To aid in assessing the hazard associated with airborne radio-cerium, two groups of rats were exposed by inhalation to heat-treated${}^{144}{\rm Ce}$ hydroxide aerosols resulting in body burdens of ∼3.0 (0.85-6.6) or ∼170 (79-300) μCi${}^{144}{\rm Ce}/{\rm kg}$ body weight. Following initial rapid clearance, the${}^{144}{\rm Ce}$ had an effective half-time of 130 days in lung. Based on average kinetics of whole-body and lung retention, estimated cumulative β-radiation doses to lungs of the high-level rats were 620 rads at 100 days, 1000 rads at 300 days and 1150 rads at 600 days postinhalation exposure, per μCi initial lung burden. Estimated cumulative doses to lungs to time of death averaged 5100 rads and ranged from 2400 to 8800 rads. Other organs of interest, for which β-radiation doses were calculated, are skeleton, liver and tracheobronchial lymph nodes.${}^{144}{\rm Ce}$ also concentrated significantly in kidneys. Median survival times were 135 and 393 days postexposure for the high-level and low-level rats, respectively. Radiation fibrosis of lungs was noted in 16 of 20 high-level rats that died 101 to 607 days postexposure. Five of 14 rats that died 263 to 607 days postexposure had squamous cell carcinomas of lung, two cases having metastases to other organs. No radiation-induced changes were observed in the low-level rats.

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