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Where does radon come from?

By Radon411 Editorial TeamUpdated How we research

Short answer

Radon comes from uranium, which is present naturally in nearly all rock, soil and water and decays into radium and then into radon gas. Because radon is a gas, it escapes the ground into the air, where it dilutes outdoors but can build up inside buildings that sit on the soil.

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

Radon comes from the natural decay of uranium and radium found in nearly all rocks and soils, and it is also found in ground water. In the United States, radon gas in soils is the principal source of elevated radon levels in homes. Well water adds radon in some homes, and building materials rarely cause radon problems by themselves.

How much radon the ground releases depends on its geology: the uranium content of the rocks and soils underneath, and how easily gas moves through them. Rocks that contain uranium include granite, shale, phosphate and pitchblende. Outdoors the gas quickly dilutes to very low levels; indoors, a house traps it, and it can build up. Even next-door neighbors can have very different levels.

What to do next: the source is under every home, so the level in yours can only be learned by testing it. Test the lowest lived-in level, and look up your county's predicted radon potential on the map for context while you wait.

Radon starts as uranium in the ground

Radon is made underground, as one step in a long chain of radioactive decay. Uranium occurs naturally in virtually all soil, rock and water. Over time it decays to radium, and radium decays to release radon gas. More than 99 percent of natural uranium is uranium-238, which has a half-life of 4.47 billion years, so the supply in the ground does not run out on any human timescale.

Decay chain from uranium-238 through uranium-234, thorium-230 and radium-226 to radon-222, radon's decay products and stable lead-206, with radium-226's half-life of 1600 years and radon-222's of 3.823 days.
Read this figure as text

  1. Uranium-238

    Solid

  2. Uranium-234

    Solid

  3. Thorium-230

    Solid

  4. Radium-226

    Solid

    Half-life 1600 years

  5. Radon-222

    Gas

    Half-life 3.823 days

  6. Radon's decay products

  7. Lead-206

    Solid

  • Uranium-238. More than 99 percent of the uranium found in the environment is in the form of U-238. Uranium decays by alpha particles.
  • Uranium-234. Less than one percent of natural uranium, but much more radioactive.
  • Thorium-230. An intermediate EPA names in the uranium-238 chain.
  • Radium-226. Radium decays to produce radon gas.
  • Radon-222. The most common radon isotope, and an alpha emitter. As a gas it can move up through the ground and into a building.
  • Radon's decay products. Radon gas decays into radioactive particles that can get trapped in your lungs when you breathe.
  • Lead-206. Where the chain ends. Only the final, stable atom in the chain is not radioactive.

EPA names uranium-234, thorium-230, radium-226 and radon-222 as intermediates "such as" these, so the chain has more steps than are drawn. Half-lives are shown only where a cited source states one.

The uranium-238 decay chain: uranium-234, thorium-230, radium-226 and radon-222 form along the way, and the chain ends at stable lead-206.

The chain that begins with uranium-238 passes through uranium-234, thorium-230, radium-226 and radon-222 before it ends in lead-206. Every product along the way is radioactive; only the last, stable atom is not. Radon forms in three such chains, which begin with uranium or thorium, and radium and then radon appear midway through them.

Radon itself is short-lived, with a half-life of 3.8 days. It quickly decays into short-lived radioactive atoms called radon progeny. So the radon in a house is always fresh: it is made in the ground continuously and keeps moving up out of it.

Why radon leaves the rock and the soil

Most radioactive elements stay locked in the minerals that hold them. Radon does not, because it is a noble gas: it releases from any chemical bonds that attach it, and it can travel far enough to reach groundwater or the open air. Soil is porous, so the gas works its way up through it and into anything built on top.

