Natural radioactive background as a factor for external and internal exposure of the population

According to the Regulation on basic norms of radiation protection (RBNRP-2004) the natural radiation background is a radiation field caused by natural sources. Under unchanged natural radiation background one means such a natural radiation background, which is not affected by human activity.

"Natural source" is a source of ionising radiation, existing under natural conditions – the cosmic radiation and the naturally spread radionuclides in the environment, in foods and in human body.

Life on Earth has appeared and exists in the environment of natural ionising radiation. According to the contemporary science the age of the Universe is estimated to be about 15 billion years and the Earth, as one of the planets in the Solar system, have emerged before 4.5 billion years. The solar light and heat, due to which life on Earth exists, are obtained as a result of nuclear reactions in the Sun. Nuclear reactions occur also in the galactic space.

During these nuclear reactions there appear powerful flows of highly energetic ionised particles, which form the primary cosmic rays. They consist of two components, which significantly differ by their origin as well as by the composition, intensity and energy:

- galactic cosmic rays;
- solar cosmic rays.

The intensity of the galactic cosmic rays, which reach the Earth, is constant and its influence is isotropic (uniform in all directions towards the Earth). They are composed of protons (atomic nucleuses of Hydrogen) – about 90 % and about 10 % of atomic nucleuses of light elements such as α-particles (atomic nucleuses of Helium), photons, neutrons, electrons (Lithium, Beryllium, Carbon, Oxygen, Nitrogen) which originate in the depth of the Universe as a result of emission and evaporation of matter during explosion and creation of new stars.

The intensity of the solar cosmic rays is not constant and varies depending on the solar activity. The cycle of changes in solar activity is 11 years.

When the primary cosmic rays reach the atmosphere of the Earth, the highly energetic ionising particles interact with the atoms and molecules of the air-creating multitude of electrically charged and neutral particles. They, in turn, create “avalanches” of new ionising particles in the atmosphere and thus create secondary cosmic rays, which consist of photons (γ-quanta), electrons, neutrons and other elementary particles.

The intensity of the cosmic rays depends on the altitude as well as the place (geographical latitude) of a given region of the Earth. The intensity of the cosmic rays at the poles of the Earth is about 10 % higher compared to the equatorial regions.

The Atmosphere that envelopes the Earth and spins together with it extends to an altitude of about 1 000 km. The atmosphere may be described as a “shield” which covers the Earth and absorbs the greater part of the cosmic rays, directed towards it slowing or stopping the powerful flow of highly energetic particles coming from Space. Owing to this “anti radiation shield” the ionising radiation, which falls on Earth, is significantly decreased. With the increase of the altitude the thickness of the protective atmosphere layer decreases and that is why the exposure to cosmic rays in the high mountainous areas is greater compared to that at the sea level, where the protective layer is thickest and more dense. For example, at mount Everest, the highest point on Earth (8848 m.), the influence of the cosmic rays has 20 times greater intensity than the areas at the sea level.

Earth has one more natural shield, which mitigates the impact of the cosmic rays on its inhabitants – this is the Earth’s magnetic field. Earth resembles a giant magnet whose magnetic field deflects part of the electrically charged particles coming from Space forcing them to circumvent our planet. The other part of the charged cosmic particles under the influence of the magnetic field of the Earth are deflected (whirled) in the equator area and concentrate in the regions of the geographical poles following the configuration and the direction of the magnetic field lines surrounding the Earth. That is why the intensity of the cosmic rays in the equatorial areas is minimal while in the areas of the North and South poles it is maximal. Due to the existence of the Earth’s magnetic field the intensity of the cosmic rays changes depending on the latitude. For example, at about 50 degrees northern latitude (London, Moscow, Tokyo, New York) the exposure from cosmic rays is about 30 – 40 % higher compared to areas with 15 degrees latitude on both sides of the equator.

