Toxicity Profiles
Condensed Toxicity Summary for STRONTIUM-90
NOTE: Although the toxicity values presented in these toxicity profiles were correct at the time they were produced, these values are subject to change. Users should always refer to the Toxicity Value Database for the current toxicity values.
DECEMBER 1994
Prepared by: Sylvia S. Talmage, Ph.D., D.A.B.T., Chemical Hazard Evaluation Group, Biomedical and Environmental Information Analysis Section, Health Sciences Research Division, Oak Ridge National Laboratory*, Oak Ridge, Tennessee.
Prepared for: Oak Ridge Reservation Environmental Restoration Program.
*Managed by Martin Marietta Energy Systems, Inc., for the U.S. Department of Energy under Contract No. DE-AC05-84OR21400.
Strontium-90 is a radioactive isotope of strontium that is produced in nuclear fission. It is a low energy emitter with a physical half-life of approximately 28 years. In the environment, it is accompanied by its decay product, yttrium-90, also a emitter (NCRP, 1991).
Metabolically, strontium is an analog of calcium. Strontium-90 is rapidly absorbed from the gastrointestinal tract or the lung into the bloodstream and is subsequently deposited in bone (Hobbs and McClellan, 1986). Retention in the bone is long-term, with yearly loss of the existing burden in adults of 7.5% from cortical bone and 30% from trabecular bone (Papworth and Vennart, 1984).
Oral intake at high levels of activity results in irradiation of target organs and nearby tissues. At high exposures, death results from radiation-induced hemorrhagic syndrome; at lower exposures, death results from destruction of the bone marrow. As survival times increase at lower administered activities, these effects are accompanied by neoplasms. Oral administration to miniature swine at 115 x 106 Bq/day resulted in radiation-induced hemorrhagic syndrome and death within 4 months. Lower intakes, 4.62 x 106 to 0.037 x 106 Bq/day induced effects on the hematopoietic system ranging from pancytopenia to neutropenia. Mean survival times were decreased in all exposure groups (NCRP, 1991). Subchronic exposures (5.6 x 104 to 133 x 104 Bq/day) also resulted in cytopenias in beagle dogs (NCRP, 1991). Median survival times were reduced, particularly at the two highest exposures. Doses in the latter study were estimated at 22.5 to 107 Gy. In a three-generation study of miniature swine, no effects were noted on litter size, the percentage of stillborn, or birth weight when dams received life-shortening exposures (Clarke et al., 1972). The EPA has not calculated subchronic or chronic oral reference doses (RfD) for radionuclides.
Inhalation of soluble forms of strontium-90 at high activity levels resulted in early death of beagle dogs from bone marrow hypoplasia, panleukocytopenia, terminal hemorrhage, and bacterial infection (Gillett et al., 1987; NCRP, 1991). Long-term bone burdens of 17 x 106 to 41 x 106 Bq were calculated. Acute pulmonary effects observed in mice exposed to high activity levels of insoluble strontium-90 were radiation pneumonitis and pulmonary fibrosis (Scott et al., 1987). The dose lethal to 50% of animals was 370 Gy. The EPA has not calculated subchronic or chronic inhalation reference concentrations (RfC) for radionuclides.
The primary effect in animals surviving acute effects of strontium-90 exposure is neoplasia of bone and bone-related tissues. Soft tissue carcinomas in tissues near the bone were also observed above control levels. Chronic ingestion in beagle dogs and miniature swine also produced a high incidence of myeloproliferative disease, including frank leukemia (Hobbs and McClellan, 1986). Orally administered activity levels of 0.1 x 104 to 133 x 104 Bq/day to beagle dogs, beginning in utero and continuing to 540 days of age induced primary bone sarcomas and myeloproliferative disorders at activity levels of >5.6 x 104 Bq/day (Pool et al., 1973; NCRP, 1991). Bone sarcomas and hematopoietic neoplasms were induced in F1 and F2 generations of miniature swine that chronically ingested 4.62 x 106 Bq/day or 23.1 x 106 Bq/day (Clarke et al., 1972; NCRP, 1991). These generations were exposed in utero and gradually raised to the treatment level by 6 months of age. At higher exposure activities, mean survival time was drastically reduced (to less than tumor induction time); at lower exposure activities, 0.037 x 106 to 0.925 x 106 Bq/day, no bone sarcomas were observed, and survival times were increased to approximately that of controls.
The primary effect in adult beagle dogs administered strontium-90 in a single inhalation exposure and surviving more than 2 years was an excess of bone tumors (McClellan et al., 1973; NCRP, 1991). This effect was induced in dogs with long-term retained bone burdens of >1.0 x 106 Bq/kg.
Bone tumors were induced in 2 of 7 adult monkeys administered strontium-90 by gavage (NCRP, 1991), in adult beagle dogs administered strontium-90 by intravenous injection (Mays and Finkel, 1980), and in mice administered strontium-90 by intraperitoneal injection (Nilsson and Ronnback, 1973). Soft tissue sarcomas in tissues near the bone were observed in the beagle dogs (Mays and Finkel, 1980). No tumors were observed over a 20-year period in young and adult Rhesus monkeys administered a single injection at activity levels of 0.13 x 106 to 6.21 x 106 Bq (NCRP, 1991).
The EPA has classified all radionuclides as Group A carcinogens based on their property of emitting ionizing radiation and on the weight of evidence provided by epidemiological studies of radiation-induced tumors in humans (EPA, 1994). A slope factor of 8.9E-10 (risk/Bq) was calculated for oral ingestion. The combined oral slope factor for strontium-90 and yttrium-90 is 9.7E-10 (risk/Bq). For inhalation exposure, the slope factor for strontium-90 is 1.5E-09 (risk/Bq); the combined inhalation slope factor for strontium-90 plus yttrium-90 is 1.7E-09 (risk/Bq). Because of their low penetration ability, external exposures (risk/yr per Bq/g soil) for strontium-90 and strontium-90 plus yttrium-90 are both 0.0E+00.
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