N-6 For the nonprofessional reader, the following is an explanation of the specific radiological meaning of the terms, exposure and dose. Very simply, the medical analogy would be this. A patient takes a spoonful of heart medicine -- radiologically considered, that is his exposure. Of the swallowed medicine, three-quarters are eliminated but one-quarter passes from the intestine into the circulation and is absorbed by the heart -- that one-quarter is the dose. expressed per gram of heart tissue. It would be For exposure to radiation per se, the unit is the roentgen (R), measured in air. For radionuclides (atoms which spontaneously decay - and emit radiation), the units are the bequerel (Bq), equal to 1 atomic disintegration per secofld, or the curie (Ci), 3.7 x 10 1° disintegrations per second. The microcurie (pci) and the picocurie (pCi) are respectively 1 millionth of a curie, and 1 millionth of a microcurie (27 pCi equal 1 Bq). The units of dose are the rad (for any type of ionizing radiation: 100 ergs absorbed per gram of tissue); and the rem (dose equivalent in biological effect to 1 rad of standard radiation). The particular point to remember about radiation dose is that it is per . gram of tissue. A whole-body dose of 100 rad means that every gran (on average) received 100 rad; it does not mean that the entire body received 100 rad to be distributed throughout the tissues. Both exposure and dose are referred to as resulting fron external or internal sources. An external exposure or external dose is the result of a radiation source outside of the body, e.g., fallout-contaminated soil. An internal dose would result from a source inside of the body, e.g., radioactive iodine due to the use of fallout-contaminated drinking water. In the case of radionuclides, we shall use the term "whole-body dose" in the technical sense of “committed effective dose equivalent". Committed- means the dose delivered to the body over the next 50 years from the amount of radionuclide under discussion (e.g., the amount I eat today). Effective signifies corresponding to whole-body exposure (e.g., 1 rem to the entire lungs corresponds to .12 rem whole-body). Dose equivalent in rem signifies that whatever kind of radiation is being used, its dose in rem gives the same effect as that of any other type of radiation expressed in ren. The “tissue dose" is the committed dose equivalent. [3 (The radiation guides, couched in terms of rez, are given in Note 5.] 5000bb4