What Are Normal Radon Levels? Indoor and Outdoor
What are normal radon levels is best answered as a set of ranges rather than a single figure, because radon differs sharply between outdoor air, typical indoor air, and homes in high-radon regions. The EPA puts the average indoor level at about 1.3 picocuries per liter (pCi/L) and the average outdoor level at roughly 0.4 pCi/L, while the action level for reducing radon is 4 pCi/L. Looking at the full distribution, indoor versus outdoor and region by region, gives a clearer sense of what normal really means. This guide summarizes EPA and CDC guidance current as of 2026; it is informational only, so consult a radon professional for testing and mitigation decisions and your physician for any health concerns.
What Are Normal Radon Levels?
Normal radon levels depend on where you measure. The EPA reports the average indoor radon level in U.S. homes as approximately 1.3 pCi/L, and the average outdoor level as about 0.4 pCi/L. These two averages bracket what is typical: outdoor air is the baseline, and indoor air runs several times higher because homes draw in and hold radon. A home testing near 1.3 pCi/L is at a normal, lower-risk indoor level.
Normal here means typical, not zero and not necessarily ideal. Because radon is generated continuously in soil and rock, all air contains some, and the indoor average reflects the baseline accumulation homes experience. The EPA’s action level of 4 pCi/L sits well above the indoor average, so a normal level is comfortably below the threshold at which mitigation is recommended.
Indoor Versus Outdoor Ranges
The gap between indoor and outdoor radon is one of the most important things to understand. Outdoors, radon disperses into the open atmosphere and stays low, averaging around 0.4 pCi/L. There is simply too much open air for it to concentrate. Indoors, the story changes: the slight pressure difference between a home and the ground pulls radon in through foundation cracks, sump openings, and gaps around utility penetrations, where it builds up in enclosed air.
This is why indoor levels typically run several times higher than outdoor levels, and why the indoor average of 1.3 pCi/L is the more relevant figure for homeowners. It also explains why lower floors and basements often read higher than upper stories: the closer a space is to the soil, the more radon enters. A companion guide on how radon levels are measured and what they mean covers this dynamic in detail.
Why Outdoor Air Stays Low
The outdoor average of roughly 0.4 pCi/L holds steady for a simple reason: dilution. Radon escaping from soil into the open atmosphere mixes into an effectively unlimited volume of air and disperses before it can build up. Wind and constant air movement keep outdoor concentrations low almost everywhere, which is why the outdoor figure is so much smaller and so much more uniform than indoor levels. The contrast underscores that radon is an indoor-accumulation problem, not a problem with the gas existing at all; the same radon that is harmless outdoors becomes a concern only when a building traps and concentrates it.
The Distribution Across Homes
Averages tell only part of the story, because radon levels vary widely from home to home. Many homes sit near or below the 1.3 pCi/L indoor average, but a meaningful share test higher, and some test well above the action level. The distribution is skewed: most homes cluster at lower levels, with a tail of homes reaching elevated concentrations driven by local geology and construction.
This variability means your neighbor’s result does not predict yours. Two adjacent homes can differ substantially depending on foundation condition, soil beneath each, and how each home is sealed and ventilated. Because the distribution is so spread out, the only way to know where your home falls is to test it directly rather than assume it matches the average. For where elevated readings begin to matter, see the companion guide on what level of radon is unsafe.
Normal Versus the Action Level
It is worth separating “normal” from “the threshold for action,” since the two numbers are often confused. The indoor average of about 1.3 pCi/L describes what is typical. The EPA’s action level of 4 pCi/L describes when mitigation is recommended. Between them, the EPA suggests considering action in the 2 to 4 pCi/L range, where levels are above average but below the firm recommendation threshold.
So a home can be above normal yet below the action level, which is a common situation. A reading of 3 pCi/L is higher than the typical indoor average but does not trigger a firm mitigation recommendation, though some homeowners choose to reduce it. No level is completely safe, so lower is always better, but the action level is the practical line that drives decisions.
Colorado Zone 1 Context
Colorado homeowners should be especially cautious about assuming their levels are normal, because much of the state, including the Front Range, falls within the EPA’s highest radon-potential category, often described as Zone 1. The Colorado Department of Public Health and Environment urges every household to test. In Zone 1 regions, the distribution shifts upward: a larger share of homes test above the national indoor average and above the action level than in lower-potential areas.
For homeowners across Denver, Douglas, Arapahoe, Jefferson, El Paso, Boulder, and neighboring counties, the regional geology means elevated readings are common enough that “normal” national figures may understate local risk. A home near the national indoor average is reassuring, but given the Zone 1 designation, many Front Range homes exceed it. Our overview of radon testing in Colorado explains why testing is essential across the region.
