The U.S. Office of Civil Defense illustrates facts about fallout from nuclear explosions. Radiation can make a person sick and even kill them. The three best defenses are distance, mass, and time. Get as far from the fallout as possible, which may mean going to the middle of a large building rather than near its outside; have concrete and other materials between you and fallout; and, allow for the fact that radioactive material loses its potency over time: e.g., two weeks after an explosion, it's at 1/1000th of its initial strength. Fallout shelters are the best civil defense: the federal government has a program to ensure adequate fallout shelters for all.