1 month could be assumed, similar conditions of exposure in man might be expected to cause a reduction of the mean after survival time of 9 to 10 days /rad
of gamma-ray exposure.
232.
Grahn and Sacher [G1] based their extrapolations on the linearity of
the log mean after-survival in days as a function of daily dose in rad or R,
for mean after-survival of 25 per cent or more of the control values.
This
linear trend of the mean after-survival had been shown previously to hold for
the mouse [S4]
(see Figure XV).
The coefficient of this regression is a species
constant [G9] and reflects the days of life lost per rad or the fraction lost
per day.
It may therefore allow, when two points referring to a given species
are Known, to define the slope of the curve applying to that species.
The ratio
of the life-shortening coefficients would be in a direct proportion to the ratio
of life expectancies for non-irradiated populations or to the ratio of the agespecific mortality rate slopes.
A radio susceptibility scale could thus be con-
structed for man, dog, guinea-pig and mouse, on the basis of the slopes being in
the ratio of 33:10:3:1 for life expectancies of 16500, 4000, 1400 and 500 days,
respectively.
It could therefore be deduced that the relative sensitivity of
the various species (expressed in per cent life-shortening for exposure of 1 rad)
Was approximately the same for all species and in the ratio of:
guinea-pig = 1.8;
dog = 1.25; and man = 1.
mouse = 1;
Similar conclusions were also drawn
by Sacher [S23, S14].
233.
Grahn [G6] explored further the problem of inter-species comparisons,
starting from the notion of the exponential decline of mean after-survival with
daily dose, already mentioned repeatedly (see Figure XV).
By comparing the vi-
tal statistics of two selected populations of male men and mice, he established
that the ratio of time scale to equate the two populations is 10 mouse days =
1 man year, or 1 mouse day = 36.5 man days, a factor slightly higher than he
previously used (33:1)
(20:1)
[F4].
[G1] and much higher than that used by Failla and McClement
In consideration of these other estimates Grahn selected for his
calculations a ratio of 1:30 and established that the daily dose to induce a
20 per cent reduction of life expectancy in man would be 0.65 R/day, to be compared with a 19.4 R/day in the mouse.
Calculations for man and for other spe-
cies showed that the guinea-pig would be a relatively sensitive species in a
framework defined by the mouse, dog and man.
It should be pointed out that the
life-span and radiosensitivity values used for these calculations are very much
at variance with those in the 1968 paper by the same authors [G11 and therefore
the relative sensitivity scales in the two papers do not correspond.