The two most important chains providing nuclides of significance in NORM are the thorium series and the uranium series: Another major source of terrestrial NORM is potassium 40 (K-40).
The long half-life of K-40 (1.25 billion years) means that it still exists in measurable quantities today.
At higher altitudes, the dose due to both increases, meaning that mountain dwellers and frequent flyers are exposed to higher doses than others.
For most people, cosmogenic NORM barely contributes to dose – perhaps a few tens of microsieverts per year.
A 70 kg person has 4400 Bq of K-40 – and 3000 Bq of carbon-14.) Cosmogenic NORM is formed as a result of interactions between certain gases in the Earth’s atmosphere and cosmic rays, and is only relevant to this paper due to flying being a common mode of transport.
Since most cosmic radiation is deflected by the Earth’s magnetic field or absorbed by the atmosphere, very little reaches the Earth’s surface and cosmogenic radionuclides contribute more to dose at low altitudes than cosmic rays as such.
The list of isotopes that contribute to natural radiation can be divided into those materials which come from the ground (terrestrial sources – the vast majority) and those which are produced as a result of the interaction of atmospheric gases with cosmic rays (cosmogenic).
The materials may be original (such as uranium and thorium) or decay products thereof, forming part of characteristic decay chain series, or potassium-40.
This means that material which is considered radioactive waste in one context may not be considered so in another.
Also, that which may constitute low-level waste in the nuclear industry might go entirely unregulated in another industry (see section below on recycling and NORM).
All minerals and raw materials contain radionuclides of natural origin.
The most important for the purposes of radiation protection are the radionuclides in the U-238 and Th-232 decay series.