Radon Results Range: What Homeowners Know
A radon test report can feel like a foreign language. It hands you a number in picocuries per liter, maybe a margin of error, and not much else. Understanding the radon results range, what each band of numbers actually means, is the difference between overreacting to a normal result and overlooking a genuine problem. This guide walks through how to read your report, what each portion of the range signals, and the right next step for each scenario. It summarizes EPA and Colorado Department of Public Health and Environment guidance current as of 2026 and is general information, not medical advice. For an accurate measurement, follow certified testing procedures.
How to read your radon results range
Radon results are reported in picocuries per liter (pCi/L). The full practical range homeowners encounter runs from below 1 pCi/L up into the double digits and occasionally higher. The single most important reference point is the EPA action level of 4.0 pCi/L. Below it, no mitigation is required; at or above it, mitigation is recommended.
A complete report should list the average reading, the test duration, and sometimes the high and low points during the test. The average is the figure you compare against the action level. This article focuses on interpreting that average; for how a result becomes a fix decision, our piece on acceptable radon levels covers the thresholds in detail.
The low band: below 2.0 pCi/L
Results under 2.0 pCi/L sit in the lower part of the range and are generally treated as good. The national indoor average is around 1.3 pCi/L, so a reading here is close to typical. No action is required, though it is worth remembering that no level is entirely risk-free and outdoor air averages just 0.4 pCi/L.
Homeowners in this band should still retest periodically, every two years is a reasonable cadence, and after renovations or foundation work that could open new radon entry points. A low result today does not guarantee a low result forever.
The gray zone: 2.0 to 3.9 pCi/L
This middle band is where many homeowners feel uncertain, and rightly so. The number is below the action level, so mitigation is not required, but the EPA specifically suggests considering it in this range because measurable risk remains. A reading of 3.5 pCi/L, for instance, is more than twice the indoor average.
Whether to act in the gray zone is a judgment call. Families who spend a lot of time in a basement bedroom or office may choose to mitigate, while others retest and monitor. Sealing cracks and improving ventilation can nudge gray-zone numbers lower without a full system.
The action band: 4.0 to 9.9 pCi/L
At 4.0 pCi/L and above, the report crosses into territory where the EPA recommends mitigation. Numbers in this band are common in Colorado given the region’s geology, so a 4.5 or 6.0 result is not unusual here. It is, however, a clear signal to plan a fix.
The typical solution is active soil depressurization, which routinely brings readings in this band well below the action level. Before committing, confirm a borderline result with a second test. Our radon level chart shows how this band compares to the rest of the scale.
The high band: 10 pCi/L and above
Readings of 10 pCi/L or more occupy the high end of the range and call for prompt action. The EPA characterizes long-term exposure at this level as carrying lung-cancer risk comparable to significant smoking exposure over years. Results above 20 pCi/L are very high and should be addressed quickly.
For very high numbers, you do not need a long-term confirmatory test; a quick short-term retest is enough, and you should move toward mitigation without delay. The dedicated guide on whether a radon level of 10 is dangerous addresses this band specifically.
Why your number can vary between tests
Two tests in the same home can produce different numbers, and that is normal. Radon levels rise and fall with weather, barometric pressure, season, and how sealed the house was during testing. Winter and closed-house conditions tend to push readings higher; open windows lower them.
This variability is why the EPA recommends confirming borderline results and favors long-term tests for the most representative average. A single short-term snapshot near the action level deserves a second look before you make a major decision. Consistent device placement, in the lowest livable level away from drafts, improves comparability.
Short-term versus long-term results
The test type shapes how you read the range. Short-term tests, two to seven days, are fast and useful for an initial screen or a real-estate timeline. Long-term tests, more than 90 days, capture seasonal swings and give a truer annual average. If a short-term and long-term result disagree, the long-term number better reflects your actual exposure.
For real-estate transactions, short-term professional measurements are standard because of timing, but homeowners with borderline short-term results benefit from following up with a long-term test once the deal closes.
Turning your result into a decision
Match your number to the band, then act accordingly. Below 2.0, monitor and retest. In the gray zone, weigh how the space is used and consider mitigation. At or above 4.0, plan a mitigation system. Above 10, act promptly. After any mitigation, always retest to confirm the system brought the level down.
Cost is part of the decision in the higher bands. Our guide to radon mitigation cost in Colorado gives realistic figures, and the Colorado radon testing hub explains how testing and mitigation work together along the Front Range.
