ma 25 ¢ The whole-body dose from external radiation in 1968 was essentinily due to natural background radiation. ’ An estimate of this dose is given in the table below. The variability with geographic Location should be within a factor of 2. The higher doses occur in mountain areas where man is subjected to both higher levels of cosmic ridiation and to higher levels of terrestrial radiation because of the rocky nature of the environment. vee Dose Rates of External Irradiation from Natural Sources Source Whole Body Dose Rate Cosmic Rays Ionizing Component 28 millirads/yr Neutrons 0.7 Terrestrial Radiation (including air) Total 50 79 . = The whole-body dose rates from fallout in the northern hemisphere ranged from 1 to 2 mrad per year in the period 1965-1967. Measurements in the United States in 1968 yielded estimates of one-half to one mrad per year. The highest dose rates to any part of the body from natural sources come from inhalation of the short-lived daughter products of radon. Current estimates give local dose rates of several hundred millirads per year to the bronchi, with other portions of the lung receiving smaller doses by factors of 10 (vronchioles) to 100 (alveoli). No other natural or artificial radio- nucl$e produces any significant exposure to lung tissue. It should be noted thet the whole-body dose from inhalation is negligible, since the weight of irradiated tissue is very small. There are no continuing measurements of exposure of the gastrointestinal tract by material passing through. ‘An indication of the magnitude of the dose can be obtained from the following quotations from the 1962 UNSCEY : report.