The most significant characteristic of the biological effect of radiation is that a small amount of absorbed energy may have an extremely great effect. A case of

acute radiation (e. g., a 1000 sievert dose, meaning 1000 J/kg = 1 kJ/kg absorbed energy) causes death immediately. For a man who weighs 70 kg, the total absorbed energy is 70 kJ. This energy is much smaller than the energy produced when a 1 mol substance (e. g., fuel) is oxidized (a few hundred kilojoules). The question arises: why does this small amount of energy have such a dramatic effect? This can be explained by the fact that the small amount of energy is absorbed as great impulses: the energy of each particle is about six orders of magnitude higher than the energy of the chemical bonds (see Section 2.1.2). Another important character­istic is that beacuase of the high energy of each particle, the radioactive radiation ionizes the substances independent of their chemical species. Since any electron of the substances can be ejected, the cross section of the ionization is determined by the number of electrons. This means that the cross section of the ionization of the heavier elements and substances in large quantities is dominant.

The biological effect of the radiation occurs through consecutive physical, chemical, and biological steps. Since the water content of biological systems is the largest (the human body consists of about 70% water), the basic process of the bio­logical effects of radiation is the radiolysis of water. The first step of the radiolysis of water is the ionization of water, a physical process, producing a positively charged water molecule ion. Since this molecule ion contains an unpaired electron, it is a radical:

H2O radiation > H2O+ + e2 (13.4)

The products of the ionization (Eq. (13.4)), the water molecule ion/radical and the free electron, initiate different reactions, such as:

H2O+ ! H++ • OH (13.5)

e2 + H2O! OH2 + H — (13.6)

The products of the reactions in Eqs. (13.5) and (13.6), the hydrogen and hydroxide ions, can neutralize each other:

H+ + OH2 ! H2O (13.7)

The net process is the formation of atomic hydrogen and hydroxide radicals. These radicals can combine to water again or can produce other highly reactive species as described in Eq. (13.8):

H2O! H — + OH! H2O2, O2, H2, H2O (13.8)

In the presence of oxygen, which is essential in living organisms, additional radicals can be produced. Some examples are listed in Eqs. (13.9)—(13.12):

O2 + H — ! HO2-

HO2 • + H2O2 ! H2O + • OH + O2

HO2 • + HO2!H2O2 + O2 (13.11)

HO2• + H! H2O2 (13.12)

The radicals in Eqs. (13.5)—(13.12) and the free electrons can react with each other and any molecules of the biological systems, producing additional radicals. As seen in the reactions in Eqs. (13.6)—(13.12), both oxidizing and reducing com­pounds form, causing redox reactions of the biological molecules. All these chemi­cal reactions (including the reactions with radicals, electrons, and the redox agents) change the structure of biological molecules; thus, they cannot fulfill their biologi­cal functions (biological reactions). The damage to DNA under the effect of radia­tion has to be emphasized. The chains of DNA can break, leading to somatic effects of radiation, e. g., cancer and inheritable DNA defects.

The living organisms have different ways of protecting against the radiation effect. They contain natural radical scavengers. If they are present in excess of the radiolysis product, they can protect the biological molecules, including DNA. The preventing capacity depends on the age and physical conditions of the given organ­ism. In addition, the cells have various repair mechanisms for restoring the damages of the biological molecules. This mechanism is called “immune activity.” If, how­ever, this repair mechanism fails, the undesirable effects of radiation will appear.

The protection against radiation can be assisted by chemicals, namely by com­pounds that can scavenge the radicals, e. g., by compounds containing conjugated double bonds (Vitamins A and E) or compounds that are oxidized easily (Vitamin C). Sulfur compounds can also scavenge radicals. Because of the triple bond, a cyanide ion should scavenge the radicals well; however, it cannot be used for radiation protection of living organisms because of its strong toxicity.