30 BIOLOGICAL EFFECTS OF LOW LEVEL EXPOSURES
Figure 1.20. Gompertz plots for control and hexachlorobenzene-treated female
Sprague-Dawley rats. Hexachlorobenzene administration (0.32-40 ppm
feed) was begun at about 30 days of age and continued throughout life.
Time on the abscissa refers to that period following initiation of dosing.
Although the data are consistent with concomitant longevity hormesis and
toxicity, and this model was used in the curve-fits, the figure is equivocal in
making a more definitive judgment. The theoretical lines were obtained by
simultaneous fitting of Equations 3 and 12, employing weighted
least-squares regression analysis. In the mathematical model, maximum
longevity hormesis appeared to have been reached at the lowest 0.32-ppm
dose. The toxicity parameter, y, was characterized by a logarithmic-logistic
function. The original data came from Arnold et al.100 Reprinted from
Neafsey et al.,25 p. 141, by permission of Marcel Dekker, Inc.other physician specialists. For those radiologists who joined the Radiologi
cal Society of North America after the year 1940, mortality rates were
initially lower than all other physician specialties; however, with follow-up
15-20 years later, mortality increased to values greater than ophthalmolo
gists. This is akin to the longevity hormesis-irreversible toxicity model previ
ously discussed for animal populations (Equation 12) and illustrated for y-
radiation in male and female mice (Figure 1.9).
Standardized mortality ratio (SMR) data also exist that are consistent
with the idea that exposure to low doses of ionizing radiation can be benefi
cial. The SMR is the ratio of the number of deaths from a specific (defined)
cause in one population relative to deaths from the same cause in a second
“standard” population, appropriately adjusted for gender, age, and calen
dar year.103 Forman et al.104 reported that SMRs for most cancers were
significantly less over a 22 year period in the vicinity of nuclear installations
than in non-installation areas. The investigators indicated that this was
“unlikely to be due to a protective effect of ionizing radiation” and con
cluded instead that there were likely to have been large socioeconomic and/
or environmental differences between the two groups of SMRs responsible
for the apparent anomaly.
A particularly acrimonious controversy centers around cancer SMRs for
workers at the Hanford nuclear weapons site in southeastern Washington