During evolution in an aerobic environment, multicellular organisms survived by adaptive responses to both the endogenous oxidative metabolism in the cells of the organism and the chemicals and low-level radiation to which they had been exposed.
Cells contain preventive antioxidants to suppress oxidative damage to membranes.
The biologic consequences of the low-level radiation that exceeds the background level of oxidative damage could be necrosis or apoptosis, cell proliferation, or cell differentiation. If the end points of cell proliferation, differentiation, or cell death are not seen at frequencies above background levels in an organism, it is unlikely that low-level radiation would play a role in the multistep processes of chronic diseases such as cancer. The mechanism linked to homeostatic regulation of proliferation and adaptive functions in a multicellular organism could provide protection of any one cell receiving deposited energy by the radiation tract through the sharing of reductants and by triggering apoptosis of target stem cells. Examples of the role of gap junctional intercellular communication in the adaptive response of cells and the bystander effect illustrate how the interaction of cells can modulate the effect of radiation on the single cell. The author wishes to acknowledge stimulating discussions with B. Upham, M. Wilson, C.C. Chang, and K.S. Kang concerning various ideas contained in this manuscript.