The cosmic radiation is a natural phenomenon and exposure to it cannot be avoided or be managed in any way by men. The typical value of the average annual individual dose, which is received by external exposure from cosmic radiation, for middle Earth latitudes, including Bulgaria, is 0.4 mSv/y (0.4 millisievert per year).

The Earth’s crust (soils, rocks, minerals) and the living environment (air, water, flora, fauna) contain various natural radionuclides form cosmic and terrestrial origin. The natural radioactive elements are contained also in the foodstuff and the human organism.
Natural radionuclides of terrestrial origin may be grouped in the following way:

- radionuclides from the existing in nature three natural radioactive chains of decay: uran-radiun chain with parent radionuclide Uranium-238 (half-life 4,5 billion years), thorium chain with parent radionuclide Thorium-232 (half-life 14 billion years) and actinium chain with parent radionuclide Uranium-235 (half-life 700 million years). All these chains contain 47 different radionuclides.
- radionuclides from the middle part of the periodic table of elements – 12 in total, all with half-life more than 10 billion years, from which the main are Potassium-40, Rubidium-87, Calcium-48.

The radionuclides, which are formed as a result of the primary processes during the formation of the Earth as a planet, are called “primary radionuclides”. The highest content of natural radionuclides is found in volcanic rocks (for example granite). The content of natural radionuclides is also high in sediments such as clay, phosphates and other natural minerals.

Due to the extremely large half-lives, commensurable with the age of the Earth, the parent radionuclides and the progenies of the three natural radioactive families, as well as the rest of the natural radionuclides (Potassium-40 etc.) continue to exist in the crust of the Earth up to present days.

The presence of natural radionuclides on the surface of the Earth leads to external exposure and the dose rate of the earth’s exposure is directly proportional to the amount (concentration) of the natural radionuclides in the soil. The concentration of radionuclides in the soil depends on the radioactivity of the substances from which the soil is composed and on the processes of sorption, deposition and washout of radionuclides upon the influence of natural waters on it.

The natural γ-background on the surface of the Earth is created mainly from radionuclides which are contained in the upper soil layer with thickness of about 30 cm. Due to the shielding effect of the soil the γ radiation from natural radionuclides lying at a depth greater than 30 cm. significantly diminishes and does not play a significant role for the formation of the γ - background.

35 % of the external exposure as a result of the natural γ–background from the Earth’s surface is due to Potassium-40, 25 % - of the γ emitting radionuclides from the Uranium-238 chain (mainly Lead-214 and Bismuth-214) and 40 % of the γ emitting radionuclides from the Thorium 232 chain (mainly Thallium 208 and Actinium-228). The investigations in many countries show that it is completely normal that the natural γ–background, caused by the Earth’s radiation, to differ up to 6 times for different regions of the Earth. It is common practice the natural γ–background to be measured at a height of 1 m from the surface.

The population living in the mountainous places and the regions rich on granites, minerals and soil with high concentration of natural radionuclides is subjected to higher external exposure. The γ–background in such places is higher.

When the exposure of the population to natural radionuclides is evaluated it should be taken into account that the significant part of his time the person stays in the premises where he lives and works. In principle, the walls, floors and roofs of the buildings shield (weaken) the natural γ–background. On the other side, however, the construction materials (bricks, concrete, plasters) from which the buildings and homes are made contain natural radionuclides with various concentrations. This leads to the exposure of the people living in the buildings. The γ–background inside the buildings constructed from bricks, concrete and granite, as a rule, is higher than that in the wooden constructions. The results of the conducted investigations in the world show that the γ–background inside the premises in buildings is, on the average, 20 % higher than in the open air.

Upon the assessment of the average annual effective dose for the population on Earth, the exposure from the natural radionuclides indoor and outdoor should be considered separately. It is estimated that the contemporary man spends approx. 80% of his time in buildings (19 hours per day), and 20% outdoor (5 hours per day), which means that the natural indoor exposure of a person dominates the outdoor one.
It is established that the gamma-radiation, caused by the natural radionuclides in the ground surface layer and in building materials, causes external exposure, which, averaged over the world, is estimated to be 0.5 mSv per year (0.5mSv/y). For 95% of the Earth’s population the average individual annual effective dose from the natural (coming from the ground) gamma-radiation varies between 0.3mSv/y and 0.6mSv/y.

