What Is an Acceptable Radon Level? EPA and CDC Answer
Ask what is an acceptable radon level and the honest answer has two parts. The first is that no amount of radon is known to be safe, so “acceptable” never means “harmless.” The second is that federal guidance draws practical lines anyway: a level where fixing the house is recommended, and a lower band where fixing is worth considering. This guide lays out those lines in plain language, quotes the EPA and CDC wording directly where it matters, and walks through what to do with a result in each range, how to retest, and how the numbers play out in Colorado Front Range homes. It summarizes EPA and CDC guidance 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 house.
Part of our Radon Levels guide — start there for the full picture →
The Short Answer: No Level Is Safe, but 4 pCi/L Is the Action Line
Radon in U.S. homes is measured in picocuries per liter of air (pCi/L). Three numbers frame almost every conversation about what counts as acceptable.
- 4 pCi/L is the EPA action level. The CDC summarizes it this way: “The U.S. Environmental Protection Agency (EPA) recommends fixing your home if radon levels are 4 picoCuries per liter of air (pCi/L) or higher.”
- 2 to 4 pCi/L is the “consider fixing” band. EPA’s radon zones page puts it directly: “Consider fixing if your level is between 2 and 4 pCi/L.”
- There is no safe floor. The CDC states, “There is no known safe level of radon. You should always aim to have the lowest level.” EPA’s Citizen’s Guide says the same in its own words: “EPA believes that any radon exposure carries some risk—no level of radon is safe.”
For context, the Citizen’s Guide gives two reference points: “The average indoor radon level is estimated to be about 1.3 pCi/L, and about 0.4 pCi/L of radon is normally found in the outside air.”
Put together, a practical reading is this. Below 2 pCi/L is about as low as most homes can practically be pushed, and the result is generally treated as acceptable in the everyday sense. Between 2 and 4 pCi/L is a judgment call that leans toward fixing, especially for a home where people spend a lot of time on the lowest lived-in level. At 4 pCi/L and above, federal guidance recommends fixing after a confirming test. For a broader look at the “safe” framing, see the companion guide on safe radon levels, and for a quick lookup table in both pCi/L and Bq/m³, the radon levels chart covers every range.
Acceptable Radon Levels at a Glance
The table below pairs each range with what EPA or CDC says about it, quoted where the agencies use specific wording, and a reasonable next step. The action column is general guidance, not a substitute for a professional’s read of a specific house.
| Radon level (pCi/L) | What EPA / CDC say | Practical action |
|---|---|---|
| About 0.4 | EPA: “about 0.4 pCi/L of radon is normally found in the outside air.” | Outdoor background. Indoor levels at or near this are rare and are the long-term goal Congress set for indoor air. |
| About 1.3 | EPA: “The average indoor radon level is estimated to be about 1.3 pCi/L.” | Typical U.S. home. No fix is recommended. Retest after major renovations or if the basement becomes living space. |
| Below 2 | EPA notes “most homes today can be reduced to 2 pCi/L or below.” | Generally the practical target after mitigation. Keep the result on file and retest periodically. |
| 2 to 3.9 | EPA: “Consider fixing if your level is between 2 and 4 pCi/L.” CDC: “There is no known safe level of radon.” | Confirm with a long-term test if time allows. Weigh mitigation, particularly if bedrooms or a family room sit on the tested level. |
| 4 to 8 | CDC: EPA “recommends fixing your home if radon levels are 4 picoCuries per liter of air (pCi/L) or higher.” | Run a follow-up test (long-term or second short-term). Fix if the long-term result or the short-term average is 4 or higher. |
| Above 8 | EPA: if the first short-term result “is more than twice EPA’s 4 pCi/L action level, you should take a second short-term test immediately.” | Do the second short-term test right away rather than waiting months for a long-term test, then plan mitigation. |
A result sitting right on a boundary, such as 3.9 or 4.1, deserves a careful second look. The Citizen’s Guide gives a specific example: “if the average of your two short-term test results is 4.1 pCi/L, there is about a 50% chance that your year-round average is somewhat below 4 pCi/L.” In other words, a number near the line is close to a coin flip on which side the true annual average falls. That uncertainty is the main reason follow-up testing exists.
What to Do at Each Range
The numbers only matter because of what they trigger. Here is how each band typically translates into a next step for a homeowner, buyer or seller.
Below 2 pCi/L
A result under 2 pCi/L on the lowest lived-in level is a good outcome. EPA’s guidance notes that reducing radon below 2 pCi/L is difficult, which is one reason this band is commonly treated as the practical floor. No mitigation is recommended here. Two things are still worth doing. First, save the test report with the date, device type and location. Second, plan to test again in the future. The Citizen’s Guide says that “Even if your test result is below 4 pCi/L, you may want to test again sometime in the future.” Radon can shift after foundation work, a new HVAC system, air sealing, or simply over time as soil and the building settle. For a sense of where an ordinary home tends to land, the guide on what a normal radon level looks like walks through typical results.