Normal Ranges by Region and Geology
Because radon originates in the ground, normal ranges shift with regional geology. Areas underlain by uranium-bearing soils and rock tend to produce higher indoor levels across the board, while areas with low-uranium geology see lower averages. The EPA maps the country into radon zones reflecting this potential, with Zone 1 representing the highest predicted indoor levels. In a Zone 1 area, a level that would be unusually high elsewhere may be relatively common, and the share of homes above the action level is larger.
This regional variation is why national averages are only a rough guide. Two homes of identical construction can have very different radon levels purely because of the ground beneath them. Soil permeability, moisture, and how a home interacts with the soil all influence the result. The practical takeaway is that knowing your region’s radon potential tells you how likely an elevated level is, but only a test of your specific home tells you where you actually stand.
How to Measure Your Levels Accurately
Because radon is invisible and odorless, testing is the only way to know whether your home is near normal. Short-term tests run a few days to a week and give a quick screen. Long-term tests run more than 90 days and capture seasonal variation, giving the most accurate picture of your year-round average. Continuous monitors log levels over time, showing how readings shift with weather and household activity.
Follow EPA protocols for reliable results: test the lowest livable level, keep the home closed during short-term testing, and avoid disturbing the device. Radon varies by season, often peaking in winter when homes are sealed, so a long-term test best represents your true average. If a short-term result lands near the action level, confirm it with a second test before deciding on next steps.
How Normal Levels Vary Within One Home
Even within a single house, there is no one normal number. Levels are typically highest in basements and on ground floors, closest to the soil and the foundation openings where radon enters, and lower on upper stories as the gas disperses. Seasonal patterns add another layer of variation: winter readings often run higher because sealed, heated homes both draw in more soil gas and exchange less fresh air. So a home’s normal level is really a range that depends on where and when you measure.
This is why testing guidance points to the lowest livable level and to consistent, closed-house conditions for short-term tests. Measuring a frequently occupied basement gives a more meaningful read on exposure than testing a rarely used top-floor room. Understanding this internal variation keeps homeowners from drawing false comfort from a single low reading taken in the least-affected part of the house.
Seasonal Variation in Normal Levels
Normal levels shift not just by place but by time of year, which is worth understanding when interpreting a single test. Winter typically produces higher readings because homes are sealed against the cold, heating systems intensify the pressure difference that draws soil gas indoors, and reduced ventilation lets radon accumulate. Summer, with windows open and more air exchange, often shows lower readings. A short-term test run in one season may not represent the home’s true year-round average.
This is why a long-term test, spanning more than 90 days, gives the most reliable picture of a home’s normal level: it averages across changing conditions. If you test short-term in summer and get a low number, a winter result could be higher, and vice versa. For a meaningful read on whether your home is genuinely normal, either run a long-term test or interpret a short-term result with the season in mind. Seasonal awareness keeps a single snapshot from being mistaken for the whole story.
What to Do With Your Result
If your home tests near the indoor average of 1.3 pCi/L or otherwise below 2 pCi/L, your level is normal and lower-risk; periodic retesting keeps you informed, especially after renovations. Between 2 and 4 pCi/L, your level is above average, and the EPA suggests considering action based on your circumstances. At or above 4 pCi/L, the EPA recommends mitigation regardless of how the number compares to averages.
When a level comes back high, a qualified radon professional can install a mitigation system, most commonly active sub-slab depressurization, and verify its effectiveness with a follow-up test. Knowing what normal radon levels are gives you a frame of reference, but your own test result is what matters. Given Colorado’s Zone 1 designation, testing is the responsible first step for every Front Range home.
Putting Normal Ranges Into Context
Pulling the ranges together gives a clear mental model. Outdoor air sits around 0.4 pCi/L, the realistic floor. Typical indoor air averages about 1.3 pCi/L, the baseline most homes experience. The EPA’s “consider action” range runs from 2 to 4 pCi/L, above average but below the firm recommendation. At and above 4 pCi/L, mitigation is recommended, and readings climbing into the double digits call for prompt action. Holding these reference points in mind lets you place any single result on a meaningful spectrum rather than reacting to a number in isolation.
The most important habit this context supports is testing rather than assuming. Because radon varies so much by geology, by home, and even by floor and season, no average can substitute for measuring your own home. National figures tell you what is typical; a Zone 1 designation tells you elevated levels are more likely in your area; but only a test of your specific home, on its lowest livable level under proper conditions, tells you where you actually stand. That is the step that turns general knowledge about normal ranges into a decision you can act on.
References
- Average indoor and outdoor radon levels — U.S. Environmental Protection Agency
- Radon basics and exposure risk — Centers for Disease Control and Prevention
- Colorado radon zones and testing — Colorado Department of Public Health and Environment
If you are a Front Range homeowner who wants to know how your home compares to normal radon levels, you can connect with a vetted professional through our contact page.