Arradiated by internal sources; Incorporated in body tissues. that is, by radionuclides These radionuclides gain gentrance into the body through inhalation or through constaminated food or water. Once inside they behave like their mon-radioactive counterparts. Radioactive iodine, for example, accumulates in the thyroid gland in the same fashion as stable iodine, and radioactive strontium or calcium accumulate din the bene similar to their naturally occurring non-radio°F" active counterparts. The radioactive iodine will thus deliver a dosage to the thyroid gland that is many times larger than that to the other organs or to the whole body, and the radioactive strontium and calcium will mainly irradiate the bone. ‘Because of the uneven distribution of radionuclides in the body organs, radiation exposure standards have been developed not just for the whole body, but also for individual ‘organs. In this report we will be referring to the maximum permissible whole body and lung doses. Largely as a matter of convenience, secondary or derived ‘yadiation standards have been developed. These secondary standards, which limit radionuclice concentrations or organ burdens, are often more easily employed than the primary dose standards. We shall examine two secondary standards in this