Definicija: A soft silvery-white and highly reactive metal belonging to the alkali group of metals. It is a radiation hazard, because it can occur in two radioactive forms. Caesium-134 is produced in nuclear reactors, not directly by fission, but by the reaction. It emits beta- and gamma-radiation. Caesium-137 is a fission product of uranium and occurs in the fallout from nuclear weapons. It emits beta- and gamma-rays and has a half-life of 30 years. Caesium-137 was the principal product released into the atmosphere, and hence the food chain, from atmospheric testing of nuclear weapons and from the Windscale fire and Chernobyl nuclear accidents. After the Chernobyl accident, which spread a radiation cloud across Europe, the European Commission proposed new and more restrictive limits on levels of caesium in food and drinking water.

Definicija: Mehka, srebrno-bela in visoko reaktivna kovina alkalne skupine. Nevaren je zaradi sevanja, ker se lahko pojavlja v dveh radioaktivnih oblikah. Cezij-134 nastaja v jedrskih reaktorjih, ne direktno s cepitvijo, ampak v reakciji. Oddaja beta in gama žarčenje. Cezij-137 je produkt fisije urana in se pojavlja posledično po uporabi jedrskih orožij. Oddaja beta in gama žarke in ima razpolovno dobo 30 let. Cezij-137 je bil glavni produkt testiranja jedrskega orožja v atmosferi in požara v černobilski nesreči, ki se je širil z vetrom. Po černobilski nesreči, po kateri se je oblak sevanja razširil po Evropi, je Evropska skupnost predlagala nove in bolj restriktivne omejitve dovoljene koncentracije cezija v prehrani in pitni vodi.