Should I Test for Radon: A Decision Framework
Asking “should I test for radon” is reasonable. The answer for most American homeowners is yes, and for Front Range Colorado homeowners the answer is essentially always yes. The framework below uses EPA radon zones, home construction features, and occupancy patterns to produce a clear go/no-go recommendation. This guide summarizes EPA and CDPHE guidance current as of 2026 and is not a substitute for direct testing. Three factors drive the decision: where the home sits geographically, how the home is built, and who lives in it.
EPA Zone 1, 2, and 3 — what they mean
EPA divides U.S. counties into three radon zones based on predicted indoor concentration averages.
Zone 1 counties have predicted average indoor radon concentrations greater than 4 pCi/L. EPA’s action level is 4 pCi/L. By definition, the average home in a Zone 1 county warrants mitigation. The entire Colorado Front Range falls in Zone 1, along with most of the Rocky Mountain interior, the Upper Midwest, much of New England, and Appalachia. CDPHE estimates roughly 50 percent of Colorado homes test above the action level.
Zone 2 counties have predicted averages between 2 and 4 pCi/L. Mitigation is warranted in roughly 30 to 40 percent of Zone 2 homes based on EPA modeling.
Zone 3 counties have predicted averages below 2 pCi/L. Even Zone 3 homes can register elevated readings due to building-specific factors — basement bedrooms, sump pits, foundation cracks, soil heterogeneity. EPA still recommends testing in Zone 3, just with lower prior probability of a positive.
The geographic framework is the starting point. Look up your county’s zone designation on EPA’s radon zone map, then layer the home-construction and occupancy factors on top.
Decision matrix: when the answer is clearly yes
Six conditions push the recommendation to “test now” regardless of zone.
Home in EPA Zone 1
The entire Front Range, all of Colorado except a handful of southwestern counties. Test even if no one has ever asked you to. The Surgeon General lists radon as the second leading cause of lung cancer in the U.S., and the leading cause among never-smokers.
Basement bedroom or finished basement living space
Lower levels concentrate radon two to four times more than upper floors. Any occupancy below grade warrants testing regardless of zone.
Buying or selling a home
Real-estate transaction testing should be a default, not an exception. Even Zone 3 buyers benefit from the baseline data and the negotiation leverage.
Pre-1990 construction
Older foundations often lack the polyethylene under-slab vapor barrier that became standard in the 1990s. Soil-gas pathways are more open.
Smokers or former smokers in the home
Radon and tobacco smoke have synergistic lung-cancer risk. EPA estimates the lifetime lung cancer risk for a smoker at 10 pCi/L is roughly 26 in 100. The combination is far more dangerous than either alone.
No prior test in the last five years
Radon levels drift over time. A 2020 test result is not a 2026 result. Foundation settling, sealant aging, soil moisture cycles, and HVAC changes all shift the pathway.
Decision matrix: when the answer is “yes, but lower priority”
Two conditions still warrant testing but at a relaxed pace.
Home in EPA Zone 3 with all living space above grade. Test once, ideally during a heating-season closed-house period. If result is under 2 pCi/L, retest every five years or upon life-event triggers.
Newer Zone 3 construction with active radon mitigation already installed by the builder. Confirm system function with a single 48-hour test. If under 2 pCi/L, schedule annual fan-status checks rather than annual tests.
Decision matrix: when “no” might be reasonable
Almost never. Even the limited cases below should produce a test eventually.
A high-rise apartment above the third floor, where soil-gas pathway is negligible. Some EPA guidance allows skipping radon testing above the third floor of a multi-story building. Below that height, soil gas can reach living space through elevator shafts and stairwells.
A home that just tested below 2 pCi/L in winter closed-house conditions within the last two years, with no major foundation work or HVAC changes since. The result is current enough to defer the next test by another year or two.
Front Range specifics
Denver, Douglas, Elbert, Arapahoe, Jefferson, El Paso, Adams, Boulder, and Broomfield counties all sit in EPA Zone 1. CDPHE radon data shows elevated readings clustered along the foothills and on bentonite-clay soils, with Boulder, Jefferson, and western Douglas counties showing the highest median concentrations. The eastern plains generally read lower but still produce elevated results in basements. No Front Range neighborhood is safely “off the map” for radon — pockets of high readings appear in tract subdivisions in Aurora and Thornton just as readily as in foothills custom homes.
