What Should Radon Levels Be in a Home? A Plain Guide
What should radon levels be in a home is one of the most practical questions a homeowner can ask after running a test, and the answer hinges on a single number: 4.0 picocuries per liter (pCi/L). That is the EPA action level, the point at which the agency recommends fixing the home. But the full picture is more nuanced, because no amount of radon is completely risk-free, and the official action level is a practical threshold rather than the point where harm begins. Understanding both the number and the reasoning behind it helps you read your own test result wisely. This guide summarizes EPA and Colorado Department of Public Health and Environment (CDPHE) guidance current as of 2026; it is educational only and is not a substitute for certified testing or a measurement professional.
Part of our Radon Levels guide — start there for the full picture →
What should radon levels be in a home?
The EPA recommends taking action to reduce radon if your home tests at or above 4.0 pCi/L. That is the official action level. Below 4.0 pCi/L, the EPA does not require mitigation, but because radon risk is continuous rather than a sharp cutoff, the agency suggests that homeowners consider mitigation when levels fall between 2.0 and 4.0 pCi/L.
For context, the average outdoor radon level is roughly 0.4 pCi/L, and the average indoor level in the United States is around 1.3 pCi/L. So a “good” result is as low as reasonably achievable, ideally near outdoor levels, while 4.0 pCi/L marks the line where action is clearly recommended. There is no level the EPA labels perfectly safe.
Why 4.0 pCi/L is the action level
The 4.0 pCi/L figure is a practical threshold, balancing health risk against what mitigation technology can reliably and affordably achieve. At 4.0 pCi/L, the lifetime lung-cancer risk from radon is significant, and mitigation systems can typically bring levels well below that. The EPA set the action level where intervention is both worthwhile and achievable, not at the point where risk first begins.
Radon is the second leading cause of lung cancer in the United States and the leading cause among people who have never smoked, according to EPA and Surgeon General statements. Risk rises with both the concentration and the years of exposure, which is why long-term average exposure matters more than any single reading. Our radon level chart puts the numbers in perspective.
Understanding the 2.0 to 4.0 range
Results between 2.0 and 4.0 pCi/L sit in a gray zone. They are below the mandatory action level but above what many health-conscious homeowners want, especially households with children, smokers, or anyone with respiratory concerns. The EPA’s suggestion to “consider” mitigation in this range reflects that lowering radon further always reduces risk.
Whether to act in this range is a personal risk decision informed by who lives in the home and how long they spend there. A finished basement bedroom occupied nightly is a stronger case for action than a rarely used storage area. For more on interpreting borderline numbers, see our guide to what level of radon is dangerous.
How to get a reliable result
The level you act on should come from a properly conducted test, because radon fluctuates daily and seasonally. A single short-term test is a snapshot; the EPA recommends confirming an elevated short-term result with a second test or a long-term test before committing to mitigation.
- Test the lowest livable level of the home.
- Place the device 2 to 6 feet above the floor, away from drafts and humidity.
- Maintain closed-house conditions for short-term tests, starting 12 hours before.
- Run long-term tests (90+ days) for the most representative annual average.
- Retest after mitigation and every few years thereafter.
Because levels vary with weather and house operation, a long-term average is the truest measure of the exposure that actually drives health risk.
Front Range context
Colorado homeowners face higher baseline risk than much of the country. Much of the Front Range, including Denver, Aurora, and surrounding counties, sits in EPA Radon Zone 1, where predicted average indoor levels exceed 4.0 pCi/L. CDPHE data show a large share of Colorado homes test at or above the action level, so a result over 4.0 pCi/L is common here, not unusual.
This means Front Range residents should not assume a low number; many local homes test high regardless of age or construction. The parent Colorado radon testing guide explains the regional geology, and our Denver radon testing overview covers metro specifics.
Understanding the picocurie measurement
Radon levels are reported in picocuries per liter of air (pCi/L), a unit that measures radioactive decay rather than a simple gas concentration. One picocurie represents a specific rate of radioactive disintegrations per minute in a liter of air. The reason radon is measured this way is that its health risk comes from the radiation released as it and its decay products break down, not from the gas behaving as a chemical.
When radon decays, it produces short-lived radioactive particles called radon progeny, or decay products, which can attach to dust and lodge in the lungs when inhaled. There they continue to emit radiation that damages lung tissue over years of exposure. This is the mechanism behind radon’s classification as a carcinogen, and it is why a higher pCi/L reading translates to higher long-term lung-cancer risk.
Some countries use a different unit, becquerels per cubic meter, but in the United States pCi/L is standard and the 4.0 action level is expressed in it. Keeping the unit in mind helps when comparing test kits and reports, since all should report in the same pCi/L scale for an apples-to-apples comparison against the action level.
Why radon levels vary so much
Two identical-looking homes on the same street can test very differently, which surprises many homeowners. Radon entry depends on the uranium content of the specific soil and rock beneath a home, the number and size of foundation openings, and how the house pulls air. Small differences in foundation cracks, sump configuration, or soil composition produce large differences in indoor radon.
Levels also change over time within the same home. Winter typically produces higher readings because heated homes draw air upward (the stack effect), creating suction that pulls more soil gas indoors, and because closed windows trap radon. Heavy rain, snow cover, and barometric pressure shifts can all nudge levels. This variability is precisely why a long-term test gives a truer average than any single short-term snapshot.
