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LOWLEVEL EXPOSURE TO IONIZING RADIATION: DO ECOLOGICAL AND EVOLUTIONARY CONSIDERATIONS IMPL Y PHANTOM RISKS ? PETER A. PARSONS* Introduction Many people have serious concerns about risks from low exposures of environmental chemicals and ionizing radiation. Consequently, support for the expenditure of large sums of money to reduce these perceived risks is substantial. Basically, these fears derive from the direct extrapolation of the obvious harm at high exposures of environmental agents to the very small exposures to which the public is exposed. The implicit assumption is a linear no-threshold theory, or LNT. Here I develop arguments from the field of evolutionary biology, taking into account the stressful habitats in which organisms occur, in order to challenge the validity of the LNT model on evolutionary grounds. A brief comment on risks is then provided. In a literature extending over many decades, a range of minerals and vitamins that are toxic at high exposures appear to be advantageous in small amounts compared with total absence. That is, organisms perform best at a little above zero exposure. The term hormesis has been used to describe this phenomenon, especially in the toxicological literature. A hormetic zone occurs between zero and a Zero Exposure Equivalent Point, ZEEP (Fig. 1). While hormesis is a qualitative and quantitative deviation from the LNT model, for exceedingly toxic agents the ZEEP is so close to zero that hormesis is frequently not detected. Examples include the elements copper, cadmium, lead, and mercury [I]. Ten years ago, two Policy Forum articles in Science highlighted radiation hormesis, whereby low levels of ionizing radiation were argued not to be necessarily harmful [2, 3] . Conversely, the possibility of "beneficial" effects The author is grateful to the Australian Institute of Nuclear Science and Engineering for more than 20 years offinancial and logistical support. A recent term on the Australian Ionising Radiation Advisory Council was most helpful in assisting with recent background information. Correspondence: P.O. Box 906, Unley, SA 5061, Australia * School of Genetics and Human Variation, La Trobe University, Bundoora, Australia.© 1999 by The University of Chicago. All rights reserved. 0031-05982/1999/4301-1123$01.00 Perspectives in Biology and Medicine, 43, 1 ¦ Autumn 1999 | 57 HIGH CO CO UJ Z HORMETIC ZONE ZEEP° EXPOSUREmGH Fig. 1.—The relationship between fitness and exposure to an environmental variable. The hormetic zone occurs between zero exposure and the Zero Exposure Equivalent Point, ZEEP, when the fitness falls below that at zero exposure. was suggested. Immediately prior to these articles, a whole issue of Health Physics was devoted to radiation hormesis. In an Editorial, Sagan regretted that hormesis had received little attention, but noted the conflict with conventional radiation science paradigms, in which the LNT model remains heavily supported [4] . Are there more fundamental reasons why the field of low-exposure hormesis has not been taken seriously in many quarters? While it is possible that this situation may be becoming ameliorated [5] , a major factor is undoubtedly the difficulty in finding common physiological and metabolic mechanisms underlying hormesis. In fact, The range of agents employed in such studies has been very diversified including numerous chemicals (e.g., heavy metals, PCBs, pesticides, hydrocarbons from crude oil, chemotherapeutic agents, antibiotics, ethanol, solvents such as CCl4 and chloroform , essential trace elements, and numerous other agents) and radiation. [6] At first sight, this heterogeneity of agents suggests that a satisfactory underlying explanation for hormesis is elusive. Another difficulty is that the hormetic effect is typically quite small, often only 10 to 20 percent above the origin and the ZEEP. Careful experimentation is therefore needed to explore the possibility of hormesis. Perhaps more important, the relatively small increments required to detect hormesis at exposures close to zero are often not explored for agents that are exceedingly toxic at high exposures, because of an implicit a priori assumption of the LNT model. Consequently, the LNT model is frequently assumed without sufficient investigations at low exposures, including the biological consequences of ionizing radiation. However, increasing data 58 Peter A. Parsons ¦ Low Level Exposure to Ionizing Radiation sets in both experimental organisms and in human populations now suggest deviations consistent with, or in the direction of, radiation hormesis [V-IO]. Is there a plausible unifying...

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