Lung dosimetry of inhaled radon progeny in mice

Abstract

Biological response of exposure to radon progeny has long been investigated, but there are only few studies in which absorbed doses in lungs of laboratory animals were estimated. The present study is the first attempt to calculate the doses of inhaled radon progeny for mice. For reference, the doses for rats and humans were also computed with the corresponding models. Lung deposition of particles, their clearance, and energy deposition of alpha particles to sensitive tissues were systematically simulated. Absorbed doses to trachea and bronchi, bronchioles and terminal bronchioles, alveolar-interstitial regions, and whole lung were first provided as a function of monodisperse radon progeny particles with an equilibrium equivalent radon concentration of 1 Bq m⁻³ (equilibrium factor, 0.4 and unattached fraction, 0.01). Based on the results, absorbed doses were then calculated for (1) a reference mine condition and (2) a condition previously used for animal experiments. It was found that the whole lung doses for mice, rats, and humans were 34.8, 20.7, and 10.7 nGy (Bq m⁻³)⁻¹ h⁻¹ for the mine condition, respectively, while they were 16.9, 9.9, and 6.5 nGy (Bq m⁻³)⁻¹ h⁻¹ for the animal experimental condition. In both cases, the values for mice are about 2 times higher than those for rats, and about 3 times higher than those for humans. Comparison of our data on rats and humans with those published in the literature shows an acceptable agreement, suggesting the validity of the present modeling for mice. In the future, a more sophisticated dosimetric study of inhaled radon progeny in mice would be desirable to demonstrate how anatomical, physiological, and environmental parameters can influence absorbed doses.

Appendix

Physiological parameters

n _i :

Number of airways in the ith generation

L _i :

Airway length in the ith generation (cm)

D _i :

Airway diameter in the ith generation (mm)

R _i :

Airway radius in the ith generation (cm)

V _i :

Airway volume in the ith generation (ml)

θ _i :

Branching angle in the ith generation (rad), describing the change in direction of the bulk airflow from the i − 1th generation into the ith generation

ϕ _i :

Gravity angle in the ith generation (rad), describing the inclination of the airway segment to gravity

S _i :

Total airway surface area in the ith generation (mm²)

$\bar{v}_{\text{i}}$ :

Mean flow velocity of inhaled or exhaled air in the ith generation (cm s⁻¹)

$\bar{V}$ :

Mean volumetric flow rate of inhaled or exhaled air (ml s⁻¹)

TLC:

Total lung capacity (ml), corresponding to the air volume of the lungs at maximum inhalation

TV:

Tidal volume (ml), corresponding to the air volume inhaled and exhaled during a breath

LV:

Adjusted lung volume (ml)

FRC:

Functional residual capacity (ml), corresponding to the air volume of the lungs at end-exhalation of a normal breath

f :

Respiratory frequency (min⁻¹)

B :

Breathing rate (ml min⁻¹)

W :

Body mass (g)

Aerosol and exposure parameters

d _p :

Particle diameter (cm)

r _p :

Particle radius (cm)

ρ _p :

Particle density (kg cm⁻³)

D _p :

Particle diffusion coefficient in air (cm² s⁻¹)

g :

Gravitational acceleration (cm s⁻²)

C :

Cunningham slip correction factor

μ :

Viscosity of air (kg cm⁻¹ s⁻¹)

Kn :

Knudsen number

λ _p :

Mean free path of particles in air (cm)

Stk :

Stokes number

N _n :

Number of particles per ambient air volume (cm⁻³); the subscript, n, stands for a radionuclide

λ _n :

Decay constant (min⁻¹); the subscript, n, stands for a radionuclide

C _Rn :

Equilibrium equivalent radon concentration in ambient air (Bq m⁻³)

F :

Equilibrium factor

f _p :

Unattached fraction, corresponding to the fraction of the potential alpha energy concentration of short-lived radon progeny that is not attached to ambient aerosols

GM:

Geometric mean particle size (nm)

GSD:

Geometric standard deviation

AMAD:

Activity median aerodynamic diameter (nm)

Deposition

η _I :

Inhalability

η _ET,In :

Deposition efficiency of the ET region at an inhalation phase

η _ET,Ex :

Deposition efficiency of the ET region at an exhalation phase

η _i,In :

Deposition efficiency of the ith generation at an inhalation phase

η _i,Ex :

Deposition efficiency of the ith generation at an exhalation phase

η _i :

Deposition efficiency of the ith generation at an inhalation or exhalation phase

η _i,D :

Deposition efficiency of the ith generation by diffusion

$\eta_{\text{i, D}}^{\text{E}}$ :

Deposition efficiency of the ith generation by diffusion; this considers the effect of entrance configuration

η _i,S :

Deposition efficiency of the ith generation by sedimentation

η _i,I :

Deposition efficiency of the ith generation by internal impaction

DE _i,In :

Deposition fraction in the ith generation at an inhalation phase

DE _i,Ex :

Deposition fraction in the ith generation at an exhalation phase

DE _i :

Deposition fraction in the ith generation per breath, corresponding to the fraction of the number or activity of particles of a given size that are present in a volume of ambient air before inhalation

DE _i,n :

Deposition fraction in the ith generation per breath; the subscript, n, stands for a radionuclide

Clearance

$\bar{T}_{\text{i}}$ :

Mean residence time (min), corresponding to the time required for mucous clearance of particles from the ith generation to the i − 1th

v _i :

Mucous velocity in the ith generation (mm min⁻¹)

M _i,n :

Number of particles deposited in the ith generation; the subscript, n, stands for a radionuclide

Dosimetry

A _i :

Radioactivity in the ith generation (Bq)

E _α :

Energy of alpha particles (MeV)

m _T :

Mass of the target tissue (kg)

D _R :

Absorbed dose in the region R of the lung (Gy)