Radon Levels in Home: A Homeowner Guide
Every home has some radon, and the only way to know how much is to measure it. Understanding radon levels in home settings means knowing the typical range homes fall into, the EPA action level that triggers a response, where in the house to test, and why Colorado homeowners face higher odds than most. This guide summarizes EPA and CDPHE guidance current as of 2026; it is informational only and not medical advice, and testing or mitigation decisions should be confirmed with a certified professional. Radon is measured in picocuries per liter of air, written pCi/L.
The Typical Distribution of Home Radon
Radon levels in homes vary widely, but the EPA provides useful anchors. The average indoor level across U.S. homes is roughly 1.3 pCi/L, while outdoor air averages about 0.4 pCi/L. Most homes cluster in the low single digits, though a meaningful fraction test above the EPA action level of 4 pCi/L, and some test far higher.
This spread exists because radon entry depends on local geology, soil, foundation type, and how a house is built and ventilated. Two neighboring homes can return different readings. That variability is exactly why national averages cannot substitute for testing a specific home. A reading near 1.3 pCi/L is typical; a reading above 4 pCi/L is elevated and warrants action. For a focused look at the unit behind these numbers, the guide on radon pCi/L levels explains how to read them.
The 4 pCi/L Action Level
The EPA recommends taking action to reduce radon when a home tests at or above 4 pCi/L. This figure is an action level, not a strict safety boundary, because radon carries some risk at any concentration. Below 4 pCi/L the EPA still suggests considering mitigation between 2 and 4 pCi/L, reflecting the principle that lower is always better.
The action level gives homeowners a clear decision point. At or above 4 pCi/L, the recommended response is to confirm the reading and then arrange mitigation by a qualified professional. Between 2 and 4 pCi/L, mitigation is optional but reasonable. Below 2 pCi/L, the level is low and periodic retesting is the main task. Treating 4 pCi/L as a trigger rather than a guarantee of safety is the correct interpretation.
Where to Test in the Home
Where a test sits changes the result, so placement follows a rule: measure in the lowest livable level of the home. That is usually a basement or ground-floor space the household actually uses, since radon enters from the soil and concentrates lower in the structure. Testing an unused attic would understate the exposure people actually receive.
Short-Term Versus Long-Term Tests
Short-term tests run from a few days and screen quickly, which is useful for real estate timelines. Long-term tests run more than 90 days and capture the seasonal swings radon shows, giving a more representative year-round picture. A short-term test under closed-house conditions is a reasonable first step; a long-term test or a repeat short-term test confirms borderline results.
Closed-House Conditions
For short-term tests, keeping windows and exterior doors closed except for normal entry produces a more representative reading. Radon levels rise and fall with weather, ventilation, and season, so consistent conditions matter. A single test taken during unusual weather can mislead, which is why confirmation is built into the process.
Why Colorado Homes Test Higher
Colorado is a high-radon state, and the Front Range is no exception. Much of the state falls within EPA Zone 1, the highest radon-potential category, because of the uranium-bearing geology beneath the soil. The Colorado Department of Public Health and Environment urges every home to be tested regardless of age, location, or construction.
For homeowners in Denver, Jefferson, Douglas, El Paso, Boulder, and surrounding counties, this means the odds of exceeding 4 pCi/L are higher than the national picture suggests. A new home is not automatically safer than an old one, since radon entry depends on the ground beneath the foundation. Testing the specific home is the only reliable approach. The inspectandtest Colorado radon testing guide walks Front Range residents through the process.
What Affects Radon Levels in a House
Several factors push a home’s reading up or down. Foundation type matters: homes with basements or slab-on-grade construction in contact with soil tend to draw more radon than those over crawl spaces with good ventilation. Cracks in the foundation, gaps around pipes, and sump openings create entry paths. Soil composition and moisture influence how much radon migrates toward the home.
Seasonal and behavioral factors matter too. Radon often reads higher in winter, when homes are sealed and heating creates a stack effect that pulls soil gas indoors. Ventilation habits, weather, and even barometric pressure shift the numbers day to day. These influences explain why a single reading is a snapshot, and why retesting over time gives the truest picture. For typical bands and what they mean, see radon levels in homes.
Responding to Your Home’s Reading
Once a home is tested, the response follows the action level. Below 2 pCi/L, the level is low; retest periodically and after major renovations. Between 2 and 4 pCi/L, consider mitigation, with a long-term test to confirm. At or above 4 pCi/L, confirm and arrange professional mitigation, typically a sub-slab depressurization system that vents soil gas above the roofline and usually drives levels well below 2 pCi/L.
