What Radon Levels Are Acceptable: A Plain Guide
Radon is a colorless, odorless gas that seeps up from soil and rock, and Colorado has some of the higher background levels in the country. After a test comes back, the first thing most homeowners want to know is simple: what radon levels are acceptable, and at what point should the number on the report worry them? The honest answer is that no level is completely free of risk, but public-health agencies have set practical thresholds that tell you when action is recommended. This guide summarizes EPA and Colorado Department of Public Health and Environment (CDPHE) guidance current as of 2026, and it is general information, not medical advice or a substitute for a certified radon measurement.
What radon levels are acceptable in a home?
The U.S. Environmental Protection Agency sets its action level at 4.0 picocuries per liter (pCi/L). At or above that reading, the EPA recommends installing a mitigation system. Below 4.0 pCi/L, no fix is required, but the agency is clear that risk does not vanish at that line. It encourages homeowners to consider mitigation when results fall between 2.0 and 4.0 pCi/L because even those readings carry measurable long-term lung-cancer risk.
So if you are looking for a single “safe” number, it does not exist. The closest thing to an acceptable level is the outdoor average, roughly 0.4 pCi/L, which no indoor space realistically reaches. A practical goal for most homes is to get as close to outdoor air as you reasonably can, and certainly below the 4.0 threshold.
Why 4.0 pCi/L is the action level, not a safety line
The 4.0 figure is a policy decision, not a biological cutoff. The EPA chose it because mitigation systems can reliably bring most homes below that number at a reasonable cost. Lower targets become harder and more expensive to guarantee. The World Health Organization, by contrast, suggests a reference level closer to 2.7 pCi/L, which shows that reasonable agencies draw the line in slightly different places.
What matters for a homeowner is the trend of the science: the more radon you breathe over years, the higher your risk. There is no threshold below which exposure is proven harmless. That is why the language is always “acceptable for action” rather than “safe.”
How acceptable levels compare across common readings
It helps to translate the numbers into plain terms. A reading under 2.0 pCi/L is generally considered acceptable, and mitigation is optional. Between 2.0 and 3.9 pCi/L sits a gray zone where the EPA says you may want to act. At 4.0 pCi/L and above, mitigation is recommended. Anything in the double digits should prompt prompt action and a confirmatory test.
For a deeper breakdown of how individual numbers translate into risk, our radon level chart guide lays out the ranges side by side. If your result is unusually high, the companion piece on what counts as high radon walks through the upper end of the scale.
Why Colorado homes often read higher
The Front Range sits on uranium-bearing geology, and the CDPHE reports that a large share of Colorado homes test above the EPA action level when measured. Counties such as Douglas, Jefferson, El Paso, and Boulder routinely show elevated results. Elevation, soil composition, and tight modern construction all push indoor radon up.
That regional context matters when you interpret a result. A 4.5 pCi/L reading in Denver is not a fluke; it is typical for the area. Colorado homeowners should treat radon testing as a normal part of homeownership rather than a rare event, and many local real-estate contracts now include radon contingencies.
Single test versus long-term average
A short-term test, usually two to seven days, gives a snapshot. Radon fluctuates with weather, season, and how often doors and windows are open. A reading of 3.8 pCi/L in a closed-up winter test could average lower across the year, while a summer test might understate the true winter peak.
For a number you can trust, the EPA recommends either a long-term test of more than 90 days or two short-term tests. If the first short-term result is borderline, a second test confirms whether the home truly sits below the action level. Acceptable, in other words, is a judgment you make on solid data, not a single afternoon reading.
What to do if your level is acceptable but not low
Suppose your home tests at 3.2 pCi/L. No mitigation is required, but the EPA flags this range as worth considering. You have options. Sealing foundation cracks, improving sub-slab ventilation, and increasing fresh-air exchange can all nudge the number down. Some homeowners install a system proactively to protect long-term health, especially in basements where families spend a lot of time.
Retesting every two years, and after any major renovation or foundation work, keeps you confident the level stays acceptable. Radon entry points can change as a house settles.
When a level is clearly not acceptable
Once a confirmed reading sits at or above 4.0 pCi/L, mitigation moves from optional to recommended. A standard active soil depressurization system, the most common fix, draws radon from beneath the slab and vents it above the roofline. In Colorado, that work is typically straightforward for a qualified contractor.
If your numbers are high, planning ahead helps. Reviewing radon mitigation costs in Colorado gives a realistic budget, and the Front Range radon testing hub explains how testing and mitigation fit together. Acting on a high reading is far cheaper than ignoring the long-term health cost.
