Simulation of time-related changes in dose conversion factor for indoor radon progeny

The inhalation of airborne short-lived radon progeny contributes to the most important natural radiation exposure at many working places and in homes. For the estimation of dose, a relation between radon progeny exposure and dose, the so-called dose conversion factor (DCF) is needed. The DCF in steady-state conditions can be calculated by available software that implements the ICRP 66 model (ex. LUDEP distributed by NRPB). However, the DCF for indoor radon progeny is variable with environmental changes. For example, the DCF could be affected by the working of an air cleaner or opening doors/windows. The objective of the present study is to simulate time-related changes in the DCF with such environmental changes. For this purpose, a compartment model was developed using a commercially available computer program. The following parameters were included in the model: (1) room-specific parameters such as ventilation rate and room size, (2) aerosol parameters (particle concentration, activity size distribution, attachment rate, etc), and (3) dosimetric parameters (breathing rate, dose per unit intake, etc). As an example of model applications, changes in the DCF by the working of the air-cleaner were simulated. Equilibrium equivalent radon concentration (EERC), unattached fraction (fp) and the DCF were plotted against time. The air-cleaner removed aerosol particles as well as radon progeny of both unattached and attached fractions. Thus, the working of the air-cleaner caused the decrease of EERC and the increase of fp. The DCF increased with fp. According to UNSCEAR reports, the dose due to radon progeny is calculated by multiplying EERC by the DCF. Because of the increase of the DCF, the dose was not reduced than expected from the decrease of EERC. The model was useful for simulating effects of environmental changes on the DCF