There are many different types of radiation, such as heat, light, microwaves, radio and television transmissions and X-rays. The particular kind of radiation of concern here is ionising radiation, which damages living tissue as it passes through. This is the kind of radiation associated with the nuclear industry, though X-rays are also a very typical form of ionising radiation. All matter is made up of atoms.
Some nuclei are unstable.
Occurrence Radiation occurs naturally. Essentially all elements that occur naturally have a radioactive form. As a consequence, life on earth is, and always has been, constantly exposed to radiation. One of the most widely known characteristics of radiation is its ability to damage living tissue. The amount of damage sustained increases with increased exposure.