Radioactive material which falls back to the earth after the release into the atmosphere (e.g. by nuclear weapon tests, accidents). The fallout occurs in two forms: the close fallout consists of heavy particles which fall to the ground within some days near the place of release and in an area which, depending on the weather conditions, may reach up to several hundred kilometres in the wind direction. The world-wide fallout consists of lighter particles reaching higher atmospheric layers and distributed over a wide part of the earth because of atmospheric currents.