What is Radon?

Contact: Aaron Berndt | 517-327-2618
Agency: Environmental Quality

Ra · don ( ra´don ) n. Symbol Rn A colorless, radioactive, inert gaseous element formed by disintegration of radium. Atomic number 86; atomic weight 222; melting point -71oC; boiling point -61.8oC; specific gravity (solid) 4; valence 0; half-life 3.823 days. 

Radon is a naturally occurring radioactive gas.  Radon is tasteless, odorless, and colorless. It comes from the radioactive decay (breakdown) of radium, which comes from the radioactive decay of uranium.  Both radium and uranium are found in at least trace amounts in almost any kind of soil or rock. Granites, shales, and phosphates have higher than average concentrations of uranium.  As a result, they may produce higher concentrations of radon. However, elevated radon levels can occur even in areas with low concentrations of uranium in the soil or rocks. 

Radon is not chemically reactive with most materials.  Hence, it will move freely as a gas through the ground.  The first four radioactive isotopes formed as radium decays are polonium-218, lead-214, bismuth-214 and polonium-214.  They are commonly referred to as "radon daughters" or "radon progeny."  These short-lived isotopes are not gases but are chemically active solids.  They are present in any environment where radium was once found and, like radon, cannot be detected by human senses.

The earth is the source of all radon gas in our atmosphere.  Since radium and uranium concentrations vary throughout the earth's crust, radon concentrations will also vary in a geographic area.  The amount of radon gas that escapes into the atmosphere is dependent on the depth at which it is formed and the permeability of the surrounding earth.  Radon formed in the top 10 meters of soil and rock provides the largest component of radon entering the atmosphere.  Because they are metallic particulates, radon daughters formed in the soil will not escape.

The second most important contributor to atmospheric radon is from groundwater.  Underground radon is carried in groundwater and when this groundwater surfaces most of the radon is released to the atmosphere.

Other sources of atmospheric radon are very small contributors and are largely due to human activities.  For example, there are radium-rich industrial by-products spread upon the earth, and sometimes construction materials are produced from raw materials that contain uranium or radium. 

Worldwide, over 2.5 billion curies of radon are emitted annually into the atmosphere.  Atmospheric dilution results in a typical outdoor level of between 0.2 and 0.7 picocuries per liter (pCi/l).  However, wide variations do exist. 

The above information was taken from the Introduction to Indoor Radon in Michigan:  Residential Survey Summary Report, EQC 1625 (8/97).  To request a copy of this report, please call the radon hotline at 800-RADON GAS (800-723-6642).