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Radon comes from the soil, rock, and water around us. Why do some houses have high levels of indoor radon while nearby houses do not? The reasons lie primarily in the geology of radon - the factors that govern the occurrence of uranium, the formation of radon, and the movement of radon, soil gas, and ground water.

The geology of the state of North Carolina suggests certain areas of the state in the Piedmont and Blue Ridge Mountain regions could have elevated indoor radon levels. A State Geological Society survey map of North Carolina identifies areas that contain gneiss, schist and granite rocks underlying the soils.  These are the rock types which contain higher concentrations of uranium and radium, the parent radionuclides of radon gas.

Radon gas (Chemical symbol Rn-222) is a result of the decay of Radium-226 (Ra-226) in soil. Radon gas can travel some distance through the soil because of its 3.8 day half-life.

As part of Congressionally mandated activities under the Indoor Radon Abatement Act, the US Environmental Protection Agency (EPA) with assistance from the US Geological Survey (USGS) has produced a map of Radon Zones for the State of North Carolina.

This map is designed to represent the average county-by-county potential for radon in air for the entire state of North Carolina. Because the map provides only county wide predictions requiring a number of assumptions about the geology and living habits of county residents among others it cannot be used to predict the radon level in any individual home or on any individual tract of land. The only way to actually determine home radon levels is to perform a test.

The most important characteristics of soil that determine how much radon is emitted and how far it can travel are the soil's radium content, the moisture in the soil and how easily the gas can travel through the soil (permeability). The moisture in the soil and permeability depend on the age and composition of the soil. EPA and USGS included all of these characteristics in their production of the Map of Radon Zones, along with survey data on uranium in soil, house architecture information and actual radon measurements. Based on the results from the Map of Radon Zones, there are eight North Carolina counties that qualify as Zone One counties. These counties are projected to have average radon in air levels greater than 4 pCi/L on average. They are:

The state of North Carolina can be divided into 3 broad regions, the Coast or Coastal Plain, the Piedmont and the Mountain Regions as shown in the map below:

The Coastal region includes those counties lying in the Coastal Plain. The Piedmont region can be further divided into sub-regions based on geological formations. Piedmont One is the region closest to the coast, including the Raleigh and Carolina belt formations. Piedmont Two is the next region west and includes the Charlotte and Milton Belts. Finally, the Piedmont Three region lies next to the Mountain region and includes the Inner Piedmont, Sauratown, and King's Mountain Belt formations. The Mountain region includes the Blue Ridge Belt formation among others.

The EPA Map of Radon Zones shows the general trend of radon levels in the state. The levels tend to be lower in the Coastal Plain, then increase in the middle area of the state - the Piedmont, and increase more in the mountains. There are exceptions to this general such as Rockingham County, a Zone One county amidst generally Zone Two counties. Other exceptions are Wake County, Warren County and Franklin County, which are Zone Two counties with Zone Three counties nearby. These exceptions are often due to the different types of rock formations and geological features in the area.

More information about the Geology of Radon

For information on natural radioactivity in the environment from nuclides such as uranium or Potassium-40 (Chemical symbol K-40) for the United States, see the USGS website.

If you have additional questions about this information, contact Dale Dusenbury, NC Environmental Radiation Specialist.