What the rest of your report tells you
The average reading gets the attention, but a good report includes other useful fields. The test duration confirms whether the result is a short-term snapshot or a longer average. Some reports show the high and low readings during the test, which reveal how much the level swung, useful context for judging whether a borderline average is stable or volatile. The device type and calibration date speak to reliability.
Professional reports for real-estate transactions often include closed-house condition verification and tamper checks, which matter because an improperly conducted test, with windows left open, can understate the true level. When you read your report, glance past the headline number to these details. They tell you how much weight the average deserves and whether a confirming test is warranted.
Comparing your range to typical homes
Homeowners often want to know how their number stacks up against everyone else’s. Nationally, the average indoor reading is about 1.3 pCi/L, and outdoor air sits near 0.4 pCi/L. Against that, anything in the low single digits is common, and a meaningful minority of homes exceed the 4.0 action level. In Colorado, the distribution shifts upward; a 5 or 6 pCi/L result that would be notable elsewhere is routine along the Front Range.
This regional context helps you read your own range without alarm. A Colorado result in the action band reflects the region’s uranium-bearing geology, not a uniquely defective home. The comparison that matters most, though, is not your home against the national average but your average reading against the 4.0 action level. That single comparison drives the decision, while the broader context simply sets expectations.
Margin of error and what it means
Radon measurements carry uncertainty, and many reports express it as a plus-or-minus figure or a percentage. A result of 4.2 pCi/L with a margin that could place the true value just below or above 4.0 is genuinely borderline, and that uncertainty is a reason to confirm with a second test rather than treat the single number as definitive. Continuous monitors and laboratory analyses both have inherent variability.
This is not a flaw in the test; it is the nature of measuring a fluctuating gas. The practical response is to treat results near the action level as a range rather than a precise point, and to confirm borderline numbers. A reading deep in the high band, say 15 pCi/L, has enough margin that the uncertainty does not change the conclusion. Near the threshold, the margin matters and a retest resolves it.
Interpreting results in a real-estate context
Radon results carry extra weight during a home sale, where a number above 4.0 pCi/L often triggers negotiation. Buyers may request that the seller install mitigation or provide a credit. Because of the stakes and the tight timeline, real-estate radon tests are usually short-term professional measurements with documented closed-house conditions and chain of custody, which makes the result harder to dispute.
If you are buying along the Front Range, expect radon to come up, since so many Colorado homes test high. A result in the action band is not a deal-breaker; it is a known, fixable condition with a predictable cost. Understanding the range lets you respond calmly: a 5 pCi/L result means plan mitigation, not walk away. Our guide on mitigation cost in Colorado helps you put a number on the fix during negotiations.
How seasonal timing shifts your range
The time of year you test can move your result by a meaningful margin. Winter readings tend to run higher because homes stay sealed, heating systems alter indoor pressure, and the cold intensifies the stack effect that draws soil gas upward. Summer readings often run lower, with more open windows and air exchange. A home that reads 3 pCi/L in July might read 6 pCi/L in January.
This seasonality matters most for borderline results. A summer short-term test landing just under the action level could understate the true winter peak, so a heating-season follow-up or a long-term test gives a more honest picture. For real-estate transactions, the timing is dictated by the deal, but homeowners testing on their own schedule should consider that a winter test captures the more conservative, protective end of the range.
The practical guidance is to weight the higher, heating-season figure when results conflict, and to favor long-term testing for the truest annual average. Reading your range with the testing season in mind prevents a favorable summer snapshot from masking a genuinely elevated winter level.
Tracking your results over time
A single result is a snapshot; a series of results is a story. Keeping a record of each test, its date, the device used, the conditions, and the average, lets you see whether your home’s radon is stable, rising, or falling. A jump between tests can flag a new foundation crack, a renovation that opened a pathway, or a failing mitigation system. A steady low trend is far more reassuring than one isolated reading.
For homes with mitigation systems, periodic retesting confirms the system still works, since fans can degrade over years. The EPA suggests retesting every two years and after any major change to the home. Treating radon as something you monitor over time, rather than a one-time box to check, gives you durable confidence that your results stay in the acceptable range.
Ultimately, a radon report is only as useful as your ability to read it. Knowing which band your average falls into, understanding why the number can vary, and confirming borderline results turns a confusing figure into a clear next step. That clarity is what lets Front Range homeowners respond proportionately, neither shrugging off a genuine problem nor overreacting to a normal one.
References
- A Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- Radon in Colorado — Colorado Department of Public Health and Environment
- Radon and Indoor Air — American Lung Association
If you are along the Front Range and want help interpreting your radon results range or connecting with a vetted testing professional, reach out through our contact page.