Whats a Radon Test: A Plain-Language Explainer
Whats a radon test in practical terms? It is a measurement of radon-222 concentration in indoor air, expressed in picocuries per liter (pCi/L), used to decide whether a home needs mitigation. The test can be a 48-hour electronic monitor reading, a 2- to 7-day mail-in charcoal kit, or a 90-day to one-year alpha-track detector — each appropriate for a different situation. This guide summarizes EPA and Colorado Department of Public Health and Environment (CDPHE) guidance current as of 2026 and walks through what the test does, how to choose between options, and what the results mean. Nothing here is medical advice or a substitute for professional radon assessment.
Whats a Radon Test in One Paragraph
A radon test is a measurement of how much radon gas is in a home’s indoor air. Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. It seeps into homes through foundation cracks, slab penetrations, sump pits, and crawl space soil. The gas is colorless, odorless, and chemically inert — there is no way to detect it without a measurement device. EPA designates 4.0 picocuries per liter (pCi/L) as the action level above which mitigation is recommended. CDPHE estimates that roughly half of Front Range Colorado homes test above 4.0 pCi/L because of the region’s uranium-bearing geology.
The test produces a single number — the average radon concentration during the measurement period. The number drives the mitigation decision. Below 2.0 pCi/L, no action is typically needed. Between 2.0 and 4.0, EPA recommends consideration of mitigation. At or above 4.0, mitigation is recommended. Above 10.0, mitigation is urgent. The radon testing pillar for Colorado homeowners walks through the full testing decision and Colorado-specific risk patterns.
Why Radon Testing Matters
EPA estimates radon causes about 21,000 lung cancer deaths in the United States each year, making it the second leading cause of lung cancer after smoking and the leading cause among non-smokers. The CDC has reached similar estimates. The risk is dose-dependent: long-term exposure to elevated radon raises lung cancer risk meaningfully; short-term exposure to a single elevated reading does not. The case for testing is that the only way to know whether a home has elevated radon — and therefore whether the homeowner is accumulating elevated long-term exposure — is to measure.
Colorado’s geology makes testing especially important. The uranium-bearing soils and rocks across much of the Front Range, the central mountain corridor, and parts of the western slope produce naturally elevated radon entry into homes. EPA Zone 1 designation covers most of the populated state. CDPHE survey data shows roughly half of tested Front Range homes above 4.0 pCi/L. Specific homes within Zone 1 vary widely — neighbors with similar homes can have very different radon levels because foundation construction, ventilation patterns, and soil contact each affect entry rates.
Buyers in real-estate transactions on the Front Range almost always include radon testing in the inspection contingency. Mitigation, if needed, runs $1,200 to $2,000 for a typical sub-slab depressurization system installed by a licensed Colorado mitigator. The cost of testing ($135 to $200 for an inspector CRM test) is small relative to the mitigation cost and far smaller than the long-term health risk of ignoring elevated levels. The radon health risks guide walks through what the evidence shows about lung cancer risk.
Test Types Explained
Three test types dominate residential radon testing. Short-term passive charcoal canisters expose for 48 hours to 7 days and ship to a lab for analysis. Continuous radon monitors (CRMs) measure radon electronically in real time and produce 48-hour reports on the device. Alpha-track long-term detectors expose for 90 days to a year and ship to a lab for analysis.
Each test type has a different cost, timeline, and accuracy profile. Charcoal canisters are cheapest ($15 to $35) but slowest end-to-end (7 to 14 days). CRMs are mid-priced ($135 to $200 for an inspector test, $150 to $400 for a consumer device) and fastest (2 to 4 days end-to-end for a 48-hour test). Alpha-track detectors are mid-priced ($25 to $50) and slowest (3 months to over a year) but produce the most representative annual average.
The right test depends on the situation. Real-estate buyers under contingency time pressure use the inspector’s CRM. Homeowners doing baseline testing without time pressure can use any test type. Homeowners following up on a borderline short-term result often use a long-term alpha-track detector for confirmation. Homeowners verifying a mitigation system after installation use either a short-term CRM (immediate confirmation) or a long-term alpha-track (seasonal confirmation).
Short-Term vs Long-Term Testing
Radon levels vary substantially across seasons — typically higher in winter when homes are sealed up against cold and lower in summer when ventilation is higher. A short-term test produces a snapshot of the test period; a long-term test produces an annual average. A short-term test in February may read 6 pCi/L while a long-term test averaged across the year may read 3.5 pCi/L. Both are correct in their respective measurement windows; the long-term result better reflects typical year-round exposure. EPA recommends long-term testing when feasible and short-term testing when speed is required.
The Closed-House Protocol
All short-term tests require a closed-house protocol that begins 12 hours before the test starts and continues throughout. Windows and exterior doors stay closed except for normal entry and exit. Whole-house fans, attic fans, and extended bathroom fan cycles stay off. The HVAC system runs in normal mode. The protocol prevents ventilation from artificially lowering the indoor radon concentration during the test period.
