The effect of small doses of radiation on the body
Intensive tests of nuclear weapons in the middle of the 20th century, the use of atomic energy, ionizing radiation in the national economy led to an increase in the radiation background on the planet. These processes have led to a change in emphasis in radiobiological research. They began to pay more attention to studies of the effects of radiation in relatively small doses, which are prolonged in time.
What radiation doses are considered small?
There is no unanimity among scientists on this issue, but the majority believe that the range of small doses is above the natural background and exceeds it ten times. The upper limit of the low dose range is less certain because there is a large difference between different organisms in radiosensitivity. The measure of the upper limit of small doses is the dose of radiation at which 50% of individuals of a given species die within 30-60 days (LD50 \ 30) or 100% during the same time (LD100 / 30). The range of small doses is limited “from above” by a value that is 2 orders of magnitude (one hundred times) less than LD50 \ 30 for a certain type of living creatures (organisms). In the case when small doses are attributed to a person, then we are talking about doses of 4-5 rad (0.04 - 0.05 Gy) in a single exposure.
What is the biological effect of low doses of radiation?
To answer this question, it is necessary to turn to how the effect of ionizing radiation is realized at the level of individual ionizing particles (quanta) when interacting with DNA (DNA in this situation is considered as a target). Even one single hit to a biological target (interaction) can lead to irreversible damage to the gene (to mutation). Changes in genetic information can lead to cell death. Thus, ionizing radiation is not the only physical agent known to mankind that has no threshold of effect. Because even with the smallest exposure (one ionizing particle), serious biological consequences can occur (of course, with a very low probability). A direct conclusion from all of the above is that any additional exposure to the existing radiation background to the body is harmful and dangerous.
But not so simple. The probabilistic nature of the action of radiation is carried out only on those biological processes that are directly related to the functioning of the genetic apparatus of the cell. Such effects develop according to the "all or nothing" principle (the ionizing particle either hit or did not hit the "target"). With an increase in the radiation dose, the number of such elementary events increases, and not their magnitude. All other biological effects of radiation depend on the magnitude of the dose received - with an increase in the radiation dose, the expressiveness of the effect increases. For example, with an increase in the radiation dose, the duration of the cell division delay increases.
Moreover, at low doses of radiation, the levels of which border on the natural background, scientists record a stimulating effect of radiation. This effect is manifested in an increase in the frequency of cell division, accelerated germination and improved similarity of seeds, and even in an increase in crop yields. The hatching of chickens increases (their mortality decreases when they hatch from eggs). Chickens gain weight better, and chickens improve egg production. The resistance of animals to bacterial and viral infections increases. Thus, not only in plants, but even in animals (even in radiosensitive mammalian species), a range of doses is isolated that cause stimulation of vital activity (1-10-25 rad). Scientists call this effect hormesis. It should be noted that for probabilistic (stochastic) effects, that is, mutations, the phenomenon of hormesis has not been proven.
Under such conditions, the application of the theory of thresholdless action of radiation is significantly limited and is justified only for stochastic effects.
On the other hand, many scientists have shown that there is a threshold in the action of radiation even for stochastic effects. These include, for example, an increase in the incidence of leukemia and cancer (which results from damage to chromosomes). In the range of significant radiation doses (from 20 to 30 rad), a linear dependence of the frequency of long-term effects on the radiation dose is clearly recorded. With decreasing doses, it is increasingly difficult to establish such a relationship, and if we take into account that there is a natural level of cancers and leukemias (their occurrence is not associated with radiation and radiation), then it is extremely difficult to establish the dose-effect relationship. Under such conditions, to determine the effects of small doses of radiation, that is, to establish the reliability of a scientific experiment, it is necessary to increase the number of experimental animals thousands of times. In this case, it is necessary that animals (for example, mice) be a homogeneous population, which is extremely difficult to achieve. In addition, it is very difficult for such a number of animals to create uniform (uniform) environmental conditions. Considering the above, we can conclude that experimental verification without a threshold, or threshold concept of the effect of radiation on the body, is an extremely difficult task, and today this issue has not been resolved.
Regarding the threshold concept of the effects of radiation, it should be added that this concept has significant theoretical and experimental confirmation. The main content lies in the fact that there are entire systems in the cell that are responsible for the restoration of damage to the genetic apparatus. These DNA (chromosome) repair systems are called repair (repair) systems. These systems are extremely effective and have a powerful reserve of functional resistance to stress caused by the repair of damaged DNA. Based on the knowledge about the repair systems in the cell, it is concluded that at low doses of radiation (when relatively small damage to the genetic apparatus is observed), the repair (restoration) systems manage to completely eliminate gene damage. Only when the dose (radiation power) is increased above a certain level, the systems for restoring the genetic apparatus simply do not have time (cannot cope) to restore the damaged DNA. The consequences of radiation (effects) are recorded as an increase in genetic damage.
How to understand the presence of two opposite concepts of the action of low doses of radiation?
According to some scientists (for example, V.A. Baraboy), there is an explanation that explains the expediency and meaningfulness of the two concepts. It is necessary to pay attention to the fact that, despite the presence of powerful DNA repair systems, they cannot completely eliminate damage to the genetic apparatus (of both radiation and non-radiation nature). The systems for restoring the genetic apparatus of the cell were formed along with the emergence of life on Earth. Along with living organisms, systems of restoration (protection) of the genetic apparatus of a cell, an organism from the mutagenic influence of the environment (including the radiation background) have evolved.
On the other hand, full restoration of the altered genetic information is not in the interests of the biological species. Because the conditions of life on Earth are gradually and constantly changing. In the face of changing living conditions (environment) for a biological species, it is a vital need to be able to adapt to changes. In conditions when the species 100% protects its heredity, it loses the ability to adapt and as a result, in the changed conditions of life, it will die. In such a situation, it becomes obvious that for a biological species it is extremely important to preserve a certain number of mutant individuals, which, under the changed conditions of life, would be more adapted for existence due to better adaptation. Thanks to these individuals, in already changed environmental conditions, the species can successfully reproduce and, ultimately, preserve the species (prevent extinction).
Based on these assumptions, it can be concluded that, despite the presence of the most powerful systems for restoring (protecting) the genetic apparatus of the cell, under conditions of natural radiation (in a broad sense - mutagenic) background, mutant individuals arise among populations of all types of living beings. The mutation process takes place continuously. Thus, mutant organisms are the "raw material" due to which natural selection is carried out and organisms (species) that are most adapted to environmental conditions are preserved.
It turns out that the repair systems eliminate not all, but only part of the DNA damage. A certain amount of damage is not restored and is the beginning of mutations that occur at a frequency that is most beneficial for the population of a particular species. Thus, even the natural background radiation, which coexists with life on Earth for billions of years, plays the role of a "supplier" of mutations. The threshold is thus absent or below the background. This mutagenic role of radiation and over the background area of low doses of radiation. Reparative systems eliminate the bulk of mutations, with the exception of biologically necessary ones. Therefore, within the limits of small radiation doses, there is no linear (direct) dependence in the dose-effect relationship, but a wavelike dependence is observed or the curve reaches a plateau. Only proceeding from a certain dose value (for each type of organism it is unique), the dose-effect relationship has a linear relationship - there is a linear increase in DNA damage, which is an indicator of the transition from low doses of radiation to already significant values at which the reserve the possibilities of cell repair systems.
Following this explanation, it can be concluded that within the limits of small doses of radiation, effects of stimulating the physiological functions of cells or the whole organism (hormesis) are possible, as well as mutagenic effects that are comparable to the action of the natural mutagenic background.