What Is a Radon Test for a House: A 2026 Plain-Language Guide
A radon test for a house measures the concentration of radon gas in indoor air, reported in picocuries per liter (pCi/L) in the United States. Radon is a colorless, odorless, radioactive gas produced by uranium decay in soil. It enters homes through foundation cracks, sump pits, and slab penetrations, then accumulates in lower levels. EPA’s action level is 4 pCi/L. This guide summarizes EPA and CDPHE guidance current as of 2026 and is not a substitute for direct professional consultation. The Front Range — EPA Zone 1 — has roughly 50 percent of homes testing above action level per CDPHE estimates, which is why this topic matters more in Colorado than in most parts of the country.
What the test physically measures
Radon-222 has a half-life of 3.8 days. As it decays, it emits alpha particles and produces a chain of short-lived decay products (often called radon progeny or radon daughters) including polonium-218, lead-214, bismuth-214, and polonium-214. These decay products attach to airborne particles and can be inhaled into the lungs, where their continued decay deposits alpha radiation directly onto bronchial tissue. The damage accumulates with cumulative exposure and is the mechanism by which radon causes lung cancer.
Radon test devices measure either the parent radon-222 directly or its decay products, depending on the technology. Charcoal canisters and electret ion chambers capture decay products and measure their resulting gamma emission or ionization. Continuous radon monitors using alpha spectroscopy or pulse ionization measure the parent gas directly. Both approaches report results in the same pCi/L units after appropriate conversions.
Why pCi/L and what the number means
One picocurie equals 0.037 disintegrations per second. One pCi/L therefore means 0.037 alpha-particle events per second per liter of air. At EPA’s 4 pCi/L action level, an occupant breathes air with roughly 0.15 alpha emissions per second per liter — emissions that the lungs cannot filter or excrete. Over decades of exposure, this rate of damage accumulates into measurable lung-cancer risk.
EPA published lifetime lung-cancer risk tables based on assumed 70-year exposure at constant concentration. At 4 pCi/L, a never-smoker faces roughly 7 in 1,000 lifetime lung-cancer risk. A current smoker at the same level faces 62 in 1,000 — nearly ten times higher due to synergistic damage. At 2 pCi/L the never-smoker risk drops to about 4 in 1,000. At 20 pCi/L it climbs to 36 in 1,000 for a never-smoker and 260 in 1,000 for a smoker.
The numbers translate the abstract pCi/L value into concrete risk terms most homeowners can act on. The companion should I test for radon framework covers risk-based decision-making in depth.
How the test interacts with home conditions
Radon entering a home depends on three things: soil concentration, building condition, and building pressurization. Soil concentration varies geologically. Uranium-rich soils — common across the Front Range — produce more radon than uranium-poor soils. Building condition determines pathway: cracks, sump pits, slab pours without vapor barriers, and plumbing penetrations all serve as entry points. Building pressurization drives gas flow — homes with negative interior pressure relative to soil draw radon in continuously.
This means the same soil under two adjacent homes can produce very different indoor readings. A 1985 ranch home with multiple slab cracks and a sealed-but-aging foundation may read 8 pCi/L. The 2018 home next door with a polyethylene vapor barrier and tighter construction may read 2 pCi/L. The geology is identical. The home is the variable. Neighbor readings are therefore not a substitute for testing your own home. The radon in Colorado overview covers Front Range geological patterns and home-specific factors.
Test types available
Three main test types cover almost every situation a homeowner faces.
Short-term passive (2 to 7 days)
Charcoal canister or electret ion chamber. $20 to $50 including lab analysis. Best for screening and real-estate transactions. Results in 5 to 10 days from kit purchase.
Long-term passive (90 days to 1 year)
Alpha-track detector. $30 to $80 including lab analysis. Best for understanding seasonal variability and confirming short-term results. More representative of annual average exposure.
