Cosmic radiation

Cosmic rays originating from the sun, stars and other major events in outer space are continuously striking the Earth. The majority of these cosmic rays are absorbed by the Earth atmosphere but the more energetic radiations interact with the atoms in the atmosphere creating energetic neutrons. Other energetic rays such as gamma rays can make it through the atmosphere to the Earth's surface.

Radioactive rocks

The Earth contains radioactive rocks. Since the Earth was formed 4 billion years ago many of the short-lived radioactive rocks have decayed. However radioisotopes with long half lives such as Uranium-235, Uranium-238 and Thorium-232 still remain along with the radioisotopes formed from the decay of these long lived isotopes.
During the radioactive decay of Uranium Radon gas is produced which seeps through rocks underground and introduced into the atmosphere. Radon gas is radioactive and accounts for a large proportion of the natural background radiation.

Human body and food

Small traces of radioactive materials are present in the human body. These come from natural radioactive sources such as Carbon-14 in the air we breathe. Fruits and crops take in radioactive materials from the soil as well as Carbon-14 from the air. These are introduced into the body when they are eaten or via the food chain when meat or milk from animals grazing on the crops are consumed.

Medical sources

Humans are exposed to radiations by medical procedures such as x-rays and radiotherapy.

Nuclear power stations

Major incidents from nuclear power stations have released radiations into the environment. Nuclear waste from power station also accounts for a small proportion of artificial background radiation.

Nuclear weapons

Nuclear weapon testing in the 1950s and 1960s resulted in an increase of radiation in the environment.