2 to 4 pCi/L
This is the band where “acceptable” becomes a personal decision. EPA does not say these homes must be fixed. It says to consider fixing. Several factors push the decision toward mitigation:
- The result came from a basement or ground floor that people use daily, such as a bedroom, office or playroom.
- Household members smoke. The Citizen’s Guide says smoking combined with radon is an especially serious health risk.
- Planned work, like finishing a basement, will move people closer to the soil.
Before spending money, confirm the number. A long-term test of more than 90 days captures seasonal swings far better than a two- to seven-day test, and it settles whether the year-round average is really in this band. A detailed walk-through of one common reading in this range is in the guide on a 2.5 pCi/L radon level.
4 pCi/L and above
At or above the action level, the federal recommendation is to fix the house once the result is confirmed. EPA’s testing steps in the Citizen’s Guide are specific:
- 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 either a long-term test, for a better picture of the year-round average, or a second short-term test if results are needed quickly.
- If a long-term test was used, fix the home if the result is 4 pCi/L or more. If a second short-term test was used, consider fixing if the average of the first and second test is 4 pCi/L or higher.
Mitigation in most Front Range homes means a sub-slab or sub-membrane depressurization system with a fan that vents soil gas above the roofline. Budget ranges and what drives them are covered in the guide to radon mitigation system costs. The CDC adds a practical caution worth repeating: fixing high radon levels “requires technical knowledge and special skills so you should rely only on a qualified professional for these repairs,” and homeowners should “Test radon levels again after any repairs to be sure they worked.”
Very high results
Readings well above 8 pCi/L are not unusual in parts of Colorado. EPA’s guidance for a first short-term result more than twice the action level is to take a second short-term test immediately rather than waiting through a long-term test.
Why “Acceptable” Is a Policy Line, Not a Health Threshold
The 4 pCi/L action level is easy to misread as the point where radon “becomes dangerous.” That is not how EPA or CDC describe it. Both agencies frame radon risk as continuous: the more radon and the longer the exposure, the higher the lung cancer risk, with no point below which risk drops to zero. The CDC states plainly that “Reducing radon inside your home or building will always help reduce your risk of lung cancer, even when the level in your home is less than 4 pCi/L.”
So why draw a line at 4? The action level reflects a balance between risk reduction and what mitigation can reliably achieve at reasonable cost. EPA 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.” The 4 pCi/L line identifies homes where fixing clearly makes sense. The 2 to 4 band acknowledges that many homes below the line can still benefit.
The CDC also notes that radon is “the second leading cause of lung cancer deaths in the United States after cigarette smoke,” and that EPA and the Surgeon General’s office “estimate radon is responsible for more than 21,000 lung cancer deaths each year in the United States.”
Other countries and international health bodies use their own reference levels, and some of these are set lower than the U.S. action level. For a U.S. home, EPA’s numbers are the ones that drive real estate negotiations, mitigation standards and contractor guarantees, so this guide sticks with them.
How Testing Shapes the Number You Get
A radon result is a measurement of one location over one window of time. Understanding how the test was run is the difference between reading a number correctly and overreacting or underreacting to it.
Short-term versus long-term tests
The Citizen’s Guide divides tests into short-term (two to 90 days) and long-term (more than 90 days). A long-term test “will give you a reading that is more likely to tell you your home’s year-round average radon level.” Short-term tests are faster and are the norm during real estate transactions, but radon levels vary from day to day and season to season. For timing details on each method, see the guide on how long a radon test takes.
Where the test was placed
EPA guidance is to test the lowest level of the home that is lived in or could be lived in. A basement storage room that no one uses may read higher than the family room upstairs, but the number that matters for most decisions is the one from where people actually spend time. The Citizen’s Guide adds that if living patterns change and a household “begin[s] occupying a lower level of your home (such as a basement) you should retest your home on that level.” The guide to radon levels in basements explains why the lowest level usually reads highest and how to interpret it.
Closed-house conditions
Short-term tests assume closed-house conditions: windows and exterior doors kept closed except for normal entry and exit, starting before the test begins. Open windows dilute radon and produce a falsely low result.
Continuous monitors
Electronic continuous radon monitors report hourly or daily readings, which helps show patterns and spot interference. Consumer-grade digital monitors can be useful for watching trends over months, though professional devices used in real estate testing are calibrated and documented to stricter standards. The guide to radon monitors covers the differences. Cost comparisons across kits, monitors and professional tests are in the guide to radon test costs.
Retesting: When a Second Number Is Worth Getting
Many households test once and file the result. A few situations call for testing again regardless of the first number.
- After mitigation. The CDC says to test again after repairs to be sure they worked. A post-mitigation test, usually run at least a day after the system starts, confirms the fan and piping are pulling the level down.
- After major renovation. New basement finishing, foundation repair, a new furnace or major air sealing can all change how soil gas enters and moves through a house.
- When the lowest level comes into use. A basement turned into a bedroom changes which number matters.
- When the first result was borderline. Anything from roughly 3 to 5 pCi/L is close enough to the line that a long-term test is worth the wait if the timeline allows.
- Periodically, even when low. EPA says homeowners may want to test again in the future even after a result below 4 pCi/L.
