long-term gas testing uses alpha-track detectors over 90 days to one year. The detector contains a small piece of plastic film (CR-39 or similar) mounted in a passive housing that allows gas diffusion but limits other particle entry. Alpha particles from radon decaying near the film etch tracks; the lab counts the tracks under microscopy and reports time-averaged concentration. Cost: $30 to $60. Long-term testing is EPA’s preferred method for accurate risk assessment because the averaging window captures seasonal variation.
The role of the radon mitigation system
A radon mitigation system, specifically sub-slab depressurization, intercepts the gas before it enters the home. A PVC pipe runs from a suction point under the slab to a fan that exhausts soil gas through the roof. The fan creates negative pressure beneath the slab, redirecting the stack-effect pressure gradient that would otherwise draw gas inward. Reduction of indoor concentration is typically 50 to 90 percent. Cost: $1,500 to $3,000 for a typical single-family home. Post-mitigation testing verifies system performance. Our radon abatement system guide covers the SSD design.
Testing pre-purchase, occupied, or new-construction homes
Pre-purchase testing follows EPA’s real estate protocol: a short-term test with confirmation or two simultaneous tests under closed-house conditions, typically using a professional CRM for documented chain of custody. Occupied-home testing for personal information can use a DIY kit. New-construction homes with Radon-Resistant New Construction (RRNC) features should still be tested post-occupancy because the passive system may need to be activated into an active SSD system depending on measured levels.
Why radon gas testing is required in some states
Several states require radon testing on residential real estate transactions or on certain commercial properties. Illinois requires disclosure of any known radon test results above 4 pCi/L. New Jersey requires radon-resistant new construction in designated high-radon areas. Florida requires testing in some commercial transactions. Colorado does not statewide-require testing, but local jurisdictions may. EPA encourages testing in all 50 states regardless of statutory requirements, with Map Zone 1 areas (including the entire Front Range) being the highest priority.
Putting gas testing in context
Radon gas testing is straightforward technically and inexpensive financially: a $20 charcoal canister is a defensible test method. The decision points are timing (short-term vs long-term), placement (lowest livable level under closed-house conditions), and follow-up (confirmation testing for elevated short-term results). The decision points are not hard once the homeowner understands the gas itself. The 21,000 annual US lung cancer deaths attributed to radon by EPA make the test one of the highest-return health investments a homeowner can make. The parent radon testing in Colorado guide covers the full decision framework.
How outdoor radon compares to indoor levels
Outdoor radon concentration averages around 0.4 pCi/L globally and is not a meaningful exposure source for most people. Indoor concentration in homes without mitigation typically runs 5 to 20 times higher than outdoor levels, depending on building envelope tightness, soil radon flux, and ventilation rate. The reason outdoor radon is not a health concern is dilution: open air disperses the gas before significant decay-product accumulation occurs. Indoor accumulation is the public health issue. This is why ventilation strategies (opening windows, increasing HRV operation) reduce indoor radon, though dedicated mitigation systems are more energy-efficient.
Why gas testing matters more in tight homes
Modern energy-efficient construction tightens the building envelope, reducing natural air-exchange rates from the 1 air change per hour typical of older homes to as low as 0.1 air changes per hour in passive-house construction. The reduced air exchange concentrates any soil-gas entry inside the building. Net effect: a passive-house home on radon-rich soil can register higher indoor levels than a leaky 1960s home on the same soil. Energy-efficient construction is not a problem in itself; it is a reason to plan radon-resistant features in design and to test more diligently after occupancy.
Front Range homeowners with elevated gas-test results who want a connection to a vetted mitigation contractor can reach out through our contact page.
References
- EPA radon gas information and health resources — U.S. Environmental Protection Agency
- CDC radon gas health resources — Centers for Disease Control and Prevention
- NIEHS radon gas research — National Institute of Environmental Health Sciences
- CDPHE Colorado radon program — Colorado Department of Public Health and Environment