What Is Considered High Radon: A Plain Guide
When a radon report lands in your inbox, the number rarely comes with context. You see a figure in picocuries per liter and have to decide whether it is fine, borderline, or alarming. So what is considered high radon? The short version: at or above the EPA action level of 4.0 pCi/L, a reading is treated as high enough to act on, and the higher the number climbs from there, the more urgent the response. This guide explains where the thresholds sit, how to read your result in context, and why Colorado homes often land on the higher end. It summarizes EPA and Colorado Department of Public Health and Environment guidance current as of 2026 and is general information, not medical advice.
What is considered high radon?
The EPA defines 4.0 pCi/L as its action level, the point at which it recommends installing a mitigation system. Any confirmed reading at or above that figure is reasonably called “high” because it triggers the recommendation to fix the home. Readings of 10 pCi/L or more are clearly high and warrant prompt action, while results in the 20s and beyond are very high and should prompt mitigation as soon as practical.
Below 4.0 pCi/L, a reading is not classified as high, though the EPA still suggests considering mitigation between 2.0 and 4.0 pCi/L. So “high” is really a band: borderline near 4, clearly high in the teens, and severe above 20.
Putting numbers into perspective
Outdoor air averages about 0.4 pCi/L, and the national indoor average is roughly 1.3 pCi/L. Against that backdrop, a 4.0 reading is several times the typical indoor level. A 10 pCi/L result is dangerous enough that the EPA equates the long-term lung-cancer risk to that of heavy smoking exposure over years.
For a sense of how individual readings stack up, our radon level chart guide lines up the ranges, and the focused piece on whether a radon level of 10 is dangerous drills into one of the most-searched thresholds.
Why the action level is a trigger, not a cliff
It is tempting to read 4.0 pCi/L as a hard line between safe and unsafe. That is not how radon risk works. The biological risk rises gradually with exposure; there is no level proven to be risk-free. The 4.0 figure reflects what mitigation technology can reliably achieve at reasonable cost, not a sudden jump in danger.
This is why the World Health Organization suggests a lower reference of about 2.7 pCi/L. Both agencies agree on the direction: lower is better, and high readings deserve action. The exact line is a policy choice layered on top of a continuous risk curve.
How high is too high to wait?
Urgency scales with the number. A confirmed reading just above 4.0 pCi/L calls for mitigation, but you have time to plan and get quotes. A reading in the teens calls for faster action. The EPA recommends that very high readings, such as those above 20 pCi/L, be addressed quickly, and it even suggests short-term confirmatory testing rather than waiting months for a long-term test.
If your number is high, do not panic, but do not file it away either. Radon-related lung-cancer risk accumulates over years of exposure, so reducing a high level promptly protects everyone in the home.
Why Colorado readings run high
Colorado’s geology works against homeowners. The Front Range sits on uranium-bearing soils and rock, and the CDPHE reports a large share of tested Colorado homes exceed the EPA action level. Counties including Douglas, El Paso, Jefferson, and Boulder frequently show elevated results.
That context should shape expectations. A high reading in Colorado is not unusual or a sign of a uniquely defective house. It reflects regional geology. Treating radon testing as routine, and acting on high results, is simply part of responsible Front Range homeownership. The Colorado radon testing hub explains the local picture in depth.
Confirming a high reading
One short-term test can be skewed by weather, season, and how sealed the house was. Before committing to mitigation on a borderline result, confirm it. The EPA recommends a follow-up short-term test or a long-term test of more than 90 days for readings closer to the action level. For very high readings, a quick confirmatory short-term test is enough; you should not wait months to act on a dramatic number.
Consistent test placement matters. Put the device in the lowest livable level, away from drafts and humidity, with windows closed before and during a short-term test. A reliable high reading is the basis for a sound mitigation decision.
What to do about a high reading
The standard fix for high radon is active soil depressurization. A vent pipe and fan draw radon from beneath the foundation and discharge it above the roofline, before it can enter living space. Properly installed, these systems routinely bring high readings well below the action level.
Cost varies by home, but in Colorado the work is common and competitively priced. Our guide to radon mitigation cost in Colorado gives a realistic budget range. After mitigation, retest to confirm the system brought the level down as expected.
Living with a high result before mitigation
If you have a high reading and mitigation is weeks away, a few interim steps help. Increase ventilation by opening windows when weather allows, which dilutes indoor radon temporarily. Spend less time in the lowest, most-affected level. Seal obvious foundation cracks as a stopgap, though sealing alone rarely solves a high problem.
These measures reduce exposure modestly but are not substitutes for a mitigation system. Once the system is installed and a follow-up test confirms a lower level, you can use the whole home with confidence.
