[Vou. 91 1 therefore e daughter ent of soils 1? curies of ms of most neous rock il respects. rranite. Its Rubidium, andanee in the atmosowder and des in sea 1964] .WARTIN : RADIOECOLOGY AND STUDY OF ENVIRONMENTAL RADIATION 287 ingested with food and water or other nonliving materials. According to Hursh (1955), the most important natural radionuclides assimilated by man are C-14 and K-40 (Table 4). According to Libby (1955) and others, people who live between sea level and an altitude of 5000 feet above sea level in areas underlain by granite are normally exposed to about 200 mr/year of ionizing radiation from natural sources. Roughly 25% of this total is due to cosmic radiation, 50¢e to external emitters, and 25% to internal emitters. b. ‘‘Man-made’’ sources. The major man-made sources of ionizing radiation in the biosphere are radionuclides produced by atomic fission or by neutron induction and released to the atmosphere as reactor effluents or as fallout from nuclear weaponstests. Table 3. Estimated activity of naturally oceurring radionuclides in the above ground parts of an acre of green crop plants weighing 50 tons*. Radio- Activity in Radio- Activity in which are nuclides euries x 10-9 nucildes curies x 10-9 [In soil, for K-40 Rhb-87 C-14 75,600 19,900 13,500 Ra-226 U-238 H-3 400 157 156 sea water m in fresh 123 x 104 31.0 x 10738 156). Since L957), this e acre-foot water. are readily id H-3 are d C-14 in itration of ve ground tain about g only the ratlo on a ‘le erop of soil.* ym the at- ionuelides erived from * From: Fried & Heald (1959). Table 4. Estimated activity of naturally oecurring radionuclides in a man weighing 70 kilograms". Radio- Activity in Radio- Activity in nuclides euries x 10-12 nuclides curies x 10-12 K-40 C-14 190,000 85,000 Ra-266 H-3 100 to 290 6.3 to 31.5 * After: Hursh (1955), The direct effects of neutron and gamma radiation associated with the operation of a nuclear reactor are largely confined to the immediate vicinity of the reactor. Reactor effluents containing small quantities of fission products and radionuclides produced by neutron induction may be released to environmental media, but only local areas are affected. The disposal of highly radioactive reactor waste products is usually aeeomplished in a manner which prevents their release to the environment. The direct effects of neutron and other ‘‘prompt’’ radiation released by the detonation of nuclear weapons are usually confined to areas in which the effects of blast and thermal radiation are more important. The radionuclides produced by nuclear detonations in the atmosphere are—by virtue of their quantity, world-wide distribution, and potential biological significance—the most important man-made sources of radioactivity in the biosphere. The slow fission of uranium produces at least 170 isotopes of 35 lighter