Directorate for Nuclear Safety and Security

The human population is continuously exposed to ionizing radiation from several natural sources; this exposure is a continuing and inescapable feature of life on Earth. Natural ionizing radiation is considered the largest contributor to the collective effective dose received by the world population. The two pie diagrams below show respectively:
  • How the radiation is distributed between natural and non-natural sources
  • How the natural radiation is composed
Public Exposure to different radiation sources (UNSCEAR, 2008)

Natural sources can be classified in two broad categories:
  • high-energy cosmic rays incident on the Earth's atmosphere and releasing secondary radiation (cosmic contribution);
  • radioactive nuclides generated during the formation of the Earth and still present in the Earth‘s crust (terrestrial contribution).

The primary radiations that originates in outer space and impinge isotropically on the top of the earth‘s atmosphere, consist of 87% protons, 11% α particles, about 1% nuclei of atomic number Z between 4 and 26, and about 1% electrons of very high energy.
These radiations originate outside the solar system and only a small fraction is normally of solar origin, however the solar component becomes significant during solar flares, which follows an 11-year cycle.
Primary cosmic rays predominate above 25 km of altitude. The specific character of primary cosmic rays is their high energy which implies a low stopping power and higher penetration capability than is usual for heavy particles. The interactions of the primary particles with atmospheric nuclei produce electrons, γ rays, neutrons, and mesons. At sea level the dominant component of the cosmic-ray field is muons with energy mostly between 1 and 20 GeV.

Cosmic irradiation of the atmosphere produces neutrons and protons which react with Azote(N2) , Oxigen(O2) , Argon(Ar) resulting in the production of radioactive nuclides. These nuclides are produced at constant rates and brought to the earth surface by rain water.
Equilibrium is assumed to be established between the production rate and the mean residence time of these radionuclides in terrestrial reservoirs (atmosphere, sea, lakes, soil, plants, etc) leading to constant specific radioactivities of the elements in each reservoir.
If a reservoir is closed from the environment, its specific radioactivity decreases and this can be used to date marine sediments, groundwater, glacial ice, dead biological materials etc. (Link for more details)
The shorter-lived cosmogenic radionuclides have been used as natural tracers for atmospheric mixing and precipitation processes.

Terrestrial radioactivity is mostly produced by Uranium (238U and 235U), and Thorium (232Th) radioactive families together with Potassium (40K), which is a long lived radioactive isotope of the elemental potassium.
In most circumstances Radon (222Rn), a noble gas produced in the radioactive decay of the 238U progeny, is the major contributor to the total dose.

Radon is a noble gas, colorless, odorless, and radioactive. In nature it presents three isotopes: 222Rn (usually called Radon), 220Rn and 219Rn.When 226Ra (Radium), descendent of 238U chain, decays by α emission, it transmutes to its decay product 222Rn with a half-life of 3.8 days.
Radon exhibits the longest radiological half-life of the radon isotopes and it has a greater opportunity than the shorter-lived radon isotopes to escape to the atmosphere.
Radon enters the atmosphere mainly by crossing the soil-air or building material-air interface. Because soil and also most earth-building materials have 103-104 times higher gas concentrations that the atmosphere, there is a great radon concentration gradient between such materials and open air.
This gradient is permanently maintained by the generation of the 238U and 232Th series from long-lived mother nuclides and is responsible for a continuous flux of the radon isotopes into the atmosphere. The amount of activity released per surface and time unit is called the exhalation rate.
In open atmosphere, it is rapidly diluted in a large volume of air and its concentration rarely becomes a problem. But if radon enters a building, the volume of air available for dilution may be much smaller and high concentrations become possible.

In many case the soil and the bedrock beneath house are the main source but are known cases of high contents of 226Ra in the building material. (Link for more details)

After the European Commission published the “Atlas of Caesium Deposition on Europe after the Chernobyl Accident”, the Radioactivity Environmental Monitoring (REM) group of the Joint Research Centre (JRC) of the European Commission decided to embark on a European Atlas of Natural Radiation (EANR). This is in line with its mission, based on the Euratom Treaty, which is to collect, validate and report information on radioactivity levels in the environment.
This Atlas is intended to familiarise the public with the radioactive environment, to give a more balanced view of the annual dose that it may receive from natural radioactivity and to provide reference material and generate harmonised data for the scientific community.
The Atlas should be a collection of maps of Europe displaying the levels of natural radioactivity caused by different sources (indoor and outdoor radon, cosmic radiation, terrestrial gamma radiation and water). The overall goal of the Atlas is to estimate the annual dose that the public may receive from natural radioactivity, combining all the information from the different maps.