picture. In almost all cases no direct accurate quantitative data for man are available. Animal data, occasional accidents involving human exposure, theory, and opinion constitute the basic data for setting standards and estimating the degree of hazard for various levels of exposure to radiation. The fact that the fall-out radiation field is a decaying field which for local fall-out is rapid at first and then slows down, has important biological implications. This makes the time of entry into the field as well as duration of stay necessary parameters to consider in computing biological recovery factors. Each of the parameters involved in human hazard will be discussed in order to arrive at a quantitative scheme for assessing the isolated characteristic, e.g., lethality. Deviations from this ‘standardized scheme due to particular aspects of any single situation will be dis- cussed. This basic standard wili constitute a reference and may be used as a point of departure for other situations. For some of the parameters of interest sufficient data are not available. In fact, even in assessing lethal expectation, the scheme is subject to limitations but represents the best data available at this time. Lethality Expectation for Total Body Irradiation. The assessment of acute, i.e. within 30 days, lethality expectation due to exposure to radiation in a fall-out area involves several complex phenomena, both physical and biological. At present a simplified approach seems likely to yield the best general picture. Variations in the physical decay factor, the gt? law, are likely to be small and will not be considered. For the biological factors involved, quantita- tive constants which are admittedly somewhat tenuous will be applied and used without considering variations from the values commonly employed. The basic curve for predicting lethality due to instantaneous total body ganma radiation is given in Figure 12.17 17 Until the CASTLE Mitchell, H.H., A General Approach to the Problem of Estimating Personnel Dosage on Atom Bonbed Targets, The RAND Corporation, RM-1149, 1953, SECRET. 63