Testing for Radon Gas: What Homeowners Need to Know
Testing for radon gas means measuring the concentration of a specific radioactive gas in indoor air, in picocuries per liter, using detectors calibrated for that single substance. Radon is not lumped in with carbon monoxide, volatile organic compounds, formaldehyde, or general air-quality readings. It has its own measurement infrastructure because the molecule, the health risk, and the entry pathway are distinct. This guide explains why radon testing is its own category, how gas measurement works, what equipment is appropriate, and what Front Range Colorado homeowners should expect from a test. EPA, CDC, and Colorado Department of Public Health and Environment guidance current as of 2026 underpins the answers.
Why radon is a gas, and why that matters for testing
Radon (Rn-222) is a noble gas. It has no chemical reactivity worth speaking of, no smell, no taste, no color. It is heavier than air by mass but mixes through indoor air via normal convection and diffusion rather than settling to the floor like a denser combustion gas. Radon is produced by the radioactive decay of radium in soil and rock, primarily from uranium-bearing geology, and seeps into buildings through every below-grade entry path: foundation cracks, slab penetrations, sump pits, untrapped floor drains, and the soil-to-air interface beneath basement floors.
Because radon is a gas, detection methods rely on either absorbing radon onto a substrate (charcoal canister) or counting the alpha particles emitted by its decay products (alpha-track and Continuous Radon Monitor instruments). Either approach measures the time-averaged airborne concentration. The framework is summarized at the EPA radon program page.
How radon gas enters a home
The dominant entry pathway is the stack effect: warm indoor air rises through the building, creating slight negative pressure at the foundation level, which pulls soil gas (including radon) up through any available opening. Cracks in the foundation slab, gaps where utility penetrations enter through the slab, sump pits without sealed lids, untrapped floor drains, and the entire perimeter where the slab meets the foundation wall all contribute. Crawlspace homes pull radon up through the floor system above.
The geology determines how much radon is available in the soil. The building details determine how much of that available gas makes it inside. Front Range Colorado has high uranium-bearing geology and many homes have finished basements with significant slab contact area — a combination that places the region in EPA Zone 1, the highest predicted screening level, per CDPHE’s CDPHE radon program.
How radon gas testing differs from other indoor air tests
Carbon monoxide tests use electrochemical sensors that respond to CO in real time and produce alarm-grade readings. CO detection is acute-emergency framing. A spike triggers immediate evacuation and ventilation.
Volatile organic compound (VOC) tests use photoionization or metal-oxide sensors that respond to a broad class of organic compounds. VOC readings are general indoor air quality framing. Elevated VOC suggests indoor sources (cleaning chemicals, off-gassing furniture, paint) and often resolves with ventilation.
Radon gas tests use either charcoal absorption (measuring decay products in the charcoal after sampling) or alpha-particle detection (counting decays directly). Radon framing is long-term cumulative-exposure framing. A single high reading is not an emergency, but ongoing exposure produces measurable lung-cancer risk over years and decades.
This is why consumer indoor air monitors that bundle “radon detection” with CO, VOC, and PM2.5 readings often produce less-reliable radon data than dedicated radon-only instruments. The detection physics is different, the response time is different, and the measurement averaging interval is different.
Testing methods for radon gas
Short-term test kits
Charcoal canister or alpha-track detectors that run for two to seven days under closed-house conditions. Cost: fifteen to thirty-five dollars. Result: single time-averaged pCi/L number. Best for: initial screening, fast actionable data.
Long-term test kits
Alpha-track detectors that run for ninety days to a year. Cost: twenty-five to fifty dollars. Result: single time-averaged pCi/L number with seasonal variation captured. Best for: representative long-term exposure assessment.
Continuous Radon Monitors
Electronic instruments that sample air continuously and report concentration over time. Consumer-grade: one hundred fifty to four hundred dollars. Professional-grade: more. Best for: ongoing monitoring, post-mitigation verification, real-estate testing requiring traceable instrument calibration.
Certified measurement professional
Colorado-certified radon measurement professional performs the test using calibrated CRM or other approved instrumentation under EPA Protocol A or B. Cost: one hundred fifty to three hundred dollars. Best for: real-estate transactions where documented professional results carry more weight than DIY kits.
EPA Protocol A and Protocol B
The EPA defines two protocol families for radon measurement. Protocol A requires closed-house conditions throughout the test — windows and exterior doors closed except for normal entry, HVAC operating normally, beginning at least twelve hours before the test starts. Protocol A produces conservative (higher) readings reflecting peak occupancy conditions.
Protocol B allows normal occupancy without specific closed-house preparation. Protocol B is more realistic for “typical living conditions” but produces variability that can mask the home’s underlying radon level.
Most short-term consumer kits use Protocol A. Most real-estate-context certified tests use Protocol A. Long-term tests typically use Protocol B because the long sampling period smooths out short-term variability.
