Since the discovery of the x-ray, in 1895, studies of the health effects of ionizing radiation have received continuing impetus from the expanding uses of radiation in medicine, science, and industry, as well as from the peaceful and military applications of atomic energy.1 The extensive knowledge of the effects of ionizing radiation generated by these studies has prompted strategies for protection against radiation that have been influential in shaping measures for protection against other hazardous physical and chemical agents as well.
PHYSICAL PROPERTIES OF IONIZING RADIATION
Ionizing radiations differ from other forms of radiant energy in being able to disrupt atoms and molecules on which they impinge, giving rise to ions and free radicals in the process. Ionizing radiations include (a) electromagnetic radiations of short wave length and high energy (e.g., x-rays and gamma rays) and (b) particulate radiations, which vary in mass and charge (e.g., electrons, protons, neutrons, alpha particles, and other atomic particles).
Ionizing radiation, impinging on a living cell, collides randomly with atoms and molecules in its path, giving rise to ions and free radicals and depositing enough localized energy to damage genes, chromosomes, or other vital macromolecules. The distribution of such events along the path of the radiation—that is, the quality or linear energy transfer (LET) of the radiation—varies with the energy and charge of the radiation, as well as the density of the absorbing medium.2 Along the path of an alpha particle, for example, the collisions occur so close together that the radiation typically loses all of its energy in traversing only a few cells, whereas along the path of an x-ray the collisions are far enough apart so that the radiation may be able to traverse the entire body (Fig. 35-1).
Differences among various types of ionizing radiation in penetrating power in tissue.2
Because the biological effects of ionizing radiation result from the deposition of energy in exposed cells, doses of ionizing radiation are customarily expressed in terms of energy deposition (Table 35-1). On traversing a given cell, a densely ionizing radiation (e.g., an alpha particle) is more likely than a sparsely ionizing radiation (e.g., an x-ray) to deposit enough energy in a critical site, such as a gene or chromosome, to injure the cell.3,4,5,5,6 Hence an additional dose unit (the equivalent dose) is used in radiation protection to enable different types of radiation to be normalized in terms of their relative biological effectiveness (RBE). The equivalent dose (expressed in sievert [Sv]) is the dose in gray (Gy) multiplied by an appropriate weighting factor to adjust for differences in RBE; that is, 1 Sv of alpha radiation is that dose (in gray) of alpha radiation that is equivalent ...