Continuous radon monitor (CRM)
Consumer-grade $200 to $300 as a one-time purchase. Professional CRM testing $200 to $400 per visit. Best for transactions requiring fast turnaround and for ongoing home monitoring.
The companion DIY radon test guide covers each type in detail with brand comparisons.
How the result gets reported
A laboratory report for a passive test runs 1 to 2 pages and contains the home address, deployment dates, average radon concentration in pCi/L, lab certification information, and basic interpretation guidance. The pCi/L number is the time-averaged concentration over the test period. Reports rarely include uncertainty estimates by default, but reputable labs can provide them on request — typical short-term test uncertainty runs plus-or-minus 25 percent at action level.
CRM reports include the same time-averaged number plus hourly logs showing concentration variation through the test, environmental data (temperature, humidity, barometric pressure) when available, and flags for any interference events the device detected. The hourly detail makes CRM results harder to dispute in transaction contexts.
What the EPA thresholds mean
EPA establishes two reference levels and one international benchmark to consider.
4 pCi/L: EPA action level. Above this, mitigation is strongly recommended. Lifetime cancer risk at this level is far higher than acceptable risk thresholds for other indoor contaminants.
2 pCi/L: EPA consideration level. Between 2 and 4 pCi/L, EPA suggests considering mitigation. Many Front Range homeowners with readings in this range install mitigation anyway, especially when basement occupancy is high or seasonal swings might push winter readings above 4.
2.7 pCi/L: World Health Organization (WHO) reference level. WHO recommends mitigation above 2.7 pCi/L (100 Bq/m³). The WHO threshold is more conservative than EPA’s, reflecting a different statistical approach to acceptable lifetime risk.
What happens after a positive result
A reading above 4 pCi/L triggers a mitigation conversation. The standard solution on Front Range homes is sub-slab depressurization (SSD): a sealed suction pit under the basement slab, a PVC vent stack to roof, and a continuously running fan that pulls soil gas before it enters the home. Cost runs $1,200 to $2,500 in 2026. EPA recommends post-mitigation verification testing 14 to 30 days after install, and annual retests thereafter. The Colorado radon mitigation guide walks through the installation process.
Borderline results (2 to 4 pCi/L) usually warrant a heating-season retest, a long-term alpha-track detector, or a CRM deployment for several weeks. The decision becomes clearer with more data. A summer reading of 3.5 pCi/L might correspond to a winter reading of 6 pCi/L in the same home — long-term measurement reveals the truth.
The Colorado Front Range context
All nine Front Range counties — Denver, Douglas, Elbert, Arapahoe, Jefferson, El Paso, Adams, Boulder, and Broomfield — sit in EPA Zone 1. CDPHE estimates roughly half of Colorado homes test above 4 pCi/L. Foothill subdivisions in Boulder, Jefferson, and western Douglas counties tend to show the highest medians. Eastern plains homes read lower on average but still produce elevated results in basements. Front Range homeowners should treat radon testing as a standard maintenance task on par with smoke-detector battery replacement.
The main radon testing pillar covers the Colorado-specific context in depth, including CDPHE outreach programs that occasionally subsidize test kits for low-income households.
Common misconceptions
Three myths to dispel.
“Newer homes don’t have radon.” False. New construction can reduce radon entry through vapor barriers and passive SSD piping, but soil concentration drives the underlying risk. A new home on uranium-rich soil can still read above action level. Always test.
“Radon is rare.” False at least in Zone 1. CDPHE estimates 50 percent of Colorado homes test above action level. Even in Zone 3, EPA estimates 1 in 15 homes nationally has elevated radon.
“I’d smell radon if I had it.” False. Radon is odorless. The companion does radon smell piece debunks this directly.
References
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- EPA Health Risk of Radon — U.S. Environmental Protection Agency
- CDPHE Radon Program — Colorado Department of Public Health and Environment
- CDC Radon Health Information — Centers for Disease Control and Prevention
Front Range homeowners ready to test their house can connect with a vetted radon measurement professional for a 48-hour test with chain-of-custody documentation.