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to radiation. lonizing-radiation effect, in the depression of blood-forming
function and blood-forming cells, is proportional to radiation dose even down
to 5 r (Hennessy and Huff).
Other effects upon blood cells, leading to abnormal
doubling of the cell nucleus, are now reasonably established by Dobson in the
range of 0.1 to 0.3 r of single exposure, but are not yet tested for proportionality.
There is no reason to doubt the general evidence of a proportional
effect of radiation; but'it is also possible that linear extrapolations of higher
doses to the small-dose range may not give a true representation of the prob-
lem.
It is known for some kinds of cellular response to radiation that there
can be no effective change in function until two or more similar critical entities
within the cell are affected. Some kinds of observed injury, however, appear
to depend upon the effect on one critical entity per cell; and other observed
injuries may be the result of damage to any one of a number of critical functional parts. The response that depends upon a chain of two or more detrimental
changes shows a lesser apparent effect in proportion to radiation exposure at
the low dose ranges. Although this kind of irradiation response does not argue
against extrapolation of radiation effect, it may explain a factor-of-2 or -4
buffering against detectable radiation effect in the lowest exposure ranges; or
it may even have the opposite effect, because radiation effect can add onto
partially initiated dysfunctional changes in structures that otherwise would
have remained functional.
Radiation effect is most frequently estimated in animals that are
rather uniformly irradiated over the whole body. Thus, we are usually generalizing from the observed result of whole-body exposure. In some studies
(Kaplan), shielding a relatively small portion of the bone marrow from radiation may protect the animal from generation of thymic tumors. In others,
local irradiation is associated with induction of cancer in that region, quite
independently of exposure or shielding of the remainder of the body.
The problem of estimating radiation effect and making recommendations concerning it is not the simple problem of avoiding exposure at levels
at which there is a detectable or predictable response. This is especially
true when considering radiation effect through systems that allow proportional
extrapolation to very small radiation exposures. It is always important to
keep radiation exposure to a minimum; but it is also important in the understanding and evaluation of the relative importance of radiation effect to establish its place in the entire climate of factors that can modify health. Similar-and, at times, greater--effects upon health can be shown to result from a
large number of common environmental factors.
Also, the problem is not simply that of effect of body irradiation upon
health. It is necessary to evaluate the effect upon human beings of all known
phenomena resulting from the onset of the atomic age, including general socioeconomic factors related to our well-being, which are dependent upon progress
and the development of useful energy.