Skipping the closed-house protocol can drop the result by enough to produce a falsely reassuring number. An open window during a 48-hour test can lower the average by half. Buyers and homeowners should plan around the closed-house protocol; for vacant homes (between owners during a real-estate transaction) the protocol is met naturally. For occupied homes, a brief conversation with the inspector or buyer’s agent ensures cooperation.
Inspectors document the protocol in the test report. Some CRMs flag protocol violations automatically by detecting unusual hourly variation that suggests a window was opened. A test that fails the closed-house protocol is grounds to retest. The retest adds 2 to 4 days to the timeline, which is why early booking in the inspection contingency window matters.
Where to Place the Test
The test device should sit in the lowest livable area of the home — a finished basement if used as living space, otherwise the lowest finished floor. Place it 20 inches above the floor, at least three feet from exterior walls, doors, and windows, and away from drafts, heat sources, high humidity, and direct sunlight. The location should not be disturbed during the test period.
For homes with multiple levels, EPA recommends testing the lowest level used regularly. A finished basement that hosts a home office or rec room is the right test location even if the family spends most of its time on the main floor. A walkout basement that is fully finished and used as living space gets tested even if the family spends little time there. Unfinished basements and crawl spaces are not tested because they do not represent typical occupant exposure.
Placement in a kitchen, bathroom, or laundry room — areas with high humidity or air movement — can interfere with some detector types and should be avoided. Stable interior surfaces in living areas produce the most representative results. The what is a radon test explainer covers placement and protocol details further.
Reading the Result
Test results come back as a single number in picocuries per liter (pCi/L). EPA’s action level is 4.0 pCi/L. At or above 4.0, mitigation is recommended. Between 2.0 and 4.0, EPA recommends considering mitigation. Below 2.0, no action is typically needed. EPA notes that no level of radon is risk-free, and the World Health Organization’s recommended action level is 2.7 pCi/L (100 Bq/m³) — lower than the U.S. action level.
Lab reports for charcoal and alpha-track tests typically include the result number, the test period, the lab’s QA notes, and the EPA action-level comparison. CRM reports include the result number plus an hourly chart showing variation across the test period. The hourly chart helps validate the test conditions; an unusual dip suggests a window was opened, an unusual spike suggests a closed-house protocol issue or weather-driven variation.
A single test result, especially short-term, is a snapshot. Borderline results (3.0 to 4.5 pCi/L) often warrant follow-up with a long-term test before deciding to mitigate. Clear results (below 2.0 or above 6.0 pCi/L) usually do not need follow-up to support a decision. The DIY radon test accuracy guide covers result interpretation and when to retest.
What Happens After an Elevated Result
An elevated result (4.0 pCi/L or higher) calls for mitigation. The standard mitigation system on the Front Range is sub-slab depressurization (SSD): a fan-powered vent that pulls air from beneath the foundation slab and exhausts it above the roofline, reversing the pressure gradient that drives radon entry. SSD systems installed by a licensed Colorado mitigator typically reduce radon by 80 to 99 percent and bring most homes well below the action level. Cost runs $1,200 to $2,000 for a typical installation.
Post-mitigation testing is recommended within 30 days of installation to confirm the system reduced radon below action level. EPA also recommends a long-term follow-up test at 90 days to confirm the system is working across seasons. Most homeowners use a short-term CRM or charcoal kit for the immediate verification and an alpha-track detector for the seasonal verification. The radon mitigation cost guide covers system installation and verification testing in detail.
Some homeowners with borderline results choose to install a passive radon-reduction system (passive sub-slab venting without a fan) and re-test. Passive systems can lower radon by 30 to 70 percent, which may be enough to bring a borderline reading below action level. Active systems with fans are more reliable for higher-level reductions. The decision depends on the initial reading and the homeowner’s risk tolerance.
Front Range Context
EPA designates much of the Colorado Front Range as Zone 1, the highest-priority radon zone. CDPHE survey data shows roughly half of Front Range homes test above the EPA action level of 4.0 pCi/L. The Front Range counties — Denver, Douglas, Elbert, Arapahoe, Jefferson, El Paso, Adams, Boulder, Broomfield, and surrounding areas — all carry elevated baseline risk relative to the national average.
Real-estate buyers on the Front Range almost always include radon testing in the inspection contingency. The 48-hour CRM test fits the typical contingency window with several days to spare for follow-up if needed. Sellers in Front Range markets often pre-test before listing, which gives the buyer existing data to compare against the inspection-time test. CDPHE maintains a list of certified radon measurement professionals; reputable inspectors hold this certification.
Front Range homeowners not currently in a transaction should test every two years per EPA recommendation, or after any major foundation work, structural change, or HVAC change that could affect radon entry. A single negative test years ago is not a guarantee that the level is still low. Testing is cheap; mitigating without knowing whether mitigation is needed is expensive. Periodic testing is the right ongoing practice.
References
- EPA radon program guidance — U.S. Environmental Protection Agency
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- CDPHE radon testing program for Colorado — Colorado Department of Public Health and Environment
- CDC about radon — Centers for Disease Control and Prevention
Front Range homeowners with elevated test results, borderline numbers, or questions about which test type fits a specific home can connect with a vetted local inspector through our contact page for property-specific guidance.