The lung retention of uranium was determined in rats that inhaled aerosols of commercial yellowcake powders obtained from two mills (Mill A and Mill D) and whose chemical composition and solubilities in vitro were significantly different. Analysis by IR absorption indicated Mill A yellowcake contained 82% ammonium diuranate (ADU) and 18% U3O8. The Mill D powder contained 25% ADU and 75% U3O8. In vitro dissolution studies indicated for the Mill A sample, approximately 85% of the uranium had a dissolution half-time (T 1/2) of less than one day, with the remainder dissolving with a half-time of 500 days. For the Mill D sample, 25% had T 1/2 less than one day and 75% had T 1/2 of 300 days. Groups of 50 rats were exposed by nose-only inhalation to aerosols of either the Mill A or the Mill D yellowcake. Rats were sacrificed in groups of five at intervals through six months after exposure. Selected tissues and excreta samples were assayed by fluorometry to determine their uranium contents. For the Mill A yellowcake, 78% initial lung (broncho-alveolar) burden cleared with T 1/2 of 0.5 days, and 22% with T 1/2 of 240 days. For the Mill D yellowcake, 25% initial lung burden cleared with T 1/2 of 3.5 days and 75% with T 1/2 of 110 days. Thus, the lung clearance of uranium in the rat mimicked the in vitro dissolution data and supported the contention that ADU should be considered as a Class D compound (T 1/2 = 0.5 days) and U3O8 behaves in the lung as a Class Y (T 1/2 greater than 100 days) material.

A study was conducted in rats to determine solubility and subsequent metabolism of an inhaled aerosol of curium treated at high temperatures. Young adult Fischer-344 rats received a single inhalation exposure to one of three monodisperse aerosols of 244Cm2O3 (0.70, 1.3, or 2.6 micron activity median aerodynamic diameter) heat-treated at 1150 degrees C. Animals were maintained individually in metabolism cages for excreta collection and serially sacrificed in groups of two male and two female rats from 2 to 33 days after inhalation exposure. Additionally an injection study with curium citrate was done to define the systemic behavior of Cm in this rat model. The in vivo solubility was inversely related to the aerosol particle size. The relationship of the results of this study to results from other experimental inhalation studies with curium oxide aerosols is discussed, as is the relevance to bioassay interpretation and risk assessment in man.

A biokinetic model was used to simulate retention and excretion of two forms of U: ammonium diuranate (ADU), a relatively soluble form, and U3O8, a relatively insoluble form. These two U forms represent those most likely to be encountered in the U milling industry. The simulation model was compared with results from a study of aerosols of commercial refined U ore inhaled by laboratory animals. Beagle dogs were exposed by inhalation to ADU aerosols to achieve a median initial body burden of 0.058 mg U kg-1 body weight (within a range of 0.016 to 0.64 mg U kg-1), or to U3O8 aerosols to achieve a median retained body burden of 0.28 mg U kg-1 (0.030-0.81 mg U kg-1). The simulation model accurately described the accumulation of nephrotoxic concentrations of U in kidneys of animals exposed to ADU. Very small fractions of the initial body burden of U3O8 were translocated to kidney, and these fractions were overestimated by the model. The model showed general agreement with results of other laboratory animal studies and with available information from human exposures to ADU, UF6, or U3O8.