Causes of Radon in Homes: Geology and Entry Pathways
The causes of radon in homes split cleanly into two categories: the geological source (where the gas comes from) and the building pathways (how it gets indoors). Both matter for homeowners trying to understand why their house has elevated levels and what can be done about it. The geological source is largely fixed — uranium in soil and rock continues to produce radon for billions of years on human timescales — but the building pathways are addressable. Mitigation systems work by intercepting the pathways, not by reducing the source. This guide summarizes EPA, USGS, and Colorado Department of Public Health and Environment guidance current as of 2026 — consult a state-licensed mitigation professional for system design and an EPA-listed measurement professional for testing decisions.
Causes of Radon in Homes: The Two-Part Answer
The simplest answer is that radon comes from the natural decay of uranium in soil and rock beneath the building, and it enters through foundation cracks, sump pits, and slab penetrations driven by the pressure differential between warm indoor air and cooler outdoor air (the “stack effect”). The Front Range of Colorado is in EPA’s highest-risk Zone 1 because Rocky Mountain granitic rock and the Pierre Shale formation under much of Denver-area soil produce radon at higher rates than the national average. Roughly half of Colorado homes tested show indoor levels above the EPA action threshold of 4 pCi/L. Our broader radon testing guide for Front Range homeowners covers what to do once a home tests elevated.
The Geological Source
Uranium-238 in Soil and Rock
Uranium-238 is present at trace levels in virtually all soil and rock on Earth. The concentration varies by formation: granitic and metamorphic rocks typically contain 3 to 5 parts per million uranium, while sedimentary rocks like sandstone and limestone may contain 1 to 2 ppm. Some specific formations — black shales, phosphate deposits, certain igneous intrusions — can contain 10 to 20 ppm or more.
Uranium-238 has a half-life of 4.5 billion years. On human timescales, the source is effectively inexhaustible. As long as uranium is present in the soil and rock under a building, it will continue to produce radon.
The Decay Chain
Uranium-238 decays through a chain of intermediate elements before reaching radon. The key steps:
- Uranium-238 → Thorium-234 (alpha decay).
- Thorium-234 → Protactinium-234 → Uranium-234 (beta decays).
- Uranium-234 → Thorium-230 → Radium-226 (alpha decays).
- Radium-226 → Radon-222 (alpha decay, half-life 1,600 years for radium).
- Radon-222 → Polonium-218 (alpha decay, half-life 3.8 days for radon).
Radium-226 is the immediate precursor to radon-222. The 1,600-year half-life of radium means it acts as a long-term reservoir, continuously producing radon. The 3.8-day half-life of radon means the gas does not persist long enough to migrate great distances — it decays quickly enough that most of what enters a home comes from soil within a few meters of the foundation.
Diffusion Through Soil
Once radon forms in soil grains, it diffuses through the soil pore space. The diffusion rate depends on soil moisture, permeability, and temperature. Dry, permeable soils (gravel, sand) allow rapid diffusion. Wet, low-permeability soils (clay, frozen ground) slow diffusion considerably. Water in pore space tends to trap radon temporarily, releasing it when the water evaporates or is replaced by air.
The implication for homes: radon flux into a building is highest from dry, permeable soil and lowest from clay-heavy or frozen soil. Front Range homes built on glacial outwash gravel, alluvial sand, or weathered granite typically experience higher entry rates than homes built on heavy clay. Resources from epa.gov/radon describe the geological mapping at the national scale.
Building Pathways: How Radon Enters
The Stack Effect
The driving force for radon entry into a building is the pressure differential between indoor and outdoor air. Warm indoor air is less dense than cooler outdoor air. The warm air rises through the building and escapes through the upper levels — windows, attic, and roof. To replace it, lower-level air pulls in from below, including soil gas through any unsealed pathway. The effect is strongest in winter when the indoor-outdoor temperature differential is greatest.
