Monkeys, dogs and rats inhaled natural UO2 dust of approximately 1 micron MMD, at a concentration of 5 mg U/m3, 6 hr per day, 5 days per week for periods up to 5 yr. Retention curves for U in the lungs and other tissues including bone were established in a program of serial sacrifice and animal replacement over the 5-yr exposure period for the dog and monkey and over a 1-yr period for the rat. The 2 major sites of U accumulation, the lungs and tracheobronchial lymph nodes (TLN), accounted for over 90% of the U found in the body. For the dog and monkey, a rapid build-up of U occurred in the lungs and TLN during the first year of exposure. After 1 yr the lungs contained approximately 2000 μg U/g in the dog and 3600 μg U/g in the monkey, near maximal values. Unlike lung, the U content of the TLN for both species continued to rise reaching maximal values of 50,000 to 70,000 μg U/g after 4 yr of exposure. In kidney, femur, spleen and liver, U concentrations were comparatively low; after 5 yr of exposure, monkey spleen showed the highest concentration (350 μg U/g), whereas dog spleen represented the lowest (0.9 μg U/g). Alpha radiation dosages, calculated from the organ burdens of U, indicate that dose rates to the lungs and the lymph nodes of each species surpassed 0.03 rad/wk during the first few months of exposure. At 5 yr, dose rates to dog lung and TLN and to monkey lung and TLN were 1.8, 55, 3.3 and 64 rads/wk, respectively. The integrated alpha radiation dose to dog and monkey lung, after 5 yr of exposure, was estimated to be 500 and 900 rads, respectively. At this time TLN values for both species were in the order of 10,000 rads. No evidence of U toxicity was found in body weights or mortality, in the NPN levels of the blood, or in the hematologic picture. Kidney injury did not occur at the exposure level of 5 mg U/m3 (20 × TLV or 28 × MPCa). Fibrotic changes suggestive of radiation injury, however, were seen occasionally in the TLN of dogs and monkeys and in monkey lungs after exposure periods longer than 3 yr in tissues with estimated alpha doses greater than 500 rads for lung and 7000 rads for TLN. The lung and lymph node data obtained in this study show that the animal body can accumulate sufficient U from prolonged exposures to insoluble U dust at 5 mg/m3 to create potential radiologic hazards. The lung and TLN values were high enough, in fact, to anticipate radiation hazards in these tissues from exposures at or lower than the occupational TLV (250 μg U/m3) recommended by the ACGIH or the MPCa (6 × 10−11 μCi/cm3 = about 180 μg U/m3) suggested by the ICRP.
©1970Health Physics Society