What Is Considered a High Level of Radon?
Asking what is considered a high level of radon calls for more than a single number, because radon results fall along a spectrum and different bands carry different urgency. EPA’s 4 pCi/L action level is the anchor, but a reading of 4.5 and a reading of 25 demand very different responses. This guide tiers radon levels, from low to very high, explaining what each band means and what to do about it. It summarizes EPA and CDPHE guidance current as of 2026 and is informational only, not medical advice; consult EPA guidance and a certified professional for testing decisions and your physician for health questions.
What is considered a high level of radon?
Any radon reading at or above EPA’s action level of 4 pCi/L is considered high enough to warrant action. That is the formal threshold. But “high” is best understood as a range rather than a single cutoff, because the health risk and the urgency of the fix scale with the concentration. A result of 4 pCi/L crosses the action line; a result of 20 pCi/L is roughly five times the action level and represents a far more pressing situation, even though both call for mitigation.
The reference points anchor the spectrum. EPA estimates the average outdoor level at about 0.4 pCi/L and the average indoor home level at roughly 1.3 pCi/L. Against those baselines, anything above the indoor average is elevated to some degree, the 2 to 4 pCi/L band is a consider-action zone, and 4 pCi/L and up is the action zone. The dedicated explainer on the 4 pCi/L action level covers the threshold’s basis in detail.
Radon levels tiered: from low to very high
Breaking the spectrum into bands makes a result easier to interpret. Each tier below describes a typical range and the response it generally warrants, all measured in pCi/L as an average under proper closed-house testing conditions.
Below 2 pCi/L: relatively low
Readings under 2 pCi/L are relatively low, sitting between the outdoor baseline and the lower part of the indoor average range. No radon level is entirely risk-free, so this is not “zero concern,” but mitigation is generally not recommended at this level. The sensible response is periodic retesting, since radon fluctuates with season and structural changes, and a follow-up test after major renovations or HVAC work.
2 to 4 pCi/L: elevated, consider action
This band sits above the national indoor average but below the action level. EPA explicitly advises homeowners to consider taking action in this range, because reducing exposure further reduces risk. The case for mitigating strengthens when vulnerable people are involved, children, smokers, or anyone spending long hours in a basement bedroom or office. Many homeowners in this band choose to mitigate, especially in high-potential regions.
4 to 10 pCi/L: high, mitigation recommended
At or above 4 pCi/L, EPA recommends action, so this entire band is firmly “high.” A reading here is common in high-potential areas and clearly calls for professional mitigation. The health risk over years of exposure is significant enough that public-health guidance treats this range as a clear trigger to install a radon reduction system and then retest to confirm the reduction.
10 to 20 pCi/L and above: very high
Readings of 10 pCi/L and higher are very high and call for prompt mitigation. Above 20 pCi/L, the level is extreme and warrants fast action, since lifetime exposure at that concentration carries serious risk. The fix is the same proven mitigation technology used at lower elevated levels, but the priority and speed increase sharply. A very high reading should not be left unaddressed while a homeowner deliberates. For the risk-focused framing of these upper bands, the guide on how much radon is too much connects the numbers to action.
Why the bands matter more than a single cutoff
The value of tiering radon levels is that it matches the response to the severity, which a single yes-or-no cutoff cannot do. Treating every reading as simply “above” or “below” 4 pCi/L hides important distinctions. A homeowner at 4.2 pCi/L and one at 22 pCi/L are both above the action level, but their situations differ in urgency by a wide margin. The banded view tells the first homeowner to plan mitigation and the second to arrange it quickly, while telling someone at 1.5 pCi/L to simply retest periodically.
Bands also help interpret change over time. A home that tested at 3 pCi/L last year and 5 pCi/L this year has moved from the consider-action band into the action band, a meaningful shift that a single threshold check might obscure if both readings were just lumped as “around 4.” Watching which band a home falls into, and whether it is trending up or down, gives a fuller picture than any one measurement. The risk-focused companion guide on how much radon is too much connects these bands to the underlying health risk.
Why the same level can mean different things
Two homes can show identical radon numbers yet face different practical stakes, because exposure depends on how a space is used. A 6 pCi/L reading in an unoccupied crawl space matters less than the same reading in a finished basement bedroom where someone sleeps every night. EPA’s risk estimates assume substantial time spent in the measured air, so occupancy patterns shape how urgently a given level should be addressed, even though the action-level recommendation does not change.
Testing conditions and timing also affect what a number means. Short-term tests swing with weather and season, so a high short-term reading should be confirmed before major decisions, while a long-term average is the most representative single figure. A high winter reading that drops in summer still reflects real exposure during the heating season. The guide on reading radon testing numbers explains how test type and averaging factor into interpretation.
