model takes into account the exposure conditions in homes and in mines as well as the relevant physiological characteristics of the population groups. The ratio of the dose of alpha energy per unit exposure for a particular population group (men, women, children, infant) as given by the radon concentration to the dose per unit radon concentration to the miners is given by K:

K = [Dosehome/Exposurehome]/[Dosemine/Exposuremine] (1)

The K-factor includes diverse environmental and physiological factors and the use of this double ratio greatly simplifies the risk assessment for indoor radon. This chapter addresses dosimetry of radon progeny in the lung and presents the calculated K-factor values by reviewing the information available on exposure conditions in homes and mines, presenting the dosimetric model used and then presenting the resulting distributions of K-factor values that were calculated.

EXPOSURE

Introduction

The formation and decay sequence for 222Rn was shown in Figure 1-1. Because 222Rn has an almost 4-day half-life, it has time to penetrate through the soil and building materials into the indoor environment where it decays into its progeny. There is some recent evidence that in spite of its short half-life, 55 seconds, 220Rn can also penetrate into structures in significant amounts. However, the data are limited and the extent of the thoron problem is quite uncertain as discussed in a subsequent section of this chapter.

The short-lived decay products, 218Po (Radium-A), 214Pb (Radium-B), 214Bi (Radium-C), and 214Po (Radium-C'), represent a rapid sequence of decays that result in two α-decays, two β-decays and several γ-emissions following the decay. To illustrate the behavior of the activity of the radioactive products of the radon decay, the activity of each of the short-lived isotopes is plotted as a function of time for initially pure 222Rn in Figure B-1. Because 222Rn has a longer half-life than either of the four short-lived products, the progeny reach the same activity (number of decays per unit time) as the radon. The mixture then decays with the 3.8 day half-life of the radon. Each 222Rn decay results in four progeny decays so that the total activity is then the sum of these individual decay-product concentrations. The activity is the product of the decay constant (In 2/half-life) times the number of radioactive atoms. Thus, the short-lived 218Po can have an activity equal to the 222Rn because a large decay constant times a small number of atoms becomes equal to a small decay constant times a large number of atoms.

If the products formed by the decay of the radon were to remain in the air, then there would also be equal activity concentrations of 218Po, 214Pb, and other progeny. The resulting mixture is said to be in secular equilibrium. As used in the monitoring of uranium mines, an equilibrium mixture of these decay products



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