Although this model is applied carefully and conservatively, there is room for concern about the validity of the low dose exposure risks obtained in this way because a number of findings have accumulated which cannot be explained by the classical “target theory” of radiation biology. Specific cellular responses observed in response to low dose and/or low dose rate radiation have been described as the radioadaptive response, the radiation-induced bystander response, lowdose hyper-radiosensitivity, and genomic instability. All of these phenomena are considered to be responses to radiation which involve non-targeted molecules or molecules which have not interacted directly with radiation (Waldren 2004). The propagation of damaging effects from irradiated to nonirradiated bystander cells would, presumably, result in supra-linear dose-response relationships. In contrast, the expression of adaptive responses that mitigate the initial
damaging effects induced by radiation would suggest an infralinear dose-response relationship or the existence of a threshold dose, below which there would be no risk.