Straight Answer
Where Does Radon Come From?
The short version. Where does radon come from? Straight out of the ground under your house. It's produced by the slow natural breakdown of uranium in soil and rock — uranium that's been there since the earth formed. Some radon is made under every square foot of ground on the planet. What changes from one lot to the next is how much uranium is in the specific bedrock, how easily gas can travel up through the soil above it, and how much your house pulls in.
0.4pCi/L
U.S. outdoor average
1.3pCi/L
U.S. indoor average
9factors
GSA radon-potential factors
It Starts With Uranium
Radon Starts as Uranium in the Ground
Every radon atom starts life as a uranium atom. Uranium is a trace element in almost every rock and soil on earth — very little in typical sandstones and limestones, more in certain shales, granites, and carbon-rich rocks. Uranium takes about 4.5 billion years to break down, which for practical purposes means it's a permanent feature of the ground under any house.
Uranium slowly breaks down through several other elements. Somewhere in the middle, it becomes radium — a solid metal that stays stuck inside its rock grain. When a radium atom breaks apart, it releases a radon atom. Radon is the only step in the whole chain that's a gas. Because it's a gas, it can slip out of the rock grain, move up through tiny spaces in the soil, and eventually reach the surface. From underneath a house, "the surface" is the inside of your home.
For the full decay chain and how it connects to indoor air, see what is radon gas.
What Changes The Numbers
The Nine Things That Change How Much Radon a Site Produces
The Geological Survey of Alabama publishes nine "radon-potential factors" — geology-based things that push the numbers up or down. Together they explain why a house on one lot can sit at the EPA action level (4.0 pCi/L — the line where the EPA says to fix your home) while a nearly identical house half a mile away reads a harmless 1.0 pCi/L:
- 01Uranium content of the underlying rock
The starting material. More uranium in the bedrock means more radium in the weathered soil above it, which means more radon released as that radium decays.
- 02Fractured or faulted rock
Faulting creates conduits for radon movement, per the Geological Survey of Alabama. The Valley and Ridge province — the Birmingham metro's physiographic province — is defined by folded, faulted bedrock.
- 03Karst limestone and dissolution features
Karst adds permeability. Fractures, sinkholes, and small dissolution cavities give soil gas — including radon — fast paths upward from source rock to the surface.
- 04Thin soils over bedrock
Thinner soils mean shorter travel distance from source rock to a foundation. A house built on shallow soil directly over an elevated-radon unit is more exposed than the same house built on twenty feet of clay.
- 05Hillside or slope siting
Homes built into a slope commonly have a walkout lower level with more soil-contact wall surface. That geometry increases the potential entry area radon can cross.
- 06Carbonaceous shales, including coal-bearing units
Coal-bearing and carbonaceous shales are among the specific Alabama source rocks the GSA calls out as elevated-radon units.
- 07Granitic rocks
Granitic rocks carry more uranium than typical sandstones. Alabama's highest-radon-potential zones correspond to specific granitic and altered carbonaceous units.
- 08Construction and excavation
Cutting a basement into bedrock, blasting for a foundation, and disturbing near-surface soil all increase permeability. Newly built homes are not exempt from elevated readings.
- 09Soil moisture and permeability
Dry, permeable soils let radon move faster than saturated ones. Seasonal soil moisture is one of the reasons the same house tests differently in different months.
Any one of those factors can push a lot into the elevated-potential column. In the Birmingham metro, most sites carry at least three of them at once.
Why Birmingham
Why Alabama and the Birmingham Metro Read Above Average
Birmingham sits in what geologists call the Valley and Ridge — folded, cracked bedrock with long parallel ridges of hard rock and valleys carved into softer rock. In its 2025 presentation to the EPA Region 4 Radon Stakeholders Meeting, the Geological Survey of Alabama ranked the Valley and Ridge asmoderate to high for radon potential (USGS OFR 93-292-D). Three local features drive that ranking:
- Rock types that hold more uranium. The GSA specifically points to certain carbon-rich shales and granite-type rocks under Birmingham as our higher-radon source rocks — they carry more uranium than average sandstones do.
- Cracked, faulted bedrock. The Valley and Ridge is broken by faults. The GSA is explicit that those faults act like elevator shafts, letting radon travel up toward the surface faster.
- Sinkhole country (karst limestone). Parts of Jefferson and Shelby sit on limestone that's been slowly eaten away by water over millions of years. All those little cracks, dissolution channels, and cavities give radon fast paths upward.
The Appalachian Plateau, which stretches into northwest Jefferson and up through Walker and Blount counties, is ranked moderate — its sandstones and coal-bearing shales carry a bit less uranium than the Valley and Ridge average, but still more than the state as a whole.
That's why the Alabama Department of Public Health lists Jefferson and Shelby amongAlabama's 15 highest-potential radon counties. It's why the CDC's 2008–2017 data shows about 1 in 9 reported tests in Jefferson and Shelby coming back at or above the EPA action level of 4.0 pCi/L. And it's why Jefferson's highest recorded reading in that dataset was 24.6 pCi/L — more than six times the action level.
