Alpha decay occurs in heavy nuclei such as uranium or plutonium and therefore occurs in fallout. In alpha decay a helium nucleus (called an alpha particle) is ejected spontaneously. Since an alpha particle is relatively more massive than other forms of radioactive decay, it can be stopped by a thin layer of cloth or metal. A 4-MeV alpha particle can travel about 1 inch through the air. If an alpha decaying element is ingested, the alpha particle can do considerable damage to the surrounding tissue. This is why plutonium is extremely hazardous if ingested.
