What is Radon?

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 radioactive gas that arises from the natural radioactive decay of radium, which is a natural decay product of uranium.  Scientifically, "radon" is known to be radon-222, the most abundant isotope of the element radon.  The terms radon and radon-222 are often used interchangeably when referring to the indoor radon issue.

As a noble gas, radon is colorless, odorless and chemically inert and cannot be detected by human senses.  Also, since radon is not chemically reactive with most materials, it will move freely as a gas.  Radon has a radiological half-life of 3.8 days, and can move substantial distances from its point of origin.

The first four radioactive isotopes formed as radon 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 radon is found and, like radon, cannot be detected by human senses.

The earth is the source of all radon gas in our atmosphere.  Uranium is a natural part of the earth's crust.  Therefore, radium and radon are also naturally present (as noted above).  Since uranium and radium concentrations vary throughout the earth's crust, radon concentrations will also vary.  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 emanation from groundwater.  Underground radon is readily 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, call the DEQ Indoor Radon Program at 800-RADON GAS/800-723-6642, or 517-327-2618.) 

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