Radon Testing Numbers: How to Read Results
A radon test comes back, and the radon testing numbers on the report can be baffling: a unit most people have never heard of, a single figure or several, and no obvious sense of whether the result is good or alarming. Reading those numbers correctly is what turns a test into a decision. This guide explains the units, the difference between short-term and long-term measurements, how averaging works, and how to compare a result against EPA’s action level. 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.
What radon testing numbers actually measure
Radon results in the United States are reported in picocuries per liter of air, written pCi/L. A picocurie is a measure of radioactivity, so the number describes how much radioactive radon is present in each liter of indoor air. The higher the number, the more radon, and the higher the long-term health risk. Some countries use a different unit, becquerels per cubic meter (Bq/m3), but U.S. homeowners will almost always see pCi/L on a domestic report.
The reference points give the number meaning. EPA estimates the average outdoor level at about 0.4 pCi/L and the average indoor home level at roughly 1.3 pCi/L. The action level, the figure that should anchor interpretation, is 4 pCi/L; at or above it, EPA recommends mitigation. So a result of 2.0 is below the action level but above the indoor average, while a result of 8.0 is double the action level and a clear call to act. For the threshold itself, the explainer on the EPA 4 pCi/L action level goes deeper.
Short-term versus long-term results
The kind of test behind the number matters as much as the number itself. A short-term test runs from two to seven days, sometimes up to ninety, and gives a fast snapshot. A long-term test runs more than ninety days, often a full year, and captures how radon varies across seasons. Both report in pCi/L, but they answer slightly different questions: short-term tells you what radon was doing during a brief window, long-term tells you the average exposure over time.
Short-term results swing more because radon levels rise and fall with weather, season, and how the house is operated. A closed-up house in winter often reads higher than the same house ventilated in summer. EPA’s general approach is to start with a short-term test, and if the result is at or near the action level, either confirm with a second short-term test or follow up with a long-term test for a more representative average. A single short-term number, especially one near 4 pCi/L, is a prompt to confirm rather than a final verdict. The guide on when to choose a professional radon test over DIY explains how testing approach affects reliability.
Why closed-house conditions matter
Test protocols call for closed-house conditions, meaning windows and exterior doors stay shut except for normal entry and exit, typically for twelve hours before and during a short-term test. This is not about inflating the number; it is about capturing a realistic measurement rather than an artificially low one created by ventilation. Opening windows during a test dilutes indoor radon toward the low outdoor level and produces a result that does not reflect how the home normally operates. Following the protocol is what makes the number trustworthy.
Different devices, different numbers
The device behind a result shapes how to read it. Passive kits, such as charcoal canisters and alpha-track detectors, absorb or record radon over the test period and are then sent to a lab, which reports a single integrated average. Charcoal devices suit short-term testing; alpha-track detectors are common for long-term testing. Continuous radon monitors, by contrast, are electronic instruments that sample the air repeatedly and log readings throughout the test, reporting an average plus the hour-by-hour pattern.
Continuous monitors add useful context a passive kit cannot. Because they record over time, they reveal whether radon spiked during certain hours, and many include tamper detection that flags whether a window was opened, which protects the integrity of a real-estate test. Passive kits are inexpensive and adequate for an initial homeowner screen, while professional and transactional testing often relies on continuous monitors for the extra detail and tamper resistance. Whichever device produced the number, the headline figure is an average concentration to be compared against the action level. The guide on professional radon testing explains when the added rigor is worth it.
How averaging works
Radon numbers are about averages, not instantaneous readings. A continuous monitor records radon many times over the test period and reports the average, sometimes alongside the high and low readings it captured. A passive kit, by contrast, integrates exposure over the whole period and yields a single average figure when the lab analyzes it. Either way, the headline number represents the average concentration during the test window, which is the appropriate basis for comparison against the action level.
Averaging explains why two tests of the same home can differ. A short-term test in January and another in July may produce different averages because the underlying radon varied, not because either test was wrong. This is also why a long-term annual average is the most representative single number for estimating real exposure. When results disagree, the longer test and the more recent test under proper conditions carry more weight. To see how these averages compare against typical and elevated benchmarks, the guide on what counts as a high radon level tiers the numbers.
Comparing your number to the action level
The practical step is mapping your number onto EPA’s framework. At or above 4 pCi/L, EPA recommends mitigation; this is the clearest signal to act. Between 2 and 4 pCi/L, EPA suggests considering action, since no level is risk-free. Below 2 pCi/L, the level is relatively low, though periodic retesting remains sensible because radon fluctuates over time and with structural changes.
