Biological Effects of Ionizing Radiation

The study of the biological effects of ionizing radiation started practically at the same time as the discovery of X-rays in 1895. Since the techniques and methods accepted today to quantify radiation dose were absent at that time, first findings and studies were barely qualitative. Nonetheless, harmful effects to man and to laboratory animal species from ionizing radiation were already observed in the early years of the 20th century, when lack of data was a shared concern and there were no radiological protection standards. The first quantitative studies in experimental radiobiology developed during the 1920s, with the results of the epidemiological studies on the survivors of the atomic bombing of Hiroshima and Nagasaki, and data obtained from studies on patients exposed to radio therapeutic treatments, currently provide a large amount of information on the health effect of ionizing radiation which is the base of safety standards for occupationally exposed individuals. Biological effects of ionizing radiation are a consequence of the ionization of atoms of biomolecules, which might cause chemical changes and alter or eradicate its functions. As illustrated in Fig. 2.1, energy transmitted by radiation may act directly causing ionization of the biological molecule or may act indirectly through the free radicals resulting from the ionization of the water molecules that surround the cell. Due to ionization, proteins can lose the functionality of its amino groups and modify its behavior, thus increasing its chemical responsiveness; enzymes would be deactivated; lipids will suffer peroxidation; carbohydrates will dissociate; and nucleic acids chains will experiment ruptures and modifications of structure. But from all possible combined alterations, DNA is the primary target for radiation because it contains genes/chromosomes that hold information for cell functioning and reproduction that are critical to cell survival.