Where radon goes once it forms
Where it ends upWhat happens there
Outdoor airFound at very low levels; outdoors it quickly dilutes and is generally not a problem
GroundwaterRadon released from rock and soil moves into groundwater and surface water as well as air
The air in buildingsIt can accumulate indoors and, over time, become a serious health hazard
Decay productsIt quickly decays into short-lived radioactive atoms, the radon progeny

Natural deposits of uranium and radium, not man-made sources, produce most of the radon in the air. Radon is in the soil and air everywhere, in varying amounts.

Which rocks and soils give off more radon

Uranium is nearly everywhere, but not in equal amounts. Rocks known to carry it include granite, shale, phosphate and pitchblende. Elevated radon also turns up where uranium or thorium is concentrated, which can include almost any mining or milling operation that handles metals or phosphates.

What makes the ground under a building a stronger or weaker radon source
FactorWhy it matters
Uranium in the rock and soilMore uranium, thorium and radium in the ground means more radon produced beneath the building
How easily gas moves through the groundPermeable soil lets radon migrate toward and into the home
Uranium near the surfaceUranium is present in the soil nearly everywhere in the United States
Local geology and waterIn some areas radon dissolves into ground water and is released into the air when the water is used

Radon from groundwater and wells

Water picks up radon on its way through rock. High radon in a water supply is more likely when the supply draws on ground water, such as a private well or a public system fed by wells. Most public water comes from surface water, from lakes, rivers and reservoirs, and radon in water is not usually a problem when its source is surface water.

Soil and water as sources of radon in a home
SourceHow the radon reaches the air you breatheHow big a share
Soil gas under the houseMoves up through the ground and in through openings in the foundationThe main cause of radon problems
Well water or a ground-water supplyReleased into the air when water is used, especially for cooking or showeringUsually a much smaller risk than radon from the soil

Soil and the water supply are the two main sources of radon in indoor air, and the air is the one to test first. If the air is high and the water comes from a well, test the water too: see does radon come from well water.

Building materials and granite countertops

Stone and masonry come from the same crust as the soil, so some carry a little uranium too. Granite, brick and marble contain natural radioactivity; brick contains uranium and thorium; and fly ash, a waste from burning coal, is added to certain types of concrete. When that radioactivity decays it releases radon. The radiation released from these materials is typically very, very low.

Building materials as a radon source
MaterialWhat is known
Granite, including countertopsAny sample can hold varying amounts of uranium, but the data do not show that countertop granites are significantly increasing indoor radon levels
BrickContains uranium and thorium, at low levels
ConcreteSome additives such as fly ash may be naturally radioactive, but even with them the radiation levels in concrete are very low
Building materials in generalThey rarely cause radon problems by themselves

The principal source of radon in homes is the soil in contact with basement floors and walls, and indoor radon is a more common and far larger health risk than radiation from building materials. There is no generally accepted radiation test for countertops, so the test that matters is a radon test of the air. The radon in granite guide covers countertops in depth.

Questions about where radon comes from

What is the main source of radon in houses in the United States?

The soil. Radon gas in soils is the principal source of elevated radon levels in U.S. homes; well water and building materials add far less.

How is radon formed?

By radioactive decay. Uranium in rock and soil decays to radium, and radium decays to release radon gas. The chain then continues through radon's short-lived decay products to stable lead.

Does radon come from granite?

Granite can give off some radon, because any sample may hold varying amounts of uranium. The levels from natural rocks such as granite are not typically high, and the main source of radon in homes is the soil under them.

What type of soil has the most radon?

Soil with more uranium, thorium and radium in it, and soil that lets gas move easily. No soil type rules radon out: elevated levels have been found in every state.

Does radon come from outside air?

Outdoor air holds a little, about 0.4 pCi/L on average in the U.S. against about 1.3 pCi/L indoors, by EPA's estimate. Outdoors it dilutes quickly; indoors it builds up because the house traps it.

Where did the name radon come from?

From radium. Friedrich Ernst Dorn discovered the gas in 1900 while studying radium's decay chain. It was first named niton, from the Latin nitens, shining, and has been called radon since 1923.

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