Irradiation refers to the exposure of a body, substance or organism to ionising radiation. It occurs when radioactive substances outside of the organism irradiate from a distance without there being any direct physical contact between the body and the radioactive substance.
When the person moves out of the field of radiation, the exposure ceases. It is therefore possible to protect oneself from a source of radiation by limiting exposure time, monitoring distance from the source and using protective screens (for example the walls of a house in the event of taking shelter). The irradiated person does not become radioactive.
The main source of irradiation is natural (mainly from radon) and is expressed in particular units of measurement (Becquerel, Sievert, Gray).