HUMAN RADIATION INJURY proportion of contaminant from most surfaces. That remainingis firmly fixed and is notlikely to become airborne easily. Ifa personnel decontamination center is established, it should be relatively mobile and isolated from more permanent buildings where definitive care is given. This stems from the fact that contamination can only be transferred, not destroyed, and the decontamination area is likely to become quite “hot” in a relatively short time. Tinned goods can be eaten with complete safety and it is highly unlikely that city water systems outside the area of blast damage will be contaminated soon after a burst. One thing appears to be certain—anyeffects from internal radiation will be long range and will be of no concern in the acute period. Total body radiation from gamma rays, and skin irradiation from beta emitters will be the chief radiological concern at early times following an explosion. 6.36 Evasive Action; Protection From Fallout Some warning of possible fallout will be available and the falling radio-active material mayactually be visible. As stated, the pattern of fallout will depend on wind velocities and other weather conditions, and the pattern is thus difficult to predict under the best of circumstances. However, it will be apparent that in closer-in areas, fallout may not occur for sev- ern! minutes after the blast and this period may extend to several hours at greater distances and with slower wind velocities. Thus, there issome time for evasive action. Consideration might be given to evacuating the area if possible fallout patterns have been investigated and are believed to be predictable. Or it may be possible to take shelter. Sufficient time probably would be available to allowrelatively complete preparation for an extended stay in adequate shelters with storing of sufficient food and water to allow some advantage to be taken of the decay of fission product radiation with safer evacuation of an aren a few days after the fallout. Facilities may, for the most part, be essentially intact, such as water, power,fire-fighting equipment, etc. In this sense, at least, one is im- . 99 measurably better off than within the area of blast and thermal damage. With regard to effectiveness of shelters in the fallout area, the following estimates have been released. A frame house would reducethe total dose received by one-half, and a brick or concrete structure would be more effective. A basement would reduce the total exposure to one-tenth of its value. Ina shelter of thickness equivalent to three feet of earth, the dose would be reduced to one five-thousandth of its value, affording complete protection in the most heavily contaminated areas. It should also be noted, on the other hand, that while the decay of fission product radiations is extremely rapid over the first few minutes after detonation, the rate of decay becomes considerably less rapid in the succeeding hours (2). Thus, with fallout occurring some hours after the blast, if adequate shelter is not available, exrlier evacuation may be better than relying on partial shelter and on rapid decay of the radiation field. Starting at 1 hour after the blast, a given dose rate will fall to about 44 percent of its value by 1 hour later. However, at 10 hours after the blast, a given dose rate will fall by only 11 percent of its value in a period of 1 hour, e. v., the dose rate ut 11 hours will be 89 percent of what it was at 10 hours. Such statements as “more than 80 percent of the radiation dose from atomic debris will be delivered within 10 hours of the explosion time” are true only if fallout occurs immediately after the cletonation. If the maximum fallout and thus maximum exposure rates in a fallout area have not occurred for severn! hours, the rate of fallout in the area obviously will not be as rapid as it would be for earlier fallout material. 6.4 Estimation of the Severity of Exposure to Gamma Radiation 6.41 Predictions From Physical Estimates of Dose If the absolute sensitivity of man to radiation were known, andif it were feasible to cetermine the dose to groups under catastrophe