Radon Testing House: 2026 Front Range Placement Guide
Radon testing house procedures look simple on the surface (place a device, wait, send to a lab) but the details determine whether the result is accurate or misleading. This guide summarizes EPA, CDPHE, and Colorado state guidance current as of 2026 and is not a substitute for professional consultation when results approach or exceed action levels. The Front Range sits almost entirely in EPA Radon Zone 1, where the predicted average indoor radon level exceeds 4 picocuries per liter (pCi/L). Testing is the only way to know what a specific house is producing. This article walks through the full testing process for short-term, long-term, and continuous monitor methods.
How to Run Radon Testing on a House Correctly
Radon testing a house starts with choosing the right test type for the goal. Short-term tests (2 to 90 days) are appropriate for initial screening; charcoal canisters, alpha-track detectors, and continuous radon monitors all qualify as short-term when run for less than 90 days. Long-term tests (greater than 90 days) capture seasonal variation and produce the most reliable single estimate of annual average exposure. Continuous radon monitors (CRMs) record hourly readings and produce both a time-averaged result and a record of variability across the test period.
For real estate transactions, EPA Protocols for Real Estate Transactions require a short-term test of at least 48 hours under closed-house conditions, with placement in the lowest livable level. For homeowner screening outside a transaction, a longer test (90 days or more) is preferred when time permits. The pillar resource on radon testing in Colorado walks through the state-specific context for Front Range residents.
Test Placement Inside the House
Placement controls accuracy. Place the test device in the lowest livable level of the home, defined as a basement that is used or could be used as a living space, or the lowest occupied floor if there is no basement. Avoid kitchens, bathrooms, laundry rooms, and other high-humidity or high-airflow areas. Place the device 20 inches to 6 feet above the floor, 12 inches from exterior walls, and at least 12 inches from forced-air supply registers and return grills.
Avoid placement near windows, exterior doors, or direct sunlight; temperature swings and air movement bias the result. Place the device on a stable surface (shelf, end table, bookcase) where it will not be disturbed. Do not move the device during testing. The CDPHE website provides detailed placement guidance specifically for Colorado homes, which often have unique basement and walkout configurations. The companion guide on how to test a home for radon covers the homeowner walkthrough.
Closed-House Conditions
EPA Protocols require closed-house conditions throughout the test period for short-term tests under real estate scenarios. Closed-house conditions mean windows and exterior doors closed except for normal entry and exit, no operation of whole-house fans or window air conditioners, and HVAC operated in normal heating or cooling mode (not in “fan only” mode set to continuous bypass). For tests longer than 48 hours, closed-house conditions should begin at least 12 hours before the test starts to allow the home to equilibrate.
Closed-house conditions matter because radon concentration varies dramatically with ventilation. A home tested with windows open will show a lower reading than the same home tested closed, but the closed reading better represents normal winter occupancy when radon exposure is highest. Front Range winters extend roughly seven months of the year, making closed-house conditions the relevant exposure scenario for most of the calendar.
Test Duration and What It Captures
Short-term tests of 2 to 7 days produce a snapshot of radon during the test period. They are useful for initial screening and for real estate transactions where time is limited. The trade-off is that radon varies day-to-day with weather, barometric pressure, and HVAC operation; a single short-term test can over- or underestimate the long-term average by a factor of two or more. Repeating short-term tests in different seasons reduces this uncertainty.
Long-term tests of 90 days to one year produce a time-averaged result that captures seasonal variation. Alpha-track detectors are the most common long-term test technology; they sit in place for the entire test period and integrate exposure into a single result. The trade-off is the long wait. Continuous radon monitors split the difference: they record hourly readings over a defined test period (typically 2 to 7 days) and produce both an average and a chart showing variability. CRMs are the gold standard for real estate transactions and post-mitigation verification because the hourly record can detect test tampering. The neighboring guide on radon sampling methodology covers each device type in detail.
EPA Action Levels and What They Mean
EPA recommends action at 4 pCi/L or higher. At this level, EPA’s lifetime lung cancer risk for never-smokers is approximately 7 in 1,000; for smokers, it is approximately 62 in 1,000. The 4 pCi/L threshold is not a safe-versus-unsafe line; it is the level at which mitigation becomes cost-effective relative to the health risk. EPA notes that even levels between 2 and 4 pCi/L warrant consideration of mitigation, particularly in homes with smokers or young children.
WHO recommends an action level of 100 Bq/m3, equivalent to roughly 2.7 pCi/L, reflecting more recent epidemiological data on low-dose risk. Some states and Canadian provinces have adopted action levels below 4 pCi/L. For Front Range homeowners, treating any reading above 2 pCi/L as a candidate for retest or mitigation aligns with the most current health-protective guidance. CDPHE publishes detailed action-level guidance specific to Colorado climate and housing stock.
