19/ writing in the Military Surgeon, has used a factor of 10% per day recovery for a 20-day period, and according to him this scheme breaks down beyond this point. Although some biological recovery certainly takes place, there is also a degree of permanent residual damage from which an animal never recovers, or at best, recovers at a rate so slow as to be diffi- cult to measure. From a quantitative point of view, 20% of the total incident gamma radiation received is considered tobe an irreparable equivalent and 80% to be reparable according to the factors given above, i.e., 20% per day by E. H. Smith and 10% per day by F. McLean. A quentitative approach to lethality estimates may be made by using the following: (1) ‘Instantaneous lethal curve, such es Figure 12. (2) Total dose and rate of administration. (3) Biological recovery rate, using 10% per day and assuming a first order recovery process. (4) Irreparable recovery fraction, using 20% of total gamma dosage. (5) Fission product decay law, I * 1,078, where I, = radiation dose rate at time ¢ I, 1 t = radiation dose rate at wit time = time The final equation in a fission product fall-out field then pecomes20/ R_,,(t) eff to 0.2 z(t) * *o - (2) 1.2 -Bt + 0.8t,""e e Bt [ ra*. 3° 19/ McLean, F.C., et al., Extension to Man of Experimental Whole-Rody Irradiation Studies. Some Military and Civil Defense Considera- tions, Military Surgeon 112, 1953. 20/ Latter, A.L., The RAND Corp., Feb. 1955, Personal communication. 66