Radiation levels today are considered no different from normal anywhere on earth. It's a bustling city of over 1m residents. Population returned to pre-1945 levels by mid-50s. The bomb was detonated in mid-air, to maximize the physical 'shock' effect, and most of the radioactivity rose up with the mushroom cloud. Compared to (eg) Chernobyl, where tons of radioactive material was involved, the bomb that dropped on Hiroshima contained relatively small amounts of radioactive material. Also, Hiroshima was hit with uranium, less deadly than the plutonium used in Nagasaki.