Let us compare the radiation we are concerned about with another type of radiation. Just as we measure radioactivity in roentgens we measure heat in calories. But, if I then said that it was going to be over the next million years, at the rate of one calorie per year, you would realize that you were in greater danger of freezing to death than of burning to death if you were depending upon that pipe for heat. A mere two hundred calories suddenly inflicted upon one point of the skin would create a bit of a sting, but hundreds of thousands might be comfortably absorbed from a heating pad over an appropriate period of time. It is the same with radiation.
Most isotopes give off their energy so rapidly that they are like flash bulbs.
Flash and they are gone. It just happens right in the vicinity of the bomb. Others are like regular light bulbs that give off their light and heat for some period of time before they burn out. They may travel a long way from the bomb as fallout before they dissipate their energy. For these we need a shelter to protect us if we are in their vicinity.