Stochastic effects are those that occur by chance and consist
primarily of cancer and genetic effects. Stochastic effects often
show up years after exposure. As the dose to an individual increases,
the probability that cancer or a genetic effect will occur also
increases. However, at no time, even for high doses, is it certain
that cancer or genetic damage will result. Similarly, for stochastic
effects, there is no threshold dose below which it is relatively
certain that an adverse effect cannot occur. In addition, because
stochastic effects can occur in individuals that have not been
exposed to radiation above background levels, it can never be
determined for certain that an occurrence of cancer or genetic
damage was due to a specific exposure.
While it cannot be determined conclusively, it often possible
to estimate the probability that radiation exposure will cause
a stochastic effect. As mentioned previously, it is estimated that the probability
of having a cancer in the US rises from 20% for non radiation
workers to 21% for persons who work regularly with radiation.
The probability for genetic defects is even less likely to increase
for workers exposed to radiation. Studies conducted on Japanese
atomic bomb survivors who were exposed to large doses of radiation
found no more genetic defects than what would normally occur.
Radiation-induced hereditary effects have not been observed in
human populations, yet they have been demonstrated in animals.
If the germ cells that are present in the ovaries and testes and
are responsible for reproduction were modified by radiation, hereditary
effects could occur in the progeny of the individual. Exposure
of the embryo or fetus to ionizing radiation could increase the
risk of leukemia in infants and, during certain periods in early
pregnancy, may lead to mental retardation and congenital malformations
if the amount of radiation is sufficiently high.
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