HYPOTHESES ON LONGEVITY HORMESIS 23TIME (YR)Figure 1.10. Representative survivalship functions for eastern chipmunks (Tamias
striatus) living under natural conditions in the wild. These curves
characterize data for female animals of known birthdate at location site 101
in Crawford County, Pennsylvania. Empiric polynomial functions were fitted
to the three sets of population data: • = controls; ■ = animals receiving a
single dose of 200 R ionizing radiation; and o = animals receiving a single
dose of 400 R. Time on the abscissa began at capture. Note the improved
survival of irradiated animals. Thompson et al.47 equated Roentgens with
rads; however, a more appropriate conversion factor is 1 R is approximately
0.95 rad with respect to muscle tissue.91 The original data came from Tryon
and Snyder.92 Reprinted from Thompson et al., p. 276, by permission of
Marcel Dekker, Inc.
sis; values above 5 indicate net toxicity. Initially, longevity hormesis domi
nates, but it dissipates over time. By the end of the study there was relatively
minor toxicity. In the work from which the original chipmunk data were
obtained,92 the investigators also administered 900-1700 R to 16 captive
chipmunks. Within 30 days, all but one died. So, whereas 400 R is benefi
cial to longevity, 900-1700 R is highly toxic. By way of comparison, the
LD 50 for 7 -radiation in female laboratory mice (RF strain) is approximately
736-1053 rad.93
Figure 1.12 illustrates another example of a system (employing methylene
chloride exposure to female hamsters) in which longevity hormesis, in the
absence of toxicity, is produced.25 94 A logarithmic-logistic equation was
used to characterize X, and consequently, the larger the dose, the greater the
longevity hormesis. In stark contrast, Figure 1.13 illustrates the effect of
methylene chloride exposure (same ppm in the air) on the Gompertz plots of
female rats.25 94 No longevity hormesis here, only irreversible toxicity. This
time a logarithmic-logistic equation was used to characterize irreversible
toxicity. These two figures exemplify one of the major problems in toxicol
ogy and risk assessment —interspecies extrapolation.