Abstract
Current popularity of 125I for research applications such as protein iodination is leading to a rapid increase in the use of this isotope. An ingestion risk laboratory classification based upon comparison of turnover of radioactive material with permissible daily intake figures predicts that ingestion risks have increased by factors of 1000 compared with the use of soft beta labelled materials. This classification system is described. Thyroid counting has revealed detectable thyroid burdens in the entire exposed population and some burdens approximate to the maximum permissible level for constant exposure while typical values are one-tenth of this level. These results are presented and summarized. Methods of minimizing the ingestion risk are discussed.