HVAC operation, exhaust fans, and clothes dryer venting all amplify the stack effect by removing additional air from the building. Houses that are tightly sealed at the upper levels but leaky at the basement create the strongest soil-gas pull.
Common Entry Pathways
Six pathways account for most radon entry into homes:
- Cracks in concrete slabs. Even hairline cracks (less than 1/16 inch) pass measurable soil gas under stack-effect pressure. Wider cracks pass more.
- Construction joints. The seam where a slab meets a foundation wall is rarely sealed at construction and is a common entry point.
- Sump pits. Open sump pits provide a direct soil-to-interior pathway. A standard cover does not seal the pit; a radon-rated cover with a one-way pressure relief is required.
- Floor drains. Open floor drains connect to soil through the building’s perimeter drain system. Trap primer or one-way trap covers reduce the pathway.
- Slab penetrations. Plumbing, electrical, and HVAC penetrations through the slab create gaps between pipe and concrete. These gaps must be sealed to prevent soil gas entry.
- Crawl spaces. Unsealed crawl spaces with exposed soil floors can let large amounts of radon-laden air into the building above. A 6-mil polyethylene vapor barrier sealed at the perimeter substantially reduces this pathway.
Well Water as a Secondary Source
Homes with private well water can have a secondary radon exposure pathway. Groundwater in uranium-rich formations dissolves radon, and the gas is released when the water is aerated during showers, dishwashing, or laundry. Public water supplies typically aerate the water before distribution, so this pathway is rare for municipal-water customers. Well water in some Colorado mountain counties shows elevated dissolved radon. Treatment is straightforward: granular activated carbon filtration or whole-house aeration systems remove the dissolved gas before it enters the home.
Well-water radon contributes about 10 pCi/L of dissolved gas per 100,000 pCi/L in water, by EPA estimate. The pathway is rarely the dominant cause of indoor levels but can add meaningfully on top of soil-gas entry.
Why the Front Range Is EPA Zone 1
EPA classifies counties into three zones based on average indoor radon measurements:
- Zone 1. Predicted average indoor radon above 4 pCi/L. Highest risk.
- Zone 2. Predicted average indoor radon between 2 and 4 pCi/L. Moderate risk.
- Zone 3. Predicted average indoor radon below 2 pCi/L. Lower risk.
All of the Front Range — including Denver, Douglas, Elbert, Arapahoe, Jefferson, El Paso, Adams, Boulder, and Broomfield counties — is classified Zone 1. The geological reason: the Rocky Mountains’ uranium-rich granitic rock and the widespread Pierre Shale formation under Denver-area soil both produce radon at higher rates than the national average. Colorado Department of Public Health and Environment publishes county-level data confirming the Zone 1 designation. Resources at cdphe.colorado.gov/radon map specific neighborhoods.
Building Features That Affect Indoor Levels
Two homes on adjacent lots with the same geology can have very different indoor radon levels. The variability comes from building features:
- Foundation type. Slab-on-grade, basement, and crawl space foundations have different entry profiles. Basements with finished living space typically test highest because more living space is below grade.
- Building tightness. Tighter buildings retain radon at higher concentrations because the air exchange rate with outdoors is lower. Newer energy-efficient homes can paradoxically test higher than older drafty homes despite better construction.
- HVAC operation. Forced-air heating creates pressure imbalances that can either pull more soil gas in or push it out, depending on the return duct configuration.
- Concrete slab quality. Older slabs with more cracks pass more soil gas. Newer slabs with vapor barriers and gas-permeable layers (radon-resistant construction) pass less.
- Use of below-grade space. A finished basement bedroom or playroom exposes occupants to higher indoor concentrations than a basement used only for storage.
Our companion piece on what causes radon gas in homes walks through the building-side factors in additional depth.
Why Levels Vary Seasonally
Indoor radon concentrations are typically higher in winter than in summer. Three factors contribute:
- Winter stack effect is stronger due to larger indoor-outdoor temperature differential.
