Health risks by low and lowest radiation doses

As the biological effects depend on the distribution of dose in time and space, we must differentiate between risks of concentrated and diluted radiation, of low dose and very low dose. A different shape of dose-effect curves with low and very low doses is possible as 2 different mechanisms may lead to damage: The singular type (1 event) leads to the effect and the multiple type (only several events together lead to the effect). For very low diluted doses (represented by burden of natural radiation or exposure of population by power production from nuclear fission) a practically innocuous for multiple mechanisms dose could exist. For low (acute) doses, represented by medical exposure the minimum of necessary energy for multiple reactions will be mostly exceeded. The risks factors are estimated under the postulation of a linear dose effect relation. They can be reality or overestimation and also underestimation, as low doses under certain circumstances could be relatively more efficient than high doses by protection of cells with sublethal damages, mitotic multiplication of damage. (orig.)