Is Radon Testing Necessary: A Decision Framework for Homeowners
Is radon testing necessary? For homeowners in EPA Radon Zone 1 counties — which includes every county on Colorado’s Front Range — the EPA’s answer is yes, and the agency recommends testing every home regardless of foundation type or building age. The decision framework rests on three numbers: the EPA action level of 4 picocuries per liter (pCi/L), the Zone 1/2/3 county classification, and the home’s specific construction and ventilation characteristics. This guide summarizes EPA and Colorado Department of Public Health and Environment (CDPHE) guidance current as of 2026 and is general homeowner education only — consult your physician for symptoms and a certified professional for specific testing decisions.
Part of our Radon Levels guide — start there for the full picture →
Is Radon Testing Necessary? The Direct Answer
Yes, for any home in EPA Radon Zone 1, which includes all nine Front Range counties and the great majority of Colorado. EPA’s longstanding position — published in “A Citizen’s Guide to Radon” and reinforced in every subsequent national risk assessment — is that radon is the leading cause of lung cancer among non-smokers, and that testing is the only way to know a home’s radon level because the gas is colorless, odorless, and tasteless.
For homes in EPA Radon Zone 2, testing is strongly recommended; many of these counties have substantial fractions of homes above the action level even though the county average is below it. For homes in EPA Radon Zone 3, the EPA still recommends testing for new construction and at point of sale because individual home variability can exceed the county average significantly. The Surgeon General’s National Health Advisory in 2005 — still cited in current EPA publications — recommends that all homes be tested for radon.
What Is the EPA Action Level?
The EPA action level is 4.0 pCi/L (picocuries per liter), and it is the threshold at which the agency recommends that homeowners take action to reduce radon. The number is a long-term annual average, derived from a balance between achievable mitigation in residential construction and lung-cancer risk reduction. EPA notes that there is no fully “safe” level of radon — the dose-response curve does not have a clean threshold — and homeowners with results between 2 and 4 pCi/L may still want to consider mitigation, particularly for homes with young children, smokers, or full-time basement occupants.
The action level corresponds to a calculated lifetime lung-cancer risk for a non-smoker exposed at that level across a 70-year residence of roughly 7 deaths per 1,000 people. The same exposure for a smoker is meaningfully higher because radon and smoking exposure act multiplicatively rather than additively on lung-cancer risk. This is the data underpinning the EPA’s recommendation that smokers especially should test and mitigate.
Colorado’s Zone 1 Designation
The EPA classifies every Colorado Front Range county as Zone 1 — the highest predicted-radon classification. CDPHE data are consistent with this: roughly half of Colorado homes tested have shown radon levels above the 4 pCi/L action level, and the median tested-home value in many Front Range counties sits at or above the action level. The high-altitude, uranium-rich geology of the Front Range produces consistently elevated indoor radon when soil gas can migrate into building cavities through foundation cracks, slab penetrations, and sump pits.
Front Range homeowners therefore start with a strong prior that their home is likely above the action level. The question is rarely “should I test” but “when should I test and which kind of test should I use.” For homeowners on the broader Front Range planning a test, the Colorado-specific radon testing pillar guide walks through the choices.
When to Test
Four scenarios commonly drive a radon test. First, at the point of sale during a real-estate transaction — a 48-hour short-term test is routinely added to the home inspection at $150 to $200. Second, after any home is purchased or moved into, regardless of prior testing — initial baseline testing should be performed in every new occupancy. Third, after any structural change to the home that might affect radon entry or ventilation — a remodel, a new HVAC system, a basement finishing project, or a foundation repair. Fourth, on a periodic basis (every two to five years for unmitigated homes that have tested below the action level; every two years for mitigated homes to verify the mitigation system is still functioning).
EPA generally recommends short-term testing (2 to 7 days, using a charcoal canister or short-term electret) for initial screening, with a long-term follow-up test (90+ days, using an alpha-track detector) when the short-term result is between 2 and 8 pCi/L. Results above 8 pCi/L typically don’t need a long-term confirmation before mitigation. The DIY radon test guide covers consumer test selection.
What Test Types Tell You What
Short-term tests give a snapshot — what was radon doing during a 48-hour to 7-day window with windows and doors closed. They’re useful for real-estate timing and initial screening. Long-term tests give an integrated annual-style average that’s closer to actual lifetime exposure. Continuous radon monitors (CRMs) — used by certified professionals during real-estate testing in many states — give hour-by-hour data that document closed-house conditions and identify tampering. The continuous radon monitor guide covers consumer CRM options.
What to Do With the Results
EPA’s recommendations are straightforward. If the long-term average is at or above 4 pCi/L, install a radon mitigation system (typically active sub-slab depressurization for slab-on-grade or basement homes, sub-membrane depressurization for crawl spaces). If between 2 and 4 pCi/L, consider mitigation, particularly for homes with young children, smokers, or basement-level living space. If below 2 pCi/L, retest every two to five years.
A professional radon mitigation system in Colorado typically costs $1,200 to $2,500 installed. The system uses a sub-slab fan to draw soil gas out from beneath the foundation and vent it above the roofline, maintaining negative pressure relative to the home. Post-mitigation re-testing is essential to confirm the system is reducing indoor radon below 4 pCi/L. For more on local pricing, see the Colorado radon mitigation guide.