High radon and lung-cancer risk in plain terms
The reason high readings matter is the link to lung cancer. Radon is the leading cause of lung cancer among people who have never smoked, and the second leading cause overall after smoking, according to EPA and surgeon-general assessments. The risk is cumulative: it builds with the concentration you breathe and the years you breathe it. A high reading does not cause harm overnight, but sustained exposure at elevated levels meaningfully raises lifetime risk.
For smokers, the danger compounds. The combination of smoking and high radon exposure produces a risk far greater than either factor alone. This is why public-health messaging pairs radon mitigation with smoking cessation. Understanding the mechanism, fine radioactive particles depositing in lung tissue and damaging cells over time, makes clear why “high” is defined by long-term exposure rather than any single day’s reading.
How high readings happen in seemingly normal homes
Homeowners are often baffled when a tidy, modern home tests high. The explanation lies underground and in how the house breathes. Soil rich in uranium produces more radon; foundation cracks, sump pits, crawlspaces, and gaps around utility penetrations let it in; and the pressure difference between the warm interior and the cooler soil actively draws soil gas upward, a phenomenon called the stack effect.
Tightly sealed, energy-efficient homes can read higher precisely because they exchange less air with the outdoors, allowing radon to accumulate. So a high reading is not a sign of neglect or a defective house. It reflects geology and building physics. Recognizing this removes the stigma some homeowners feel and reframes a high result as a solvable engineering problem rather than a personal failing.
Seasonal swings and high readings
High readings are most common in winter. Cold weather intensifies the stack effect, homes stay sealed, and heating systems alter indoor pressure, all of which pull more radon from the soil. A home that tests at 3 pCi/L in summer might read 6 or 7 pCi/L in January. This seasonality is why a single warm-weather test can understate the true peak exposure.
For an honest read on whether a home is high, a long-term test spanning at least 90 days, or a test conducted during the heating season, gives a more representative picture. If a short-term summer test lands near the action level, a winter follow-up often clarifies the situation. Acting only on a mild-season snapshot can leave a genuinely high home undetected.
High radon and the real-estate transaction
High readings surface constantly in Front Range home sales. Buyers commonly request a radon test during the inspection period, and a result above the action level frequently becomes a negotiating point. Sellers may agree to install mitigation before closing or credit the buyer to do so afterward. Because Colorado homes test high so often, this scenario is routine rather than exceptional, and experienced agents treat it as a normal step.
For buyers, the key is not to panic at a high number. A reading of 8 or 10 pCi/L is a known, fixable condition with a predictable cost, not a reason to abandon a home you otherwise want. For sellers, testing and mitigating ahead of listing removes a common obstacle and can protect the asking price. Either way, a high reading is best handled as a practical line item, with the cost of the fix factored into the deal.
Steps to take after a high result
A confirmed high reading calls for a clear sequence. First, confirm the number if it is borderline, though for clearly high readings a quick confirmatory short-term test suffices and you should not delay. Second, contact a certified mitigation professional and get a design and quote. Third, have the system installed, usually a one-day job for a standard home. Fourth, retest after installation to verify the level dropped below the action level.
While you wait for mitigation, you can reduce exposure modestly by increasing ventilation when weather allows and spending less time in the most-affected lowest level. These are stopgaps, not solutions. The durable fix is the mitigation system, and the post-installation test is what proves it worked. Acting promptly on a high reading protects everyone in the home during the years that radon risk would otherwise accumulate.
What mitigation achieves for high readings
The encouraging part of a high result is how effectively it can be fixed. A well-designed active soil depressurization system routinely reduces high readings to well below the action level, often to 1 to 2 pCi/L. The system intercepts soil gas beneath the foundation and vents it harmlessly above the roofline before it can enter living space. Most homes are mitigated in a single day.
After installation, a follow-up test confirms the drop, and a system monitor lets you verify the fan is working over the years. Maintenance is minimal. For homeowners staring at a high number, this is the key reassurance: high radon is among the more straightforward home hazards to remediate, and the result is durable. The investment also tends to pay back at resale, since a documented mitigation system is a selling point in radon-prone Colorado.
It is worth keeping the result in perspective. A high reading is unsettling, but radon is among the more straightforward home hazards to identify and correct. Unlike structural or foundation problems, the cause is well understood, the fix is standardized, and the outcome is verifiable with a follow-up test. Homeowners who approach a high number methodically, confirm, mitigate, retest, end up with a quieter mind and a protected home rather than a lingering worry.
References
- What Is Radon Gas? Is It Dangerous? — U.S. Environmental Protection Agency
- Radon in Colorado — Colorado Department of Public Health and Environment
- Radon and Indoor Air — American Lung Association
If your Front Range home tested high and you want help interpreting the result or connecting with a vetted mitigation professional, reach out through our contact page.