The methods of measuring radiation and radioactivity, purely physical events, were discussed earlier. In studying the effect of radiation on living organisms, a biological event, the crucial data are the amount of energy absorbed by a specific amount and type of tissue . This requires first measuring the amount of energy left behind by the radiation in the tissue and, second, the amount and type of tissue. 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. 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. 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. It does not directly measure the biological effect of that radiation.