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UCRL-3644
of a few hundred roentgens has not changed; but extensive information on effects
of lower levels of radiation has recently appeared. This knowledge requires a
re-evaluation of the cost to humans of radiation exposure in terms of (a) genetic
effects, (b) shortening of life span, (c) induction of cancers, (d) destruction of
tissue, (e) congenital malformation, and (f) effects upon young individuals.
All
these effects appear to be proportional to the exposure to radiation, and have
been largely responsible for a recent downward revision in maximum allowable exposure to radiation.
The Genetic Evaluation of Radiation Effect
Up to about 1946, estimations of the genetic effects of radiation had
placed the quantity necessary to double the mutation rate per generation in the
fruit fly at about 50 r (Muller, Stern)--but with some uncertainty, so that the
true value might have been 80 r or 35 r. At that time, there had been relatively
little comprehensive evaluation of the range of genetic sensitivity to radiation
in mammals or man. At the present time, the mutation rate per generation
for the fruit fly is known to be doubled over the natural rate by about 50 r
(Stern).
Through genetic study of irradiated mice (Russell), the amount re-
quired to double mutation rate per generation in the mouse is partially established also at approximately 35 to 80 r. Wright has estimated from evidence
now available that the mammalian mutation rate may be doubled by as little as
3 r or as much as 300 r. The best current estimates place the mammalian
mutation-doubling dose of radiation at about 50 r.
As an approximation, each species appears to form in natural circumstances about one new mutation in a generation time. The fruit fly lives a
short time in about the same radiation environment (estimated roughly at 0.1 r/yr)
as man. In its life span of 20 to 30 days, it can accumulate only the minute
quantity of 0.008 r.
Thus, if 50 r in the fly produces an additional number of
mutations equal to those which occur naturally, radiation can account for only
a part of the natural mutation frequency, namely, the fraction
0.008
1
“50 6,000 ��
Hence, at background radiation, only one observed mutant in 6,000 is suspect
of being induced by radiation. In humans, the life span up to average reproduction age is about 30 years, lived in the same environment of 0.1 r/yr, or
a total of about 3 r by average reproduction age.
Thus, if 50 r is estimated
to double the human mutation rate, radiation from natural sources may be
expected to account for
3 1
;
50° 17?
or approximately 6% of the naturally occurring mutations.
If we accept the
lowest possible value of 3 r for the mutation-doubling dose, we would have as
the fraction attributable to radiation