Radon is a naturally occurring radioactive gas found everywhere on Earth, from the soil beneath your feet to the water you drink. It forms underground as uranium breaks down in rock and soil, then migrates upward through cracks and pores until it reaches the surface. The average outdoor air contains about 0.4 pCi/L of radon, a trace amount that’s roughly one-tenth of the level considered worth addressing indoors. But certain rocks, soils, groundwater sources, and enclosed spaces like caves concentrate radon far beyond that baseline.
How Radon Forms Underground
Radon doesn’t exist on its own. It’s a product of uranium’s slow radioactive decay, a chain reaction that unfolds over billions of years. Uranium-238, the most common uranium isotope in Earth’s crust, gradually transforms through a long sequence of intermediate elements before eventually becoming stable lead. Radon-222 is one stop along that chain, and it’s the isotope that matters most for human exposure because it has a half-life of about 3.8 days, long enough to escape from rock and accumulate in enclosed spaces before it decays.
Two other radon isotopes occur naturally. Thoron (radon-220) comes from the thorium-232 decay chain and has a half-life of only about 56 seconds, which limits how far it can travel. Actinon (radon-219), from uranium-235, lasts less than 4 seconds and is rarely a concern. Radon-222 dominates because it persists long enough to move through soil, dissolve in water, and seep into buildings.
Rocks With the Highest Radon Levels
Radon production depends entirely on how much uranium a rock contains. Granite, shale, and limestone are the most common culprits, though not all formations of these rocks are equally rich in uranium. Phosphate rocks also contain very high levels of naturally occurring radioactive material from both uranium and thorium decay.
The variation between rock types can be enormous. In Pennsylvania, geological formations rich in carbonate rock (like the Axemann, Bellefonte, and Nittany Formations) produce indoor radon concentrations 220 to 250 percent higher than areas sitting on sandstone formations like the Stockton. Granite samples from Egypt’s Eastern Desert have shown radon concentrations ranging from roughly 33,000 to over 251,000 Bq/m³, hundreds of times above what you’d find in outdoor air. Even building materials can be a source: concrete made from uranium-rich “alum shale” has been documented releasing radon at concentrations around 1,000 Bq/m³.
Soil: The Main Pathway to the Surface
Most radon that reaches the atmosphere passes through soil first. How much arrives at the surface depends on the soil’s physical properties, especially its permeability. Loose, coarse, dry soil lets radon migrate upward easily. Clay-heavy or waterlogged soil traps it below ground, slowing its release.
Two forces drive radon through soil. When there’s a temperature or pressure difference between the ground and the air above it (or inside a building), radon gets pulled upward by bulk air movement, a process called advective transport. When no pressure difference exists, radon spreads more slowly through simple diffusion. This is why radon entry into homes often spikes in winter: heated indoor air creates a slight vacuum at the foundation, drawing soil gases upward through cracks in concrete, gaps around pipes, and other openings.
Particle size, porosity, moisture levels, and temperature all interact to determine a given patch of soil’s radon output. Two houses on the same street can have very different radon levels if their foundations sit on slightly different soil compositions.
Groundwater vs. Surface Water
Radon dissolves readily in water, and groundwater picks up far more of it than lakes or rivers do. Groundwater sits in direct contact with uranium-bearing rock for extended periods, giving radon time to accumulate. Measurements from a study in Kenya found groundwater radon levels ranging from about 0.6 to 18 mBq/L, while surface water ranged from just 0.06 to 1.5 mBq/L.
The difference comes down to aeration. Rivers and streams constantly churn and expose water to open air, allowing dissolved radon to escape into the atmosphere. Groundwater, sealed underground with no air exchange, holds onto its radon until it’s pumped to the surface. This is why private wells drilled into granite or other uranium-rich bedrock can deliver radon directly into a home’s water supply, releasing it into indoor air when you shower, wash dishes, or run a faucet.
Caves, Springs, and Volcanic Systems
Natural enclosed spaces act as radon traps. Caves, mine shafts, and volcanic tunnels all concentrate the gas because they’re surrounded by rock and have limited ventilation. A study of a volcanic cave system found radon concentrations ranging from 17 Bq/m³ in winter to 882 Bq/m³ in summer, with an annual average of 169 Bq/m³. The seasonal swing happens because warm outside air in summer sits above cooler cave air, creating a “cap” that prevents ventilation. In winter, the temperature relationship reverses, flushing radon out more effectively.
Thermal and mineral springs can also carry elevated radon. Water heated deep underground passes through uranium-bearing rock before surfacing, and the radon it absorbs comes along for the ride. Some historic “radon spas” in Europe were actually marketed for their radioactive water before the health risks of radon exposure were understood.
Regions With Naturally High Radon
Certain geological formations produce consistently elevated radon across large areas. In the United States, one of the best-documented examples is the Reading Prong, a band of ancient metamorphic rock stretching from Pennsylvania through New Jersey and into New York. Counties overlying the Reading Prong have significantly higher indoor radon than surrounding areas. In a study of nearly 900,000 Pennsylvania radon measurements collected over 24 years, Reading Prong counties (Berks, Lehigh, and Northampton) had a median indoor radon concentration of about 192 Bq/m³, roughly triple the median in Philadelphia, which sits on different geology.
Similar high-radon zones exist wherever uranium-rich bedrock sits near the surface. The Appalachian Mountains, parts of the upper Midwest, and areas of the northern Great Plains all have elevated radon potential. Internationally, regions of Scandinavia, the United Kingdom’s granite-heavy southwest, and parts of central Europe are well-known radon hotspots. The pattern is consistent: wherever the local geology is rich in uranium or radium, radon follows.
How Much Radon Is in Outdoor Air
Even in the open atmosphere, radon is always present. The EPA estimates the average outdoor radon concentration at 0.4 pCi/L (15 Bq/m³). This is considered a safe background level, well below the 4 pCi/L (150 Bq/m³) action level that triggers a recommendation for mitigation indoors. Most countries around the world have adopted action levels similar to that 4 pCi/L threshold.
Outdoor radon stays low because it disperses quickly into the atmosphere. The concern is always about enclosed spaces, whether natural or human-made, where the gas can accumulate faster than it dissipates. Your geology determines the starting concentration, and your ventilation determines how much of it you actually breathe.