Unsafe Radon Levels in Home: What to Know
Understanding unsafe radon levels in home air helps you act on test results instead of guessing what they mean. Radon is a naturally occurring radioactive gas that you cannot see, smell, or taste, and it can build up indoors to levels that raise long-term health risk. Knowing the numbers turns an abstract worry into a clear plan.
This guide explains what counts as an unsafe radon level, how risk increases as readings climb, and what to do when your home tests high. It is informational only and not medical advice. Radon testing and risk thresholds follow methodology and guidance from the EPA and CDC, and Colorado-specific guidance comes from the Colorado Department of Public Health and Environment.
How Radon Is Measured
Radon is measured in picocuries per liter of air, written as pCi/L. The higher the number, the more radon is present and the greater the long-term exposure risk. Testing devices, whether short-term kits or long-term monitors, report results in this unit so you can compare them against established thresholds.
Because radon levels rise and fall with weather, season, and how a home is operated, a single reading is a snapshot. The EPA recommends confirming an elevated short-term result with a second test before deciding on action. A long-term test of more than ninety days gives the most representative picture of typical exposure.
Radon enters through the soil beneath a home, so the lowest livable level usually reads highest. Test there for the most relevant result. Our guide to understanding radon levels explains how to interpret readings in context.
What Counts as an Unsafe Radon Level
The EPA sets an action level of 4 pCi/L. At or above that reading, the EPA recommends taking action to reduce radon in the home. That is the most important number to remember, and it is the threshold most professionals use to decide whether mitigation is warranted.
It is important to understand that 4 pCi/L is an action level, not a line between safe and dangerous. The EPA is clear that no level of radon is completely risk-free, because radon risk rises continuously with exposure. Even readings between 2 and 4 pCi/L carry some risk, and the EPA suggests considering action in that range.
Above 4 pCi/L, the risk increases as the number climbs. A reading of 8 pCi/L represents roughly double the exposure of 4, and readings of 20 pCi/L or higher are considered seriously elevated and call for prompt mitigation. The relationship is straightforward: more radon over time means more risk.
Radon Risk Tiers and What They Mean
Thinking in tiers helps you gauge urgency. The ranges below reflect common interpretations of EPA guidance.
- Below 2 pCi/L. Relatively low, though no level is entirely risk-free. Periodic retesting is still wise.
- 2 to 4 pCi/L. The EPA suggests considering action to lower the level, especially for long-term occupancy.
- 4 to 10 pCi/L. At or above the action level. Confirm with a follow-up test and plan mitigation.
- 10 to 20 pCi/L. Clearly elevated. Mitigation is strongly advised without delay.
- Above 20 pCi/L. Seriously elevated. Treat as a priority and arrange professional mitigation promptly.
These tiers guide urgency rather than define a hard safety line. The consistent message is that lower is better and any reading at or above 4 pCi/L deserves action.
Why Colorado Homes Deserve Extra Attention
Colorado has geology that tends to produce higher indoor radon, and much of the state falls into the EPA’s highest-priority radon zone. The Colorado Department of Public Health and Environment encourages testing across the Front Range and beyond because elevated readings are common in the region.
This regional context means a Colorado homeowner should not assume a low reading based on a neighbor’s result. Radon varies house by house depending on soil, foundation, and construction. Testing your own home at the lowest livable level is the only way to know your exposure.
If you live in Denver, Boulder, Fort Collins, or Colorado Springs, treat radon testing as routine maintenance rather than an unusual step. Retest every few years and after major renovations or changes to your foundation or HVAC system.
What to Do When Results Come Back High
A high reading is manageable. Radon mitigation is a well-established field with a strong track record of reducing indoor levels. The path from an elevated result to a safer home follows a few clear steps.
First, confirm the result. If a short-term test reads at or above 4 pCi/L, run a follow-up test to verify before committing to mitigation. Second, consult a qualified radon mitigation professional, who can evaluate your foundation and design a system. The most common approach, sub-slab depressurization, uses a fan to draw radon from beneath the slab and vent it safely above the roofline. Our overview of radon mitigation systems explains how this works.
Third, retest after the system is installed to confirm levels have dropped. A properly designed system typically brings readings well below the action level, often below 2 pCi/L. For background on the gas itself, see our explainer on what radon is and our broader radon testing guide.
How Radon Affects Health Over Time
Understanding why elevated radon matters helps put the numbers in context. Radon itself is a gas, but the health concern comes from what happens after you breathe it in. As radon decays, it produces tiny radioactive particles that can lodge in lung tissue and release energy that damages cells over time.
