Fig. 3

Schematic illustration of the relation between LNT, thresholds, and hormesis at chronic irradiation. When DDREF = 1 by acute and intensive irradiation, dose–response approximates a linear line in a limited dose range at higher doses. Even with acute irradiation, radiation effects of lower doses are nullified by powerful and efficient defense mechanisms in which adaptive response induces hormesis. When DDREF > 1 by chronic and mild irradiation, low and mild doses are under a range of adaptive response and hormesis occurs. The limit of the defense capacity determines a threshold. At high doses beyond the threshold, adverse effects appear. A linear response (dashed line) is realized transiently in a limited dose range immediately before the downturn