The sum of all radiation originating from natural sources in a certain location. Almost everything contains radioactive isotopes and can be a source of radiation — the rocks, the air, the water, even the human body and our food. Cosmic rays are also a source of radiation. These sources together form a natural background radiation, which differs from place to place and depends, for example, on the altitude or the composition of the underlying rocks. The typical annual dose received from natural background radiation in the US is about 3 mSv.