The γ-background (coming from the ground) varies during rain and snowfall. The blanket of snow decreases the γ-background with 10%, up to 20% in some areas, because of the shielding effect. Rains bring down to the ground surface natural radionuclides (radioactive aerosols), contained in the near the surface ground air (water washes down the dust particles in the air) and as a result of this, the γ-background could increase in some areas by 20% to 50% (especially in heavy rains or at the initial hours of the rain). The influence of the meteorological factors should always be taken in into account, when the γ-background of a given area is assessed, because the measured moment-values could fluctuate up to 50% for some time interval (hours or days).

And in conclusion, in the world average effective dose from external exposure for the population, due to cosmic rays (0.4mSv/y) and the ground radiation (0.5 mSv/y), is totally equal to 0.9 mSv/y.

In addition to the external exposure, the natural radiation background causes internal exposure when certain natural radionuclides get into human body by inhalation and swallowing.

The main radionuclides from the Uranium-Radium and Thorium chains, which cause internal exposure to humans, are Uranium-238, Radium-226 and Thorium-232. The radionuclides with terrestrial origin get into human body through inhalation of dust particles, lifted from the ground and by swallowing of food and water. Excluding Polonium, which accumulates in the soft tissues, all other natural alpha-nuclides accumulate mainly in human bones. The internal exposure from Thorium-232 is about 3 times less than the internal one from Uranium-238. From all known 13 Radium radioisotopes, the main contribution for the internal exposure has Radium-226, which enters the human body mainly through food.

Another main source for the natural internal exposure is Potassium-40, which enters the human body mainly through food. The natural Potassium contains 2 stable isotopes: Potassium-39 (93.08%) and Potassium-41 (6.91%) and one unstable (radioactive) isotope – Potassium-40 (0.01%) with half-life 1.3 billion years. Everywhere in nature this percentage is same for the three Potassium isotopes. Every living organisms in nature have in them Potassium, which is a typical bio-element. Living organisms, assimilating Potassium, don’t change its nuclide content.

The content of Potassium-40 in food varies in wide ranges – from 20 up to 600 Bg/kg. In the human body, depending on age, Potassium concentration varies between 1.0 and 2.5 g/kg. The Potassium is concentrated in human muscles. In young people Potassium concentration, Potassium-40 respectively, is approx. 2 times higher than in adults. The average specific activity of Potassium-40 in human body is 60 Bq/kg.

The average annual effective dose, which the Earth’s population receives from internal exposure to natural earth radionuclides (mainly Uranium-238, radium-226, Thorium-232 and Potassium-40), contained in the human body, is estimated to be 0.3Sv/y.

Cosmogeneous radionuclides are formed mainly as result of the interaction between the cosmic rays and the atomic nucleuses in the air and to a lesser degree by the interaction with atomic nucleuses, contained in the Earth’s crust. There are about 20 cosmogeneous radionuclides known, the main being Tritium, Carbon-14, Berilium-7, Sodium-22, Phosphorus-32, Sulphur-35 and Chlorum-36.

Upon estimation of the internal exposure from cosmogeneous radionuclides, the radioactive impact of Carbon-14 and Tritium should be taken into account (for Carbon-14 the annual effective dose from internal exposure is about 0.001 mSv/y, and for Tritium is much less). The internal exposure caused by radionuclides with cosmogeneous origin, is negligible, compared with the internal exposure due to radionuclides with terrestrial origin.

The major contribution to the internal exposure of the Earth’s population has the natural radioactive element Radon (Rn-222) with atomic number 86 in the Mendeleev periodic table. The average annual effective dose, obtained by a person as a result of radon inhalation and its short-living progenies in air, is estimated to be 1,2 mSv/y. Hence the natural internal exposure, due to Radon and its progenies (1.2mSv/y) is 4 times greater than the internal exposure, due to the natural radionuclides with cosmogeneous and terrestrial origin. (0.3mSv/y).