Location adds context to the comparison. Colorado’s Front Range sits largely in EPA Radon Zone 1, the highest-potential category, and CDPHE reports that a large share of Colorado homes test above the action level. A result of 5 or 6 pCi/L, unremarkable to find in a Front Range basement, still calls for mitigation regardless of how common it is locally. The regional picture in the Colorado radon testing guide helps frame what local numbers tend to look like.
Reading numbers in a real-estate transaction
Radon numbers carry particular weight during a home sale, and reading them correctly affects negotiation. In a transaction, a short-term test is common because of time pressure, and the result is usually compared directly against the 4 pCi/L action level written into many contract contingencies. A reading at or above that figure typically lets a buyer request that the seller install mitigation before closing or provide a credit to cover it. A reading below it generally satisfies the contingency.
Because a single short-term transactional test drives a financial decision, the conditions behind the number matter even more. EPA’s home buyer and seller guidance emphasizes proper closed-house conditions and tamper resistance, which is why continuous monitors are favored for these tests. A buyer reading a transactional result should confirm the test followed protocol and, where time allows, consider a follow-up test after moving in to establish a longer-term average. The number on a transaction report is a snapshot under specific conditions, and understanding that keeps it from being over- or under-interpreted.
Units beyond pCi/L
While U.S. reports use pCi/L, homeowners occasionally encounter the international unit, becquerels per cubic meter (Bq/m3), on imported devices or in research. The conversion is straightforward: 1 pCi/L equals about 37 Bq/m3, so EPA’s 4 pCi/L action level corresponds to roughly 148 Bq/m3. Knowing this prevents confusion if a device or document reports the metric figure, and it helps when comparing U.S. guidance against international references that use Bq/m3. For domestic decisions, though, pCi/L against the 4 pCi/L action level is the framework that matters.
What to do once you understand the number
Reading the number is step one; acting on it is step two. If a short-term result lands at or above 4 pCi/L, confirm with a second short-term test or a long-term test, then arrange professional mitigation if the elevated reading holds. Sub-slab depressurization, the most common system, vents soil gas above the roofline and typically drives levels down close to the outdoor baseline. Always retest after mitigation; the post-mitigation number is what proves the system worked.
For results between 2 and 4 pCi/L, weigh mitigation against your household and how the lower levels are used. Below 2 pCi/L, retest periodically and after significant changes to the home. Because EPA estimates long-term exposure raises lung-cancer risk, an elevated, confirmed number deserves a real response. When a result calls for action, consult EPA guidance and a certified radon professional rather than guessing at the next move.
What can throw off your numbers
Several factors can distort a radon number, and recognizing them prevents misreading a result. The most common is failing to maintain closed-house conditions, which lets ventilation pull the reading artificially low. Placing the test device improperly also skews results: it should sit in the lowest livable level, away from drafts, exterior walls, windows, and direct sunlight, and at a height representative of the breathing zone rather than on the floor or near a heat source.
Severe weather during a short-term test can also move the number, since storms and rapid pressure changes affect how much radon enters. A test run during an unusual weather event may not reflect typical conditions. Tampering, whether deliberate in a transaction or accidental through an opened window, is another source of error, which is why continuous monitors with tamper detection are favored for high-stakes tests. When a number seems surprising, checking whether these factors were controlled is the first step before acting on it.
Tracking numbers over time
A single radon number is a snapshot; tracking numbers over time tells a fuller story. Because radon fluctuates with season and house operation, a home benefits from periodic retesting, and comparing readings across years reveals trends. A level that creeps upward might signal new foundation cracks or changes in how the home is heated and ventilated. A level that drops after mitigation confirms the system is working, and continued monitoring catches any decline in that system’s performance.
EPA recommends retesting every couple of years, after major renovations, after changes to the heating and cooling system, and when a lower level of the home becomes occupied. Keeping a simple record of each test, the number, the date, the device, and the conditions, makes these comparisons meaningful. Over time, that record turns isolated numbers into a clear picture of how a home’s radon behaves, which supports better decisions than any single reading could. The action-level explainer provides the benchmark to track against.
References
- EPA guide to interpreting radon test results — U.S. Environmental Protection Agency
- CDPHE Colorado radon testing guidance — Colorado Department of Public Health and Environment
- CDC radon testing information — Centers for Disease Control and Prevention
Front Range homeowners who want help interpreting results or testing properly can reach a vetted local radon professional through our contact page.