What Are High Levels of Radon?
When a radon test reads above the average, the useful question is not just whether the level is high but what to actually do about it. Knowing what are high levels of radon, and matching each tier to a clear next step, turns an intimidating number into an action plan. This guide summarizes EPA and CDPHE guidance current as of 2026; it is informational only and not medical advice, and any testing or mitigation choice should be confirmed with a certified professional. Radon is measured in picocuries per liter of air, written pCi/L, and the EPA action level is 4 pCi/L.
High Levels, Organized by Action
A high radon level is any reading at or above the EPA action level of 4 pCi/L, well above the roughly 1.3 pCi/L average indoor concentration. Rather than memorizing tiers in the abstract, it helps to pair each band with what the EPA suggests you do. The response, not just the number, is what protects the household.
The bands break down cleanly: 2 to under 4 pCi/L is below action but worth considering; 4 to under 10 pCi/L calls for confirmation then mitigation; 10 to under 20 pCi/L calls for prompt mitigation; and 20 pCi/L and above calls for urgent action. Each band has a sensible playbook, described below. None of this is a health assessment; concerns about symptoms belong with a physician.
What to Do at 2 to 4 pCi/L
This range sits below the action level but above the comfortable zone. The EPA suggests that homeowners consider mitigation when results land between 2 and 4 pCi/L, because no level is risk-free and reducing exposure is always beneficial. There is no obligation to act, but the option is reasonable, especially for households with long daily occupancy in the tested space.
The practical step here is a long-term test to confirm where the home truly sits, since short-term tests vary with weather and season. If a long-term measurement confirms a result in this band, a homeowner can weigh the modest cost of mitigation against the benefit of pushing the level toward the outdoor baseline. For how thresholds are decided, the guide on at what level radon should be mitigated covers the reasoning.
What to Do at 4 to 10 pCi/L
At or above 4 pCi/L, the EPA recommends taking action. The first step for a result in this most common high band is to confirm it. Short-term tests are screenings, so a second short-term test or a long-term test verifies that the home genuinely exceeds the action level rather than catching an unusual spike.
Once confirmed, the step is mitigation by a qualified professional. The standard fix, sub-slab depressurization, draws radon from beneath the foundation and vents it safely above the roofline, typically bringing levels well below 2 pCi/L. This band is routine for Colorado, where much of the Front Range falls in EPA Zone 1, and it is very treatable. The inspectandtest Colorado radon testing guide describes what local mitigation involves.
What to Do at 10 to 20 pCi/L
A reading in this band is notably high, and the action shifts from “confirm then decide” to “move toward mitigation promptly.” The level is far enough above 4 pCi/L that delaying months for a long-term test is hard to justify. Many professionals will proceed to mitigation planning while treating the initial reading as actionable.
The mitigation approach is the same in principle as the lower band, but the urgency is higher because cumulative exposure builds faster at higher concentrations. After a system is installed, a post-mitigation test confirms it brought the level down. Homeowners in this band should prioritize scheduling the work rather than letting it sit on a to-do list. For context on how home readings distribute across these bands, see radon levels in the home.
What to Do at 20 pCi/L and Above
Very high readings demand urgent action. At 20 pCi/L and up, the EPA advises arranging mitigation as soon as possible, and a single short-term test is sufficient grounds to act, no waiting for a long-term measurement. Some homes in uranium-rich geology test into the dozens of pCi/L, and these cases warrant the fastest response.
The fix remains professional mitigation, and at these levels a well-designed system still achieves large reductions. The reason for urgency is purely about limiting cumulative exposure: every month a home sits at 25 pCi/L adds meaningfully more exposure than a month at 5 pCi/L. Acting quickly is the responsible response, followed by verification testing to confirm the system works.
Why the Action Tiers Make Sense
Radon is a leading cause of lung cancer according to the EPA and CDC, and the risk scales with both concentration and time. That is precisely why the response tiers exist. A home at 4 pCi/L and a home at 25 pCi/L both have a problem, but the math of cumulative exposure means the higher home should not wait. The tiers translate that risk math into a practical schedule for action.
What homeowners control is the level in their home and how fast they reduce it. They do not control the geology, and they cannot self-diagnose health effects, which is a question for a doctor. Matching the response to the reading is the part fully within reach.
Confirming, Mitigating, and Re-Testing
Across every band, three principles hold. Confirm low and moderate readings with longer or repeat testing, because short-term results vary. Mitigate with a qualified professional once a result is confirmed or, for very high readings, without delay. Re-test after mitigation and periodically thereafter, since systems can lose effectiveness and home changes alter radon entry.
