Radon Level Chart: What Homeowners Need to Know
A radon level chart turns a confusing test number into a clear plan. Radon is measured in picocuries per liter, written pCi/L, and a chart maps each range to what it means and what to do about it. The single most important reference point is the EPA action level of 4.0 pCi/L, but the ranges above and below it matter too. This guide summarizes EPA and CDPHE guidance current as of 2026 for Front Range homeowners. It is educational only; consult a certified radon professional for testing and mitigation decisions and a physician about any health concerns.
The Radon Level Chart at a Glance
Here is a plain-language radon level chart based on EPA guidance:
- 0 to 1.9 pCi/L — Low. Below the U.S. indoor average; retest periodically. Risk is reduced but not zero.
- 2.0 to 3.9 pCi/L — Moderate. The EPA suggests considering mitigation, especially for occupied lower levels.
- 4.0 to 9.9 pCi/L — Elevated. At or above the EPA action level; mitigation is recommended.
- 10 to 19.9 pCi/L — High. Prioritize mitigation promptly.
- 20 pCi/L and above — Very high. Mitigate as soon as practical.
For reference, the average indoor level in the U.S. is about 1.3 pCi/L and outdoor air averages roughly 0.4 pCi/L. The chart is a guide, not a hard safety line, because the EPA is clear that no level of radon is completely risk-free. Think of the bands as a way to translate a bare number into a sense of urgency rather than as sharp boundaries between safe and unsafe. To understand how these numbers connect to actual exposure, see our companion guide to radon levels.
Why 4.0 pCi/L Is the Key Number
The 4.0 pCi/L action level is where the EPA recommends homeowners install a mitigation system. It is not a threshold below which radon is safe; rather, it reflects a practical point at which mitigation is both clearly beneficial and reliably achievable with standard systems. Below 4.0, the EPA still recommends considering mitigation between 2.0 and 3.9, because reducing exposure further continues to lower lung cancer risk.
Think of 4.0 as the recommended line of action, not a guarantee of safety on either side. A reading of 3.8 is not meaningfully safer than 4.1; both warrant attention, particularly if people sleep or spend long hours in lower levels of the home. The action level exists partly because mitigation technology can dependably bring most homes below it, making 4.0 a realistic target as much as a risk threshold.
What Each Range Means for Your Health
Radon decays into radioactive particles that lodge in lung tissue when inhaled and emit radiation that damages cells over time. The EPA identifies radon as a leading cause of lung cancer, second only to smoking. Risk rises with both the level and the years of exposure, which is why a chart range is best read alongside how long you have lived in the home and how much time is spent in the lowest levels.
A home reading 12 pCi/L delivers far more cumulative exposure than one at 2 pCi/L over the same period, roughly in proportion to the difference in the numbers. Smokers exposed to radon face a substantially higher combined risk than either factor alone. None of this means a single elevated test is an emergency, but it does mean elevated ranges deserve prompt action rather than indefinite delay, since the harm accumulates quietly year after year.
How Reliable Is a Single Number?
A radon level chart is only as good as the measurement behind it. Radon fluctuates with weather, season, and how the house is operated, so a two-day short-term test is a snapshot rather than a verdict. Levels are often higher in winter when homes are sealed up and the heating system pulls soil gas in more strongly through the stack effect.
For decisions that matter, a long-term test of more than 90 days, or a follow-up short-term test, gives a more dependable figure to place on the chart. Continuous electronic monitors show daily patterns and help separate a true average from a temporary spike caused by a storm or a closed-up week. Many homeowners install an electronic radon detector for ongoing awareness after their initial test, which turns a one-time number into a trend they can watch.
Front Range Context for Reading Your Chart
Most Front Range counties sit in EPA Zone 1, the highest radon-potential category, so elevated results are common across Denver, Boulder, Jefferson, Douglas, and El Paso counties. A reading in the 4 to 10 range is not unusual here, and seeing your number land in an elevated band is not a sign that something is unusually wrong with your house; it reflects the region’s geology. Local mitigation contractors are experienced and readily available, and the standard fix is well understood. A high result on the chart is a reason to act, not to panic, since the remedy is routine. Our pillar on radon testing in Colorado explains why the region’s geology pushes readings up.
What to Do Based on Your Result
Match your action to your chart range:
- Below 2.0: retest every couple of years and after major renovations
- 2.0 to 3.9: consider mitigation, especially for occupied basements
- 4.0 to 9.9: arrange mitigation; the EPA recommends it
- 10 and above: prioritize mitigation promptly
The standard remedy is active soil depressurization, which routinely brings levels under 2.0 pCi/L regardless of where you started on the chart. After any system is installed, a post-mitigation test confirms it worked and gives you a new, lower number to anchor to.
