oem . eee Vol. 66 2 te ee ee aeeeee ee peers Gamma RADIATION Exposures FRoM FaLtoutr et renee oo ene sO ry, 589 ” ship for timed doses versus biological effects; yet there are sufficient convincing data to pernit an attempt at estimating the effect of this phenomenon. Blair (6, 7,) Smith (8), Davidson (9), rt. and others have made extensive analyses of existing data on the effects of timespaced doses for several species oi animals. t a’ + accumulated radiation dose for persons living normally in a contaminated area (Graph 3). Since Graph 3 is based on an essumed dose rate of 1 r per hour at the time of fafiout, the accumulated doses may be linearly extrapolated to any other dose rate at fallout. For example, if fallout befins at three hours after detonation and the ar et een vr o | « o e © o: o 5 Le L ae oe & “ug 1 Len Jecgrnemciarenrneentee . Graph 4. ° ne T™ | 10 poy OURATION OF EXPOSURE TIMED DOSES AND BIOLOGICAL EFFECTS It has been recognized that, in general, the longer the period over which a given radiation dose is delivered, the less is the resultant biological effect, except for such aspects as the genetic. Since past experiments usually have been designed for other purposes, the data from these do not readily elucidate the rate of repair or the proportions of reparable and irreparable damage resulting from differently timed doses. Varying relationships have been demonstrated, depending upon the species or even the strain of animal, as well as the criteria selected for study, such as skin damage, life shortening, and LD 50 values. Our present knowledge does not permit establishment of a precise overall relationoat po . ey 1000 fs Ratio of total accumulated equally fractionated daily gamma whole-body doses to a one-day exposure * to produce the same whole-body effects. é dose rate at that time is 10 r per hour, then about 90 r might be accumulated by personnel continuing to live normally in the contaminated area. ~~ st pt 100 Generally, the recovery rate for larger mammals, such as dogs,is significantly less than for mice. One estimate places the half-time recovery for man at four weeks (9). The most conservative estimate of the effect of time-spacing of doses, for application to the problems under discus- sion, is that of Davidson. On the basis of his analysis, a plot has been constructed (Graph 4) of accumulated, equally fractionated daily doses versus an acute ex- posure which would result in the same whole-body effect (death or sickness). This analysis indicates, for example, that if a radiation exposureis divided into equal daily doses, the total amount accumulated over eighty days would be twice the amount required by a one-day exposure to produce death or sickness. The calculations necessary to incorpo- rate the factor of timed doscs into those for radiological decay, weathering, and shield- ing are rather tedious. An approximation