radiation exposure, a measure of the ionization produced in air by x-rays or gamma rays. It is the sum of the electric charges on all ions of one sign that are produced when all electrons liberated by the radiation in a volume of air are completely stopped, divided by the mass of air in that volume. The unit of exposure is the roentgen. Acute radiation exposure is exposure of short duration to intense ionizing radiation, usually occurring as the result of an accidental spill of radioactive material. Exposure of the whole body to approximately 10,000 rad (100 gray) causes neurological and cardiovascular breakdown and is fatal within 24 hours. A dose between 500 and 1200 rad (5 and 12 gray) destroys GI mucosa, produces bloody diarrhea, and may cause death in several days. A dose of 200 to 500 rad (2 to 5 gray) destroys the blood-forming organs and may cause death in a few weeks.
