Radon Levels Chart: pCi/L, Bq/m³ and What Each Range Means
A radon test result arrives as a single number, usually in picocuries per liter (pCi/L), and the natural question is what that number means for the house. A radon levels chart answers that by placing the result into a range and pairing each range with a recommended next step. This guide lays out a chart in both pCi/L and becquerels per cubic meter (Bq/m³), explains where the EPA action level and the “consider fixing” band come from, shows how short-term and long-term results are read differently, and puts the numbers in Colorado Front Range context. It summarizes EPA, CDC and USGS material current as of 2026 for general education. It is not medical advice. Health questions belong with a physician, and testing or mitigation decisions belong with a certified radon professional who has seen the actual house.
Part of our Radon Levels guide — start there for the full picture →
The Radon Levels Chart in pCi/L and Bq/m³
Radon is measured as radioactivity per unit of air. In the United States the unit is the picocurie per liter. Most of the rest of the world uses becquerels per cubic meter. The conversion is fixed: 1 pCi/L equals 37 Bq/m³. That makes it easy to translate a result from an imported monitor or an international reference into the numbers EPA uses, and back again.
The chart below combines the reference points that appear in EPA’s A Citizen’s Guide to Radon with the standard unit conversion. The averages and the action level come from the EPA guide. The Bq/m³ column is simple arithmetic, rounded.
| Result (pCi/L) | Approx. Bq/m³ | What it means | Typical next step |
|---|---|---|---|
| About 0.4 | About 15 | EPA’s figure for radon normally found in outside air | Reference point only; indoor levels this low are uncommon |
| About 1.3 | About 48 | EPA’s estimate of the average indoor radon level | No action recommended by EPA; retest after major changes to the house |
| Under 2 | Under 74 | Low compared with EPA reference points, but EPA states no level is safe | Retest periodically and after renovation or HVAC changes |
| 2 to under 4 | 74 to under 148 | EPA’s “consider fixing” band | Confirm with a long-term test; weigh mitigation, especially in a lived-in basement |
| 4 | 148 | EPA action level | Follow-up test, then fix the home if confirmed |
| 8 | 296 | Twice the action level | EPA says take a second short-term test immediately; plan for mitigation |
| 10 to 20 and above | 370 to 740 and above | Well above the action level | Confirm quickly and contact a certified mitigator |
Two things about this chart deserve emphasis. First, the bands are a decision aid, not a measure of where risk starts and stops. Radon risk rises with exposure in a continuous way, so a house at 3.8 pCi/L and a house at 4.2 pCi/L are not in meaningfully different situations. Second, the chart is only as good as the test behind the number. A two-day test taken during a warm, windy week can come in lower than the house’s year-round average, and a test in a closed-up basement in January can come in higher. The rest of this guide covers how to read a result with those limits in mind. For a basement-specific version of the same numbers, see the basement radon level chart.
Where the 4 pCi/L Action Level and the 2 to 4 Band Come From
The EPA guide states that the agency recommends fixing a home if the radon level is 4 pCi/L or more. That figure is called the action level, and it anchors almost every radon chart in circulation. The guide is careful to frame it as a practical decision point rather than a safety line. EPA states that any radon exposure carries some risk and that no level of radon is safe. Even levels below 4 pCi/L pose some risk, and lowering the level lowers the risk of lung cancer.
That framing explains the second band on the chart. In its risk tables, the EPA guide marks the range between 2 and 4 pCi/L with the instruction “consider fixing between 2 and 4 pCi/L.” The same tables note that reducing radon levels below 2 pCi/L is difficult. Elsewhere, the guide says radon levels can be reduced in most homes to 2 pCi/L or below. Taken together, those statements suggest why the 2 to 4 band exists: it is a range where mitigation can still produce a measurable improvement, even though the result is under the formal action level.
The CDC’s radon page carries the same message in plain terms. It says there is no safe level of radon and that reducing radon inside a home will always help reduce the risk of lung cancer, even when the level is less than 4 pCi/L. CDC also advises contacting the state radon office if a test shows levels above 4 pCi/L, because that office can help find a qualified or state-certified radon contractor.
