How to Test Radon Levels: A Homeowner Walkthrough Guide
Knowing how to test radon levels is one of the more useful pieces of homeowner knowledge for Front Range residents. EPA places all Front Range counties in Radon Zone 1, the highest-risk designation, and CDPHE estimates that approximately half of Colorado homes test above the action level of 4 pCi/L. The good news is that homeowner testing is reliable, inexpensive, and well-defined by EPA protocols. A short-term passive test kit costs fifteen to thirty dollars and produces a lab-certified result in two to three weeks total. This guide walks through the full process from buying a kit to interpreting the result and deciding next steps.
This guide summarizes EPA, CDPHE, and CDC guidance current as of 2026. Consult a certified radon professional for testing decisions in high-risk or post-mitigation scenarios.
The Two Categories of Home Radon Tests
Homeowner testing falls into two categories: passive and continuous. The choice depends on what the homeowner is trying to learn and how much they want to spend.
Passive Tests
Passive tests use materials that absorb or record radon decay products during the exposure period and are then analyzed by a lab. The two most common passive methods are charcoal canisters and alpha-track detectors. Charcoal canisters are used for short-term testing of 48 to 96 hours. Alpha-track detectors are used for long-term testing of three to twelve months. Both are sold as kits with prepaid lab postage. Prices run fifteen to thirty dollars for short-term and twenty-five to forty dollars for long-term.
Continuous Tests
Continuous radon monitors (CRMs) are electronic devices that record radon concentration hour-by-hour over a test period. They display the running average on a screen and store hourly readings for download. CRMs cost one hundred fifty to three hundred dollars for consumer models such as Airthings Wave Plus or Corentium Home. Professional-grade CRMs used by inspectors cost six hundred dollars and up. The continuous data is more informative than passive averages but the equipment cost is higher.
Both categories produce reliable results when used correctly. For the homeowner first-time test, passive short-term is the standard recommendation. For a property under contract with a 10- to 14-day inspection contingency, short-term continuous testing is preferred because the result is available immediately. Our radon sampling methods overview covers the trade-offs in more depth.
Choosing Test Placement
EPA protocols specify placement of the test device in the lowest livable level of the home. For most Front Range homes with finished basements, that means the basement. If the basement is unfinished and rarely occupied, the test goes on the first floor of habitable space. The principle is to measure radon where occupants spend significant time, not in a part of the home that does not represent typical exposure.
The device sits on a flat surface at breathing height, typically two to six feet above the floor. It should be at least three feet from exterior walls, doors, and operable windows. It should be at least twelve inches from interior walls. It should not be in a closet, kitchen, bathroom, or laundry room because cooking, showering, and washing affect humidity and airflow in ways that can skew results.
Closed-House Conditions
EPA short-term testing protocols require closed-house conditions for twelve hours before the test starts and throughout the test period. Closed-house means windows and doors stay closed except for normal entry and exit. HVAC systems can operate as they normally would. Whole-house fans should not run during the test. The closed-house requirement matters because open windows dilute indoor radon and produce artificially low results that do not reflect typical conditions.
Front Range homeowners testing in spring or fall may find closed-house conditions inconvenient because mild weather invites open windows. Testing during winter or summer when HVAC systems run continuously is often easier.
The Short-Term Passive Test Procedure
The standard short-term test runs 48 to 96 hours and uses a charcoal canister.
1. Buy the Kit
Charcoal canister kits are available from home improvement retailers, online retailers, state radon programs, and ALA. The Colorado Radon Program at CDPHE periodically offers low-cost or free kits. Verify the kit’s lab is listed as certified under the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB).
2. Verify the Closed-House Condition Window
Plan to maintain closed-house conditions starting twelve hours before opening the canister. If a major weather event such as a storm is expected, postpone the test because barometric pressure swings affect radon levels.
3. Open and Place the Canister
Remove the canister from its sealed package. Note the exact opening time on the included form. Place the canister in the chosen location.
4. Wait the Required Period
Leave the canister in place for the manufacturer-specified period, usually 48 to 96 hours. During this time, maintain closed-house conditions and normal HVAC operation. Do not move the canister.
5. Close and Ship the Canister
At the end of the test period, reseal the canister immediately. Note the closing time. Complete the form. Mail the canister to the lab using the prepaid postage. Mail it promptly because radon decay products in the canister decay further over time and delayed mailing reduces accuracy.
