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Where is radon found?

By Radon411 Editorial TeamUpdated How we research

Short answer

Radon is found in rock, soil and groundwater across the country, because it forms wherever uranium in the ground breaks down. It is present at low levels outdoors and can build up to much higher levels inside buildings, where it collects instead of dispersing.

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

Radon escapes from the ground into the air. Outdoors it quickly dilutes to very low concentrations; it is higher indoors and wherever ventilation is minimal, with the highest levels in places like mines, caves and water treatment facilities. For most people the greatest exposure happens at home, where they spend much of their time.

Inside a building, levels are usually higher in basements, cellars and living spaces in contact with the ground. Radon is found all over the United States, and any home can have a radon problem: new and old homes, well-sealed and drafty homes, and homes with or without basements.

Radon sometimes enters a home through well water, and most of the risk from radon in water comes from radon released into the air during showering and other household use. Radon entering through the soil is usually a much larger risk than radon entering through water.

What to do next: look up your county on the radon map to see its , keeping in mind that a prediction for an area is not a reading for any building. The only way to know the level in your home is to test it, starting in the lowest lived-in level.

In the ground, in water and in the air

Radon is found wherever there is uranium, and uranium is found in all rocks and soils. From there the gas spreads into water and air, so radon turns up in places as different as a well, a cave and a living room. It is present in nearly all air, and everyone breathes some every day, usually at very low levels.

Where radon is found, and how much
PlaceWhat to know
Rock and soilMade there by the decay of uranium; found in bedrock, soil and groundwater
Groundwater and wellsHigh levels in a water supply are more likely when it comes from ground water, such as a private well
Outdoor airAbout 0.4 pCi/L on average in the U.S., by EPA's estimate, and 5 to 15 Bq/m3 worldwide; generally not a problem, because it quickly dilutes
Mines, caves and water treatment facilitiesWhere the highest levels are found, because ventilation is minimal
Homes, schools and officesNormally higher indoors than outdoors; about 1.3 pCi/L on average in U.S. homes, by EPA's estimate

Elevated radon also turns up where uranium or thorium is concentrated, which can include almost any mining or milling operation handling metals or phosphates. How uranium becomes radon is explained in where does radon come from.

Where radon is found in the United States

Everywhere, in some amount. Radon is found all over the country, and high levels have been found in every state. Nearly 1 in 15 U.S. homes is estimated to have elevated radon, by EPA's estimate. Problems are more common in some areas, but any home can have one.

Bar chart of how many U.S. counties are in EPA radon Zone 1, Zone 2 and Zone 3, and how many have no zone on file, from the EPA Map of Radon Zones.
Read this figure as text

Predicted potential

Counties per EPA radon zone
ZoneBarCountiesShare of all counties
Zone 1Highest predicted potential1,07133.1%
Zone 2Moderate predicted potential1,03432.0%
Zone 3Lower predicted potential1,03332.0%
No zoneNo EPA zone on file932.9%

3,231 counties in all.

An EPA radon zone describes predicted potential across a whole county. It is not intended to determine whether any individual home should be tested, and homes with elevated levels occur in all three zones. EPA recommends testing every home regardless of zone, and testing is the only way to know the radon level in a specific building.

Counties in each EPA radon zone: a prediction of potential for an area, not a measurement of any building.
What the three radon zones predict
ZonePredicted potential
Zone 1Highest potential; average indoor radon levels may be greater than 4 pCi/L
Zone 2Moderate potential; average indoor radon levels may be between 2 and 4 pCi/L
Zone 3Low potential; average indoor radon levels may be less than 2 pCi/L

States differ a great deal. Minnesota's health department, for example, estimates that the state's average radon level is more than three times the U.S. level and that more than two in five Minnesota homes have levels posing a significant health risk. Your own state's program is listed under state radon programs.

In which buildings radon is found

Any kind. Any building can have high levels of radon: new and old homes, well-sealed and drafty homes, office buildings and schools, and homes with or without basements. Schools and workplaces can have radon problems as well as homes.

Radon in buildings, by the numbers
MeasureValueBasis
Range in homes, schools and officesFrom 10 Bq/m3 to more than 10,000 Bq/m3WHO's range
U.S. average indoor levelAbout 1.3 pCi/LEPA's estimate
U.S. homes with elevated radonNearly 1 in 15EPA's estimate

Where you spend the most time matters most. For most people the greatest exposure happens at home, though indoor workplaces can add to it. Two buildings side by side can differ considerably, so a neighbor's result is not a good guide to yours.

Where radon is found inside a home

Closest to the ground, most often. Radon levels are usually higher in basements, cellars and living spaces in contact with the ground, and basements and first floors typically have the highest levels. Upper floors are not exempt: considerable radon can also be found above the ground floor.

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
Where soil gas gets into a house.

That is why a test goes in the lowest lived-in level: the basement if it is used often, otherwise the first floor. Well water is the other way in, though radon from the soil is usually a much larger risk. More detail is in where is radon found in homes.

Questions about where radon is found

Where is radon most commonly found?

In soil and rock, and indoors on the lowest levels of buildings that sit on them: basements and first floors. Across the country, the zone map shows where high indoor levels are more likely, but elevated homes have been found in all three zones.

Is radon found in every state?

Yes. High radon levels have been found in every state.

Where can radon be found in nature?

In rocks, soil and groundwater, and at low levels in outdoor air. The highest levels occur where ventilation is minimal, such as mines, caves and water treatment facilities.

Is radon found in apartments and upper floors?

It can be. Levels are usually highest near the ground, but considerable radon can also be found above the ground floor, and testing is recommended for all homes below the third floor.

Is radon found in drinking water?

In some water, mainly ground water. High levels are more likely in supplies drawn from ground water, and most public water comes from surface water. Most of the risk comes from radon released into the air when water is used. See does radon come from well water.

Where is radon found on the periodic table?

Among the noble gases: radon is a naturally occurring, radioactive noble gas. The radon element guide covers its place on the periodic table.

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