Reading the Chart in Working Levels and Becquerels
Most U.S. homeowners see radon expressed in pCi/L, but you may encounter other units, and a quick translation prevents confusion. In many other countries, radon is reported in becquerels per cubic meter, abbreviated Bq/m³. As a rough conversion, 1 pCi/L equals about 37 Bq/m³, so the EPA action level of 4.0 pCi/L corresponds to roughly 148 Bq/m³.
You may also see references to working levels, an older unit tied to radon decay products rather than the gas itself. For practical home decisions, pCi/L is the unit that matters in the United States, and the 4.0 action level is the figure to anchor to. If a device or report gives you Bq/m³, divide by about 37 to get the pCi/L equivalent and place it on the chart. Keeping the units straight ensures you are comparing your reading to the correct threshold rather than misreading a number that looks alarmingly high simply because it is in a different scale.
Using the Chart for Different Parts of the Home
A single chart reading reflects wherever the test sat, which is usually the lowest livable level. But radon concentration varies by floor, so a basement reading and a second-story reading can land in very different bands of the chart. The standard protocol places the test in the lowest area people regularly use, because that captures the highest realistic exposure.
How you interpret the chart can shift with how you live in the home. Consider these cases:
- Finished basement bedroom — a basement reading directly reflects sleeping exposure, so treat it seriously
- Unused basement — a high basement reading still matters, since gas migrates upward, but main-floor levels may be lower
- Slab-on-grade home — the main living floor is the lowest level, so its reading is the one that counts
The chart band tells you the action; where you spend your time tells you how urgently that action applies. A high basement reading in a home where children sleep downstairs warrants faster action than the same number in a sealed-off, unused cellar, though both ultimately call for mitigation.
When to Call a Professional
Call a certified radon professional if your chart places you at or above 4.0 pCi/L, if you want a dependable long-term measurement, or if you are buying or selling a home. A qualified measurement provider can confirm the reading, place the test correctly, and explain exactly where your number falls, and a certified mitigation contractor can then install a system matched to your foundation and verify the result. Retest after mitigation and periodically thereafter, since levels shift with the seasons and a single chart reading captures only one moment in time. Used this way, the chart is less a one-time verdict than a running reference you return to over the years, translating each new measurement into a clear sense of whether you are in good shape or due for action. For Front Range homeowners in a high-radon region, keeping that reference handy makes radon a managed number rather than a forgotten worry.
Using the Chart to Track Progress After Mitigation
A radon level chart is not only for diagnosing a problem; it is also the scorecard for confirming a fix. After a mitigation system is installed, a post-mitigation test gives you a new reading to place on the chart, and a successful system should drop you from an elevated band into the low range, frequently below 2.0 pCi/L. Comparing the before and after numbers on the same scale makes the system’s effect tangible rather than abstract.
The chart remains useful beyond that first confirmation. Periodic retests over the years let you watch for any drift back toward the action level, which could signal a worn fan or a change in the home. If a future reading creeps upward on the chart, it prompts a check of the system before levels become a concern again. In this way the chart turns radon management into something you can monitor over time, the same way you might track any home-maintenance metric. For Front Range homeowners who have mitigated, keeping an eye on where successive readings land is the simplest way to confirm the protection is holding.
Common Mistakes When Reading a Radon Chart
A radon level chart is a helpful tool, but a few interpretation errors trip up homeowners and can lead to the wrong decision. Recognizing these pitfalls keeps your reading honest and your response proportionate.
The mistakes to avoid include:
- Treating 4.0 as a hard safety line — a 3.9 is not meaningfully safer than a 4.1, and the EPA suggests considering action below 4.0
- Trusting a single short-term test — a two-day reading can spike high or low; confirm before deciding
- Ignoring where the test sat — a reading only represents the level at that location and floor
- Confusing units — a Bq/m³ figure looks far larger than the pCi/L equivalent
- Assuming a low reading is permanent — levels change with seasons, renovations, and how the home is used
Each of these errors pushes a homeowner toward either false alarm or false reassurance. The chart works best as a guide to urgency and next steps, not as a verdict to be read literally to a single decimal point. A reading near the action level calls for confirmation and likely mitigation regardless of which side of 4.0 it lands on, and even a comfortably low reading warrants periodic retesting. Reading the chart with these cautions in mind turns a bare number into a sound decision rather than a misleading one.
References
- What Is Radon and Action Levels — U.S. Environmental Protection Agency
- Health Risk of Radon — U.S. Environmental Protection Agency
- Radon Levels in Colorado Homes — Colorado Department of Public Health and Environment
- Understanding Radon Risk — American Lung Association
If your radon reading lands in an elevated range and you are on the Front Range, get in touch through our contact page to connect with a vetted local radon professional.