**2. Biological effects of radiation exposure**

It is well-known fact that radiation can be harmful to the human body. Biological and medical knowledge about the effects of ionizing radiation has been gained since the beginning of the last century and is currently extensive but not yet complete. They include observations of clinical, experimental, and above all, group investigations, which are necessary to demonstrate an increased frequency of those diseases that are clinically indistinguishable from spontaneous diseases (e.g., cancer).

The health effects caused by radiation exposure fall into two groups: stochastic effects and deterministic effects (tissue reactions).

The *stochastic effects* of radiation are those effects that we do not know with certainty that they will manifest after exposure; they are manifested only with a certain probability. This includes an increased risk of cancer and hereditary diseases. On the other hand, deterministic effects appear only above a certain level of exposure (dose), which is relatively high. In both cases, the effects may affect a person exposed (somatic effects) or his offspring (genetic effects), as shown in **Figure 3**.

**Figure 4.** *Comparison between stochastic and deterministic effects.*

The *stochastic effects* are caused by the mutations (changes in the genetic information of the cell) and are characterized by a threshold-free, linear dose-response relationship. The dose dependence of these effects is statistical in nature, and therefore, the designation stochastic effects (probable, accidental) have been introduced for them. The size of the radiation dose does not change the severity of the individual's manifestation, but in the population, it changes the frequency of the additional appearance of malignant neoplasms and hereditary damage. Thus, with the dose, the likelihood of injury increases for the individual.

The exposure above certain threshold results in deterministic effects where the severity of the body reaction is roughly proportional to the exposure (**Figure 4**). Exposure to radiation under this level causes no impact.
