Radon Air Test: What Homeowners Need to Know
A radon air test measures radon-222 gas concentration in indoor air, expressed in picocuries per liter (pCi/L). The test is what most homeowners and real estate professionals mean when they talk about radon testing, but a parallel and less common category exists: radon-in-water testing for homes on private wells. For the vast majority of Colorado residential transactions, the air test is the standard and the only test ordered. Water testing for radon is rare and typically warranted only when the home draws from a private well and air testing has already returned elevated readings. This guide draws on EPA, CDPHE, and CDC guidance current as of 2026 and is not a substitute for professional radon measurement or certified mitigation advice.
What a radon air test measures
The test quantifies the concentration of radon-222, a radioactive noble gas produced by the decay of uranium-238 in soil and bedrock. The radon migrates upward through soil and enters homes through foundation cracks, slab penetrations, sump pits, and other below-grade openings. Once inside, radon decays into short-lived radioactive progeny that attach to airborne dust and can be inhaled. Long-term inhalation of radon and its progeny is the second-leading cause of lung cancer in the United States after smoking, according to EPA and CDC. The air test measures the concentration of the parent radon-222 in indoor air; the inhalation risk comes mostly from the progeny, but progeny levels track parent radon under typical home conditions, so the parent measurement is sufficient for risk assessment.
Air testing versus water testing: when each applies
Air testing is the standard residential measurement. Every home that has been built, regardless of its water source, contains air that could be contaminated by radon from the surrounding soil. The air test is therefore universally applicable and is the default for pre-purchase inspections and for routine homeowner testing. Water testing applies in a much narrower set of cases. Radon dissolves in groundwater, and homes drawing from private wells in radon-rich geologic zones can have measurable radon in their water supply. When this water is used for showering, dishwashing, or other activities that release dissolved gas into indoor air, the water becomes an additional radon source. Public water supplies typically do not have meaningful radon because the water passes through open reservoirs and aerators that release dissolved gas before delivery.
Most Front Range homes are on municipal water, which means water testing is generally unnecessary. Properties drawing from private wells in mountain or rural Front Range areas are the exception. Homes in Elbert, Douglas (rural), or Jefferson (foothills) counties on private wells should consider water testing as a follow-up if air testing shows persistently elevated radon despite a working mitigation system.
How an air test works mechanically
Three device categories dominate residential radon air testing. Continuous radon monitors are electronic devices that sample air continuously and record hourly concentrations. They typically use a pulsed ionization chamber or a photodiode to detect alpha particles emitted by radon decay. Charcoal canisters are passive devices containing activated charcoal that adsorbs radon over 2-7 days; the canister is then sealed and shipped to a laboratory for gamma spectroscopy analysis. Alpha-track detectors contain a polymer film that records permanent etch tracks from alpha particles over 90 days or longer. Electret ion chambers measure the discharge of a precharged electret caused by air ionization from radon decay.
The choice of device depends on the test purpose. Pre-purchase inspections almost always use continuous radon monitors because they produce immediate results and document closed-house conditions. Long-term homeowner testing typically uses alpha-track detectors because they capture seasonal variation. Initial screening on a budget often uses charcoal canisters because they are inexpensive. The full landscape of devices is covered in the radon detector device explainer.
Closed-house conditions for an air test
Short-term air tests (less than 90 days) require closed-house conditions for at least 12 hours before the test and throughout the test. All exterior doors and windows stay closed except for normal entry and exit. The HVAC system operates as usual. Whole-house fans, attic fans, and continuous exhaust fans should not run during the test. The closed-house requirement exists because open windows dilute indoor radon and produce artificially low readings. The test is supposed to capture the radon level that would build up under normal winter living conditions when windows are closed.
Long-term tests (90 days or longer) capture annual or seasonal averages and do not require strict closed-house conditions throughout. The longer averaging window reduces the impact of any specific day’s ventilation pattern. This is why long-term tests are often considered more representative of true exposure than short-term tests, although short-term tests are practical for pre-purchase decisions.
Placement of the air test device
The device sits in the lowest livable level of the home. For a finished basement, the test occurs in the basement. For an unfinished basement that is not used as living space, the test occurs on the first floor. Placement is at breathing height, typically 2-6 feet above the floor, away from drafts, humidifiers, exterior walls, and direct sunlight. The lowest-level rule reflects the fact that radon enters from the ground and concentrates near the foundation; a basement reading is typically the highest reading in a home, and the EPA action level applies to the lowest livable level. The broader Colorado radon testing guide covers placement protocols in more detail.
