Relationships Between Chemical and Biological Effects of Ionizing Radiations

Abstract

In radiobiology we are interested in chemical effects of ionizing radiations only in so far as they may be involved in biological effects.2 As background for discussion of this relationship, let us briefly review the general state of our knowledge (or ignorance) of mechanisms of radiobiological actions: 1. The first event in such an action is, of course, the absorption of radiant energy by molecules in the biological object or its medium. The molecules which thus acquire energy in excess of normal are said to be activated. Some absorb so much energy that electrons are ejected; these molecules are said to be ionized. Others absorb less energy; electrons are merely shifted to other orbits; such molecules are said to be excited. The foregoing physical processes are at present fairly well understood, and many aspects of them can be described quantitatively. 2. After the absorption of energy, our knowledge of the radiobiological action is practically zero until the biological effect occurs. This effect—depending upon the biological object irradiated, the conditions of irradiation, the technic of observation, and a multitude of other factors—may be the mutation of a gene, the breaking of a chromosome, an increase in permeability of a membrane, an inhibition of cell division, a leukopenia, induction of a neoplasm, death of an animal, or any one of many other radiobiological phenomena. In all these diverse cases, however, there is no apparent resemblance between the biological effect and the initial physical phenomena associated with the absorption of energy. 3. From the foregoing considerations, it is evident that, during the so-called “latent period” which intervenes between the energy absorption and the observation of biological effect, a chain of important connecting events must occur. So far as I am aware, the specific nature of these events is in all cases unknown, as is also the number of successive events in the chain. However, in general it appears highly probable—and most physicists and radiobiologists have long considered it almost axiomatic—that at least the early events in the chain are chemical. Indeed, it is exceedingly difficult to imagine a mechanism of radiobiological action which does not involve chemical changes. The biological end-effects are changes in either physiology or morphology, these terms being used in the broad sense. The physiological changes clearly are changes in cell chemistry or are the results thereof. Moreover, when we recall that biological structures are built by chemical means, we can hardly imagine a morphological change which does not involve chemical change. Let us also consider the early part of the mechanism of radiobiological action. The high-energy radiations can ionize any type of molecule, and we may be confident that many kinds of molecules in the cell are thus highly activated under irradiation. Once a pair of ions is formed, they may do one of two things: