Age-dependent inhalation doses to members of the public from indoor short-lived radon progeny

Abstract

The main contribution of radiation dose to the human lungs from natural exposure originates from short-lived radon progeny. In the present work, the inhalation doses from indoor short-lived radon progeny, i.e., ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po, to different age groups of members of the public were calculated. In the calculations, the age-dependent systemic biokinetic models of polonium, bismuth, and lead published by the International Commission on Radiological Protection (ICRP) were adopted. In addition, the ICRP human respiratory tract and gastrointestinal tract models were applied to determine the deposition fractions in different regions of the lungs during inhalation and exhalation, and the absorption fractions of radon progeny in the alimentary tract. Based on the calculated contribution of each progeny to equivalent dose and effective dose, the dose conversion factor was estimated, taking into account the unattached fraction of aerosols, attached aerosols in the nucleation, accumulation and coarse modes, and the potential alpha energy concentration fraction in indoor air. It turned out that for each progeny, the equivalent doses to extrathoracic airways and the lungs are greater than those to other organs. The contribution of ²¹⁴Po to effective dose is much smaller compared to that of the other short-lived radon progeny and can thus be neglected in the dose assessment. In fact, 90 % of the effective dose from short-lived radon progeny arises from ²¹⁴Pb and ²¹⁴Bi, while the rest is from ²¹⁸Po. The dose conversion factors obtained in the present study are 17 and 18 mSv per working level month (WLM) for adult female and male, respectively. This compares to values ranging from 6 to 20 mSv WLM⁻¹ calculated by other investigators. The dose coefficients of each radon progeny calculated in the present study can be used to estimate the radiation doses for the population, especially for small children and women, in specific regions of the world exposed to radon progeny by measuring their concentrations, aerosol sizes, and unattached fractions.

Appendices

Appendix 1: Age-dependent dose coefficients for members of the public exposed to short-lived radon progeny

Age-dependent dose coefficients (Sv Bq⁻¹) for members of the public exposed to short-lived radon progeny, i.e., ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po, both for the unattached mode (1 nm) and for progeny in the nucleation (50 nm), accumulation (230 nm) and coarse (2,500 nm) modes are presented in Tables A1–A16 (see supplementary electronic material).

Appendix 2: Calculation of the radon dose conversion factor (DCF) from individual dose coefficients of progeny

This appendix illustrates the derivation of Eq. (4) from dose coefficients into two different units, i.e., in the unit of Sv (J m⁻³ h)⁻¹ and Sv WLM⁻¹, so that the DCFs given in different units can be easily converted. In the calculation of DCFs, the following steps were performed:

• 1. The dose coefficients per potential alpha energy were calculated;

• 2. The contribution of each progeny to the potential alpha energy was calculated, according to the ratios of concentrations of the nuclides and the particle sizes;

• 3. The dose contribution per exposure of each progeny in each size was calculated;

• 4. All the contributions were added.

Furthermore, the ratio of the activity concentrations in the air (C _j,i in Bq m⁻³) and the ratio of the four aerosol sizes (unattached to coarse; f _pj ) are given in Table 1.

The dose coefficients (Sv Bq⁻¹) are summarized in Table 10.

Table 10 Effective dose coefficient (Sv Bq⁻¹) of each radon progeny at four different sizes

For a convenient application in practice, the dose conversion factors of radon progeny are calculated in the units of Sv (J m⁻³ h)⁻¹ and Sv WLM⁻¹ (Table 11) where 1 WL = 2.08 × 10⁻⁵ J m⁻³ and 1 WLM = 2.08 × 10⁻⁵ J m⁻³×170 h = 3.54 × 10⁻³ J m⁻³ h.

Table 11 Derivation of dose conversion factor in two different units