Toxicity of 99Tc: can it represent a risk to man?

The movement through the environment and the toxicity to plants of 99Tc have been studied. However, information is scanty on 99Tc toxicity to mammals even though this is the decisive criterion for assessing the consequences of releases of Tc. A critical tissue could be the thyroid because of its preferential accumulation of Tc and the developing organism because of its greater radiosensitivity; moreover, this toxicity might be enhanced under conditions of a low iodine (I) diet. These questions were studied in rats given large amounts of 99Tc in either a normal or an I-deficient diet for several months starting 2 wk before mating. Newborns were continued on these diets after weaning. The parameters determined were: occurrence of pregnancy and litter size, triiodothyronine and thyroxin in serum, uptake of 131I by the thyroid 24 h after injection, histology of the thyroid and concentration of Tc in tissues of the mothers and their decendants. Thyroid damage, as well as the effects on pregnancy, could be observed after amounts of 10 micrograms Tc g-1 food. Iodine deficiency only slightly influences the 99Tc toxicity. The chemical and radiological toxicity of 99Tc to rats is small. Consequently, it seems unlikely that contamination levels in the environment would ever reach levels that could lead to serious non-stochastic effects, even in the developing organism.