fall-out studies were conducted, it was thought that the dose required
to cause lethality in 50% of exposed individuals within 30 days - hereinafter abbreviated as 1p 07 should perhaps be somewhat greater than
indicated in Figure 12.
After the CASTLE fall-out studies, however,
the possibility that the curve values may be somewhat high was raised.
The biological effectiveness of radiations from various sources as well
as the geometry of both source and receiver are important.
Considering
the present state of uncertainty regarding each of the parameters in-
volved, it is considered that the original curve should be accepted as
it is.
One should be conservative with regard to raising the values
needed to cause a particular per cent lethality.
Radiation exposure in a fall-out field is not instantaneous.
It is therefore necessary to evaluate various time-dose relationships
in terms of lethal effectiveness.
Protraction and fractionation of a
Biven dose of radiation reduces the lethal effectiveness to a limiting
value determined by the non-recoverable fraction of total dose.
In evaluating the lethal effect of exposure to intermittent or
continuous radiation over a period of time, the term “effective roentgen'
will be used.
This means that any time-dose relationship under con-
sideration will be referred back to its equivalent effect on the in-
stantaneous effect curve and the dose will be said to be equivalent to
s0 many instantaneous roentgens.
For example, if 600 r are given over
a period of time so that 50% of the exposed group can be expected to
die, the effective roentgen value will be 400 r.
The decrease in lethal expectancy associated with protraction
of the dose delivery period is due to the fact that the body recovers,
at least partially, from the effects of a given exposure.
This biologi-
cal recovery factor has been experimentally examined in lower animals.
According to E. H. smith, 20/ the recovery rate for all animals studied
may be taken to be 29% per day.
The design of the experiments was such
that results were not sensitive to this parameter.
F. McLean,
18/ smith, E.H., Informal Study for the Chief, AFSWP, 1953.
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