What Causes Radon: A Plain-Language Geology Guide
Radon is not manufactured, not emitted by appliances, and not the result of building materials degrading. It is a natural product of radioactive decay in the Earth’s crust — specifically, the slow decay of uranium-238 (present in essentially all soil and rock at variable concentrations) into a chain of intermediate radioactive isotopes that includes radon-222. The geology underneath a house determines how much radon enters it, with granite, shale, phosphate-bearing sediments, and uranium ores producing the highest emanations. This guide walks through the source chemistry, the soil-and-rock geology, and the entry pathways from soil to indoor air. Based on EPA, USGS, and CDPHE geological data current as of 2026; not medical advice.
The Direct Answer: Uranium-238 Decay Underground
Radon-222 is produced by the radioactive decay of radium-226, which is itself produced by the decay of uranium-238 — the most abundant uranium isotope in the Earth’s crust. Uranium-238 has a half-life of approximately 4.5 billion years, meaning the original uranium present when the Earth formed is still decaying today at roughly its original rate. That decay continuously produces radium-226 (half-life roughly 1,600 years), which in turn produces radon-222 (half-life 3.8 days). Each step of this decay chain releases ionizing radiation; the radon step is the one that matters for indoor-air exposure because radon, unlike its solid parent isotopes, is a gas that can migrate.
The complete decay chain from uranium-238 to stable lead-206 has fourteen steps. Radon is step eight. The key feature of radon-222 in this chain is its physical state at room temperature: a noble gas, chemically inert, capable of diffusing through soil pore space and entering buildings through foundation cracks, sump openings, and slab penetrations.
Where Uranium Is Found: Geology Driving Radon Emanation
Uranium concentration varies widely across geologic formations. Most soils contain trace uranium (1 to 3 parts per million is typical), and the radon they produce is correspondingly modest. Certain rock types and formations contain significantly elevated uranium concentrations, and those formations produce correspondingly elevated radon emanation into overlying soil and structures.
Granite
Granitic bedrock can contain 4 to 40 parts per million uranium — substantially above average crustal concentration. The Front Range Colorado granites (Pikes Peak granite, Boulder Creek granite, the Idaho Springs Formation) are well-documented uranium-bearing rocks. The decay of this uranium produces continuous radon emanation that, where overlying soils are permeable, supports elevated indoor radon in homes built on these formations.
Shale and Black Shale
Marine shales (particularly black shales rich in organic matter and trace metals) often contain elevated uranium. The Pierre Shale and Niobrara Formation across portions of the Great Plains and Colorado contain measurable uranium and contribute to elevated regional radon. Buildings constructed on or near outcrops of these formations show elevated radon in EPA Radon Zone mapping.
Phosphate-Bearing Sediments
Phosphate deposits often contain elevated uranium because of the geochemical co-precipitation of phosphate and uranium in marine environments. Florida’s phosphate-mining region (Polk and surrounding counties) and Idaho’s Phosphoria Formation both show elevated regional radon partly attributable to phosphate-uranium geochemistry. Even processed phosphate fertilizers and gypsum byproducts retain trace uranium and contribute marginally to soil radon.
Uranium Ores and Mining Tailings
Areas with uranium mining history — the Colorado Plateau, including portions of western Colorado, southeastern Utah, northwestern New Mexico, and northern Arizona — show some of the highest indoor radon concentrations recorded. Mining tailings used historically as construction fill (the Grand Junction, Colorado uranium-mill-tailings cleanup is the textbook case) produced extreme indoor radon in homes built on or with the tailings. Federal remediation programs have addressed many of these legacy sites, but residual elevated radon persists in some communities.
From Soil to Indoor Air: The Entry Pathways
Soil radon does not reach indoor air spontaneously — it requires both a driving force and an entry pathway. The driving force is typically the stack effect: warm indoor air rises in winter, drawing slightly negative pressure at the foundation level relative to outdoor and soil pressure. This pressure differential pulls soil gas (including radon) through any available pathway into the building.
Foundation Cracks
Any crack in a slab, foundation wall, or footing provides an entry path. Hairline cracks that look insignificant visually can pass meaningful soil-gas volumes when negative pressure exists. Older homes with settled foundations and visible cracks are particularly vulnerable.
Slab Penetrations
Plumbing, electrical, and HVAC penetrations through a basement or crawl-space floor slab create deliberate openings that, if unsealed, allow soil gas entry. Sump openings, floor drains, and unsealed pipe penetrations are common pathways.
