Inhalation studies show that dogs, monkeys and rat9 can breathe a natural uranium dioxide (UO,) aerosol of approximately 1 pm mass median particle diameter (MMD), at a mean concentration of 5 mg U/m3(25 x TLVS or 28 x MpC,$), for periods as long as 5 yr with little evidence of serious injury (L. J. LEACH et al., Health Phys. 18, 599 (1970)).Some of these animals were observed for protracted postexposure periods during which pulmonary neoplasia developed in a high percentage of the dogs examined 2-6 yr after exposure.Pulmonary and tracheobronchial lymph node fibrosis, consistent with radiation effects, apparently dose dependent, and more marked in monkeys than in dogs was also noted.No evidence of uranium toxicity was found in records of body weights, mortality, various hematologic parameters or the histologic condition of the kidneys.

Monkeys, dogs and rats inhaled natural UO, 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.

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