Radon (Rn-222) is a radioactive inert (noble) gas, heavier than the air. It is a result of alpha-decay of the atomic nucleus of Radium (Ra-226), which is a progeny in Uranium-Radium chain, with the parent radionuclide Uranium 238. After the radioactive decay of the gaseous Radon-222 (3.8 days half-life) a whole chain of new products in solid aggregate state are formed, namely Polonium-218, Polonium-214, Bismuth-214 and Lead-214 (short-living Radon progenies with half life under 30 minutes). These Radon progenies stick to the existing microscopic dust particles in the air (aerosols) and when inhaled, they are retained in the human respiratory system.

The internal exposure is precisely a result of inhalation of short-living progenies, not the inhalation of the radon itself, which is an inert (noble) gas. From the radiation protection point of view all noble gases are a source for external exposure, because by nature they are chemically inert gases and do not participate in biochemical reactions with substances contained in human tissues and organs. The noble gases are also easily exhaled from the lungs. As a result of many scientific investigations it was established, that 98% of the human internal exposure is due to inhaling of radon progenies, which influence the bronchial epithelium in the lungs, and only 2% of the exposure is due to Radon-222 itself.

The natural Uranium is spread in the whole Earth’s crust and its content is estimated to be approx. 3.10^-4 % (average about 3 g per ton of soil). The Radon, respectively, as a member of the Uranium-Radium chain, is distributed everywhere on the Earth – in soils, rocks, minerals, waters, air. Part of the Radon, which is generated in the natural chain Uranium-Radium-Radon, gets into the air from the ground (soils, rocks and water). The Radon distribution by air into the environment by diffusion and convection depends on some natural and climate factors – atmosphere pressure, air temperature, rainfalls, wind, topography and other characteristics.

The Radon concentration in the air decreases with altitude. In addition, the Radon concentration in air changes with the seasons – the highest values are registered during summer months. During twenty-four-hour period radon concentration also varies (the maximum is during the night, minimum – during the day).

Typical average Radon concentration in open air near the ground is 10 Bq/m³. As a rule, the Radon concentration in buildings is higher and depending on the building materials. The typical Radon concentration in buildings is 40 Bq/m³.
Thus, the annual effective dose from internal exposure for population, due to Radon (1.2 mSv/y) and to other radionuclides with cosmogeneous and terrestrial origin (0.3 mSv/y), is totally equal to 1.5 mSv/y.

Hence the world average for annual effective dose from both external and internal exposure, due to the natural radioactive background is 2.4 mSv/y. 50% of this dose (1.2 mSv/y) is due to Radon and its short-lived progenies, contained in the air, 37.5% (0.9mSv/y) – due to external terrestrial and cosmic rays, and 12.5% (0.3mSv/y) – due to radionuclides with terrestrial origin, incorporated in the human body.

All the data, presented here, is taken from published reports of The Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), established by The United Nations (UN) in 1955. In the UNSCEAR activities take part over 60 countries, including Bulgaria, which periodically presents the necessary data and estimates the whole world population exposure, caused by the naturally existing radioactive sources, as well as the men-made technologically enhanced sources of ionising radiation. In Table 1 are shown, in compact form, some generalized data from the UNSCEAR Report for 2000 about the population exposure due to natural radioactive sources.

In Table 2 are shown generalized data for the exposure of the European population, due to natural radiation background.

The external exposure due to natural radioactive sources in Europe is estimated to be approx. 1mSv/y. About 40% of this annual effective dose is caused by cosmic rays, and the other 60% - by gamma-rays in buildings and outdoors (the exposure in buildings is about 2-3 times higher than exposure outdoors). The rest of the average annual effective dose for the European population is caused by Radon. The internal exposure by Radon for countries such as France, Swiss and Finland is 4 to 7 times greater than the external exposure due to other natural radioactive sources.