How to Know If There Is Radon in Your House: Testing Guide
How to know if there is radon in your house has exactly one answer: by testing. Radon is invisible, odorless, and tasteless. The gas produces no sensory cues that a homeowner or visitor could detect. Symptoms of radon exposure also do not appear until years or decades after long-term inhalation, in the form of lung cancer. The only way to know whether a home has elevated indoor radon is to measure the air concentration with a calibrated device. This is true for new construction, old construction, basements, slab-on-grade homes, and elevated mountain properties alike. For homeowners in Colorado, where the entire state is rated EPA Radon Zone 1, the prior probability of an elevated reading is high enough that testing is essentially mandatory. This guide draws on EPA and CDPHE guidance current as of 2026 and is not a substitute for professional radon measurement or certified mitigation advice.
The single answer: testing
Radon-222 is a radioactive noble gas produced by the decay of naturally occurring uranium in soil and bedrock. The gas has no smell, no color, no taste, and no immediate physiological effect at residential concentrations. Long-term inhalation causes lung damage that can lead to cancer, but the latency between exposure and disease is years to decades. Acute symptoms are absent at any home-level concentration. The only practical way to determine whether radon is present in a specific home at a specific concentration is to measure it with a device designed for radon detection.
Why sensory detection is impossible
Radon’s chemical properties make it undetectable to the human senses. The gas is colorless because radon atoms do not absorb visible light at room temperature concentrations. The gas is odorless because radon is chemically inert and does not bind to olfactory receptors. The gas is tasteless because it does not interact with taste receptors. The radioactivity that makes radon dangerous is invisible alpha decay, which deposits energy at the cellular level when inhaled but produces no light, heat, sound, or vibration that human senses could detect. Other indoor hazards like natural gas (smell), carbon monoxide (often detectable by alarm), and mold (smell, visible growth) have some sensory or detectable signal. Radon has none.
What testing tells you
A radon air test produces a concentration reading in picocuries per liter (pCi/L). The EPA action level is 4 pCi/L. Concentrations below 2 pCi/L are considered low risk. Concentrations between 2 and 4 pCi/L are in a gray zone where mitigation is recommended for households with children, smokers, or other risk factors. Concentrations at or above 4 pCi/L trigger a mitigation recommendation. Concentrations above 10 pCi/L are significantly elevated and warrant prompt mitigation. The reading is the only objective signal available; it converts radon presence from an invisible question into a measurable answer.
The Colorado Zone 1 baseline expectation
The EPA classifies every Colorado county as Zone 1, the highest radon risk category. CDPHE data shows a significant percentage of tested Colorado homes register above 4 pCi/L. The Front Range counties of Denver, Boulder, Larimer, Adams, Arapahoe, Jefferson, Douglas, Elbert, and El Paso all have elevated baseline rates. Mountain counties are similar or higher due to granitic bedrock with elevated uranium content. The practical implication is that Colorado homeowners should assume their home could have elevated radon until testing demonstrates otherwise. New construction is no guarantee of low radon; older homes with passive radon-resistant features sometimes test high, and newer homes with active mitigation systems sometimes test low. Only the test settles the question for any specific property. The broader Colorado radon testing guide covers the state-level context in more detail.
When to test your home
EPA and CDPHE recommend several testing moments. New homeowners should test within the first year of moving in. Renters should ask landlords about prior testing and consider testing themselves if results are unavailable. Pre-purchase buyers should test during the inspection period. Homeowners who have lived in their homes for years should test every two years even if a prior test was low. Homeowners should retest after major renovation, foundation work, HVAC replacement, or any change to the building envelope. Newly installed mitigation systems should be verified with a post-mitigation test 30 days after installation and retested every two years thereafter to confirm continued performance.
Where to test
Place the test device in the lowest livable level of the home. For a finished basement, the test occurs in the basement. For an unfinished basement that is not used as living space, the test occurs on the first floor. Place the device at breathing height, typically 2-6 feet above the floor. Keep it away from drafts, humidifiers, exterior walls, and direct sunlight. Avoid placing the device in kitchens or bathrooms where humidity can affect results. The placement protocol exists because radon concentration varies within a home and the goal is to capture the air that occupants breathe. The EPA placement standard is universally followed by certified inspectors.
Short-term test or long-term test
For an initial check on a home where radon status is unknown, a short-term test of 2-7 days is appropriate. If the short-term result is below 2 pCi/L, the home is likely low risk. If the short-term result is between 2 and 4 pCi/L, a long-term test of 90 days or longer gives a more confident estimate. If the short-term result is at or above 4 pCi/L, EPA recommends a follow-up confirmation test, which can be either short-term or long-term depending on the urgency. For pre-purchase decisions, the short-term test is the standard because the inspection contingency timeline does not accommodate a 90-day measurement.