Retest and Mitigation Decision
A single short-term result above 4 pCi/L should be confirmed with a second test before committing to mitigation. The second test can be a long-term test (most accurate for understanding actual exposure) or a follow-up short-term test in a different season. Two short-term tests can be averaged to produce a more reliable estimate. If both tests exceed 4 pCi/L, mitigation is recommended. If one test exceeds 4 pCi/L and the other is below, average the two and treat any average above 4 pCi/L as a mitigation recommendation.
Mitigation systems for residential homes typically use sub-slab depressurization: a fan draws air from beneath the slab through a vent pipe that exhausts above the roof line. Cost runs $1,200 to $2,500 for a typical Front Range home with a slab-on-grade or basement foundation. Crawlspace homes use slightly different methodology (membrane sealing plus depressurization) and run $1,800 to $4,000. Post-mitigation testing is required to confirm the system reduced radon below 4 pCi/L; most professional mitigators include the post-mitigation test in the project price.
When to Retest Without a Trigger
EPA recommends retesting every two years even after a low initial result. Radon concentration changes over time with foundation cracks, soil moisture changes, HVAC modifications, and structural settling. A home that tested at 2 pCi/L five years ago may test higher today. After any major renovation that alters the foundation or HVAC, retest within a few months of completion. After installing or modifying a mitigation system, retest immediately to confirm performance.
How to Schedule and Execute a House Test
Order a test kit from the Colorado Radon Program (free for eligible homeowners) or buy a charcoal canister or alpha-track kit at a hardware store ($15 to $40). For higher accuracy, rent or buy a continuous radon monitor ($150 to $300 to buy; some libraries and county health departments loan them free). Place the device per EPA protocols. Maintain closed-house conditions throughout the test. Mail in the device or download the CRM data when the test completes. Compare the result against 4 pCi/L (EPA action) and 2 pCi/L (consideration). Retest or mitigate based on the result.
Why the Front Range Sits in Radon Zone 1
EPA classifies counties into three radon zones based on predicted average indoor radon levels. Zone 1 counties have predicted average indoor radon levels 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. The Front Range counties (Denver, Douglas, Elbert, Arapahoe, Jefferson, El Paso, Adams, Boulder, Broomfield) all sit in Zone 1 due to the underlying granite and uranium-bearing soils of the Colorado Mineral Belt.
The Zone 1 classification does not mean every home will test high; individual homes vary widely based on foundation type, soil permeability under the slab, HVAC operation, and construction quality. CDPHE survey data from sampled homes across Colorado shows averages of roughly 2 to 8 pCi/L depending on county, with substantial individual variation. The zone classification means testing is essential because the probability of a high reading is significantly elevated compared to other regions of the country.
Seasonal Variability in Test Results
Radon concentrations vary substantially with season in Colorado due to extreme indoor-outdoor temperature differentials in winter. During winter heating season, the stack effect (warm indoor air rising through the building envelope) creates negative pressure at the lowest level, drawing soil gas (including radon) up through any opening in the slab. During summer cooling season, the stack effect reverses or weakens, and outdoor air infiltration dilutes radon. The result is that winter readings are typically 30 to 100 percent higher than summer readings in the same home.
This seasonal variability has implications for testing strategy. A summer-only test may underestimate the home’s typical exposure, particularly because Front Range residents spend more time indoors during winter due to extended cold weather. A long-term alpha-track test spanning at least one full heating season captures the high-exposure period accurately. Real estate transaction tests under EPA Protocols use closed-house conditions to approximate winter exposure regardless of test season.
How HVAC Operation Affects Test Results
HVAC operation during the test period affects results in predictable ways. Continuous fan operation (set to “on” rather than “auto”) increases air mixing throughout the home and tends to lower the basement reading by mixing with upper-floor air. Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) actively exchange indoor and outdoor air and lower indoor radon proportionally. Whole-house humidifiers and air conditioners that draw return air from upper floors can also reduce lower-floor readings.
EPA Protocols require operating HVAC in normal mode (not continuous fan) during the test period to capture typical exposure conditions. Tests performed with continuous fan or HRV operation may underestimate the radon level that occupants typically experience. Conversely, tests performed with HVAC off entirely may overestimate. The “normal operation” standard means setting the thermostat and ventilation as the occupants typically would for the season and leaving the system to operate automatically.
References
- EPA Radon Information and Action Levels — U.S. Environmental Protection Agency
- Colorado Department of Public Health and Environment Radon Program — CDPHE
- CDC Radon Public Health Information — Centers for Disease Control and Prevention
- American Lung Association Radon Guide — American Lung Association
- EPA Consumer Guide to Radon Reduction — U.S. Environmental Protection Agency
Front Range homeowners with a recent high reading or planning a pre-purchase radon test can reach out through our contact page for a connection to a certified local radon professional.