What are the sources of natural radioactivity ?

Radioactivity is part of the universe. It can be found everywhere and its presence is not subject to human intervention. Right from the beginning, some five billion years ago, the earth's crust and atmosphere was made up of elements that were stable and others that were radioactive.

 

Since then, natural radioactivity has decreased continuously by the transformation of many radioactive atoms into stable elements. Among the 340 different atoms present in nature today, 70 have unstable radioactive nuclei (~radionuclides). Around 20 so-called primordial radionuclides still exist. The others, with lifetimes shorter than the age of the earth, have disappeared.

Presently, the main natural radionuclides are potassium 40 and those coming from the three radioactive families of uranium-238, uranium-235 and thorium-232. These radioactive elements are found in the atmosphere, the ground, water and in living organisms, including human beings.

• Natural atmospheric radioactivity

Natural atmospheric radioactivity is mainly due to the presence of radon-222, part of the natural radioactive decay chain of uranium-238. Radon is responsible for the majority of public exposure to natural radiation and its concentrations vary greatly from location to location. They depend on the concentration of uranium-238 in the ground, the porosity of the soil, construction materials and the effectiveness of ventilation in buildings, where radon gas can accumulate. This gas and its daughter products, produced though successive radioactive decays, can lodge in the respiratory system.

                                                                                                                 

• Natural radioactivity of soil and rock

Many elements found in the earth's crust are radioactive, such as uranium and thorium. Radioactivity levels vary greatly depending on the type of soil or rock. For instance, it is 5 to 20 times higher in granitic than in sedimentary materials.

 

• Natural radioactivity of water

The level of radioactivity in natural waters depends more on the chemical characteristics of the water than the geology of the areas from which the water drains. Mineralized spring water is more radioactive than surface water, while some groundwater is rich in dissolved radon gas.

 

• Natural radioactivity of the human body

The radioactivity of the human body is about 8400 Bq for a person weighing 70 kg. It mainly comes from radioactive elements present in the food we eat. Once ingested, the radionuclides are lodged in body tissues and bones. The human body has an average radioactivity of 4500 Bq from potassium-40 and 3700 Bq from carbon-14.