How to obtain a radon test kit
Three main sources provide test kits to Colorado homeowners. CDPHE periodically offers free or low-cost radon test kits through county public health departments. Hardware stores and online retailers sell charcoal canister kits for roughly $25, including lab analysis. Professional home inspectors deploy continuous radon monitors and produce detailed reports for $125-$200. The choice depends on the testing context. For pre-purchase due diligence, hire a certified inspector. For routine homeowner monitoring, a hardware-store charcoal kit is sufficient. For long-term assessment, an alpha-track detector is appropriate.
What “knowing” means in the radon context
Knowing whether a home has radon, at a specific concentration, with statistical confidence, requires testing under proper conditions. A single short-term test gives a reasonable initial answer. Two consecutive tests in agreement give a more confident answer. A long-term test gives the best estimate of annual exposure. Mitigation reduces the concentration to a verified post-mitigation level. The full sequence from initial unknown through testing, decision-making, and verification is the standard radon management process for residential properties. The broader single-family radon test guide covers this sequence for detached homes.
Common misconceptions about detecting radon
Several misconceptions circulate. The first is that radon detector beep alarms appear in stores like carbon monoxide detectors. Such devices exist but are far less common, more expensive than a simple test kit, and not the standard for residential measurement. The second is that high radon will produce visible mineral deposits or stains. It will not; radon and its decay products at residential concentrations do not produce visible signs. The third is that you can smell radon. You cannot. The fourth is that newer homes do not have radon. Some newer homes do; passive radon-resistant features reduce average risk but do not eliminate it. The fifth is that radon is a basement-only problem. Slab-on-grade homes without basements can also have elevated radon, particularly if the slab has cracks or penetrations.
What to do if the test result is elevated
An elevated test result is actionable, not alarming. The recommended response sequence is straightforward. Retest with a follow-up short-term or long-term test if the initial result was borderline. If both tests confirm elevation, hire a certified Colorado mitigation contractor to design and install an active soil depressurization system. The typical Front Range mitigation cost is $1,200-$2,000 installed. Verify the system with a post-mitigation test 30 days later. Retest every two years to confirm continued performance. CDPHE maintains a registry of certified mitigators serving Colorado counties.
Why radon-resistant new construction matters
Many newer Colorado homes are built with passive radon-resistant new construction (RRNC) features. These include a gas-permeable layer of gravel under the slab, a polyethylene sheet vapor barrier, sealed and caulked openings in the slab, and a vertical PVC pipe routed from below the slab to the roof. The passive system relies on natural pressure differential to vent radon outdoors. Passive RRNC reduces average indoor radon by roughly 50 percent compared to unmitigated baseline construction, but it does not guarantee a safe result. If post-construction testing shows elevated radon, an inline fan can be added to the existing pipe to convert the passive system into an active soil depressurization system. The cost of fan addition is typically lower than a full retrofit because the pipe is already in place.
Front Range neighborhood differences
Within the Front Range, radon test results vary by neighborhood for reasons related to soil composition and foundation type. Neighborhoods on alluvial deposits typically test lower than neighborhoods on granitic bedrock. Hillside homes with cuts into bedrock often test higher than nearby slab-on-grade homes. Basement-having homes test higher than slab-on-grade homes for obvious reasons. Buyers comparing homes should not assume that neighbor results predict their specific home’s result. Every home needs its own test. CDPHE maintains county-level data but does not publish neighborhood-level data, partly because the individual home variation is too high for finer-grained predictions to be useful.
What it costs to find out
The cost of finding out whether your home has radon is low compared to almost any other home concern. A charcoal canister kit from a hardware store costs around $25 including lab analysis. A long-term alpha-track detector costs $25 to $50. A professional inspection with a continuous radon monitor add-on is typically $125 to $200. CDPHE periodically distributes free or low-cost test kits through county health departments. The financial barrier to testing is essentially zero. The much larger cost comes only if testing confirms elevated radon and mitigation is warranted, in which case a Front Range active soil depressurization system typically costs $1,200 to $2,000 installed. The decision sequence — test inexpensively, mitigate only if needed — is straightforward.
What to do if you do not own the home
Renters in pre-purchase or move-in situations sometimes wonder whether they should test even when they do not own the property. The answer is yes, with caveats. EPA encourages renters in any home, particularly in Zone 1 states like Colorado, to inquire about prior testing and to consider commissioning a test for their own peace of mind. Colorado does not require landlord disclosure of radon, but some leases include radon clauses and some landlords have voluntary testing programs. If a renter tests and finds elevated radon, the question of who pays for mitigation becomes a lease negotiation rather than a regulatory matter. The CDPHE radon program publishes guidance for renters considering testing.
References
- EPA Radon Information — U.S. Environmental Protection Agency
- CDPHE Radon Program — Colorado Department of Public Health and Environment
- CDC Radon Topic Page — Centers for Disease Control and Prevention
- American Lung Association Radon — American Lung Association
Homeowners in the Denver metro or Front Range who want help connecting with a vetted radon testing professional can get in touch here.