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Where is radon most common?

By Radon411 Editorial TeamUpdated How we research

Short answer

Radon is most common where the soil and rock carry more uranium, the areas EPA's Map of Radon Zones marks as highest potential, and indoors it is usually highest in basements and ground-contact rooms. Elevated levels have been found in every state and in all three zones, so any home can have a problem.

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

Where radon is common depends on the ground and on the building. Radon comes from the natural decay of uranium found in nearly all soils, and the uranium content and permeability of the rocks and soils under a building drive how much reaches it. EPA's Map of Radon Zones sorts areas by , but it is a prediction for an area, and it is not meant to decide whether an individual home should be tested.

Indoors, radon is usually highest in basements, cellars and living spaces in contact with the ground. Outdoors it quickly dilutes to very low levels and is generally not a problem, while places with little ventilation, such as mines and caves, have the highest levels of all.

No area or floor gives an all-clear. High radon levels have been found in every state, homes next door to each other can have different levels, and considerable radon can be found above the ground floor.

What to do next: look up your county on the radon map to see its predicted potential, then test your own home in the lowest lived-in level.

Where radon is most common, from the ground up

Radon comes from the natural decay of uranium found in nearly all soils. Where it is most common depends on the scale you look at: the region and its geology, the type of place, and the part of a building.

Where radon concentrations run highest
ScaleWhere radon is most commonWhy
Kinds of placeMines, caves and water treatment facilities have the highest levelsRadon builds up where there is little ventilation
BuildingsIndoors more than outdoorsOutdoors radon quickly dilutes to very low levels; indoors it is trapped and builds up
Within a buildingBasements, cellars and living spaces in contact with the groundThey are closest to the soil the gas comes from
WaterGroundwater: wells, springs and boreholesThese normally carry more radon than surface water from reservoirs, rivers or lakes
RegionsAreas whose rocks and soils carry more uranium and let gas move easilyThe uranium content and permeability of the ground drive how much radon reaches a building

No scale gives an all-clear. High radon levels have been found in every state, and radon levels can vary greatly from one home to the next, even next door.

Regions with the greatest potential

EPA's Map of Radon Zones sorts areas of the country by their potential for elevated indoor radon: , the highest potential, where average indoor levels may be greater than 4 ; Zone 2, moderate, between 2 and 4; and Zone 3, low, below 2. It is a prediction for an area, not a reading of any building.

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.

How many counties fall in each EPA radon zone, from the zone data this site holds. A prediction of potential, not a measurement.

What makes some ground produce more radon

Radon in a building depends first on the ground under it: the uranium content and permeability of the underlying rocks and soils. Levels can run higher in homes built on soil rich in uranium, thorium and radium, and in a few areas radon dissolves into groundwater and is released into the air when the water is used.

What sets the radon level in a building
FactorWhat it does
Local geologyThe uranium content and permeability of the rocks and soils under the building
Routes into the buildingCracks in floors and at floor-wall junctions, gaps around pipes or cables, small pores in hollow-block walls, cavity walls, sumps and drains
Building materialsSome radon comes from the materials themselves, though they rarely cause radon problems on their own
Air exchangeHow much indoor air trades with outdoor air, set by construction, ventilation habits and air-tightness

Where radon is most common inside a home

Inside a house, radon is usually highest on the lowest levels: basements, cellars and living spaces in contact with the ground, because they are closest to it. But considerable radon can be found above the ground floor too, so an upper floor is not automatically low.

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 routes soil gas takes into a house; each one is at or near the lowest level.

That is why a test goes in the lowest lived-in level: the basement if it is used regularly, otherwise the first floor. See where a radon test should be placed and where radon is found in homes.

How common elevated radon is in U.S. homes

Nearly 1 out of every 15 homes in the U.S. is estimated to have an elevated radon level, EPA says. The national average indoors is about 1.3 pCi/L, against about 0.4 pCi/L in outdoor air.

Radon levels for scale
MeasurepCi/LBq/m3
National average, outdoor air0.414.8
National average, indoor air1.348.1
EPA's action level4148

Measured results by county, where states report them, are on each county's radon report, shown apart from the predicted zone so the two are never confused.

Questions about where radon is found

Where is radon most commonly found?

In the air of buildings, especially their lowest floors, and in places with little ventilation such as mines and caves. Outdoors it quickly dilutes to very low levels.

Which part of the country has the most radon?

EPA's Zone 1 areas have the highest predicted potential, but high radon levels have been found in every state. The radon map shows the zone for every county.

Is radon more common in basements?

Yes. Levels are usually higher in basements, cellars and living spaces in contact with the ground, though considerable radon can also be found above the ground floor.

Is radon more common in well water than city water?

Groundwater, including private wells, normally has more radon than surface water from reservoirs, rivers or lakes. Air from the soil is still usually the much larger source in a home. See does radon come from well water.

Can a home in a low-radon area have high radon?

Yes. Homes with elevated levels have been found in all three EPA zones, and levels vary greatly from home to home.

Where is the most radon in the world?

Radon is highest in places like mines, caves and water treatment facilities. In homes, schools and offices it can range from 10 Bq/m3 to more than 10,000 Bq/m3, and levels vary considerably even between neighboring buildings.

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