Page "Helium" Paragraph 4

On Earth it is thus relatively rare — 0. 00052 % by volume in the atmosphere.

Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements ( thorium and uranium ), as the alpha particles emitted by such decays consist of helium-4 nuclei.

This radiogenic helium is trapped with natural gas in concentrations up to 7 % by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation.