Dust particle size effects on absorbed fraction values in the anterior nose

The respiratory tract model introduced in Publication 66 of the International Commission on Radiological Protection (ICRP) includes consideration of extrathoracic airways, referred to as the ET region. This region comprises the anterior nose and the posterior nasal passages, larynx, pharynx, and the mouth. The deposition of inhaled particles in the airways depends on the thermal and aerodynamic properties of the particles and equations are presented to calculate the deposition efficiency in the various regions of the tract. In its dosimetric model the ICRP assumes that none of the energy emitted by the deposited source is lost in the particles itself (i.e., no self attenuation) and the deposition is assumed to be on the inner surface of the airway of the anterior nose. Therefore, the effects of various dust particle sizes on the energy deposition in the epithelium as characterized by the absorbed fraction in the anterior nose region were not addressed in ICRP 66. Since radioactive particulate matter is carried in air within dust particles, this subject should be addressed. In this paper the effects of dust particle sizes (various equivalent volume diameters) on the absorbed fraction to the anterior nose are studied using a truncated cone model for the anterior nose. The results indicate that attenuation in the dust particles has a significant effect on the electron absorbed fraction. This effect depends on the dust particle size and the energies of the electrons emitted by the radionuclides carried within dust particles.