Radioactive isotopes of an element behave chemically in the same way as its non-radioactive isotopes. They can thus deplete or enrich in plants, animals and in the human body. Such an enrichment chain exists e.g. in the case of iodine. Iodine is finally enriched in the human thyroid gland via air – grass – cow – milk. These enrichment processes are known and can be calculated. To avoid higher radiation exposure in the corresponding organs due to the enrichment chains, the admissible release values for such radioactive substances are fixed correspondingly low. The dose limit values prescribed by laws and regulations must not be exceeded – not even by enrichment effects.