Why Neighbors Differ
Why Next-Door Houses Test So Differently
Radon isn't a street-wide number — it's a house-by-house number. Two homes fifty feet apart can sit over different cracks in the bedrock, on different soil depths, or on slightly different rock types. And the houses themselves are different: one has a walk-out basement cut ten feet into bedrock, the other is slab-on-grade; one is tightly sealed with newer windows, the other leaks air like a screen door; one runs its HVAC constantly, the other opens windows every spring and fall.
All that adds up to a simple fact ADPH puts right on the front page of its Alabama radon site: radon levels can vary greatly from home to home, even next door. A neighbor's clean test doesn't clear your house. Their high test doesn't condemn yours. The only way to know your number is to measure it in your house.
Outside vs Inside
Outdoor Radon vs Indoor Radon: Dilution vs Buildup
Radon is being made under everyone's feet, everywhere on earth, all the time. Outside, it mixes into the open air as fast as it comes up, so the average U.S. outdoor level is only about 0.4 pCi/L (EPA). That's the low background everyone lives with.
Inside, the same radon comes up under the foundation, but now the building acts like a lid. Warm indoor air rises and escapes out the top of the house. That leaves a slight vacuum down at the foundation, which pulls soil gas (including radon) into the building faster than the house's normal air changes can dilute it. The average U.S. indoor level is about 1.3 pCi/L — roughly three times what's outside. In houses over higher-radon soil, with tight construction and constant heating, that ratio can climb well into double digits.
That's the whole story of "indoor radon": the ground produces it at a natural rate, and the building above lets it pile up. A mitigation system (a small fan and pipe that pulls the gas out from under the slab — the trade calls it "sub-slab depressurization") breaks the buildup half of that equation by actively venting the gas outside before it can enter the house. For how mitigation actually works, seefixing radon.
What About Well Water
Well Water: A Real But Usually Small Second Source
For most homes, almost all the indoor radon comes from the soil directly under the house. But there's a second path worth being honest about: your water. Radon can dissolve in water. In certain kinds of bedrock wells, groundwater can carry it in meaningful amounts. When you use that water indoors — showering, running the dishwasher, running the washer — some of the dissolved radon comes out into the air.
This mostly matters for homes on private wells drawing from those specific rock types. Homes on public water usually get much less from the water side, because most municipal treatment releases the dissolved radon before it reaches your tap. Even in homes with waterborne radon, the soil-gas path usually dominates. Treating just the water won't fix a soil-gas problem, and treating just the soil-gas paths won't fix a real water problem. It's not either-or — but it's also not usually a 50/50 split.
What To Do Next
What Any of This Means for a Homeowner
None of the geology changes what you actually do next. The uranium in the bedrock, the cracks under your yard, the sinkhole limestone, the soil depth — all of it adds up to a single number in a single house. The only way to read that number is to measure it.
For the levels and what each tier means, seewhat is a safe radon level. For a metro-wide county and ZIP breakdown, seeAlabama radon levels by ZIP code. For what a real 48-hour continuous-monitor test looks like, seehow radon testing works. If a test comes back elevated, see fixing radon. And for the underlying chemistry, see what is radon gas.
Find out what your specific site produces — $295 flat.
FAQ
Frequently Asked Questions
Where does radon come from in a house?
Almost all indoor radon comes from the soil and rock directly under the house. A small share can come from well water (radon dissolved in groundwater, released when water is agitated indoors — showers, laundry) and, rarely, from certain natural-stone building materials. The soil-gas path dominates in almost every home.
Do all houses have radon?
Every house has some radon in it — the question is how much. The average U.S. outdoor level is about 0.4 pCi/L, and the average indoor level is about 1.3 pCi/L (EPA). Above 0 is normal. Above the EPA action level of 4.0 pCi/L is where the U.S. Surgeon General and EPA recommend the home be fixed. There is no zero, and there is no fully safe indoor level.
Why do next-door houses have different radon levels?
Radon depends on nine factors the Geological Survey of Alabama lists, and only a few of them are constant across a street. The uranium content of the bedrock at each specific footprint, the fracture pattern under it, the soil depth, the foundation type, the tightness of the slab, the interior HVAC pattern, and how heavily each house is closed up during heating season all vary house to house. ADPH puts that observation on the front page of its Radon in Alabama site: radon levels can vary greatly from home to home, even next door.
Why does Alabama specifically have radon issues?
Alabama's radon potential is driven by its geology. The Geological Survey of Alabama identifies altered carbonaceous rocks, granitic rocks, and coal-bearing shales as the state's higher-radon source units, and points to faulting and karst permeability as the mechanisms that move radon from those source rocks up into homes. Both features are present under the Birmingham metro, which is why Jefferson and Shelby appear on ADPH's list of Alabama's 15 highest-potential counties.
Is well water a significant source of radon?
For most homes on public water, no. Public water systems typically release most dissolved radon at treatment before it reaches the customer. For homes on private wells drawing from certain bedrock aquifers, waterborne radon can be a secondary source — released into indoor air when water is agitated (showers, dishwashers, washing machines). It is usually a much smaller contributor than soil gas, but when it is present, mitigation involves treating the water as well as the soil-gas entry paths.