Page 333 - gas transport in porous media

CHAPTER 20

RADON TRANSPORT

BILL W. ARNOLD
Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185, USA

20.1 INTRODUCTION

Radon is a naturally occurring, chemically inert, radioactive gas that exists in soil and
rock of all geological environments. Several radioisotopes of radon occur naturally,
but the most environmentally significant is 222 Rn (radon), which has a half-life of
3.8 days. This value of the half life and the gaseous nature of 222 Rn allows relatively
rapid migration over considerable distances in both soils and the atmosphere before
decay.
The radioisotope 222 Rn occurs in the decay chain of 238 U (uranium), with 226 Ra
(radium) as its immediate parent. The relatively long half life of 226 Ra (1600 years)
means that radon will be produced at a fairly constant rate by material containing
226 222
Ra, over time periods relevant to environmental concerns. Rn decays by alpha
particle emission to 218 Po (polonium), and to other solid progeny, including 214 Po
and 214 Pb (lead).
The alpha emitting progeny of radon are the primary agents with direct impact on
human health. Although alpha particles do not penetrate deeply into tissue, radon
progeny that have been inhaled into the lungs are in close contact with lung tissue
and substantial radiation dose is deposited by the decay of these progeny. Exposure
to high concentrations of radon gas is known to cause lung cancer in underground
uranium miners (Chaffey and Bowie, 1994). In addition, chronic exposure to more
moderate concentrations of radon in dwellings may be a concern as a significant cause
of lung cancer in the general population (Lubin and Boice, 1997).
Exposure to radon gas is ubiquitous, both indoors and outdoors. Radon present
in dwellings and other structures is largely the result of natural contamination from
soils over which the building is constructed. Indoor concentrations of radon gas are
generally greater than outdoor concentrations because of limited air exchange and
lack of dilution by mixing with uncontaminated air from higher in the atmosphere.
Concentrations of radon gas within homes vary significantly, depending on radon

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© 2006 Springer.