Radiation is a general term used to describe the emission or transfer of energy, as waves or particles, through the air or other substances. Familiar kinds of radiation are the sun's rays, microwaves used for cooking in microwave ovens, and radar used to track or guide planes. Ionising radiation from radioactivity is caused by the disintegration of a small piece of matter known as a nucleus. The nucleus is itself a constituent part of another small particle known as an atom.
All matter is made up of atoms.
There are various forms of ionising radiation depending on the nucleus and form of decay involved. There are a number of sources of ionising radiation and all can interact with the human body to cause damage. Possible damage to human body by ionising radiation depends on a number of factors. Materials that emit alpha radiation outside the body are harmless, since they are already stopped by the skin. But when these materials enter the body through inhalation or swallowing, they may be harmful.