Radon Levels Chart: pCi/L Thresholds Explained
After a radon test, most homeowners want one thing: a chart that tells them whether their number is fine or a problem. A radon levels chart organizes the picocurie-per-liter readings into bands and pairs each with what the EPA recommends. The anchor for the whole chart is the EPA action level of 4.0 pCi/L, the point at which the agency recommends fixing a home. This guide summarizes EPA and CDC guidance current as of 2026 to walk through the threshold bands, what each means, and how radon risk scales with the number. This is general information, not medical advice; consult your physician for health questions and a certified professional for testing decisions.
The Radon Levels Chart at a Glance
Radon is measured in picocuries per liter of air, abbreviated pCi/L. The reading from a test places a home into a general band on the chart. While radon risk is continuous rather than divided into hard zones, organizing levels into bands helps homeowners interpret a result and decide what to do.
- 4.0 pCi/L and above — At or above the EPA action level. The EPA recommends fixing the home with a mitigation system. The higher the reading, the more urgent the action.
- 2.0 to 3.9 pCi/L — Below the action level but not negligible. The EPA notes risk remains and homeowners may consider mitigating, especially in high-radon regions.
- Below 2.0 pCi/L — Closer to typical indoor levels. Mitigation is generally not recommended, though no level is entirely risk-free.
The outdoor air average is roughly 0.4 pCi/L, and the EPA estimates the average indoor level around 1.3 pCi/L. Those figures provide useful reference points for reading any test result against typical conditions. The radon testing hub for Front Range homeowners puts these numbers in regional context, which matters because Colorado’s baseline runs higher than the national average.
Why 4.0 pCi/L Is the Anchor
The 4.0 pCi/L action level is the single most important number on the chart, but it deserves careful explanation. It is not a safety threshold below which radon is harmless and above which it is dangerous. Radon risk rises continuously with exposure, so a home at 3.9 pCi/L is not meaningfully safer than one at 4.0. Instead, 4.0 is a practical decision point that the EPA set based on what is achievable through mitigation and a reasonable balance of risk and cost.
The agency is explicit that lower is better, and it encourages homeowners to consider action even in the 2.0 to 4.0 range. Reaching outdoor-level concentrations indoors is generally not achievable with mitigation, so the goal is to get as low as practical, typically well below 4.0 and often below 2.0. Understanding the action level this way prevents the common mistake of treating 3.9 as a clean bill of health. The number is guidance for a decision, not a verdict on safety.
Understanding the pCi/L Unit
The unit on every radon chart, the picocurie per liter, can feel abstract, so a brief explanation helps. A curie is a measure of radioactivity, and a picocurie is one-trillionth of a curie. Expressed per liter of air, pCi/L describes how many radioactive decays from radon are happening in a given volume of the air you breathe. A higher number means more radioactive particles, which means more of the energy that can damage lung tissue over time.
Some countries use a different unit, the becquerel per cubic meter, and the rough conversion is that one pCi/L equals about 37 becquerels per cubic meter. American homeowners reading a chart will almost always see pCi/L, with the 4.0 figure as the familiar action point. The unit matters less than the relationship it captures: the higher the pCi/L, the greater the cumulative radiation dose to the lungs, and the stronger the case for reducing it. Keeping that simple proportionality in mind makes any radon chart easier to interpret without getting lost in the physics.
How Levels Vary Within a Single Home
A radon chart reports a number, but a home does not have one uniform radon level throughout. Concentrations are typically highest in the lowest level, such as a basement, where soil gas enters and air exchange is limited, and lower on upper floors. This is why testing protocol calls for measuring in the lowest livable level rather than a second-floor bedroom: the goal is to capture the level where people spend meaningful time and where the reading best reflects the home’s radon entry.
Levels also shift over time within the same home. They tend to rise in winter when the house is sealed and the heating system intensifies the stack effect, and they can change with barometric pressure, rainfall, and soil moisture. A single short-term reading is a snapshot of those conditions during the test window. Reading a chart wisely means recognizing that the number reflects a particular spot at a particular time, which is why a long-term test or a confirmatory second test gives a more representative figure for a borderline result. The chart bands stay the same; what changes is how confidently a single reading places the home within them.
How Risk Scales With the Number
The chart is ultimately about health risk, and that risk scales with both the radon level and the years of exposure. The EPA identifies radon as the leading cause of lung cancer among non-smokers and the second leading cause overall, behind smoking. The agency’s risk estimates show that the chance of radon-induced lung cancer climbs as the average level rises, and it climbs much faster for people who also smoke, because radon and tobacco smoke compound each other.