The EPA guide also mentions a longer-range goal. It notes that the U.S. Congress has set a long-term goal that indoor radon levels be no more than outdoor levels, and that while this goal is not yet technologically achievable in all cases, most homes today can be reduced to 2 pCi/L or below. That is a useful way to think about the bottom rows of the chart. The 0.4 pCi/L outdoor figure is the ideal, not a realistic target for most basements.
Some international bodies use lower reference levels than EPA does, in the neighborhood of 100 Bq/m³, which works out to roughly 2.7 pCi/L. Those figures come from different policy frameworks and are not part of EPA’s guidance. For a homeowner in the United States, the practical effect is simply to reinforce what EPA already says: lower is better, and the 2 to 4 band is worth taking seriously. For more on what counts as typical, see what a normal radon level looks like.
What the EPA risk tables say about each level
The EPA guide includes two risk tables, one for people who have never smoked and one for people who smoke. Each row estimates how many people out of 1,000 could get lung cancer if exposed to that level over a lifetime. For people who never smoked, the guide’s figures are about 7 per 1,000 at 4 pCi/L, about 4 per 1,000 at 2 pCi/L, and about 2 per 1,000 at the 1.3 pCi/L average indoor level. At 20 pCi/L the figure rises to about 36 per 1,000. For people who smoke, the numbers are far higher at every level: about 62 per 1,000 at 4 pCi/L and about 260 per 1,000 at 20 pCi/L. The guide notes that these are lifetime estimates drawn from EPA’s risk assessment, and that former smokers fall somewhere between the two tables.
Those tables are the clearest explanation of why a radon chart cannot be read in isolation. The same number carries a different weight for a household that includes a smoker. The CDC page describes radon as the second leading cause of lung cancer deaths in the United States after cigarette smoke, and says the EPA and the Surgeon General’s office estimate radon is responsible for more than 21,000 lung cancer deaths each year.
Reading Short-Term and Long-Term Results on the Chart
A radon chart assumes a number that reflects how much radon the household actually breathes over time. Real tests approximate that number in different ways, and the EPA guide explains how much weight each kind of test deserves.
According to the guide, short-term tests stay in the home for two days to 90 days, depending on the device. Charcoal canisters, alpha track detectors, electret ion chambers, continuous monitors and charcoal liquid scintillation detectors are the types most commonly used. Long-term tests stay in place for more than 90 days, usually with alpha track or electret detectors. The guide explains that because radon levels vary from day to day and season to season, a short-term test is less likely than a long-term test to show the year-round average.
EPA’s three-step testing sequence
The EPA guide lays out a sequence for deciding whether to fix a home:
- Take a short-term test. If the result is 4 pCi/L or higher, take a follow-up test to be sure.
- Follow up with a long-term test or a second short-term test. A long-term test gives a better picture of the year-round average. A second short-term test is the faster option. The guide says the higher the initial short-term result, the more certain one can be that a short-term follow-up is the right choice, and if the first result is more than twice the 4 pCi/L action level, a second short-term test should be taken immediately.
- Decide. If the follow-up was a long-term test, EPA says to fix the home if the result is 4 pCi/L or more. If the follow-up was a second short-term test, EPA says to consider fixing the home if the average of the first and second test is 4 pCi/L or higher.
That averaging step is where many homeowners misread their results. Two short-term results of 3.4 and 4.8 pCi/L average to 4.1 pCi/L, which crosses the line. Two results of 3.2 and 4.2 average to 3.7, which falls in the consider-fixing band. Neither pair should be read by picking the higher or the lower number.
Results close to 4 pCi/L
The EPA guide openly acknowledges that short-term tests are less definitive when results sit near the action level. It gives an example: if the average of two short-term test results is 4.1 pCi/L, there is about a 50 percent chance that the year-round average is somewhat below 4 pCi/L. The guide follows that example with a reminder that any radon exposure carries some risk and that lowering the level reduces it. In practice, a result in the 3 to 5 pCi/L range is a strong argument for a long-term test, unless a real estate deadline makes that impossible.