- Windows and doors are closed more in winter, reducing dilution by outdoor air.
- Frozen surface soil traps radon in lower soil layers, increasing concentration available to migrate into the building.
Short-term radon tests run in winter therefore tend to read higher than the annual average. EPA recommends long-term tests (90 days or more) to capture an accurate annual average for comparison to the action threshold.
What Mitigation Addresses
Mitigation systems do not address the geological source. They cannot remove uranium from the soil or stop radium from decaying into radon. What they do is intercept the entry pathways: by creating negative pressure beneath the foundation slab, the system pulls soil gas into the pipe rather than into the building. The gas is exhausted above the roof line where it disperses to outdoor background levels.
This is why the system runs continuously. The geological source is continuous; the stack-effect pressure differential is continuous; the only effective response is continuous interception. A mitigation system that operates only intermittently allows indoor levels to rebuild quickly. Our piece on radon mitigation system design covers the engineering response in depth.
Specific Geological Formations Under the Front Range
Three geological formations contribute disproportionately to Front Range radon levels. The Pierre Shale formation underlies much of the Denver Basin and stretches from the foothills out to the eastern plains. Pierre Shale is a dark marine shale deposited during the Cretaceous period, containing trace uranium concentrations elevated above the global average. The shale weathers to clay-heavy soil, but uranium remains in the soil matrix and continues producing radon for billions of years on human timescales.
The Precambrian granitic rocks of the Front Range itself — exposed in the foothills and underlying much of the western suburbs — contain higher uranium concentrations than sedimentary rocks. Boulder Creek granite, Silver Plume granite, and the Pikes Peak batholith are all in this category. Homes built directly on or near these formations can have indoor radon levels well above the EPA action threshold even with modern construction.
Glacial outwash deposits along the South Platte and Cache la Poudre rivers create highly permeable soils derived from upstream granitic source rock. The combination of trace uranium content plus high permeability produces favorable conditions for radon migration into overlying buildings. Adams, Weld, and Boulder counties all have substantial glacial outwash deposits.
Soil Moisture and Seasonal Variation
Soil moisture affects radon flux in counterintuitive ways. Dry soil allows easier radon diffusion through the pore space, increasing the gas available to enter the building. Wet soil traps radon temporarily in water-filled pore space, reducing entry rates. Frozen surface soil acts as a cap that traps radon in deeper soil layers — but those deeper layers continue producing gas, and pressure builds up over the winter season.
The net effect is that indoor radon levels typically rise during the winter heating season due to two factors operating together. The stronger stack effect pulls more soil gas in, and the frozen surface acts as a temporary cap that concentrates radon in soil immediately around the foundation. After the spring thaw, surface diffusion resumes and indoor levels typically decline. This seasonal pattern is why EPA recommends long-term testing (90 days or more) to capture an accurate annual average.
Why Some Homes Test Lower Than Neighbors
Two homes on adjacent lots can test very differently. The variation comes from building features rather than geology in most cases. A home with a tight foundation slab, sealed sump cover, sealed floor drains, and minimal cracks may test well below the action threshold even on highly radon-prone soil. A home with an open sump pit, multiple slab cracks, and unsealed floor drains can test high even on moderately radon-prone soil.
Foundation type matters too. Slab-on-grade homes typically test lower than basement homes on the same lot because slab-on-grade homes have less below-grade living space. Crawl-space homes can test very high or relatively low depending on whether the crawl-space soil is sealed and ventilated. Multi-story homes have a stronger stack effect than single-story homes, which can increase soil-gas pull into the lower levels.
References
- EPA Radon Zones Map — U.S. Environmental Protection Agency
- Colorado Radon Program — Colorado Department of Public Health and Environment
- CDC Radon Information — Centers for Disease Control and Prevention
- American Lung Association Radon Resources — American Lung Association
Front Range homeowners curious about their property’s radon profile can connect with a vetted local inspector to coordinate testing and discuss mitigation if levels are elevated.