The biological effects and health risks of exposure to low doses of natural or man-made agents, including ionizing radiation, chemicals, drugs or pesticides, that would otherwise cause cancer and degenerative diseases at high doses, remain ambiguous and are the subject of intense debate. Human epidemiological studies would be ideal to predict such deleterious effects; however, these studies are limited due to the necessity of very large cohorts (several millions) to generate data with an acceptable level of confidence. Furthermore, epidemiological surveys examine the effects of exposures that happened several years earlier, and therefore may be biased by many variables during the intervening time until overt detrimental health effects are expressed. Although great uncertainties about a causal relationship between low dose exposure and harmful health effects exist, it is nevertheless clear that for most agents, there is little direct evidence of risk at low doses. However, to meet societal concerns, a linear no-threshold model has been advocated to predict the biological effects of low dose environmental exposures. In that model, it is assumed that harmful effects increase as a function of exposure dose, and the smallest dose has the potential to cause a small increase in risk to humans. Further, the effects of sequential exposures are presumed to be additive.
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