EFFECTIVE DOSE SCALING FACTORS FOR USE WITH URANIUM SERIES CASCADE IMPACTOR DATA: A REASSESSMENT USING THE IMBA CODE

Abstract

Air sampling with a multi-stage cascade impactor enables one to assess airborne radioactivity as a function of particle size, significantly enhancing the accuracy of the dose assessment. The application of cascade sampling data to inhalation dose assessments can require more computational effort if something other than a mono-sized distribution per impactor stage is to be considered. To overcome this limitation, Kim et al. (Health Phys 89:359-374; 2005) introduced the concept of an effective dose scaling factor SF(E) enabling one to consider more realistic impactor stage radioactivity distributions (uniform, linearly decreasing, or linearly increasing variations with particle size). The SF(E) is the ratio of the effective dose given under a uniform or linearly changing radioactivity distribution across the particle size interval to that given for a mono-sized radioactivity distribution for the same impactor stage. The latter approach can initially be used (which requires less computational effort) followed by a rescaling of the effective dose either upward or downward by the SF(E) value. In this earlier study, the LUDEP code was employed which utilizes the ICRP 66 human respiratory tract model along the radionuclide biokinetic models given in ICRP Publication 30. In the present study, inhalation dose coefficients and effective dose scaling factors were reexamined for several radionuclides of the (238)U series using the IMBA program, which employs more recent and physiologically realistic biokinetic models published by the ICRP. An update of the effective dose scaling factors is thus the primary focus of this study rather than an extensive inter-comparison of the IMBA and LUDEP codes. Inhalation dose coefficients calculated by the two programs differ by up to a factor of 5 for Type F (238)U and (234)U, but are within only 2% of each other for Type S radionuclides. The ICRP 69 biokinetic model of uranium predicts retention in bone and kidneys that is slightly higher than predicted in ICRP Publication 30, but is significantly higher in the liver and other soft tissues by up to 1 or 2 orders of magnitude. Nevertheless, effective dose scaling factors generated using the IMBA program are nearly identical to those calculated by the LUDEP program as their magnitude is primarily dictated by the dependence of particle deposition and lung clearance with particle size, and less by the systemic biodistribution of the radionuclide following absorption to blood. For both codes, greater than 10% re-scaling of the effective dose is required for third-stage (4.5 to 12 microm) and filter-stage (0.03 to 0.35 microm) particles in the approximation of uniform or linearly decreasing radioactivity distributions per particle stage. For linearly increasing distributions, greater than 10% corrections in the effective dose are found irregularly across impactor stage, radionuclide, and solubility class, especially for rather steep (1:5) impactor stage activity ratios.