Humans receive a significant portion (˃50%) of the total dose attributed to all the natural radiation sources from indoor radon (222Rn), thoron (220Rn), and their progeny. While progeny contributes an overwhelming part to the dose, in most surveys, only radon gas is measured because of the simplicity of measurement. Progeny concentration is usually estimated by multiplying gas concentration with an assumed factor, called the equilibrium factor, and taken from literature. Recently, results of the measurements of equilibrium factors for 222Rn and 220Rn were reported from various parts of the globe. In India, many such studies have been conducted in the current decade. The studies show a wide variation of equilibrium factors which suggests that they depend on environmental factors and measurement conditions. Therefore, they should be determined site specifically if accurate site-specific dose estimation is targeted. This paper summarizes concepts, definitions, and methods to determine equilibrium factors and reviews literature about reported equilibrium factors worldwide, focusing on data reported from India.

Chitkara University School of Engineering and Technology Chitkara University Solan Himachal Pradesh 174103 India

Czech Technical University Prague Faculty of Nuclear Sciences and Physical Engineering Brehova 7 Praha Czech Republic

Department of Physics H N B Garhwal University Badshahi Thaul Campus Tehri Garhwal 249 199 India

Retired from German Federal Office of Radiation Protection Berlin Germany

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