Although his experience with radioactivity has been brief, man probably
already knows more about the effects of radiation than he knows about
the effects of many other contaminants that alter his environment. Even
so, he knows far less than he needs to know to make certain that atomic
energy is wisely managed in the future.
There has been neither time nor opportunity, for example, to gather
radiation-effects data on more than a few hundred of the 1,500,000 kinds
of living organisms inhabiting the earth. Nor is it possible to predict
the extent to which life can adjust itself to environmental changes
resulting from scarcely perceptible alterations of natural radiological
balances. Also undetermined is the relation between environmental
changes and the biological exchanges making up the often mentioned, but
insufficiently understood, “balance of nature”.
The case of carbon-14 is an example of a permanent man-made modification
of the environment. From the early ages of the earth, carbon-14 has been
created in the upper atmosphere by the transmutation of nitrogen in
cosmic-ray reactions. Carbon itself is an almost universal component of
living matter, and the ratio between stable carbon and radioactive
carbon is believed to have been unchanged for thousands of years. It is
this circumstance that permits the use of carbon-14 as a tool for
“dating”, or determining the ages of, fossil remains, prehistoric
artifacts, and geologic formations. But carbon-14 also is produced in
nuclear fusion, and the testing of thermonuclear devices after 1952
produced an estimated increase of 4% in the amount of carbon-14 on
earth. This is enough to disturb the natural equilibrium. Since the
half-life[12] of carbon-14 is some 5800 years, the addition will be a
factor of environmental consideration for scores of human generations.
Nuclear tests, although not the only sources of man-made radioactivity,
have been until now the most significant ones and the only sources
touching large areas of the earth. The total product of nuclear testing
is small in relation to the natural burden of radioactivity, raising the
level of radiation to which all life is subject by a factor of one-tenth
or less. But it is the unknown element, the degree to which fallout
radioactivity may introduce new influences into the environment, that
gives concern.[13]
[Illustration: _One of the last cows of the herd exposed to fallout by
the world’s first atomic detonation in New Mexico in July 1945,
photographed in 1964. The calf is her 15th to be born in 15 years. The
cow, believed about 20 years old, has been under observation by
scientists, who found she suffered little apparent effect, although the
fallout caused some hair to turn gray (see light patches on back). Other
cows in the herd died natural deaths._]
When a nuclear device is detonated, the release of energy is due to the
fission of uranium or plutonium atoms or to the fusion of hydrogen
atoms. At the instant of fission, some 75 radionuclides, or fission
products, are created.
From these primary fission products, about 100 other radionuclides may
be formed, some existing only for microseconds and others for thousands
of years. The radionuclides of significance to biologists are those that
exist long enough—no matter how brief the time—to have an impact on a
biological system.
Factors of biological transport and concentration of long-lived
radionuclides make efforts to assess possible environmental effects
particularly difficult. It has been asserted, for example, that probably
every living cell formed since the early 1950s contains some of the
radionuclides produced by nuclear testing. No one knows the significance
of such a condition, if it indeed exists. It is certain only that some
of the long-lived radionuclides already placed in the environment will
be detectable there for hundreds of years and hence will continue to
provide material for biological studies.