The main sources of radiation are not the potentially dangerous nuclear objects in cases of emergency but rather the natural sources. A person receives the main dose of radiation from natural radiation sources like outer space.
The human organism can be irradiated in the following ways:
– by gamma rays that exist in the outer space, on the surface of the Earth and in construction materials
– by gasiform element of radon that can penetrate the atmosphere
– by radiation that transits from roots into plants and further gets into the human organism through food.
The first way is the external irradiation by sources outside the human body. In this case, X-rays and gamma radiation need to have a relatively high energy to get through human body, and some high-energy beta rays need to be able to penetrate the skin surface layers. The closer to the sky, the more powerful outer space emission affects us. By the way, because of the strong geomagnetic field the citizens of southern countries close to the equator experience less effect of radioactive emission than those of northern ones. Our planet is also the source of radioactive emission as it contains such radioactive substances as uranium, radium, thorium etc. Such radioactive elements as potassium and carbon are also present in the human body.
The set of all numerous radionuclides that affect us and are present in the earth, water, air and living organisms is called natural radioactive background. Natural radiation sources comprise 70% of the total dose received by humans from all radiation sources, including nuclear objects. There is no such thing as the natural material that contains no radioactive elements.
The level of natural radioactive background directly depends on the quantity of natural radioactive elements present in the environment. For example, tundra gamma emission due to peracidity of soil is much less than that of the middle zone. Basic natural radioactive elements of the earth are potassium, rubidium, uranium and thorium. There are three existing forms of potassium but only one of them is radioactive potassium-40. As a matter of fact, potassium is an essential part of the human body as it takes an active part in metabolism.
Natural radionuclides such as uranium and thorium can be found everywhere: in water, minerals and soil. While uranium is mostly accumulated in moss, thorium prefers blueberries, heather, lichen and tomatoes.
The second way of irradiation is penetration of gasiform element of radon into the atmosphere. As a result, the radon gas is inhaled by people and the products of its dissolution are precipitated in respiratory tracts.
Radon has no smell, colour or taste. It is 7.5 times heavier than air and dissolves into plumbum and polonium. There is especially much radon in rooms with stuffy air. Its quantity in the room directly depends on the contents of radon in soil underneath the house and the materials used for its building. In moderate climate zones the concentration of radon inside closed rooms is about 8 times higher than outside. For example, in the ’70-s in some buildings in Sweden and Finland the detected concentration of radon was 5 000 times higher than that in the air outside.
The most widely used building materials (wood, brick, concrete) emit small quantities of radon. There is much more of it in pumice and granite.
Radon gets into our apartments and houses with water and natural gas. Its concentration is very high in water from deep wells and artesian bores. Radon disappears after the water is boiled. If the stove works on gas, most of radon will be concentrated in the kitchen. But its concentration sharply decreases when there is a cooker hood above the stove.
The third way of irradiation is the radiation transition from roots into plants and its further penetration into the human body through food. In order to decrease the number of radioactive elements in our everyday food to the maximum extent, the following factors should be taken into account:
1. The most polluted are plants, roots of which grow in the upper layers of soil. Therefore, there are less radioactive elements in fruit, as the roots of the fruit trees grow deeper in the soil.
2. Plants receive fewer radionuclides from black soil than from marshy and sandy soil.
3. Lichens, moss and mushrooms accumulate radioactive substances very quickly.
4. Strontium and caesium are actively accumulated in our favourite dill and parsley.
5. Flour contains more radionuclides than grain. So there are more of them in coarse-ground flour than in fine-ground flour.
6. Food processing sharply reduces the quantity of radionuclides.
7. Peeling potatoes eliminates up to 40% of strontium and caesium with the peel.
8. Squeezing the juice from plants or fruit automatically eliminates radioactive elements leaving them in oilcake.
9. When meat is polluted with radionuclides, then the most dangerous are bone broths because strontium is accumulated in bones. By the way, beef is the most polluted meat product while pork the least polluted one.
10. In order to get rid of radioactive elements in milk it should be processed. For example, only 5% of strontium is left in cream and only 9% of caesium – in sour cream.
11. If a lake or river is polluted, do not eat near-bottom fish such as catfish or bullheads. They live in the very bottom layers, where most of the radionuclides are accumulated.
12. Sea fish is much purer than river or lake one as radiation effect in sea water is much weaker.