Renovations matter too. Finishing a basement, sealing or opening foundation areas, adding an HVAC system, or changing how the home is ventilated can raise or lower radon. The practical lesson is that radon is not a one-time number; it is worth retesting periodically and after significant changes to the home, as our guide to radon readings in homes explains.
What to do at each result level
Your action depends on the confirmed long-term average. At or above 4.0 pCi/L, the recommendation is clear: install a radon mitigation system through a certified contractor and retest to confirm it worked. Between 2.0 and 4.0 pCi/L, weigh mitigation based on occupancy and household risk factors. Below 2.0 pCi/L, no action is recommended, though retesting periodically is wise.
Mitigation, usually a sub-slab depressurization system that vents soil gas above the roofline, reliably reduces radon, often below 2.0 pCi/L. The cost is modest relative to the health benefit, and a post-mitigation test verifies success.
How indoor radon compares to other benchmarks
Putting the 4.0 pCi/L action level in context helps homeowners gauge their result. Outdoor air averages about 0.4 pCi/L, the natural background everyone breathes. The average indoor level across U.S. homes is roughly 1.3 pCi/L. So the action level of 4.0 pCi/L is about ten times outdoor background and several times the typical indoor average, which is why the EPA recommends action there.
The World Health Organization has suggested an even lower reference level than the U.S. action level, reflecting the view that risk continues below 4.0 pCi/L. This international perspective reinforces the EPA’s own point that no level is risk-free and that lower is better. A reading of 3.0 pCi/L is not an all-clear; it is simply below the threshold at which U.S. guidance recommends mandatory action.
For practical decision-making, think of the numbers as a gradient rather than a switch. A result near outdoor background is excellent, a result in the 2.0 to 4.0 range invites consideration based on household factors, and a result at or above 4.0 calls for mitigation. The goal is to push exposure as low as reasonably achievable, especially in rooms where people spend long hours.
Common testing mistakes that skew results
A test only guides good decisions if it is conducted correctly, and several common errors distort results, usually toward falsely low readings that give dangerous reassurance. Knowing them protects the integrity of your measurement.
- Opening windows or doors during a short-term test, diluting radon.
- Placing the device in a kitchen, bathroom, or near a draft or vent.
- Testing on an upper floor instead of the lowest livable level.
- Running fans or HVAC settings that disturb closed-house conditions.
- Disturbing or moving the device mid-test.
Following EPA protocol, lowest livable level, 2 to 6 feet above the floor, away from drafts and humidity, closed-house conditions for short-term tests, prevents most of these errors. Because the consequence of a falsely low reading is leaving a real hazard in place, careful technique matters as much as the number the test produces. For interpreting borderline numbers, our guide to what is considered high radon adds detail.
Deciding whether to mitigate
Once you have a reliable long-term average, the decision tree is clear. At or above 4.0 pCi/L, install mitigation through a certified contractor and retest to confirm it worked. Below 2.0 pCi/L, no action is recommended, though periodic retesting is wise. The judgment call lives in the 2.0 to 4.0 range, where personal factors decide.
Weigh who lives in the home and how they use the space. A household with children, smokers, or anyone with respiratory concerns, or a basement bedroom occupied every night, leans toward acting even in the gray zone. A rarely used storage area with the same reading is a weaker case. The principle is to reduce exposure where people actually spend time.
Mitigation is reliable and not especially disruptive, typically a sub-slab depressurization system that vents soil gas above the roofline and usually drops levels well below the action threshold. Given the modest cost relative to the health benefit, many homeowners in the gray zone choose to mitigate for peace of mind, then verify the result with a post-installation test.
When to call a professional
Call a certified radon measurement professional when you want a defensible, tamper-resistant result, especially during a home sale, or to confirm an elevated DIY reading. Call a certified radon mitigation contractor for system installation when your confirmed level is at or above 4.0 pCi/L. CDPHE provides information on certified professionals in Colorado.
Always retest after a system is installed to confirm the level dropped, and retest every few years or after renovations that change airflow, since radon can change over time. For health questions, consult your physician, particularly if household members smoke.
References
- EPA Citizen’s Guide to Radon and the 4.0 pCi/L action level — U.S. Environmental Protection Agency
- CDPHE radon program for Colorado homeowners — Colorado Department of Public Health and Environment
- EPA information on the health risk of radon — U.S. Environmental Protection Agency
Frequently asked questions
What radon level is acceptable in a home?
The EPA recommends action at or above 4.0 pCi/L. Below that, no mitigation is required, but because radon risk is continuous, the EPA suggests considering action between 2.0 and 4.0 pCi/L. Lower is always better, ideally near the outdoor average.
Is a radon level below 4.0 completely safe?
No level of radon is completely risk-free. Below 4.0 pCi/L the EPA does not require action, but risk continues at lower levels, which is why the agency suggests considering mitigation between 2.0 and 4.0 pCi/L and aiming as low as practical.
Should I act on a single radon test?
Not for an expensive decision. Radon fluctuates daily and seasonally, so the EPA recommends confirming an elevated short-term result with a second short-term test or a long-term test before installing mitigation.
Are Front Range radon levels usually high?
Often, yes. Much of the Denver metro and Front Range sits in EPA Radon Zone 1, where predicted average indoor levels exceed 4.0 pCi/L. A result above the action level is common locally regardless of a home's age.
If you want a certified radon test or a mitigation referral anywhere along the Front Range, get in touch through our contact page to connect with a vetted local professional.
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 — $16.98 |
 Airthings Corentium Home | Digital monitor with a long-term average. | Amazon — $142.99 |