The companion radon in Colorado overview covers state-specific patterns in more depth, including the CDPHE outreach programs that occasionally offer free or subsidized test kits. The main radon testing hub walks through the broader Colorado-specific context for new homeowners.
What the test actually involves
A first-pass screening test is straightforward. Buy or order a passive 48-to-96-hour charcoal canister or electret ion chamber kit. Place it in the lowest occupied living area of the home, on a stable surface 20 inches above the floor, away from drafts, exterior walls, and fans. Maintain closed-house conditions — windows and exterior doors closed except for normal entry — for 12 hours before deployment and throughout the test. Ship to the lab in the prepaid envelope. Results in 5 to 10 business days. Total cost: $20 to $50 for the kit, including lab analysis.
A more decisive test uses a continuous radon monitor (CRM), either a homeowner-grade unit purchased for $200 to $300 or a professional unit deployed by a certified measurement professional for $200 to $400 including the visit. CRMs log hourly readings and distinguish valid tests from interference events like extended window opening or HVAC anomalies. The DIY radon test guide covers the home-kit options in detail.
Interpreting the result
EPA establishes two thresholds.
4 pCi/L is the action level. Above this, mitigation is strongly recommended. The lifetime lung-cancer risk at 4 pCi/L for a never-smoker is roughly 7 in 1,000; for a smoker, roughly 62 in 1,000. Both are far above acceptable risk thresholds for other regulated indoor contaminants.
2 pCi/L is the consideration level. Between 2 and 4 pCi/L, EPA suggests considering mitigation. Many Front Range homeowners with readings in the 2-to-4 range install mitigation anyway, especially when basement occupancy is high or when the seasonal swing might push winter readings above 4.
Below 2 pCi/L is generally considered acceptable. Retest every two to five years unless conditions change.
What about a “test once and forget it” approach
That approach undertests the home. A single test at one point in time captures a single point in time. Radon levels respond to season, weather, occupancy, foundation condition, and HVAC behavior. The most defensible long-term strategy is one of three patterns. Pattern one: short-term test every two years, ideally in heating season. Pattern two: long-term alpha-track detector annually rotating through different rooms. Pattern three: permanent CRM logging continuously. The right pattern depends on the home’s first-test result and the family’s tolerance for variability in the data.
EPA Zone Maps and what your zone means
The EPA Map of Radon Zones divides every U.S. county into one of three predicted-risk tiers based on indoor measurements collected during the National Residential Radon Survey and supplementary state data. Understanding the map is the first analytical step before any testing decision, but the map alone never settles the question for an individual home.
Zone 1 counties carry the highest predicted potential, with modeled average indoor concentrations above 4 pCi/L — the EPA action level. The entire Colorado Front Range sits in Zone 1, including Denver, Boulder, Jefferson, Douglas, Arapahoe, El Paso, Adams, Larimer, and Weld counties, along with the foothills communities running from Fort Collins south to Pueblo. The uranium-bearing granitic and sedimentary formations underlying the Front Range generate the radium decay chain that feeds soil-gas radon, and the relatively dry, fractured soils favor gas migration into basements.
Zone 2 counties carry moderate predicted potential, with modeled averages between 2 and 4 pCi/L. Much of the central United States, parts of the southern Rockies, and pockets of the Pacific Northwest fall into Zone 2. Some Colorado plains and Western Slope counties land here.
Zone 3 counties carry the lowest predicted potential, with modeled averages below 2 pCi/L. Coastal Southeast, much of the Gulf Coast, and scattered counties across the country fall into Zone 3.
The critical caveat: zone designation describes predicted potential, not actual indoor levels in any specific home. EPA, CDC, and CDPHE all stress that homes within the same zone vary widely — one Zone 1 house may register 1.5 pCi/L while the house across the street registers 18 pCi/L, driven by foundation condition, soil heterogeneity, HVAC pressure dynamics, and occupancy patterns. For that reason EPA recommends testing every home in every zone, including Zone 3, rather than treating the map as a permission slip to skip the test. The zone tells you the prior probability; the test tells you the answer.
References
- EPA Radon Zone Map — U.S. Environmental Protection Agency
- CDPHE Radon Program — Colorado Department of Public Health and Environment
- EPA Health Risk of Radon — U.S. Environmental Protection Agency
- CDC Radon Health Information — Centers for Disease Control and Prevention
Front Range homeowners ready to test can connect with a vetted radon measurement professional for a 48-hour closed-house test with chain-of-custody documentation.