What Does Radon Test for: A Plain-Language Guide
What does a radon test actually measure? It measures the concentration of radon-222 gas in indoor air, typically reported in picocuries per liter (pCi/L). The radon-222 itself is the gas the test instrument detects. The actual lung-cancer agents are not the gas — they are the short-lived radon decay products that the gas produces continuously inside your lungs and on indoor surfaces. This guide summarizes EPA and CDC guidance current as of 2026 — consult your physician for any respiratory symptoms and a certified mitigation professional for elevated test results. Understanding what the number on a radon report represents is the first step in interpreting it correctly.
The substance measured: radon-222 gas
Radon-222 is a naturally-occurring radioactive gas with a half-life of 3.8 days. It forms from the decay of radium-226, which itself comes from uranium-238 in soil and rock. Granite-rich bedrock, like the Pikes Peak granite under much of the Front Range, produces measurable radon continuously. The gas migrates through soil pores, enters buildings through foundation cracks and slab penetrations, and accumulates in indoor air because it is denser than the surrounding atmosphere.
A radon test instrument — a charcoal canister, alpha track detector, electret ion chamber, or continuous radon monitor (CRM) — collects or measures radon-222 atoms or their immediate decay products over the test period. The reported result is an average concentration over the test duration, expressed in pCi/L or, in some international contexts, in becquerels per cubic meter (Bq/m³). One pCi/L equals 37 Bq/m³.
The agents that cause harm: radon decay products
Radon-222 by itself is chemically inert and breathed out about as easily as it is breathed in. The harm comes from its short-lived decay products: polonium-218, lead-214, bismuth-214, and polonium-214. These are solid metal atoms with very short half-lives that emit alpha radiation. Polonium-218 and polonium-214 in particular emit high-energy alpha particles that damage the cells lining the bronchial airways.
The decay products either attach to indoor aerosols (smoke, dust, water vapor) or remain “unattached.” Both forms can be inhaled and deposit in the bronchial tree. Once in the airway, the alpha emissions continue, and the cumulative dose over years of exposure drives the lung-cancer risk. EPA’s epidemiology framework attributes radon exposure to roughly 21,000 lung cancer deaths annually in the United States, second only to smoking. The American Lung Association tracks similar figures.
Why the test measures the gas, not the decay products directly
Most consumer and short-term professional radon tests measure radon-222 gas concentration rather than measuring the decay products themselves. The gas is easier to capture in a standardized sampling medium. Charcoal canisters adsorb radon-222 onto activated charcoal; the canister is sealed and sent to a lab for gamma-ray counting of the decay products that formed inside the sealed canister. Alpha track detectors register alpha particles striking a polycarbonate film over the test period.
Continuous radon monitors (CRMs) measure radon-222 directly through pulsed ionization or alpha spectrometry. They produce hourly readings that show how concentrations rise and fall through the test period. Both gas-based and decay-product-based measurements correlate well in stable indoor conditions, so the gas measurement is a reliable proxy for exposure risk.
Units: pCi/L and what the number means
One picocurie per liter (pCi/L) represents about 2.2 disintegrations per minute per liter of air. The EPA action level is 4 pCi/L — at or above this level, EPA recommends mitigation. The EPA consideration level is 2 pCi/L — between 2 and 4 pCi/L, EPA suggests considering mitigation. Below 2 pCi/L, EPA does not specifically recommend action, though no level of radon exposure is considered “safe.”
Outdoor air typically averages 0.4 pCi/L. Indoor air in homes without elevated radon typically averages 1.3 pCi/L. Elevated homes can reach 10, 20, 50, or in rare cases over 100 pCi/L. CDPHE radon-zone designations for Colorado place much of the state in EPA Radon Zone 1 (predicted average above 4 pCi/L), and CDPHE reports roughly half of tested Colorado homes show levels above the action level.
For more on the testing process itself, see our companion in-batch piece on radon testing — what is it. The parent radon testing in Colorado guide covers the Front Range context comprehensively.
