levels can be stated in terms of the REM, which is related to the Roentgen, a unit often used for measuring x–ray dosages. A radiation exposure of about five hundred REM of either gamma rays or neutrons absorbed over a person's entire body (a so-called "whole body" dose) would kill half the people so exposed within a few weeks or less. A radiation dose of about 1,000 REM would kill almost all the people exposed. The prompt radiation is released so rapidly that there would not be time for people in the vicinity of the explosion to take cover in shelters or behind buildings.

Delayed radiation from the fallout of a nuclear explosion could deliver lethal doses to people who remain in the open where radioactive debris has settled long enough for them to receive a total dose of roughly 500 REM. The ranges of distances indicated in Table 2–1 for radioactive fallout are based on the assumptions that the wind velocity in the area is about five miles per hour, and that exposed people remain within the area for one hour, for yields less than one kiloton, increasing to twelve hours for a yield of one megaton. These distances are the most uncertain of any shown in the table, since they depend strongly on the local weather conditions, the amount and characteristics of the surface material that would be picked up in an explosion's fireball and later deposited on the ground, the extent to which people would be able to take cover or leave the area quickly after an explosion, and many other factors.

The distances indicated in Table 2–1 for severe and moderate blast damage and cratering are considerably more predictable than the distances for severe damage by radiation. A peak overpressure of ten pounds per square inch would be likely to cause very severe damage to almost all residential and office buildings, and moderate damage to heavily reinforced concrete buildings. Three pounds per square inch would cause severe damage to wood frame residential buildings.

To summarize, the human casualties and property damage that could be caused by nuclear explosions vary widely for different types of explosions detonated in different places. Nevertheless, it is clear that under a variety of circumstances, even a nuclear explosion one hundred times smaller than the one that destroyed Hiroshima could have a terrible impact on society.