Common Reasons Homeowners Skip Testing (and Why Not To)
Three patterns recur. “My neighbor’s home tested fine” — radon levels vary significantly home-to-home even within the same neighborhood and the same construction era, because the gas finds entry through specific cracks and penetrations unique to each foundation. A neighbor’s test result is not informative for your home. “My home is new construction” — many new Colorado homes test above the action level despite radon-resistant new-construction features, because the features only work if installed correctly and the home was tested after construction. “I’ve lived here 20 years and feel fine” — radon-related lung cancer has a 20- to 50-year latency, and the absence of acute symptoms is not informative about cumulative exposure.
How to Order a Radon Test
Two paths. Consumer kit: order a charcoal-canister or short-term electret kit ($15 to $50 including lab analysis) from a NRPP-listed or NRSB-listed provider, follow the closed-house protocol, deploy in the lowest occupied level of the home, mail to the lab. Results in 5 to 10 business days. Professional CRM testing: hire a NRPP-certified or NRSB-certified radon measurement professional ($150 to $300) who deploys a continuous radon monitor for 48 hours, with documented closed-house conditions. Real-estate transactions typically use the CRM path because of the tamper-resistance documentation.
Colorado Zone 1 Specifics
EPA’s Map of Radon Zones places every Colorado Front Range county in Zone 1, defined as a county with predicted average indoor radon concentrations greater than 4 pCi/L. CDPHE’s longitudinal tracking of voluntary residential test results corroborates this — across roughly 60,000 Colorado test results compiled over the last two decades, the median residential test value sits at or just above the 4 pCi/L threshold in Boulder, Larimer, Weld, Adams, Jefferson, Denver, Arapahoe, Douglas, and El Paso counties. The 75th percentile in some Front Range zip codes exceeds 8 pCi/L, and individual Boulder County results above 30 pCi/L are not rare.
The driver is geology rather than housing age. The Pikes Peak granite batholith, the Front Range fault zone, and the uranium-bearing shales of the Denver Basin produce a consistent radon-progenitor source across the region. New construction performs only marginally better than older construction on average — passive sub-slab vent stacks installed under IRC Appendix F are designed to allow easy retrofit to an active mitigation system, not to eliminate the problem at the time of construction. Front Range buyers should treat new-construction homes the same as resale homes for testing purposes.
When Retest Is Warranted and Mitigation Trigger Thresholds
Retest warrants three specific scenarios beyond the routine 2- to 5-year cadence. First, after any building modification that could affect radon entry or building pressure — new HVAC, basement finishing, foundation repair, sump-pit modification, or weatherization sealing. Second, after any seasonal change in occupancy or operation that materially changes ventilation — switching from open-window operation to year-round HVAC, installing a whole-house fan, or finishing a previously unused basement for full-time occupancy. Third, when an initial short-term result falls in the 2 to 8 pCi/L gray zone, where EPA recommends a long-term (90+ day) confirmation test before deciding on mitigation.
EPA’s two key thresholds are the 4 pCi/L action level (the level at which the agency formally recommends installing a mitigation system) and the 2 pCi/L consideration level (the level below 4 pCi/L at which mitigation may still be worth considering, particularly for homes with smokers, young children, or full-time basement-level occupants). The dose-response relationship between radon exposure and lung-cancer risk is roughly linear with no clean threshold — there is no fully safe level. Front Range Colorado homeowners whose long-term average tests between 2 and 4 pCi/L can reasonably choose mitigation for risk reduction even though EPA does not formally require action at that level. Most professional radon mitigation systems can reduce indoor radon to below 1 pCi/L when properly designed and verified by post-mitigation testing.
References
- EPA Radon page and Citizen’s Guide — U.S. Environmental Protection Agency
- CDPHE Colorado Radon Program — Colorado Department of Public Health and Environment
- CDC radon health information — Centers for Disease Control and Prevention
- American Lung Association radon information — American Lung Association
Frequently asked questions
Is radon testing necessary in Colorado?
Yes. The EPA classifies every Front Range county as Zone 1 — the highest predicted-radon classification. CDPHE data show roughly half of tested Colorado homes have radon above the 4 pCi/L action level. The Surgeon General's National Health Advisory recommends every home be tested.
What is the EPA radon action level?
4.0 picocuries per liter (pCi/L). At or above this annual average, the EPA recommends installing a mitigation system. Between 2 and 4 pCi/L, mitigation is worth considering — particularly for homes with young children, smokers, or full-time basement occupants. There is no fully safe level of radon.
How often should I retest my home for radon?
For unmitigated homes that tested below the action level, every two to five years. For mitigated homes, every two years to verify the mitigation system is still working. After any structural change — a remodel, new HVAC, basement finishing, or foundation work — retest before drawing conclusions about the new equilibrium.
Should new construction homes be tested?
Yes. Many new Colorado homes test above the action level even when radon-resistant new-construction features (vapor barrier, sub-slab vent pipe) are present, because the features only work when installed correctly. Test every new home after construction and before assuming the passive system is sufficient.
Front Range homeowners ready to schedule a professional radon test alongside their general home inspection can reach out through our contact page.
Test your home for radon
Radon is invisible and odorless, so a test is the only way to know your level. A short-term kit gives a first reading in days; a digital monitor tracks it over time.
| Product | Why | Buy |
|---|---|---|
 AirChek Charcoal Radon Test Kit | Lab fee usually included; EPA short-term protocol. | Amazon — $21.95 |
 First Alert RD1 Radon Test Kit | Widely stocked; 48-hour charcoal test. | Amazon — $16.98 |
 Airthings Corentium Home | Digital monitor with a long-term average. | Amazon — $142.99 |