Radon in a house comes primarily from the soil and rock beneath the foundation. It’s a naturally occurring radioactive gas produced when uranium in the ground decays, and it seeps upward through the earth and into your home through cracks, gaps, and openings in the lowest level of the structure. This single source, soil gas, accounts for the vast majority of indoor radon. It’s responsible for an estimated 21,000 lung cancer deaths in the United States every year.
How Radon Forms Underground
Uranium exists naturally in most soils and rock formations, though concentrations vary widely by location. As uranium slowly breaks down over time, it produces radium, which in turn decays into radon gas. Because radon is a gas rather than a solid, it migrates through tiny air spaces in soil and gravel and works its way toward the surface. Outdoors, it disperses harmlessly into the atmosphere. The problem starts when a building sits on top of that soil and traps the gas indoors.
Why Your House Pulls Radon In
Your home doesn’t just passively collect radon. It actively draws it from the ground. Air pressure inside a house is typically lower than the pressure in the surrounding soil, a difference created largely by indoor-outdoor temperature gaps. Warm air rising inside the house (sometimes called the “stack effect”) creates a slight vacuum at the lowest levels. That vacuum pulls soil gases, including radon, in through any available opening in the foundation.
Anything that increases this pressure difference makes the problem worse. Running exhaust fans, clothes dryers, fireplaces, or furnaces all push air out of the house, which strengthens the suction pulling soil gas in from below. The effect is strongest during cold months when heating systems run continuously and windows stay shut.
Specific Entry Points in the Foundation
Radon doesn’t need large openings. It enters through surprisingly small pathways, including:
- Cracks in concrete floor slabs, even hairline cracks invisible to the eye
- Floor-to-wall joints, where the slab meets the foundation wall
- Construction joints and control joints deliberately built into the concrete
- Gaps around service pipes (plumbing, electrical, gas lines) that penetrate the slab or walls
- Sump pump pits, which create a direct opening to the soil below
- Cavities inside block walls, where hollow concrete blocks allow gas to travel upward through the wall itself
- Gaps in suspended floors over crawl spaces
No foundation is perfectly sealed. Even a newly built home on a poured concrete slab has joints and penetrations that provide entry routes.
The Role of Geology
Where you live has a major influence on how much radon is available in the soil beneath your house. Certain rock types are associated with significantly higher radon potential. According to USGS research, the highest-risk geologies include granite and other uranium-bearing igneous rocks, black shales from the Carboniferous period, soils formed over carbonate (limestone) bedrock especially in areas with cave systems, and glacial deposits derived from uranium-rich source material.
In regions that were never glaciated, indoor radon levels tend to correlate directly with the bedrock underneath. In glaciated areas like the northern United States, the picture is more complicated: the type, thickness, and permeability of glacial deposits left behind by ice sheets matter more than what’s in the bedrock below them. A home built on loose, gravelly glacial soil may have high radon simply because the soil is permeable enough for gas to travel easily.
Building Materials as a Minor Source
Concrete, brick, natural stone, granite countertops, and gypsum all contain trace amounts of naturally occurring radioactive elements, and they can emit low levels of radon. The CDC notes, however, that this contribution is small compared to soil gas entering through the foundation. If your home has elevated radon, the materials in your walls and floors are almost certainly not the primary cause.
Well Water Can Release Radon Indoors
Homes on private wells have an additional source. Groundwater flowing through uranium-bearing rock dissolves radon, and when that water comes out of your showerhead or faucet, the gas escapes into the air. The general ratio is roughly 10,000 to 1: for every 10,000 picocuries per liter (pCi/L) of radon in your water, normal household use adds about 1 pCi/L to the air inside the home. That means your well water would need to contain around 40,000 pCi/L of radon to push indoor air levels to the EPA’s action threshold of 4 pCi/L on its own.
For most homes, well water is a secondary contributor. But in rare cases, particularly in areas with very high waterborne radon, showers and running taps can be a meaningful source. Municipal water systems are not a concern because radon dissipates during treatment and storage before it reaches your tap.
Why Radon Levels Spike in Winter
Seasonal variation in indoor radon is well documented. Winter concentrations can be two to five times higher than summer levels. Measurements in Romanian apartment buildings found winter readings of 70 to 168 Bq/m³ compared to 17 to 30 Bq/m³ in summer. A Greek study of 25 homes showed winter averages of 137.5 Bq/m³ versus 96.1 Bq/m³ in summer.
Several factors converge during cold months. Windows and doors stay closed, dramatically reducing ventilation. Heating systems increase the upward movement of warm air, strengthening the pressure differential that pulls soil gas in. And energy efficiency measures, such as weatherstripping and insulation upgrades, can further seal the building envelope. The same dynamic can occur during extreme summer heat when homes are sealed for air conditioning, though winter remains the peak season in most climates.
How Radon Is Removed From a Home
The most common and effective fix is called active soil depressurization. A contractor drills a small hole through the basement floor slab, inserts a pipe connected to a fan, and vents the pipe to the outside of the house. The fan runs continuously, creating a low-pressure zone beneath the slab that intercepts radon before it enters the living space and expels it outdoors where it disperses harmlessly.
This system works by flipping the pressure relationship between your house and the soil. Instead of the house pulling gas in, the fan pulls it away. Some newer homes are built with a passive version of this system (a pipe without a fan that relies on natural airflow), which can be upgraded later by adding a fan if radon levels are still too high.
The EPA recommends taking action if your home tests at or above 4 pCi/L. The World Health Organization sets a more conservative reference level of 2.7 pCi/L where feasible. Testing is the only way to know your level, since radon is odorless and invisible, and two houses on the same street can have very different readings depending on their foundation conditions and the soil directly beneath them.