This compounding effect is important. A given radon level carries a substantially higher lung-cancer risk for a smoker than for a non-smoker, which is why smokers in high-radon homes have particular reason to mitigate. For specific health-risk questions, including personal risk factors, a physician is the right source. The chart indicates when to act on the home; it does not diagnose health outcomes for an individual. Translating a level into a personal medical risk is a conversation for a doctor, not something to read off a table.
Reading Your Own Result in Context
A single number tells only part of the story, and several factors shape how to interpret it. The type of test matters. A short-term test of two to seven days gives a snapshot that can be skewed by weather and how the home was operated during the test. A long-term test over more than ninety days reflects the year-round average more accurately, since radon levels swing with seasons, ground moisture, and heating patterns.
Test conditions matter too. A valid short-term test requires closed-house conditions, with windows and exterior doors kept shut, and the device placed in the lowest livable level. A result obtained with windows open or in an upper floor understates the home’s true potential. When a borderline short-term result lands near 4.0, a confirmatory test, ideally a long-term one or a second short-term test, gives a more reliable basis for a decision. The radon inspection guide explains how the measurement is taken and which conditions produce a trustworthy reading.
What the Chart Means for Colorado Homes
The chart carries extra weight in Colorado, where the EPA designates most of the state, including Denver and the Front Range, as Zone 1, the highest radon-potential category. A large share of Colorado homes test at or above the 4.0 action level, so landing in the action band is far from unusual here. The Colorado Department of Public Health and Environment provides state-specific guidance reflecting this elevated baseline.
For Front Range homeowners, the practical implication is to test, take the chart seriously, and not assume a low result is permanent. Because Colorado’s geology produces elevated radon broadly, even a home that initially tests below the action level can warrant periodic retesting, particularly after foundation work or basement finishing that may change how soil gas enters. The chart is a snapshot; in Zone 1, it is wise to refresh that snapshot over time rather than treating one good reading as a permanent guarantee.
Common Misreadings of the Chart
Several misconceptions trip up homeowners reading a radon chart. The first is treating 4.0 as a hard safety line, when it is a practical action point on a continuous risk curve. The second is assuming a result below 2.0 means zero risk; it means lower risk, not none. The third is comparing a short-term snapshot directly to a long-term average as if they were equivalent, when in fact a short-term test can read higher or lower than the home’s true yearly average depending on conditions.
A fourth common error is testing in the wrong place, such as an upper floor or a rarely used room, which can produce a falsely low number that does not reflect where people actually spend time. Reading the chart correctly means pairing the number with the test type, the test location, and the conditions during the test. A 3.5 pCi/L short-term reading taken with a window cracked open in a basement bedroom tells a very different story than the same number from a sealed, properly conducted long-term test, and a thoughtful homeowner weighs that context before deciding.
From Chart to Action
Translating a chart band into a decision is straightforward once the number is reliable. A result at or above 4.0 pCi/L calls for mitigation, and the higher the reading the sooner it should happen; very high readings warrant prompt action. A result between 2.0 and 4.0 puts the choice in the homeowner’s hands, weighing factors like whether anyone in the home smokes, how much time is spent in lower levels, and local radon prevalence. A result below 2.0 generally needs no action beyond periodic retesting.
In every band, the response is rational rather than alarmist. Radon is fixable with a reliable mitigation system, and elevated levels are a manageable home-improvement project, not a reason to abandon a property. The chart’s job is to tell a homeowner where they stand; from there, testing confirmation and, where warranted, professional mitigation handle the rest. The radon remediation guide explains the fix for homeowners whose result lands in the action band.
It is also worth remembering that the chart describes a population-level guideline, not a personal diagnosis. The EPA’s bands are built from epidemiological studies of lung-cancer risk across large groups exposed to radon over many years. For any individual, factors like smoking history, total years of exposure, and time spent in the lower levels of the home shape the actual risk far more than a single reading does. The chart is a sound tool for deciding whether to act on a home, but it should never be read as a forecast for a specific person, which is a question only a physician can address. Used in that spirit, as a guide to home decisions rather than a personal prognosis, the radon levels chart does exactly the job it is meant to do.
References
- EPA Action Level for Radon Explained — U.S. Environmental Protection Agency
- Health Risk of Radon — U.S. Environmental Protection Agency
- About Radon and Lung Cancer Risk — Centers for Disease Control and Prevention
- Colorado Radon Levels and Guidance — Colorado Department of Public Health and Environment
Front Range homeowners unsure how to read their result can connect with a vetted local inspector through our contact page for a reliable confirmatory radon test.