Ionizing radiation
There is a little radiation all around us all the time, which people's bodies are used to, but larger amounts of radiation can make people sick or kill them. Natural ionizing radiation is produced by radioactive decay of some chemical elements, such as uranium. Stars and other things in outer space also make radiation. See cosmic ray. Some isotopes that are radioactive only stay radioactive for much less than a second. Other things can stay radioactive for thousands of years.