For most typical radiological contamination situations involving the inhalation of radioactive material, organ dose commitments are calculated using published inhalation dose conversion factors. Inhalation dose conversion factors are available for activity on particles of less than lOum in diameter; however, inhalation dose conversion factors for large particles are not available in the published literature. Accordingly, for the purpose of this dose calculation a set of large-particle inhalation dose conversion factors is developed. For particles greater than 10 um diameter, essentially all of the inhaled material is initially deposited in the nasal or nasopharyngeal region of the respiratory tract (Reference 17). This material is removed by chemical (absorption) and mechanical processes, and goes either to the blood, with subsequent transfer to other body organs, or to the gastrointestinal (GI) tract, with subsequent transfer to the blood and further transfer to other body organs or bodily elimination by excretion. The "pulmonary, clearance classification" of the inhaled material (roughly analogous to the solubility of the material) determines the amount of the material! that is initially transferred to the blood or the GI tract; for example, 99 percent of an "insoluble" material will go to the the GI tract with only 1 percent going to the blood, whereas a "very soluble" material will be divided evenly between the GI tract and the blood (Reference 16). As ingestion plays a major role in the fate of an inhaled large particle, the largeparticle inhalation dose conversion factors used for the present dose calculation are obtained by modifying ingestion dose conversion factors. This modification is necessary to properly account for that portion of the inhaled material that is initially transferred to the blood, rather than to the GI tract. The metabolic data needed to construct inhalation/ingestion dose conversion factors are given in ORNL/NUREG/ TM-190 (Reference 16) for the most radiologically significant radionuclides. The TM-190 dose conversion factors, based on the most current dosimetry information, are _used in conjunction with these. unmodified ingestion dose For those radionuclides not addressed in TM-190, conversion factors (Reference 18). 21 are taken from NUREG-0172