Natural Radioactivity and Radiation
2026 Editorial Revision
Since the dawn of time, living organisms have been immersed in a flux of natural radiation. Radioactivity is the main source of exposure to this radiation. On a hospitable planet like Earth, this exposure is modest and has not prevented life from developing…
THE PATHWAYS OF NATURAL RADIOACTIVITY:
The radioactive atoms responsible for natural radioactivity have either been present in the rocks of the Earth’s crust since the formation of the Earth or are continuously produced by cosmic radiation. The first category includes uranium, thorium, their decay products, including radon, which passes into the atmosphere, and potassium-40. Cosmic radiation mainly produces carbon-14, which enters vegetation. Cosmic rays and radiation from rocks are responsible for external exposure in humans, while radionuclides inhaled from the air or ingested through water and food cause internal exposure.
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In France, the total dose of exposure to natural radiation is 4.5 millisieverts (mSv) per year per inhabitant, compared with 1.6 mSv from medical examinations.
Overview of the average exposure of the French population. Adapted from Bilan de l’exposition de la population française aux rayonnements ionisants (p. 22), by the French Institute for Radiological Protection and Nuclear Safety (IRSN), 2021.
This is an average, and exposure varies from one person to another depending on altitude, the nature of the soil, housing, how it is ventilated, the frequency of air travel, or whether or not a person practises mountaineering…
We are constantly bombarded by particles from cosmic radiation, hundreds of which pass through us every second. Rocks such as granite, a symbol of permanence and durability, contain traces of slightly radioactive uranium. Sitting on a block of granite or passing close to one means being exposed to the gamma rays emitted by uranium and its decay products.
RADIOACTIVITY IN ROCKS:
Most naturally occurring radioactive elements are found in rocks. This excerpt from the chart of nuclides shows the sequence of radioactive decays from uranium-238 to radon. The decay products of uranium-238, present in trace amounts in these ores, emit alpha and beta radiation that remains trapped in the rocks, while gamma rays escape. The sixth decay product of uranium, radon, is a gas. Some of it escapes from the ground, releasing radioactivity into the atmosphere.
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Through our diet and by breathing, we take in radioactive elements that were produced by cosmic radiation or that date back to the formation of the Solar System. We ourselves are radioactive! Eight thousand potassium-40 and carbon-14 atoms decay every second in the body of an 80 kg person, corresponding to 100 Bq/kg.
The main source of natural radioactivity is a noble gas, radon, produced in the uranium decay chain. Radon, which is chemically inert, would pose little danger if its decay products, which are also radioactive, did not contaminate the lungs. In addition to the nature of the soil, the construction materials of a dwelling and its ventilation are determining factors in radon exposure.
THE ROLE OF RADON
The final stages of the uranium decay chain, starting from radon. This radioactive gas can escape from rocks and enter the atmosphere. Radon, which is short-lived (its radioactive half-life is 3.8 days), rapidly transforms into radioactive lead-210, which has a half-life of 22 years, before eventually becoming stable lead-206. If the rock is deep underground, radon does not have time to reach the atmosphere and the radioactivity remains in the ground. If the rock is close to the surface, the decay products of radon released into the atmosphere attach themselves to aerosols and can be deposited in the lungs.
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It should be emphasised that an exposure of 2.4 mSv represents a low dose. In some parts of India, China or Brazil, natural exposure reaches 10 or even 50 millisieverts per year (20 times the French average). The fact that living organisms have adapted to these conditions, developing without apparent difficulty for millions of years even in these areas of higher exposure, suggests that radiation doses of a few millisieverts are indeed low.
– UNSCEAR 2000: SOURCES OF IONIZING RADIATION: Man-made sources
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