Receiving a one-sievert dose of radiation in a day is enough to make you feel ill, according to Dedon. "At one to three, you have damaged bone marrow and organs, and you'll really be sick. At three to six you add hemorrhaging, and more infection," he says. "From six to ten, at that level death is something like 90 percent. And above ten, they just call that incapacitation and death."