Chapter 68: Ionizing Radiation

INTRODUCTION

Radiation is one of the most studied and best-characterized environmental exposures. Studies of the health effects of ionizing radiation have been motivated by the uses of radiation in medicine, science, and industry, as well as from the peaceful and military applications of atomic energy.¹ A main objective of these studies is to characterize risks of radiation exposure for the purpose of controlling these risks. An additional objective is to identify factors determining susceptibility to increased risk for cancer subsequent to radiation exposure.

Ionizing radiation causes chemical changes in cells and damage in DNA that may increase the risk of developing certain health conditions as an acute or late effect. Ionizing radiation can come from natural sources, such as radon, and manmade sources, such as medical imaging or therapeutic treatment. Nuclear power plant accidents and atomic weapon explosions have also led to releases of high levels of ionizing radiation. Thus, longitudinal studies of the atomic-bomb survivors in Hiroshima and Nagasaki have been a critical source of information on the consequences of radiation exposure. Being exposed to very high doses of ionizing radiation can cause damage to tissues and organs that becomes evident within a few days of exposure, while low to moderate doses can induce late effects on apparently undamaged tissues, such as cancer and cataracts. As one of the best-characterized environmental exposures, the extensive knowledge of the effects of ionizing radiation generated by both experimental and observational studies has led to numerous etiological insights and identified susceptible populations and windows of exposure. The research evidence has prompted policy and 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 radiation is a type of high-energy radiation that has enough energy to remove an electron (negative particle) from an atom or molecule, causing it to become ionized. Ionizing radiations include (a) electromagnetic radiations of short-wavelength 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, with some clustering at the ends of its track, 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; beta particles, which consist of electrons, can penetrate up to 2 cm of ...