Natural carbon-14 (C-14) is generated by a (n,p)-reaction of neutrons of cosmic radiation with nitrogen -14 in the upper atmosphere. Measurements of wood from the 19th century resulted in about 230 Becquerel C-14 per kilogram carbon. This natural (pre-industrial) ratio between the radioactive carbon-14 and the stable carbon-12 in the atmosphere is today influenced by two opposed effects:

  • The massive generation of CO2 by burning fossil C-14-free energy carriers leads to an increase in the proportion of C-12. Thus the pre-industrial ratio of C-14 to C-12 is reduced. In the mid-50s, this so-called Suess effect resulted in a five percent reduction of C-14 activity per kg carbon in the atmosphere.

  • Nuclear weapon tests in the atmosphere and disposal from nuclear facilities cause the proportion of C-14 in the atmosphere to increase.

The natural concentration of C-14 in the human body leads to a C-14-activity of about 3 kBq. The resulting effective dose amounts to 12 ยตSv/year.