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There are three naturally occurring radon (Rn) isotopes: 219Rn (actinon), 220Rn (thoron) and 222Rn, which is commonly called radon. Radon-222 is a natural radioactive gas produced by the radioactive decay of radium (226Ra), which in turn is derived from the radioactive decay of uranium. Uranium is found in small quantities in all soils and rocks, although the amount varies from place to place. 222Rn (radon) occurs in the uranium-238 decay series, has a half-life of 3. 82 days and provides about 50% of the total radiation dose to the average person. Radon concentrations1 in outdoor air are generally low (4 to 8 Bq m-3) whilst radon in indoor air ranges from less that 20 Bq m-3 to about 110, 000 Bq m-3 with a population-weighted world average of 39 Bq m-3. Country averages range from 9 in Egypt, 20 Bq m-3 in the UK, 46 Bq m-3 in the US, 108 Bq m-3 in Sweden and 140 Bq m-3 in the Czech Republic (1). Radon in soil air (the air that occupies the pores in soil) commonly varies from 5 to 50 Bq L-1 but may be <1 or more than 2500 Bq L-1. The amount of radon dissolved in ground water ranges from about 3 to nearly 80, 000 Bq L-1. synopsis describes the kinds of rocks and unconsolidated deposits that radon is associated with; how radon moves through the ground and into buildings; the associated health risks, and how to produce radon hazard maps. It is an updated summary of (2).
J.D. Appleton (Thu,) studied this question.