Radiation Doses and Dose Limits

•Radioactive material has always been a natural part of the earth. It has existed for millions of years in the crust of the earth, in building materials, in the food we eat, the air we breathe, and in nearly everything that surrounds us. Radiation from these materials, as well as cosmic radiation from the sun and universe, makes up the natural background radiation to which we are constantly exposed. On the average, persons are exposed to about 300 millirem per year from natural sources (NCRP Report No. 101).

•The guidance from NCRP Report No. 116, Limitation of Exposure to Ionizing Radiation, states that for life saving or equivalent purposes, workers may approach or exceed 50,000 mrem to a large portion of the body. Emergency exposures are considered once-in-a-lifetime. This is below the threshold for the acute radiation syndrome, discussed later.

•If an individual is exposed to more than 100 rem at one time, predictable signs and symptoms will develop within a few hours, days, or weeks depending on the magnitude of the dose. About half of all people exposed to a single dose of 350 rem will die within 60 days (LD50/60) without medical intervention. The large doses used in medicine for radiation therapy, while higher than this dose, are given to only part of the body and are typically given over a period of weeks.

•Heart catheterization is a skin dose; barium enema is an effective dose. (NRPB Report R-200, 1986)

•The dose limits are highlighted in orange.