With the exception of potential large scale nuclear accidents, natural ionizing radiation is considered to be the largest contributor to the collective effective dose received by the world population.
The human population is continuously exposed to ionising radiation from several natural sources that can be classified into two broad categories: high energy cosmic rays incident on the Earth’s atmosphere and releasing secondary radiation (cosmic contribution); and radioactive nuclides generated during the formation of the Earth and still present in the Earth’s crust (terrestrial contribution). The terrestrial contribution is mainly composed of the radionuclides of the uranium and thorium decay chains together with radioactive potassium. In most circumstances, radon, a noble gas produced in the radioactive decay of uranium, is the most important contributor to radiation exposure.
Natural radionuclides, both terrestrial and cosmogenic, migrate in the environment through different pathways: air, water, rock, soil and the food chain. Radionuclides may then enter the human body through ingestion (food and drinking water) and inhalation giving, so-called, internal exposure. External exposure is due to cosmic radiation and radiation from terrestrial radionuclides present in soil, rocks and building materials.
To gain a clearer overview of the natural sources of radioactivity, the Joint Research Centre of the European Commission launched the European Atlas of Natural Radiation with the collaboration of more than 60 institutions. Intended as an encyclopaedia of natural radioactivity, the Atlas describes the different sources of this kind of radioactivity and represents the current state of knowledge on this topic. It also contains maps of Europe that show the levels of natural background radiation originating from various sources.
You may download the full publication of European Atlas of Natural Radiation from the JRC Publications Repository. More information can be also found on the Digital Atlas of Natural Radiation.