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What causes radon in a house?

By Radon411 Editorial TeamUpdated How we research

Short answer

Radon in a house comes from uranium breaking down in the soil and rock beneath it. The gas moves up through the ground, and because air pressure inside a home is usually lower than in the soil around the foundation, the house draws it in through cracks and gaps, where it can build up.

Next step: a test is the only way to know your home's level. How to test your home

EPA's Citizen's Guide says radon comes from the natural decay of uranium that is found in nearly all soils, and that it typically moves up through the ground to the air above and into your home through cracks and other holes in the foundation. Your home traps radon inside, where it can build up. The Consumer's Guide to Radon Reduction explains the pull: because of the difference in pressure, your home acts like a vacuum, drawing radon in through foundation cracks and other openings.

So three things together cause radon in a house, as Minnesota's Department of Health describes it: the radon levels in the soil (the source), how radon enters the home (the pathways), and the pressure differences between the outside air and the inside air that drive radon into the home. Well water and building materials can add some, but the Consumer's Guide says radon gas in soils is the principal source of elevated radon levels in homes in the United States.

What to do next: you cannot see or smell the cause, so test the lowest lived-in level of the home to learn the level, and see your county's on the radon map while you wait.

Where the radon comes from

Radon is made in the ground. Minnesota's Department of Health puts the chain simply: uranium breaks down to radium, and radium eventually decays into the gas radon. ATSDR adds that because radon is a noble gas, it releases from any chemical bonds that attach it, and it may travel far enough to reach groundwater or the air.

Where the radon in a house comes from
SourceWhat the agencies say
Soil and rock under the houseRadon gas in soils is the principal source of elevated radon levels in homes; radon from soil gas is the main cause of radon problems
Well waterSometimes radon enters the home through well water; high levels in the water supply are more likely when its source is ground water
Building materialsIn a small number of homes the building materials can give off radon, but they rarely cause radon problems by themselves
Outdoor airAbout 0.4 pCi/L of radon is normally found in the outside air

Why a house pulls radon in

A house is not sealed from the soil, and it usually runs at lower pressure than the ground around it. Minnesota's Department of Health says homes commonly operate at a lower (negative) pressure compared to the outside air, and that some of the replacement air comes from the soil. It names three things that drive those pressure changes.

The three pressure effects Minnesota's Department of Health says bring radon into a home
EffectWhat happens
Stack effectWarm air rises to the upper portion of the home and is lost to the outside air; make-up air enters the lower part of the home, and some of it comes from the soil
Down wind draft effectStrong winds can blow over the top of the home, pushing and pulling air into and out of the house
Vacuum effectAppliances (water heaters, fireplaces, clothes dryers, older furnaces) and exhaust fans remove air from the home, which can drive soil gas in as make-up air enters the lower part of the house

How radon gets in

Cross-section drawing of a house with a basement and a crawl space, numbered at the seven ways EPA lists for soil gas to get in: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, the water supply.
Read this figure as text
House in cross-section showing where soil gas entersGround levelSoil and rockBasementSolid floorCrawl spaceSuspended floor1234567
  1. Cracks in solid floors
  2. Construction joints
  3. Cracks in walls
  4. Gaps in suspended floors
  5. Gaps around service pipes
  6. Cavities inside walls
  7. The water supply
The ways radon enters a house, as EPA's Citizen's Guide lists them.

The Citizen's Guide lists the routes: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, and the water supply. CDC's page on how radon gets into your home adds the sump hole or opening. Minnesota's list of common pathways includes cracks in the concrete slab, floor-wall joints, an open sump pit, or a crawl space. More detail is in how does radon get into a house.

Why one house has more radon than the next

WHO says the radon concentration in a building depends on the local geology, for example the uranium content and permeability of the underlying rocks and soils; the routes available for radon to pass from the soil into the building; the radon exhalation from building materials; and the rate of exchange between indoor and outdoor air. The National Cancer Institute says radon levels can be higher in homes that are well insulated, tightly sealed, and/or built on soil rich in the elements uranium, thorium, and radium.

Questions about what causes radon

What would cause radon in a house?

Uranium in the soil and rock under the house breaks down into radon, and the house's lower air pressure draws the gas in through cracks and gaps in the foundation, the Citizen's Guide and the Consumer's Guide explain.

What causes radon gas in your house to build up?

The house traps it. The Citizen's Guide says your home traps radon inside, where it can build up; outdoors, WHO says, radon quickly dilutes to very low concentrations.

Can radon come from well water?

Yes, sometimes. The Citizen's Guide says radon can enter through well water, and that radon entering through the soil is usually a much larger risk than radon entering through water. See does radon come from well water.

Can building materials cause radon in a house?

In a small number of homes, the Consumer's Guide says, building materials such as granite and certain concrete products can give off radon, although they rarely cause radon problems by themselves. See radon in granite.

Why is radon higher in the basement?

Because the basement is in contact with the ground. WHO says radon levels are usually higher in basements, cellars and living spaces in contact with the ground.

Can a professional find what is causing radon in my house?

A mitigation contractor can trace air movement. The Consumer's Guide says a contractor can use chemical smoke to find the source and direction of air movement, and that the sources of air flow show possible radon routes.

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