EPA action level: where 4 pCi/L came from
The 4 pCi/L action level is not a “safe” threshold — it is the level at which EPA judged mitigation reliably feasible and cost-effective in the early 1980s. Below 4 pCi/L, mitigation systems were considered harder to validate as effective. The number has been retained as the action level since EPA’s first radon guidance in 1986 and reaffirmed in subsequent EPA publications.
The 2 pCi/L consideration level reflects the fact that lung-cancer risk does not drop to zero below 4 pCi/L. Risk is a continuous function of cumulative exposure. EPA’s “A Citizen’s Guide to Radon” makes the no-safe-level point explicitly. Homeowners with results between 2 and 4 pCi/L should consider mitigation, especially when occupants are nonsmokers (radon and smoking interact multiplicatively) or when children spend significant time in basements.
Test types and what each detects
Short-term tests (charcoal canister, electret short-term, CRM in 48-hour mode) typically run 2-7 days. They measure recent indoor radon concentration but can be affected by short-term weather and HVAC changes. EPA recommends short-term tests for initial screening and for real estate transactions.
Long-term tests (alpha track, electret long-term) run 90 days to 1 year. They produce an annual-average estimate less affected by short-term variability. EPA recommends long-term tests for confirming initial results and for understanding actual annual exposure. The choice between short and long depends on the question being asked.
Continuous radon monitors are the professional-grade option, providing hour-by-hour data, tamper detection, and rapid turnaround. CDPHE and EPA both recognize CRM results as valid for real estate timelines. For more, see accurate radon testing protocols.
What the test does not measure
A radon test measures radon-222 in indoor air at one location over the test period. It does not measure: radon in well water (a separate test); thoron (radon-220) from soil and stone — typically minor in residential contexts but tested separately when relevant; radon in adjacent crawl spaces or attached structures unless a test is placed in each; cumulative lifetime exposure of the occupants. Each of these is a separate measurement.
Radon concentrations also vary widely room to room and floor to floor. A basement test typically reads higher than an upstairs test because the gas enters from soil and is denser than air. EPA recommends testing on the lowest livable level of the home — basement if finished, ground floor otherwise.
Colorado context: Zone 1 designation
EPA’s Radon Zone map designates counties by predicted average indoor radon levels. Zone 1 counties have predicted averages above 4 pCi/L. Zone 2 counties have predicted averages between 2 and 4 pCi/L. Zone 3 counties have predicted averages below 2 pCi/L. Most of the Colorado Front Range — Adams, Arapahoe, Boulder, Broomfield, Denver, Douglas, El Paso, Jefferson — is Zone 1.
CDPHE’s own testing data shows roughly half of tested Colorado homes exceed 4 pCi/L. That makes radon testing nearly mandatory for any Front Range home purchase, regardless of seller disclosure. EPA’s Citizen’s Guide to Radon and CDPHE’s radon page both recommend testing every two years for occupied homes and after any major HVAC or foundation work.
Interpreting your test result
Above 4 pCi/L: EPA recommends mitigation. Schedule a confirmation test (short-term repeat or CRM) within a few weeks, then engage a NRPP- or NRSB-certified mitigation contractor. Mitigation typically reduces indoor radon by 50-99% and costs $1,500-$3,500 for a single-family home on the Front Range.
Between 2 and 4 pCi/L: Consider mitigation, especially if occupants are nonsmokers or if children spend extensive time at lower-floor levels. Retest after a year or seasonal change to confirm. Risk reduction from mitigation in this range is real but smaller in absolute terms than from elevated readings.
Below 2 pCi/L: No specific EPA action recommended. Continue periodic testing every 2-5 years, especially after HVAC or foundation changes. CDC and the American Lung Association both note that no level of radon is risk-free, but very low results align with national-average exposure.
References
- A Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- Radon information — Colorado Department of Public Health & Environment
- Radon and lung health — American Lung Association
- Radon information — Centers for Disease Control and Prevention
Front Range homeowners with elevated radon test results can reach out through our contact page for an introduction to NRPP- and NRSB-certified mitigation contractors and qualified Colorado inspectors who handle radon retests.