6. Receive the Result
Most labs return results within two to three business days of receipt by mail and email. The result reports the radon concentration in pCi/L averaged over the test period.
The Long-Term Passive Test Procedure
Long-term tests use alpha-track detectors that record radon decay products on a plastic film over three to twelve months. The longer averaging period captures seasonal variation that short-term tests miss. Long-term tests are recommended for homeowners who want a more representative annual average and for follow-up testing after a borderline short-term result.
Placement and protocols are similar to short-term testing, but closed-house conditions are not required for long-term tests because the long averaging period smooths out daily variation. Long-term tests cost slightly more than short-term but provide stronger data for action decisions in borderline cases.
The Continuous Monitor Procedure
A continuous radon monitor is plugged in or battery-powered and placed in the chosen location. The device samples radon continuously and records hourly readings. After 48 hours or longer, the device reports a running average. Some consumer CRMs continue measuring indefinitely and stream data to a smartphone app.
The advantage of a CRM is immediate, ongoing data. The homeowner sees the result without waiting for lab analysis. The disadvantage is equipment cost and the need to confirm the device is calibrated. Consumer CRMs are factory-calibrated but should not be used for real-estate transaction decisions where regulators may want a certified lab result. For transaction-grade testing, our accurate radon testing guide covers the calibration and protocol requirements.
Interpreting the Result
The EPA action level is 4 pCi/L. Results at or above this level warrant mitigation. EPA also identifies 2 pCi/L as a consideration level where homeowners may want to mitigate even though it is below the action threshold. Below 2 pCi/L, no action is required, but periodic retesting every two years remains good practice.
The result on a short-term test reflects only the test period. Radon levels vary seasonally and weather-dependently. A borderline result of 3 to 5 pCi/L on a short-term test should be followed up with either a second short-term test in a different season or a long-term test that averages annual conditions. CDPHE specifically recommends follow-up testing for borderline results in Colorado because seasonal swings here are pronounced.
What to Do After a High Result
Results above 4 pCi/L should trigger mitigation. The standard approach is sub-slab depressurization installed by a certified mitigator. Front Range installations typically run twelve hundred to twenty-five hundred dollars and reduce indoor radon to below 4 pCi/L, often below 2 pCi/L. Our radon vent system architecture guide covers what the installation involves.
After mitigation, post-installation testing confirms the system is performing as designed. EPA protocols require a short-term test starting at least 24 hours after the mitigation system has been running. Periodic retesting every two years confirms continued performance over time.
Common Testing Mistakes
Several patterns produce unreliable test results. Placing the device in a basement that the family rarely uses underestimates exposure to occupants. Placing the device too close to a wall or in a confined space produces non-representative readings. Opening windows during a short-term test dilutes indoor air. Delaying shipment of a charcoal canister after closing reduces accuracy. Testing during a major storm produces atypically high or low readings.
The corrective action is straightforward: follow the EPA protocol carefully, retest if conditions during the first test were not closed-house, and use a follow-up test for borderline results before committing to mitigation. The cost of a second test is low compared with the cost of either unnecessary mitigation or missed exposure.
Testing in a Real Estate Transaction
Buyers in Front Range markets routinely add a radon test to their inspection contingency. The test must be completed and results obtained within the contingency window, typically ten to fourteen days. Continuous monitors are preferred for transaction testing because results are immediate and the device is tamper-resistant (some professional models log when they are moved or unplugged). Most home inspectors offer radon testing as an add-on service for one hundred fifty to two hundred dollars including a continuous monitor and certified result.
Front Range Specific Considerations
Colorado’s combination of granite-based geology and tight winter construction produces elevated indoor radon across the Front Range. Denver, Jefferson, Boulder, El Paso, Larimer, and Douglas counties all sit in EPA Radon Zone 1. CDPHE estimates suggest roughly half of Colorado homes test above the 4 pCi/L action level. Testing is correspondingly common, mitigators are widely available, and most local home inspectors offer radon testing as a standard add-on service.
References
- EPA radon testing protocols — U.S. Environmental Protection Agency
- CDPHE radon program for Colorado homeowners — Colorado Department of Public Health and Environment
- CDC radon testing guidance — Centers for Disease Control and Prevention
- American Lung Association radon resources — American Lung Association
Front Range homeowners weighing radon testing as part of a pre-purchase inspection can connect with a vetted local inspector for guidance on protocols and follow-up if needed.