This is a long-term, cumulative risk rather than an immediate one. There are no short-term symptoms from radon exposure, which is part of what makes it easy to ignore. The EPA identifies radon as a leading cause of lung cancer, and the risk grows with both the concentration and the number of years of exposure. A moderately elevated level sustained over many years adds up.
Because the risk is cumulative and silent, testing is the only way to know your exposure, and reducing elevated levels is a preventive measure. This is informational context, not medical advice. For questions about your personal health or risk, consult a healthcare provider, and rely on the EPA and CDC for the underlying methodology.
Short-Term Versus Long-Term Testing
Choosing the right kind of test affects how confident you can be in a reading. The two main approaches serve different purposes, and using them in sequence gives the clearest picture.
Short-term tests run from a few days to about ninety days and give a quick snapshot. They are ideal for an initial screen or a real estate transaction where time is limited. Because radon fluctuates with weather and how the home is operated, a single short-term result can run higher or lower than your true average, which is why the EPA recommends confirming an elevated short-term reading with a second test.
Long-term tests run longer than ninety days and capture variation across seasons, producing a more representative average of your actual exposure. They take patience but reduce the chance of being misled by a temporary spike or dip. A common strategy is to start with a short-term test, and if the result is at or above the action level, follow with either another short-term test for a fast confirmation or a long-term test for a fuller picture before deciding on mitigation.
Reducing Radon Risk at Home
If your home tests high, the path to lower levels is well established and effective. Knowing the options helps you move from worry to action with confidence.
The most reliable solution is a professional mitigation system, typically sub-slab depressurization, which uses a fan to draw radon from beneath the foundation and vent it safely above the roofline. Properly designed systems routinely bring elevated readings down well below the action level, often below 2 pCi/L. This is the dependable fix for any reading at or above 4 pCi/L.
Supporting steps can help but rarely solve a significantly elevated reading on their own. Sealing foundation cracks, gaps around pipes, and a sump pit reduces entry routes and improves the efficiency of a mitigation system. Improving ventilation can dilute radon modestly. These measures work best as complements to a professional system rather than substitutes. Whatever you do, retest afterward to confirm the change actually lowered your levels, since verification is the only way to know the home is safer.
If your home tested high or you want help arranging a radon test, reach out through our contact page.
Where and How to Test for Radon
Getting an accurate radon reading depends on testing the right place the right way. Because radon enters from the soil and varies with conditions, careful placement and protocol make the difference between a useful result and a misleading one.
Test at the lowest level of the home that is lived in or could be lived in, which is usually the basement or ground floor. That is where radon concentrates most, so it reflects the highest realistic exposure. If a basement is finished and occupied, test directly in the room people actually use rather than relying on a reading from an upper floor.
Place the test device away from drafts, exterior doors, windows, and vents, and position it at a height where air is normally breathed. For a short-term test, maintain closed-house conditions by keeping windows and exterior doors shut, opening them only for normal entry and exit, as the protocol requires. Disturbing these conditions can push a reading artificially low.
Follow the device instructions precisely for duration and handling, then send the test to the lab promptly if it requires analysis. Confirm any elevated short-term result with a second test before deciding on mitigation, and consider a long-term test for the most representative average. Testing correctly is the foundation of every decision that follows, so it is worth the small effort to do it right and to retest periodically over the years you live in the home.
Radon Testing as Routine Home Care
The most useful mindset for radon is to treat testing as part of regular home maintenance rather than a one-time event. Because levels change over time and exposure is cumulative, periodic testing is what keeps your understanding current and your household protected.
Even a reassuring result has a shelf life. Radon can rise as a home settles, as the foundation develops new cracks, or as changes to heating and ventilation alter how soil gas moves. Retesting every few years catches any drift before it becomes a long-term exposure problem.
Certain events warrant a fresh test regardless of timing. Major renovations, especially work affecting the foundation, slab, or HVAC system, can change radon levels. Finishing a basement or converting lower-level space into a bedroom or office increases the hours people spend where radon concentrates most, so test the newly occupied area directly.
If you have a mitigation system, retesting confirms it keeps working as the fan ages. Because Colorado falls largely within the EPA’s highest-priority radon zone, Front Range homeowners benefit most from this routine approach. Folding radon testing into the rhythm of home upkeep, alongside checking smoke detectors and servicing the furnace, ensures the place where your family lives stays within a reasonable range year after year.