The Front Range context for high readings
High radon readings are not unusual in Colorado. The Front Range lies largely within EPA Radon Zone 1, the highest-potential category, because of uranium-bearing soils and rock, and CDPHE reports that a large share of Colorado homes test above the 4 pCi/L action level, a rate well above the national average. A reading in the high band is therefore common across Denver, Douglas, Arapahoe, Jefferson, El Paso, and neighboring counties.
Common does not mean acceptable. A frequently seen 5 or 6 pCi/L result still warrants mitigation, and the regional prevalence is precisely why CDPHE and EPA urge every Front Range home to be tested. Local frequency should normalize testing and fixing radon, not normalize ignoring it. The regional Colorado radon testing guide details what local results tend to look like and why the geology drives them.
How outdoor and average levels frame “high”
Understanding what counts as high is easier with the baselines in view. EPA estimates outdoor air at about 0.4 pCi/L and the average U.S. home at roughly 1.3 pCi/L. Against those references, the 2 to 4 pCi/L band is already several times the outdoor baseline and above the typical home, which is why EPA flags it for consideration. The 4 pCi/L action level is about ten times outdoor air, and the very high readings of 20 pCi/L and above are roughly fifty times the outdoor baseline.
Seeing the numbers this way clarifies that “high” is relative to a low natural floor, not an arbitrary line. It also sets a realistic target for mitigation. A system cannot quite reach the 0.4 pCi/L outdoor level, but it can routinely pull an elevated home down to around 2 pCi/L or below, moving it from a high band into a low one. The goal is not perfection but getting safely under the action level and as close to the baseline as practical, which the standard mitigation approach reliably achieves.
Beyond the number: occupancy and testing quality
Two factors decide how seriously to treat a high reading beyond the figure itself. The first is occupancy, already noted: a high level in a heavily used basement bedroom carries more real exposure than the same level in an unused space, because EPA’s risk estimates assume substantial time in the measured air. The second is the quality of the test that produced the number. A reading taken with windows open, or with a device disturbed mid-test, can mislead in either direction.
Reliable interpretation therefore depends on a properly conducted test under closed-house conditions, ideally confirmed when the result sits near a band boundary. A high short-term reading should be verified with a follow-up before major spending, while a high long-term average is already representative. Pairing the raw number with how the space is used and how carefully the test was run is what turns a figure into a sound decision about whether and how urgently to mitigate.
What to do at each level
The response follows the band. Below 2 pCi/L, retest periodically. Between 2 and 4 pCi/L, weigh mitigation against your household, leaning toward action for vulnerable occupants. At or above 4 pCi/L, confirm an elevated short-term reading and then arrange professional mitigation. For very high readings of 10 pCi/L or more, act promptly. In every elevated case, sub-slab depressurization, the standard system, vents soil gas above the roofline and typically lowers levels near the outdoor baseline. Retest after mitigation to verify success. Because EPA estimates long-term exposure raises lung-cancer risk, a confirmed high reading deserves a real response; consult EPA guidance and a certified radon professional to choose the right system.
High readings and the path to a fix
A high reading is the start of a clear, well-trodden path rather than a dead end. Once an elevated level is confirmed, a certified radon professional evaluates the home’s foundation and designs a mitigation system suited to it. For most homes the answer is active sub-slab depressurization, which draws soil gas from beneath the slab and vents it above the roofline. Variations exist for crawl spaces and unusual foundations, but the principle is the same: intercept the gas before it enters and route it safely outdoors.
The results are dependable across the high bands. Whether a home reads 5 pCi/L or 25 pCi/L, a properly designed system typically brings the level down below the action threshold and often near 2 pCi/L. Higher starting levels do not mean the fix is less effective; they mean the reduction is larger. After installation, a post-mitigation test confirms the new level, and a simple gauge on the system shows it continues working. Knowing this path exists is what makes a high reading manageable rather than frightening.
How often to retest at each level
Retesting frequency sensibly tracks where a home falls on the spectrum. A home well below 2 pCi/L can be retested every few years and after any major change, since the margin to the action level is comfortable. A home in the 2 to 4 pCi/L band, closer to the threshold, benefits from more attentive monitoring, especially if occupancy of lower levels increases. A mitigated home that once read high should be retested periodically to confirm the system keeps levels down.
Certain events reset the clock regardless of the prior level: major renovations, foundation work, changes to the heating and cooling system, or finishing a basement can all alter how radon enters and accumulates. EPA recommends retesting after such changes and roughly every couple of years in general. Matching retest frequency to the home’s level and history turns a one-time measurement into ongoing assurance, which matters in a high-potential region like the Front Range where levels can shift over time.
References
- EPA radon action level explanation — U.S. Environmental Protection Agency
- CDPHE Colorado radon levels and testing — Colorado Department of Public Health and Environment
- CDC radon health risk overview — Centers for Disease Control and Prevention
If your Front Range home tested high, get connected with a vetted local radon mitigation professional through our contact page.