During a home sale, the EPA guide notes that the results of two short-term tests can be used in deciding whether to mitigate, including two tests run side by side about four inches apart. Those transaction tests are usually run by a professional with a continuous monitor. For a sense of what those services and kits run, see radon testing cost, and for do-it-yourself options, the guide to choosing a radon level test kit.
Test conditions that move the number
The EPA guide also explains how to run a test so the result can be read against the chart. For short-term tests, windows and outside doors should be kept closed as much as possible during the test. For tests lasting two or three days, they should be closed at least 12 hours before the test begins. The guide advises against running short tests during unusually severe storms or periods of unusually high winds. The kit belongs in the lowest lived-in level of the home, in a room used regularly but not a kitchen or bathroom, at least 20 inches above the floor, and away from drafts, high heat, high humidity and exterior walls.
What Each Range Means and What to Do Next
With the test type and conditions in mind, here is how each range on the chart usually translates into a decision.
Below 2 pCi/L
A confirmed long-term result under 2 pCi/L is low compared with EPA’s reference points. EPA does not recommend mitigation at this level, though its guidance is clear that no level is risk-free. The sensible follow-up is to retest after anything that changes how air moves through the house: finishing a basement, adding a sump, replacing the furnace, tightening windows or adding an addition. A single short-term result under 2 is encouraging but not conclusive, especially if it was taken in summer.
2 to under 4 pCi/L
This is the band EPA labels “consider fixing.” Whether to mitigate here often depends on how the space is used. A finished basement with a bedroom or a home office gets far more exposure hours than an unfinished laundry area. Households with a current or former smoker face a higher risk at the same level, according to EPA’s tables. A long-term test is the best way to confirm where the house really sits within this band. Basement-specific considerations are covered in what counts as a safe radon level in a basement.
4 to 8 pCi/L
At or above the action level, EPA recommends a follow-up test and then fixing the home if the result is confirmed. In most existing homes the fix is active soil depressurization: a sealed pipe from beneath the slab, a fan and an exterior discharge. Planning numbers for that work are in the guide to radon mitigation system cost. CDC adds that lowering high radon levels requires technical knowledge and special skills, and recommends relying on a qualified professional for repairs.
8 pCi/L and above
Results at twice the action level or higher call for faster confirmation. EPA’s guidance is to take a second short-term test immediately when the first result is more than twice 4 pCi/L. Once confirmed, the next step is a certified mitigator. Results in the double digits are not unusual in parts of the Front Range and are usually reduced well below the action level by a properly designed system.
After mitigation
A radon chart is also the yardstick for checking a mitigation system. A post-mitigation test should land below 4 pCi/L, and many systems bring basements to 2 pCi/L or below, consistent with EPA’s statement that most homes can reach that level. Between tests, the U-tube on the vent pipe shows whether the fan is still pulling suction; the guide to reading a manometer gauge explains what normal looks like. Periodic retesting is still worth doing, because radon fans age and houses change.
Colorado and Front Range Context
A national chart applies everywhere, but the odds of landing in its upper rows are not the same everywhere. The USGS publication The Geology of Radon explains that radon comes from the natural decay of uranium in rock and soil, and that some rock types carry higher than average uranium content, including granites, light-colored volcanic rocks, dark shales and metamorphic rocks derived from them. In its discussion of radon potential across the country, the publication describes the Rocky Mountains this way: granites and metamorphic rocks in these ranges contain more uranium than the sedimentary rocks to the east, resulting in high radon in indoor air and in ground water.
That is the geologic backdrop for the Front Range. Homes in the foothills and along the mountain front sit on or near crystalline bedrock and the soils derived from it. Homes farther east on the plains sit on sedimentary deposits, though soil, construction and ventilation still vary enough from house to house that neighbors can test very differently. The USGS publication makes a related point in its own terms: radon levels in outdoor air, indoor air, soil air and ground water can be very different, and indoor air ranges from less than 1 pCi/L to thousands of pCi/L.
The USGS figures for typical levels differ slightly from EPA’s. The USGS publication says outdoor air probably averages about 0.2 pCi/L and indoor air probably averages between 1 and 2 pCi/L, while the EPA guide uses about 0.4 pCi/L outdoors and about 1.3 pCi/L indoors. The two sources were written at different times and for different purposes. This guide uses EPA’s figures in the chart because they are the ones EPA pairs with its action level, but the difference is a reminder that “average” is an estimate, not a fixed constant.