How to test so your result is reliable
Place the test kit in the lowest livable level of the home, away from drafts, exterior doors, and high humidity. Keep windows closed for 12 hours before and during a short-term test. Follow the kit instructions exactly, since deviations skew results. For real-estate transactions, a professional measurement by a certified provider carries more weight than a DIY kit.
Continuous radon monitors give you ongoing visibility if you want to watch the number over time rather than rely on a one-time snapshot. Whichever method you choose, consistency in placement and conditions is what makes a reading meaningful. Avoid placing a detector on a windowsill, near a fireplace, or in a bathroom or laundry room where humidity and airflow can distort the reading. The lowest level a family actually uses, even a few hours a week, is the right spot, because that is where exposure accumulates.
Why “acceptable” depends on how you use the space
The same number can mean different things in different homes. A reading of 3.5 pCi/L in an unfinished basement that nobody enters is a lower practical concern than the identical reading in a finished basement bedroom where a child sleeps every night. Exposure is a function of concentration multiplied by time, so the hours spent in the affected space matter as much as the number itself.
This is why the EPA frames its guidance around where people live and breathe. If you are finishing a basement into living space, the threshold for action effectively tightens, because you are about to spend far more time in the part of the home where radon concentrates. Many Colorado families decide to mitigate a gray-zone reading specifically because they are converting a basement into a bedroom, office, or playroom.
Common misconceptions about acceptable levels
Several myths cloud how homeowners interpret radon results. The first is that a single low reading proves the home is permanently safe. Radon entry changes as a house settles, as foundations crack, and as renovations open new pathways, so a clean test today is not a lifetime guarantee. The second myth is that newer homes are automatically low. Tightly sealed modern construction can actually trap radon, producing higher readings than a drafty older house on the same soil.
A third misconception is that you can smell or feel high radon. You cannot. Radon is invisible and odorless, and symptoms of overexposure, primarily lung cancer, take years to appear. That delay is exactly why a measured, acceptable number on a current test is the only reliable assurance. Relying on how a home “feels” is no substitute for data.
How acceptable levels factor into a home sale
Radon comes up constantly in Front Range real-estate transactions. Buyers frequently request a radon test during the inspection period, and a result above 4.0 pCi/L often becomes a negotiating point, with the seller either installing mitigation or crediting the buyer to do so. An acceptable result, comfortably below the action level, removes a common point of friction from a deal.
For sellers, testing ahead of listing and mitigating if needed can streamline the sale and protect the asking price. For buyers, insisting on a test, regardless of what the seller says the level is, protects against inheriting an expensive surprise. Because Colorado homes so often test high, treating radon as a standard line item in the transaction is simply realistic.
How agencies arrived at their thresholds
The numbers homeowners rely on did not appear arbitrarily. The EPA’s risk assessments draw on decades of epidemiological research, including studies of underground miners exposed to high radon and later residential case-control studies that pooled data across continents. Those studies established that lung-cancer risk rises in a roughly linear fashion with cumulative exposure, with no clear threshold below which risk disappears.
Faced with a continuous risk curve and no natural cutoff, agencies had to set a practical line. The EPA balanced health protection against what mitigation could reliably and affordably achieve, landing on 4.0 pCi/L while still urging consideration of action down to 2.0. The World Health Organization weighted the health side slightly more heavily and recommended a lower reference of about 2.7 pCi/L. Both numbers describe the same underlying science; they simply strike the cost-benefit balance at different points.
What an acceptable result does not rule out
Even a comfortably low reading leaves a few things unaddressed. A short-term test taken in mild weather may understate the winter peak, so a single acceptable summer result is weaker evidence than a long-term average or a confirming winter test. Homes on private wells should also remember that waterborne radon is a separate pathway not captured by an air test; a low air reading does not speak to radon dissolved in well water.
Acceptable, in other words, is specific to what and when you measured. A thorough picture combines an air test in the lowest livable level, ideally over a long period or confirmed across seasons, and, for well-water households, a separate water test. Treating one favorable air reading as the final word can miss a real source. The goal is durable confidence that the home stays below the action level year-round, not a single reassuring afternoon number.
It also helps to keep records. Save each test result with its date, the device used, and the conditions during the test, so that future tests can be compared against a clear baseline. If a later reading jumps, that history tells you whether something changed, perhaps a new foundation crack or a renovation, rather than leaving you guessing. A documented trend of acceptable readings is far more reassuring than a lone number on a single report, and it is invaluable evidence at resale.
References
- Health Risk of Radon — U.S. Environmental Protection Agency
- Radon in Colorado — Colorado Department of Public Health and Environment
- Radon and Indoor Air — American Lung Association
If you are along the Front Range and want help interpreting a radon result or connecting with a vetted local measurement professional, reach out through our contact page and we will point you in the right direction.