Toxicological studies on safety of rare earths used in agriculture.

Studies on the toxicity and safety of a mixture of rare earth metal nitrates (Ce, La, Nd, Pr, and Sm) used in agricultural operations are reported. In mice, rats, and guinea pigs, the oral LD50 ranged from 1397 to 1876 mg/kg; absorption in the gastrointestinal tract was low. The accumulation coefficient was greater than 5. In rabbits, a topical application of a suspension of 500 mg/ml produced mild irritation of the skin and eye mucosa. Subchronic and chronic toxicity studies were done at different dose levels in monkeys (100 mg/kg) and rats (200 and 1800 mg/kg); biochemical and histopathological examination of tissues showed no abnormal or specific pathological changes. In chronic feeding studies with rats, the incidence of tumors and malignant tumors in test groups was lower than that in the control. Rat fetuses did not show any teratogenicity when the dams were orally fed up to 330 mg/kg of this nitrate mixture. Ames mutagenicity tests were negative at a 50 mg/kg level. The results indicate that an oral dose of 60 mg/kg should be considered as a no-effect level with an ADI of 0.6 mg/kg and that the level of rare earth nitrates used in Chinese agriculture is within acceptable risk or safety limits.