The phrase applied to the acute effects of the ionizing radiation on the entire, or major part, of the body when the dose exceeds 1 gray (Gy) of X-rays or gamma rays or 1 sievert (Sv) of other types of radiation.
The effects of radiation rely on the dose and duration of the exposure. Total body doses of less than 2 are not likely to be fatal to a healthy adult. At doses of 1 to 10 Gy, transient nausea and occasional vomiting can arise, but these mostly disappear quickly and are usually followed by a two to three week period of relative wellbeing. By the end of this time, the effects of radiation damage to bone marrow and immune system begin to appear, with repeated infections (which might be fatal unless treated with anti biotic drugs) and petechial (pinpoint spots of bleeding under the skin). Certain people are successfully treated with a bone marrow transplant or stem cell or by isolation in a sterile eminent until the bone marrow recovers.
With a dose of 10 to 30 Gy there is also an early onset of vomiting and nausea, but these symptoms seem to disappear after a few hours. However, damage to the gastrointestinal tract, which causes extreme and frequently bloody diarrhoea (known as gastrointestinal syndrome), and overwhelming infection caused by damage to the immune system are expected to result in death between four and fourteen days after exposure.
Acute exposures of 30 to 100 Gy cause the abrupt onset of nausea, anxiety, disorientation, and vomiting. Within hours, the victim often loses consciousness and dies due to damage to the oedema (accumulation of fluid) in the brain and nervous system; these combined effects are known as the central nervous system syndrome.