element define practically the whole problem of long range internal hazard. The analysis leading to the identification of strontium-90 as the only element requiring consideration is not sufficiently definitive to close the subject. A case in point was the unexpected finding of fodine-131 in seemingly appreciable amounts in individuals following the CASTLE series of detonations in the Pacific in the spring of 1954. The dosage delivered to individuals from this experience could not be calculated precisely because of the sparsity of the data, and further work will be needed to determine the hazard due to qts1, The relative importance of strontium-90 to strontium-89 is another case in point. The seriousness of the strcntium-90 problem is predicated on the hypothesis that this isotope will work its way into the biosphere over a considerable period of time. Should this hypoth- esis require modification, then strontium-89 may increase in relative importance. This question will be analyzed in detail in a later section dealing with the strontium-90 problem. In order to assess the biological significance of radioactive fall-out as an internal hazard, it is important to identify the pathological process which will be the limiting parameter. Among the path- Ological processes to be considered are the effects on the bone marrow leading to anemia, direct tissue destruction of gastro-intestinal and urinary structures, genetic effects, and carcinogenesis. Because of the low levels of radiation which have been known to cause carcinogenesis it seems likely that this may be the critical process. Since there is an inverse relationship between the time and the amount of material needed for the effects due to internal deposition, the search immediately centers on the long-lived isotopes which will remain fixed in the body for long periods of time. The 1949 Project GABRIEL report20/ contains the only fairly 36/ Smith, N.M., Project GABRIEL Report, Los Alamos Scientific Laboratory, November, 1949, SECRET Restricted Data. 96