A mitigated home also needs attention: fans wear out, and the manometer on the vent pipe is worth a glance now and then.
Acceptable Radon Levels in Colorado Front Range Homes
Colorado comes up often in radon conversations, and for reasonable geologic reasons. Much of the state, including the Front Range counties, falls in the higher-potential categories on EPA’s Map of Radon Zones. That map uses indoor measurements, geology, aerial radioactivity, soil and foundation data to estimate the potential for elevated indoor radon across counties.
The map has a clear limit, and EPA states it directly: “The Map of Radon Zones should not be used to determine if individual homes need to be tested. No matter where you live, test your home for radon—it’s easy and inexpensive.” Two neighboring houses on the same street can produce very different results because of foundation type, soil contact, cracks and penetrations, the HVAC system and how the house is used.
Several features common to Front Range housing tend to matter in practice:
- Basements and garden levels. Many homes have finished basements that are used as bedrooms or family rooms, which puts occupants on the level closest to the soil.
- Crawl spaces. Homes with exposed soil in a crawl space can draw soil gas directly into the house, especially if the crawl space connects to the return side of a forced-air system.
- Cold winters. Heating season tends to increase the stack effect that draws soil gas indoors, so a winter short-term test may read higher than a summer one.
- Tight new construction. Newer energy-efficient homes may have less air exchange, which can hold radon in once it enters. Some newer homes include passive radon piping that can be activated with a fan.
Because of these factors, a result in the 2 to 4 pCi/L band is common in this region, and results above 4 are not unusual. The guide to radon levels by ZIP code in Colorado discusses what area data can and cannot say about a single house. The short version holds across every source: area averages set expectations, but only a test of the specific house answers the question.
Acceptable Radon Levels in a Home Sale
Real estate transactions compress radon decisions into a few days. A buyer’s inspection period often allows only enough time for a short-term test, typically two to four days using a continuous monitor or a pair of passive devices. The Citizen’s Guide includes a separate section on home sales and recognizes that short-term testing is the practical choice in that setting.
In practice, the 4 pCi/L action level becomes the negotiating line. Results at or above 4 commonly lead to a request that the seller install a mitigation system, provide a credit, or reduce the price. Results between 2 and 4 are more often left to the buyer, who can mitigate after closing if they choose. Some practical points:
- Ask for the full test report, including device type, start and end times, placement and any tampering or interference flags.
- If a seller has an existing mitigation system, ask for the post-mitigation test and when it was run.
- If a seller offers to install a system before closing, a post-mitigation test should be part of the agreement.
- A borderline short-term result is not a final verdict. Buyers can run a long-term test after moving in and mitigate then if needed.
Common Misreadings of “Acceptable”
A few misunderstandings come up often enough to address directly.
“Under 4 means safe.” Neither EPA nor CDC says this. Both say no level is known to be safe and that levels below 4 still carry some risk.
“New homes don’t have radon.” The CDC notes radon can build up “in any home or building whether it has a basement, is sealed or drafty, or is new or old.”
“A neighbor’s result applies to this house.” EPA’s own zone map guidance says area data should not be used to decide whether an individual home needs testing.
“Mitigation always gets a house to zero.” Outdoor air itself carries about 0.4 pCi/L. A well-designed system typically brings a home well below 4 and often below 2, but zero is not a realistic target.
References
- A Citizen’s Guide to Radon (2016) — U.S. Environmental Protection Agency
- About Radon — Centers for Disease Control and Prevention
- The EPA Map of Radon Zones — U.S. Environmental Protection Agency
For the full radon picture, including testing methods, mitigation and Front Range context, start at the radon testing guide or browse the radon levels hub.
Frequently asked questions
What is an acceptable radon level in a home?
EPA recommends fixing a home at 4 pCi/L or higher and says to consider fixing between 2 and 4 pCi/L. The CDC states there is no known safe level of radon, so lower is always better. EPA estimates the average indoor level at about 1.3 pCi/L.
Is 3 pCi/L of radon acceptable?
A result of 3 pCi/L is below the EPA action level but inside the 2 to 4 pCi/L band where EPA says to consider fixing. Confirming it with a long-term test and weighing how much time people spend on the tested level is a sensible next step.
What radon level is normal outdoors?
EPA's Citizen's Guide says about 0.4 pCi/L of radon is normally found in outside air. Congress set a long-term goal that indoor levels be no higher than outdoor levels, though EPA notes this is not yet achievable in all cases.
Should a radon test be repeated if the first result is high?
Yes. EPA recommends a follow-up test when a short-term result is 4 pCi/L or higher, either a long-term test or a second short-term test. If the first result is more than twice the action level, EPA says to take the second short-term test immediately.
Do Colorado homes have higher radon levels?
Much of Colorado falls in higher-potential areas on EPA's Map of Radon Zones, but EPA says the map should not be used to decide whether an individual home needs testing. Every house should be tested because results vary widely even between neighbors.
Holding a radon number and unsure whether it counts as acceptable for your household? Send us the test details and we can help work out whether a follow-up test, a long-term test or a mitigation quote is the right next move.
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.
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 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 |