Fig. 1

Schematic illustration of hormetic and LNT responses. Magnitudes of responses and doses are arbitrary. In hormesis, e.g., in cytotoxicity and cell killing tests, low doses are beneficial to cells and high doses are hazardous to them so that the dose-response curve shows a reverse U-shaped curve when plotted on a linear scale (a, c, gray line). Thus, a threshold could be determined at the cross-point of the curve and the x-axis. On the other hand, LNT assumes that toxicity increases dose-proportionally and therefore the dose-response relationship is depicted as a linear line (a, red line). When the x-axis is logarithmic, the linear line becomes a curved one (c, red line). It is quite important to note that all responses in LNT come under the zero dose level (a, c, blue line). In hormesis, e.g., in mutagenicity and carcinogenicity tests, responses are opposite to cytotoxicity and cell killing tests (a, c) and show a J-shaped curve (b, d, grey line). LNT responses follow a linear line upward from left to right when plotting on a linear scale (b, red line) or a curved line when plotted on a logarithmic scale (d, red line). Here again, LNT responses never come under the zero dose level (b, d, blue line)