Revival: Biological Effects of Low Level Exposures to Chemical and Radiation (1992)

24 BIOLOGICAL EFFECTS OF LOW LEVEL EXPOSURES

Figure 1.11. Graphical representation of the longevity hormesis-toxicity Gompertz model

for eastern chipmunks, living in the wild, exposed to a single dose of either

200 or 400 R 7-radiation. These curves were derived from the same data set

illustrated in Figure 1.10. Time on the abscissa began at capture. Values of

A (In fix) denote differences in Napierian logarithms of age-specific mortality

rates between treated and control animal populations; an arbitrary value of 5

was added to ensure positive values throughout (for curve-fitting purposes).

Therefore, values below 5 indicate net longevity hormesis, whereas those

above 5 indicate net toxicity. Filled squares and filled circles represent A (In

Qx) data for the 200- and 400-R groups, respectively. The theoretical curves

were determined by fitting appropriate equations to the data, employing

weighted nonlinear least-squares analysis. The original data came from

Tryon and Snyder.92 Reprinted from Thompson et al.,47 p. 279, by

permission of Marcel Dekker, Inc.

Figure 1.14 illustrates the effect of amosite asbestos administration on
the Gompertz plot of exposed female rats.24 95 Although a mixed longevity
hormesis-irreversible toxicity model is consistent with the data, the weak
longevity hormetic effect, coupled with the apparent cancellation of one
effect by the other, makes model-independent interpretation difficult. This,
however, is not true for amosite administration to female hamsters at the
same exposure. Figure 1.15 clearly illustrates the effects of both longevity
hormesis and irreversible toxicity.24 96 Gompertz plots for male hamsters
follow the same pattern as female hamsters, as illustrated in Figure 1.16.
Figure 1.17 provides another example where visual inspection of Gom­
pertz plots leaves the viewer wondering just how likely the longevity horme­
sis-irreversible toxicity model really is.24’97 It illustrates plots for control and
dieldrin treated male mice. Given data variability and potential cancellation
of effects, one is tempted to merge the data, drawing a single straight line
through all data points.
Figure 1.18 is a good example of another problem. It illustrates the
influence of ethyl acrylate inhalation (75 ppm) on the Gompertz plot for
male rats.24 98 The case for the existence of longevity hormesis would seem to