How to interpret air test results
The result is a concentration in picocuries per liter of air (pCi/L). The EPA action level is 4 pCi/L. The World Health Organization recommends 2.7 pCi/L. Below 2 pCi/L is generally considered low risk, although EPA notes that no level is fully “safe.” Between 2 and 4 pCi/L, EPA recommends considering mitigation, particularly if the home has children or smokers. At or above 4 pCi/L, EPA recommends mitigation. Above 10 pCi/L, mitigation is strongly recommended and often urgent. Above 20 pCi/L, immediate mitigation and consideration of alternative living arrangements during mitigation may be warranted.
The Colorado Zone 1 air test baseline
The entire state of Colorado is classified as EPA Radon Zone 1, the highest risk category. Tests of Colorado homes register elevated radon at higher rates than the national average. CDPHE data shows roughly one in two tested Colorado homes above the EPA action level in some surveys. The baseline expectation for any Colorado air test is that an elevated result is plausible regardless of neighborhood, home age, or construction style. Newer homes built with passive radon-resistant features show lower average readings than pre-1990 homes, but a “newer” home is no guarantee of a passing result. Testing remains essential.
Why air testing is the residential standard
Four reasons make air testing the standard. First, every home contains air that could be radon-contaminated regardless of water source. Second, air testing is inexpensive relative to the information it provides. Third, air results map directly to inhalation risk, which is the dominant exposure pathway. Fourth, mitigation systems target the air pathway, so the post-mitigation verification test is also an air test. Water testing is a follow-up tool for specific situations, not a routine pre-purchase requirement.
How air testing relates to mitigation
An elevated air test triggers mitigation planning. The standard mitigation system is active soil depressurization, which creates suction beneath the foundation slab and vents radon directly outdoors through a pipe and inline fan. The system reduces indoor radon by intercepting it before it enters the home. Post-mitigation testing is also an air test, typically conducted 30 days after installation to confirm the system reduces concentrations below 4 pCi/L. Some mitigation contractors include the post-mitigation air test in the installation price; others charge separately. Either way, the air test is the verification mechanism for the mitigation work.
Seasonal variation in air test results
Indoor radon levels vary by season because the building envelope behaves differently in winter and summer. Winter heating creates a stack effect that draws air from the basement up through the home and out through the attic; this stack effect pulls radon from the soil through foundation openings at higher rates. Summer ventilation with open windows dilutes indoor radon. Short-term tests conducted in winter therefore tend to read higher than the annual average, and tests conducted in summer tend to read lower. Long-term tests average across seasons and produce a more representative annual exposure estimate. For pre-purchase decisions, the short-term winter test is the standard and the results should be interpreted with the seasonal pattern in mind.
When to retest after an initial air test
Retesting is warranted in several situations. If the initial test was conducted during severe weather, retest under more stable conditions. If the initial test result was borderline between 2 and 4 pCi/L, a follow-up long-term test produces a more confident estimate. If mitigation has been installed, post-mitigation testing verifies the system works. If two years have passed since the last test, EPA recommends retesting to confirm the original result. If the home has been renovated, particularly foundation work, retest before assuming the previous result still applies.
Tampering and data integrity in air testing
Continuous radon monitors include sensors that detect tampering during the test. The hourly data record shows whether the device was moved, whether windows were opened, and whether the device was unplugged or covered. Tampering attempts during pre-purchase testing are rare but not unknown; some sellers have been documented opening windows to dilute the reading. Reputable Colorado inspectors review the hourly data for tampering signatures before issuing the radon report. If tampering is detected, the test is invalidated and re-testing is recommended at the buyer’s discretion. The advantage of continuous radon monitors over charcoal canisters in pre-purchase testing comes largely from this tamper-detection capability.
The role of radon air testing in long-term homeowner care
Pre-purchase testing is the most common context for radon air testing, but homeowners who have lived in their homes for years should also test periodically. EPA recommends retesting every two years and after any significant home modification. Newly purchased homes should be retested after any major renovation, after foundation repair, after HVAC replacement, and after mitigation system maintenance. The cost of a homeowner-driven air test is modest, in the range of $25 for a charcoal canister kit ordered online plus a $30 lab analysis fee. Continuous radon monitor rentals from local hardware stores or through Colorado state and county programs are also available at low cost. For Front Range homeowners, the CDPHE radon program publishes resources for accessing low-cost or free test kits.
References
- EPA Radon Information — U.S. Environmental Protection Agency
- CDPHE Radon Program — Colorado Department of Public Health and Environment
- CDC Radon Topic Page — Centers for Disease Control and Prevention
- American Lung Association Radon — American Lung Association
Front Range homeowners coordinating a radon air test as part of pre-purchase due diligence can get in touch here for a vetted local inspector referral.