What is an absorbed dose of radiation?
The risk posed to a human being by any radiation exposure depends partly upon the absorbed dose, the amount of energy absorbed per gram of tissue. Absorbed dose is expressed in rad. A rad is equal to 100 ergs of energy absorbed by 1 gram of tissue. The more modern, internationally adopted unit is the gray (named for the English medical physicist L. H. Gray); one gray equals 100 rad. Almost all the documents from the time period studied by the Advisory Committee use the term rad rather than gray. It is important to realize that absorbed dose refers to energy per gram of absorbing tissue, not total energy. Someone absorbing 1 gray (100 rad) in a small amount of tissue, such as a thyroid gland, will absorb much less total energy than someone absorbing 1 gray (100 rad) throughout his or her entire body. Thus, when speaking of absorbed dose, it is crucial to know the amount of tissue being exposed, not simply the number of gray or rad.
