biological terms. analyzed. For the present, lethality is the parameter to be The lethality expectation for an exposed population is de- pendent upon the time of arrival of fall-out, the dose rate at that time, the degree of shielding effective, and the time spent in the contaminated area. The analysis used here assumes instantaneous depo- sition of fall-out for any time of arrival and physical decay character ized by the ete? law. The biological factors of recovery and residual damage previously discussed are also included. Figure 15 is a plot of 100%, 50%, 0% of lethality expressed in terms of the dose rate at time of fall-out. oped in the following manner. This figure was devel- From Figure 13 the maximum ratio of Rere!T(t ) for each (t, ) can be determined. This is shown in Table 5. TABLE 5 ty Max. Ratio Reee/i(ty) 1 2.2 3 6.0 8 11 13.0 16.4 > 9.0 15 20.6 Repe/T(ty) = Ratio of effective roentgens to the dose rate in r/hour at (t,)- (t,) = Time of arrival of fall-out, where total fall-out is assumed to occur instantaneously at any one location. The maximum ratio is solved for r(t,) with: Rape = 600 r = ID, Rope = 100 r = ID R ere 50 = COO r= ID, The LD, curve for residual damage is calculated on the basis of a total cumulative dose to infinity of 1000 r. This leaves a resid- ual of 200 r on the basis of a 20% residual damage fraction. 71 The