Front Range construction adds another layer. Many homes have basements or garden levels that are finished and used daily, which puts the lowest lived-in level, where EPA says to test, right next to the soil. Winter heating creates a stack effect that draws soil gas into the lower level, so cold-season tests often read higher than summer tests in the same house. Bedrock close to the surface and expansive clay soils can also complicate mitigation design. The regional hub for Front Range radon levels collects local guides on these patterns, and the broader radon testing section covers methods and devices.
Common Misreadings of a Radon Chart
Most mistakes with radon charts come from treating the chart as more precise than the test behind it. A few patterns come up repeatedly.
- Treating 4 pCi/L as a safety line. EPA and CDC both say there is no safe level. A result of 3.9 pCi/L is not a pass, and the chart’s 2 to 4 band exists for that reason.
- Reading one short-term test as the final answer. EPA’s own testing sequence calls for a follow-up test before deciding, except for the immediate second test it recommends at very high results.
- Taking the higher or lower of two short-term tests. EPA’s decision rule uses the average of the two results.
- Comparing tests from different floors. A basement result and a main-floor result are not interchangeable. EPA says to test the lowest lived-in level. Upper floors often read lower, which is useful context but not the number the chart is designed for. The article on radon levels in basements explains why the lower level usually reads highest.
- Mixing units. A monitor set to Bq/m³ that reads 150 is at about 4 pCi/L, not “150 times the limit.” Checking the unit on the display or lab report avoids a lot of alarm and a lot of false comfort.
- Assuming a neighbor’s result applies. The EPA guide says that while radon problems may be more common in some areas, any home may have a problem, and the only way to know is to test.
References
- U.S. EPA. A Citizen’s Guide to Radon: The Guide to Protecting Yourself and Your Family from Radon (2016)
- Centers for Disease Control and Prevention. Radon and Your Health
- U.S. Geological Survey. The Geology of Radon
Frequently asked questions
What radon level is considered acceptable?
EPA recommends fixing a home at 4 pCi/L or higher and suggests considering a fix between 2 and 4 pCi/L. EPA and CDC both state that no level of radon is safe, so lower is always better. EPA estimates the average indoor level at about 1.3 pCi/L.
How do pCi/L convert to Bq/m³?
One pCi/L equals 37 Bq/m³. The EPA action level of 4 pCi/L is about 148 Bq/m³, and 2 pCi/L is about 74 Bq/m³. Check the unit on a monitor or lab report before comparing a result to a chart.
Should two short-term radon tests be averaged?
Yes. EPA says that if a second short-term test is used as the follow-up, the home owner should consider fixing the home if the average of the first and second tests is 4 pCi/L or higher. Results close to 4 pCi/L are less certain, so a long-term test helps when time allows.
Is a short-term or long-term radon test more accurate?
A long-term test, which stays in place for more than 90 days, is more likely to reflect the year-round average because radon varies by day and season. Short-term tests, from two to 90 days, are faster and are often used during real estate transactions.
Are radon levels higher in Colorado?
USGS notes that granites and metamorphic rocks in the Rocky Mountains contain more uranium than sedimentary rocks to the east, resulting in high radon in indoor air and ground water. Individual homes still vary, so testing each house is the only way to know its level.
Holding a radon result and not sure which row of the chart it belongs in? Get in touch with the test details and we can help sort out whether a follow-up test, a long-term test or a mitigation quote makes sense.
Test your home for radon
Radon is invisible and odorless, so a test is the only way to know your level. A short-term kit gives a first reading in days; a digital monitor tracks it over time.
| Product | Why | Buy |
|---|---|---|
 AirChek Charcoal Radon Test Kit | Lab fee usually included; EPA short-term protocol. | Amazon — $21.95 |
 First Alert RD1 Radon Test Kit | Widely stocked; 48-hour charcoal test. | Amazon — $17.95 |
 Airthings Corentium Home | Digital monitor with a long-term average. | Amazon — $142.99 |