Colorado homeowners can use CDPHE resources to find qualified help, and keeping records of pre- and post-mitigation readings supports future home sales. A high radon level, handled with the right action at the right speed, becomes a solved problem rather than an ongoing worry.
Choosing Between Short-Term and Long-Term Tests
The kind of test you run affects how confidently you can act on a high reading, so matching the test to the situation matters. Short-term tests run from a few days to about a week and deliver fast results. They are ideal for an initial screen and for real estate timelines, where a quick answer is needed. Their drawback is that they capture only a brief window, and radon fluctuates with weather and season, so a short-term result can land high or low depending on conditions during the test.
Long-term tests run for more than ninety days and average radon levels across changing conditions, producing a more representative year-round figure. For results in the 2 to 4 pCi/L band, where the decision to mitigate is a judgment call, a long-term test is especially valuable because it resolves whether a borderline short-term reading reflects a genuine condition or a temporary spike.
The practical rule pairs the test to the level. Low and moderate readings benefit from long-term confirmation before committing to mitigation. High readings, particularly above 10 and certainly above 20 pCi/L, are far enough above the action level that waiting months for a long-term test is hard to justify, and a short-term result is sufficient grounds to act. Choosing the right test type keeps the response both confident and timely.
What a Mitigation System Involves
When a confirmed result calls for action, homeowners benefit from knowing what mitigation actually entails. The standard solution, sub-slab depressurization, places a pipe through the foundation slab into the soil beneath and uses a continuously running fan to draw radon out and vent it above the roofline. The system creates suction beneath the slab that reverses the pressure pulling radon indoors, intercepting the gas before it reaches living space.
A qualified installer assesses the home, determines how many suction points the structure needs, sizes the fan appropriately, and routes the vent pipe to discharge safely. Sealing major entry points, slab cracks, sump openings, and gaps around penetrations, helps the system work efficiently. The installation typically takes a single day for most homes, though complex foundations can require more involved designs.
After installation, a post-mitigation test confirms the system reduced the level, ideally well below 2 pCi/L. Many systems include a visual indicator that signals if the fan stops, since the reduction depends on continuous operation. Understanding this process demystifies mitigation and helps homeowners see why a high reading, while it demands action, is a problem with a reliable and well-established solution.
Living With a Mitigated Home
Once a system is in place and verified, the home enters a maintenance phase rather than a crisis phase. The mitigation fan runs continuously and uses modest electricity, and most systems operate for years with little attention. Periodic retesting, generally every couple of years, confirms the level remains low, and checking the system indicator catches a failed fan before radon can climb back up.
Home changes can affect a mitigated system. Major renovations, foundation work, or alterations to the home’s ventilation may warrant a fresh test to confirm the system still performs. Keeping records of the original high reading, the mitigation work, and the post-mitigation result creates a history that reassures future buyers and demonstrates the problem was properly resolved. A high radon level, once mitigated and monitored, becomes a routine aspect of home maintenance rather than a recurring source of worry.
What to Do While You Wait to Mitigate
There is often a gap between learning a home has high radon and getting a mitigation system installed, and homeowners reasonably ask what they can do in the interim. While professional mitigation is the real solution, a few temporary measures can modestly reduce levels in the short term. Increasing ventilation by opening windows and running exhaust fans dilutes indoor radon, though this is impractical to sustain and far less effective than a proper system.
For very high readings, spending less time in the lowest, highest-reading areas of the home, such as an unfinished basement, until mitigation is complete can reduce exposure during the wait. These steps are stopgaps, not fixes, and they should not delay arranging professional mitigation, which is the only reliable way to bring a high level down and keep it there.
The key is to treat the interim measures as bridges rather than destinations. Radon risk accumulates over years, so a few weeks awaiting installation does not transform a manageable problem into an emergency. The priority is to get a qualified professional scheduled and the work done, after which the temporary measures become unnecessary and the home’s level stays low through the mitigation system rather than through daily effort.
References
- EPA radon action level and mitigation recommendations — U.S. Environmental Protection Agency
- Colorado radon testing and mitigation resources — Colorado Department of Public Health and Environment
- CDC information on radon health risk — Centers for Disease Control and Prevention
Front Range homeowners facing a high radon reading can get in touch with a qualified local inspector through our contact page for confirmation testing and mitigation guidance.