where Rare = amount of instantaneous radiation required to give the same effect on lethal scale. (t) = Time of evaluation of R..,- r(t,) = Rate at time (t,) of initiation of exposure. B = recovery constant, here assumed to be 10% per day or 0.0042 per hour. Figure 13 is a graphic representation of the above equation for various times of entry into a fall-out field (t,) up to 19 hours. This scheme for calculating lethality expectation must be used with caution, and must be restricted to the job it is intended to perform. It will be noted that beyond a certain time the effective roentgen value decreases. This is not intended to signify that the individual is able to continue receiving radiation without further danage. It does signify that for a given radiation experience there is a maximum Rare at which point additional radiation will have its maximal significance with regard to acute lethal effect. It also signifies that beyond the point of maximum Rofr the amount of additional radiation needed to produce a given acute lethal effect does increase. The as- pect of damage to the individual which is a function of the total dose continues to increase during the period of radiation beyond the point of maximum Rape’ In a real sense, therefore, the biological damage factor is a function of the total irreparable dose and the R,te is an index relating lethality expectation of further radiation to the past radiation experience of the individual. However, it should not be over- looked that R.fr is bounded by the lethal dose, and that once this dose is reached, biological recovery factors no longer play a role, nor does additional radiation have significance. Figure 14 is a graphic representation of a fall-out situation occurring at H+3 hours with an initial dose rate of 40 r/br leading to a total integrated dose of 600 r which would be LD,69 if received instantaneously. The upper broken line indicates the cumulative dose 67