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Abstract

The target theory is relevant to those biological actions of radiation which proceed from the effects of individual ionizing particles, or even from the formation of single ion clusters. The work of the late Dr. D. E. Lea in particular has given us a clearer understanding of the manner in which this theory may be fruitfully applied, and placed us in a better position to understand its limitations.

Even in the case of biological effects of radiation to which the concepts of the target theory are formally applicable, there remain many outstanding questions regarding the mechanism by which the act of ionization results in biological damage. Several possibilities may be considered:

1. The structure of the biological entity—be it microscopic or sub-microscopic—breaks up in the process of being ionized.

2. Regions of high energy are formed by the ionization. The energy is then conducted away to the sensitive spots of the biological complex where it is responsible for hitherto unknown chemical reactions, or for the direct break in the biological structure.

3. The act of ionization may change some important active group of the biological substance to such an extent that the resulting effect becomes microscopically observable. Such an effect may be produced by excitation rather than by ionization, but this mechanism would seem to be much less important.