Crawl Space Floors
Houses with unsealed crawl-space floors (dirt floors or unsealed vapor barriers) provide essentially unrestricted soil-gas access to the underside of the home. Radon entering a crawl space readily migrates upward into the conditioned living space through the floor system above.
Sump Pump Pits
Sump pits are direct openings to the soil. Without a sealed sump cover and proper venting, they function as a high-area entry point for soil gas.
For broader detail on the geology-and-entry combination, see the geology and entry pathways guide.
The Front Range Colorado EPA Zone 1 Designation
EPA’s Map of Radon Zones designates counties at three risk levels: Zone 1 (highest, predicted average indoor radon above 4 pCi/L), Zone 2 (moderate, 2 to 4 pCi/L), and Zone 3 (lower, below 2 pCi/L). Colorado is overwhelmingly Zone 1 across the Front Range — Denver, Jefferson, Adams, Arapahoe, Douglas, Boulder, Broomfield, El Paso, and Elbert counties are all Zone 1. The CDPHE radon program publishes county-level testing data confirming meaningfully elevated household prevalence across the entire region.
The Zone 1 designation reflects the underlying geology: Front Range granites and overlying sedimentary units contain enough uranium to produce regionally elevated soil radon. Combined with the high prevalence of basement and slab-on-grade construction in Colorado housing, this geology produces a real population-level radon exposure that justifies systematic testing.
For context on what radon does (and does not do) at the human-health level, see the radon-testing pillar guide for Front Range homeowners.
Why House-to-House Variation Is Extreme
Two adjacent homes on the same lot, built by the same builder in the same year, can show indoor radon levels differing by an order of magnitude or more. The underlying geology — uranium concentration in the soil and bedrock — is essentially uniform across a single lot. What differs is the combination of foundation design, sealing quality, ventilation patterns, occupant behavior (window openings, HVAC operation), and stack-effect intensity.
A tightly sealed home with strong negative pressure at the foundation level concentrates radon. A leakier home with substantial outdoor air exchange dilutes it. A home with a sump pit and unsealed crawl-space floor draws more soil gas than a home with a fully sealed basement slab. The geology supplies the radon; the building envelope determines how much accumulates indoors.
What Causes Radon Inside the Home (Once It Enters)
Once radon enters indoor air, it accumulates because the source is continuous (soil emanation is essentially constant) and the dilution from outdoor air exchange in modern energy-efficient homes is limited. The half-life of 3.8 days means radon does not stay long — it decays into solid radioactive progeny (polonium, lead, bismuth) on the timescale of hours to days. Those progeny are what actually deposit radiation in the lungs when inhaled.
Indoor radon levels follow seasonal patterns. Winter levels are typically higher because closed windows reduce dilution and stack-effect-driven pressure differential maximizes soil-gas entry. Summer levels are typically lower. This seasonal variation is why long-term radon tests (3-12 months) give a more accurate annual-average reading than short-term (2-7 day) tests.
What Does NOT Cause Indoor Radon
A few common misconceptions worth clarifying. Radon is not produced by household appliances, electronics, or HVAC equipment. Radon is not emitted by building materials in any meaningful quantity (uranium content in concrete, drywall, and brick is too low to produce significant indoor radon contribution). Radon is not in the municipal water supply at concentrations that meaningfully contribute to indoor air radon for the vast majority of households (well-water radon can contribute in specific cases, but this is a small fraction of total exposure for nearly all U.S. households).
The dominant source — by far — is soil-gas entry from the underlying geology. Testing and mitigation address this dominant source.
What to Do: Test the Home
The actionable response to understanding what causes radon is testing the specific home, because house-to-house variation makes population-level geological data insufficient for individual decisions. Test kits cost $15 to $40 for consumer use; professional NRPP-certified testing costs $125 to $250 and is typically required for real-estate transactions.
If results exceed 4.0 pCi/L, mitigation through active soil depressurization (ASD) reduces indoor radon by 50 to 99 percent. The radon reduction system components walkthrough covers the standard system design.
When to Hire a Professional
Mitigation should be performed by an NRPP-certified or NRSB-certified radon mitigation professional, not as a DIY project. Improperly designed systems can fail to reduce radon or, in some cases, increase it. Front Range homeowners looking for a connection to a vetted radon professional can get in touch here.
The Stack Effect and Pressure-Driven Soil Gas Entry
Understanding why radon enters homes requires understanding the stack effect. In winter, warm indoor air rises through the building envelope, escaping through upper-level penetrations (attic vents, recessed lighting, plumbing chases). This creates a slight negative pressure at the lower levels of the home — basement, crawl space, and lower floors are at slightly lower pressure than outdoor air and surrounding soil. The pressure differential, typically a few Pascals, draws soil gas through any available foundation opening.
The stack effect intensifies in cold weather and in tall buildings with strong vertical temperature differentials. This is why indoor radon levels are typically higher in winter than in summer — the stack effect is stronger, the soil-gas entry rate is higher, and closed windows reduce outdoor-air dilution. Mitigation systems counteract this pressure differential by creating active negative pressure in the sub-slab soil, drawing radon away from the home before it can enter.
Why New Construction in Zone 1 Isn’t Safer
A common intuition is that new construction must have less radon than older homes — newer materials, better sealing, modern building practices. The intuition is wrong. Modern energy-efficient construction creates tight building envelopes with limited outdoor-air dilution. The geology supplies the radon regardless of building age. New homes in Zone 1 counties show radon levels above the EPA action level at rates comparable to (and sometimes higher than) older homes. The combination of geology and envelope tightness is the determinant, not building age alone.
This is why some Colorado jurisdictions now require passive radon-control measures in new residential construction — rough-in piping, sealed sumps, vapor barriers — to support easier active mitigation if testing later identifies elevated levels. The International Residential Code (IRC) includes Appendix F for radon-resistant new construction; some Front Range jurisdictions have adopted it, others have not.
Soil Permeability and Radon Transport
The transport of radon from soil rock matrix to building entry is controlled by soil permeability. Highly permeable soils (sandy, gravelly, glacial outwash) support fast radon transport — radon produced in deep soil reaches the soil surface and overlying buildings quickly. Low-permeability soils (clay, silt, dense glacial till) restrict radon transport — much of the radon produced decays before reaching the building.
This explains why two adjacent properties with identical underlying bedrock can show very different indoor radon levels: differences in overlying soil permeability, foundation design, and building envelope tightness combine to produce dramatically different indoor concentrations. The Front Range has a mix of soil types, including the highly permeable Castle Rock conglomerate (which supports rapid radon transport) and lower-permeability shale-derived soils.
Well-Water Radon: A Secondary Pathway
For households on private wells drilled into uranium-bearing aquifers, well water can contain dissolved radon. When the water is used (showering, dishwashing, laundering), radon volatilizes into indoor air and contributes to inhalation exposure. For most U.S. households on municipal water, this pathway is negligible. For specific well-water households — particularly in geologically uranium-rich areas — well-water radon can contribute meaningfully to indoor air radon.
EPA’s drinking-water radon threshold is 4,000 pCi/L for utilities serving more than 25 people, with a proposed alternative-MCL of 300 pCi/L for systems with mitigation programs. Private wells are not subject to these federal thresholds. Households on wells with measured high well-water radon can install aeration treatment systems or granular activated carbon (GAC) filtration to remove radon from water before household use.
How Geology Differs Between Front Range Counties
Within the Front Range, different counties show different radon signatures based on underlying geology. Counties with exposed Precambrian granites (Jefferson, Boulder, El Paso) tend toward higher household radon prevalence due to direct exposure of uranium-bearing crystalline rock. Counties with overlying sedimentary cover (Denver, Adams, Arapahoe, Douglas) show a moderating effect from sedimentary cover, though still consistently in Zone 1. Counties with very thick sedimentary cover and limited bedrock proximity (Elbert, eastern portions of El Paso) show somewhat lower average household radon, though still above national averages.
CDPHE publishes county-level testing summaries that quantify these geological patterns. The summaries are useful for population-level understanding but not substitutes for individual home testing — within any county, individual homes vary enormously based on foundation, soil, and ventilation.
References
- EPA Radon Information — U.S. Environmental Protection Agency
- CDPHE Radon Program — Colorado Department of Public Health and Environment
- CDC Radon Information — Centers for Disease Control and Prevention
- American Lung Association Radon — American Lung Association
- NIEHS Radon Health Topic — National Institute of Environmental Health Sciences
Front Range homeowners curious about their property’s actual radon level can request a referral to a vetted NRPP-certified radon professional through our contact page.