What Front Range homeowners should expect
CDPHE estimates roughly half of Colorado homes test above 4.0 pCi/L. A first short-term reading anywhere from 2 to 12 pCi/L is unremarkable. Readings above 12 are less common but documented. Readings below 2 happen and are good news but cannot be predicted by neighborhood, builder, or construction year.
Each home is its own measurement. Foundation type, slab contact area, sump pit presence, basement finish level, HVAC ducting routing, and the home’s natural ventilation rate all influence indoor concentration. Two homes side-by-side can produce different results.
Testing during real-estate transactions
Front Range real-estate contracts increasingly include radon testing as an inspection contingency. The buyer’s agent or buyer’s inspector arranges for a certified measurement professional to place a CRM in the home for the required test period (typically 48 to 72 hours minimum), produce a written report, and deliver it before the contingency deadline.
Sellers can pre-test before listing to control timeline. A pre-tested home with documented low radon (or a working mitigation system) often closes faster than a property where testing is contingent on the buyer’s schedule.
Interpreting a result
Above 4.0 pCi/L: EPA action level. Mitigation strongly recommended. Front Range mitigation typically costs twelve to twenty-five hundred dollars.
2.0 to 4.0: consider mitigation. The EPA notes any radon level carries some risk. Some homeowners mitigate; others retest or run a long-term confirmation.
Below 2.0: no mitigation generally recommended. Retest every two years.
The in-batch how to measure radon levels guide covers numeric interpretation in detail. The what is a radon detector used for guide covers ongoing monitoring use cases.
What testing does not do
A radon gas test does not measure radon in water (separate test, primarily relevant for well water — most municipal water sources are below problematic levels). It does not measure radon in soil under the home (sometimes done in pre-construction planning but not normally as part of home testing). It does not test for radon-decay products’ deposits in lungs or anywhere else in the body.
The test answers one question: what is the average airborne radon concentration in the tested space during the test period? That single answer drives the mitigation decision. Other questions require other tests.
The radioactive decay chain that produces radon
Understanding why radon shows up in homes helps explain why testing is the only reliable detection method. Uranium-238 sits in trace amounts throughout much of the earth’s crust, with higher concentrations in certain granite, shale, and phosphate-bearing geological formations. Uranium decays slowly over billions of years through a chain of intermediate radioactive isotopes — thorium-234, protactinium-234, uranium-234, thorium-230, radium-226 — eventually producing radon-222 as a noble gas with a half-life of about 3.8 days.
Because radon is gaseous and chemically inert, it migrates freely through soil pore spaces and rock fractures. It reaches the soil-to-air interface, and from there enters buildings through any available opening. The 3.8-day half-life is long enough for radon to travel from soil into homes but short enough that it does not accumulate beyond what fresh decay produces.
Radon itself decays to a series of solid radioactive daughter products (polonium-218, lead-214, bismuth-214, polonium-214). These daughters attach to dust particles and aerosols and, when inhaled, deposit in lung tissue. The decay products’ alpha and beta emissions are what produce the long-term lung-cancer risk. Radon’s contribution to lung cancer is mediated by its daughters, not by radon itself.
Why short-term tests can miss elevated homes
A short-term test averages over its duration. A home that reads 3.2 pCi/L on a two-day November test in mild weather might read 7.5 pCi/L on a two-day January test during a cold snap with high pressure suppressing soil ventilation. The home itself didn’t change; the test caught different conditions.
This is why EPA recommends following up a short-term result with either a second short-term test or a long-term test. The follow-up either confirms the original reading is representative or reveals that conditions varied. A first short-term test should be treated as informative, not final, for any borderline result.
Testing in different seasons
Winter testing typically produces conservative (higher) readings because closed-house heating concentrates indoor air. Summer testing typically produces lower readings because natural ventilation through open windows disperses radon. Spring and fall produce middle-range readings depending on weather stability.
For Front Range homeowners doing their first test, winter testing is the most diagnostic single window because it captures the conservative end of the annual range. Homes that test below 2.0 in winter almost always test below 2.0 across the year. Homes that test above 4.0 in winter may or may not be above 4.0 across the year, depending on how much summer ventilation pulls the average down. A long-term test resolves the question.
When to retest
Retest every two years for unmitigated homes. Retest after mitigation (thirty to ninety days after system installation). Retest after major foundation work, basement finishing, HVAC system replacement, or any building-envelope change that alters airflow. Retest if the home has not been tested in the past five years.
The parent radon testing in Colorado hub covers the broader Front Range context. The radon test time frame guide covers test duration specifics.
References
- EPA radon program — A Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- Colorado Department of Public Health and Environment radon program — CDPHE
- CDC radon and health — Centers for Disease Control and Prevention
Front Range homeowners considering a radon test before listing or buying can connect with a vetted radon professional through our contact page.