One hundred and seventy-eight mice, exposed by inhalation to an aerosol of <tex-math>${}^{144}{\rm CeO}{}_{2}$</tex-math> particles, were observed for their lifespan and their survival compared with those of 200 unexposed control mice. Approximately 91% of the initial whole-body burden of <tex-math>${}^{144}{\rm Ce}$</tex-math> was cleared within 6 days. The whole-body burden at 6 days was estimated to be equivalent to the initial lung burden (ILB) of which 32% was retained with an effective half-time of 7 days, 49% with a half-time of 28 days, and 19% with a half-time of 145 days. Cerium-144 retained in the lungs as a percentage of whole-body burden at death was determined through day 454 postinhalation exposure to be 84%. Mean survival time and cumulative percentage of survival were related to the estimated ILBs; ILBs of 4-5 μCi or greater resulted in 62% or more shortening of lifespan and ILBs of 3-4 μCi resulted in a 24% shortening of lifespan. The dose of beta radiation to the lung to the day of death ranged from 10,000 to 32,000 rads. No malignant primary pulmonary tumors were observed in the mice that had inhaled <tex-math>${}^{144}{\rm CeO}{}_{2}$</tex-math> particles.

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.

Two radionuclides which localize in different body organs, may elicit antagonistic, synergistic, or directly additive biological effects when administered in combination. Graded doses (range, 1.4 to 5.3 mCi/kg body weight) of the bone seeker 90 Sr were injected intraperitoneally into rats simultaneously with graded doses (range, 3.4 to 26.8 mCi/kg) of the rather diffusely localizing radionuclide 137 Cs. Times to reach acute death in 50% of the animals ranged from 9 to 18 days. Radiation doses calculated for these injected mixtures ranged from 3900 to 11,000 rad to the skeleton from 90 Sr and from 320 to 2200 rad to the whole body from 137 Cs. There is some indication from comparing the time to acute death with the calculated radiation doses, that these radionuclides acting in combination give at least an additive effect for this endpoint (lethality).