After the nuclear blasts at Hiroshima and Nagasaki there must have been fallout that blew around the world.

Question: Is there an official, agreed-upon level of radioactivity that made it around the world? How heavy was it? Given the prevailing winds, were there any after-effects suffered by continents and countries and cities next in line and for what length of time were the levels measureable?

I'm sure there are people and agencies who have tracked all this but a single question at ask.com didn't give me much information.