Irradiation is the direct result of the external exposure of a body (inert or living) to ionising radiation.
When carried out in a controlled manner, irradiation has applications in various fields such as the agro-food industry (food sterilisation and preservation) or in the medical field (radiography).
But when the irradiation conditions are uncontrolled (e.g. an accident during the transport of a radioactive source or a critical reactor), this phenomenon is quite a different matter and is considered essentially in terms of the biological and physical effects caused by high doses of radiation.
For lower doses, of the order of those received from natural radiation (cosmic rays, terrestrial radiation and the internal radioactivity of the human body), on average 2.4 millisieverts per year (2.4 mSv/year, effective dose equivalent), the term exposure is used given how difficult it is to establish a relationship between this radiation and any effects on health.
As far as the medical use of ionising radiation is concerned, the doses received are considered to be tolerable artificial radiation, i.e. which can be justified (average irradiation dose due to human activities: 0.9 to 1 mSv/year, of which 0.7 mSv/year is due to radio-diagnosis).