Mitigation should be installed by a qualified professional and verified with a post-mitigation test. Colorado homeowners can find qualified help through CDPHE. Keeping documentation of readings supports future home sales, where radon results are commonly requested.
Keeping a Long-Term View
A home’s radon level is not fixed. Foundations settle and crack, ventilation habits change, and mitigation systems can degrade, all of which alter the reading over time. Retesting every couple of years, and after foundation work or major renovations, keeps the picture current. Understanding the typical distribution, the 4 pCi/L action level, and the importance of testing the lowest livable level gives homeowners the framework to interpret whatever number their home returns.
Radon in Basements Versus Upper Floors
Because radon enters from the soil, its concentration is rarely uniform throughout a home, and that gradient shapes where and how to test. The lowest levels of a home, basements and ground-floor slabs in direct contact with the earth, typically read highest, since they are closest to the source and most exposed to the entry points where soil gas seeps in. Upper floors generally read lower as the gas dilutes with distance and ventilation.
This gradient is why the testing rule centers on the lowest livable level. A finished basement used as a family room, home office, or bedroom is where occupants spend real time at the highest concentrations, so testing there reflects the exposure that actually matters. Testing only an upper floor would understate what the household receives in the spaces it uses most. For homes with a finished basement, that space is usually the priority for both testing and, if needed, mitigation.
An unfinished basement that no one occupies is a different case. If the household never spends time there, the relevant test location is the lowest level people actually use. The principle is consistent: measure where people live and breathe, prioritizing the lowest such space, because that is where the radon exposure is greatest and most relevant to health.
New Construction and Radon-Resistant Features
A common assumption is that newer homes are safer from radon, but that is not reliably true. Radon entry depends on the soil beneath a home and the foundation in contact with it, factors that apply to new and old construction alike. A brand-new home built over radon-rich soil can test high, just as an older home can test low. Age alone is no guarantee.
That said, some newer homes incorporate radon-resistant construction features, sometimes called passive radon systems. These include a layer of gas-permeable material beneath the slab, a plastic sheet barrier, sealed foundation openings, and a vent pipe routed from beneath the slab up through the roof. A passive system can reduce radon entry, and it can often be activated by adding a fan if testing shows levels remain elevated, converting it into an active mitigation system.
Even with these features, testing remains essential, because radon-resistant construction reduces risk but does not eliminate it. A passive system may lower levels without bringing them below the action threshold, and only a test reveals whether further action is needed. Buyers of new homes in high-radon areas like the Colorado Front Range should test regardless of any built-in features, treating those features as a helpful head start rather than a substitute for measurement.
What Mitigation Does for a Home
When a home tests at or above the action level, mitigation reliably brings the number down. The standard approach, sub-slab depressurization, uses a fan and piping to draw radon from beneath the foundation and vent it above the roofline before it enters living space. By reversing the pressure that pulls soil gas indoors, the system intercepts radon at its entry point and typically reduces even elevated readings to below 2 pCi/L.
A qualified professional designs the system around the home, choosing the number of suction points and the fan size to match the structure and the severity of the problem. Sealing slab cracks, sump openings, and other entry paths supports the system. After installation, a post-mitigation test confirms the reduction, and periodic retesting maintains confidence that the level stays low. For Front Range homeowners, where elevated readings are common, mitigation is a routine and effective response rather than an exotic intervention.
Radon Levels and Home Sales
Radon often enters the conversation during a real estate transaction, since many buyers request a radon test as part of their inspection. A result at or above the action level can become a negotiating point, with buyers asking sellers to mitigate or to credit the cost. For sellers, addressing radon proactively, testing before listing and mitigating if needed, removes a potential obstacle and reassures buyers.
For buyers, a radon test during the inspection period is inexpensive relative to the value it provides, especially in a high-radon area like the Colorado Front Range where elevated readings are common. Knowing a home’s level before closing allows informed decisions about price and mitigation. A home with an existing mitigation system and documentation of low post-mitigation readings is generally a reassuring sign rather than a red flag, since it shows the radon question has already been answered and resolved. Keeping records of any testing and mitigation supports a smoother transaction for both sides.
References
- EPA radon levels and action level guidance — U.S. Environmental Protection Agency
- Colorado radon testing and Zone 1 information — Colorado Department of Public Health and Environment
- CDC radon and lung cancer overview — Centers for Disease Control and Prevention
Front Range homeowners who want their home’s radon level tested or interpreted can connect with a vetted inspector through our contact page.