On exposure to ionizing radiation, water, which is makes up more than 60% of the body, is ionized generating reactive oxygen species (ROS), such as hydrated electron (eaq−
) and hydrogen radical (•
These mayors can react withother substances in the body to initiate damage. This type of effect on living organisms is indirect. On the other hand, radiation can also directly inactivate substances in the body. Actually, both the direct and indirect effects of radiation cause biological damage. However, the difference is due to the type of radiation and physical and/or chemical conditions. Modulatory action can be explained by indirect effects.
As for the biological effects of radiation, damage such as cancer is mainly considered. When living organisms are exposed to radiation, the main effects are divided into early, intermediate and late as shown in Fig. .(5)
Although the three cannot be completely distinguished, the generation of ROS occurs between the early and intermediate stages and the influence of ROS should appear in the late stage.
Time-dependent process of biological effects after exposure of radiation.
The cation (H2O+) thus formed may give the hydroxyl radical (•OH) as follows:
The •H is generated from the reaction of hydrated ion with a water molecule as follows:
Except for these simple reactions, the direct dissociation of an excited (about 7 eV) water molecule may generate •H and •OH as follows:
H + •
OH (* shows an excited state)
Table shows G values of radiation with low ionized density at pH 3–10.(6)
Yield of primary products generated from radiolysis of water
Further, molecular oxygen usually exists in living cells, and then, eaq− or •H generated from the radiolysis of water reacts with molecular oxygen (O2) to yield superoxide ion (O2•−) or its conjugated acid, the hydroperoxyl radical (HO2•) as follows:
H + O2
Thus, the damage to living organisms from the indirect effects of radiation may be caused by the reactive species mentioned above. The effect of radiation is strengthened by the formation of many kinds of ROS and free radicals due to the existence of water in the living body. This ”oxygen effect” is 2.5–3 times that in the absence.
As for radiation damage, those reactive species mentioned above can react with DNA either directly or after metal-catalyzed transformation in the cell. As many as 100 different DNA modifications have been identified after exposure to ionizing radiation.(4)
These include single- and double-strand breaks, base modifications, abasic sites and cross-links. These lesions are primary sites for radiation-induced cell lethality, mutations and malignant transformation. The double-strand breaks in DNA are particularly important to the lethal effect of ionizing radiation. However, the biological importance of base modifications has not been clearly established.
There are two types of radiation injuries, acute radiation injury and delayed radiation injury. Both types are observed in many radiation accidents.
Acute radiation injury
When whole-body is exposed to high-dose radiation, many types of damage occur depending on the radiation-exposed dose. Such damage is known as acute radiation injury. Table summarizes the effect on living organisms of exposure to ionizing radiation.
Doses and effects on exposure to ionizing radiation
Late radiation injury
When whole-body is exposed to low-dose radiation or repeatedly exposed to low dose-rate radiation, some effects appear over a period of years or decades. Such damage is known as delayed radiation injury. Among them, radiation carcinogenesis, such as leukemia and other malignancies, is thought to have no threshold dose and is classified as a stochastic effect. Cataracts are also a form of late radiation injury, but have a threshold dose for onset and are classified as deterministic effects.