Cesium-133 is an isotope of the element Cesium (Cs). Out of the other 40 isotopes, it is the only stable one, and does not undergo decay. Since Cesium-133 is the only stable isotope, it is not harmful or radioactive. Radioactivity can be found all around us, in the air we breath to the food we digest. There are over 60 natural elements that are radioactive when they undergo decay, even the sun releasing gamma rays from the nuclear fusion produced. Natural radioactivity, that is not produced by man like nuclear power plants.