RADIATION
SAFETY TRAINING MANUAL
CHAPTER 4
BIOLOGICAL EFFECTS OF RADIATION
CHAPTER
4 Table Of Contents
A. SOMATIC AND GENETIC EFFECTS
B. INCREASE IN CANCER
INCIDENCE
C. GENETIC DAMAGE
D. EXPOSURE OF UNBORN
CHILDREN
The fact that
ionizing radiation produces biological damage has
been known for many years. The first case of
human injury was reported in the literature just
a few months following Roentgen's original paper
in 1895 announcing the discovery of x-rays. The
first case of radiation induced cancer was
reported seven years later. Early human evidence
of the harmful effects of ionizing radiation, as
a result of high exposures, became available in
the 1920s and 30s through the experience of
radiologists, miners exposed to airborne
activity, and workers in the radium industry.
However, the long term biological significance of
smaller, repeated doses of radiation was not
widely appreciated until later. Most of our
knowledge of these effects has accumulated since
World War II.
A. SOMATIC AND GENETIC EFFECTS
Biological effects
can be conveniently subdivided into two groups:
1.
Genetic effects which occur in the
reproductive cells and may be inherited.
2.
Somatic effects which arise from damage to
all cells in the body and are observable in
the individual affected.
Genetic effects
are essentially long term in nature since they
are manifested in offspring. In discussing
somatic effects, it is convenient to further
subdivide them into early (or acute) effects and
late (or chronic) effects. The terms acute and
chronic are also used to describe the period
during which the radiation exposure is carried
out. An acute exposure takes place within
seconds, minutes or hours and the early (or
acute) effects may be seen within minutes, hours
or up to a few weeks later. A chronic exposure
may extend over weeks, months or years, it may
not be continuous and the late (chronic) effects
may be produced during or after the irradiation.
Somatic effects
may also be categorized as non-stochastic or
stochastic. In some irradiations, the biological
response increases in severity as the dose
increases. Skin, for example, may only show a
slight reddening (erythema) at low doses, but
will exhibit severe gross tissue damage at high
doses. Such a response is termed non-stochastic
and usually exhibits a threshold dose below which
the response is not observed. Other irradiations
produce a response such as leukemia where the
severity is independent of dose, the disease is
either contracted or it is not. The probability
of inducing the response does depend upon the
dose. Such a response is termed stochastic.
Studies in both
early and late effects of ionizing radiation are
of great importance in the establishment of
guidelines for minimizing the risk inherent in
the use of ionizing radiation. The first
radiation protection standards were devised to
protect workers from acute radiation effects. The
present standards recommended by the
International Commission on Radiological
Protection (ICRP) are largely based on the
incidence of late stochastic effects, such as
cancer, for radiation workers and on genetic
effects for the general public.
B. INCREASE IN CANCER INCIDENCE
While the
relationship between acute effects and radiation
levels is well known, the situation for late
effects, both somatic and genetic,
is more obscure. The difficulty arises in part
because the effects are so small. Since so many
of the population (16-25%) die of cancer, small
effects due to low levels of chronic radiation
exposure are impossible to measure. As a
consequence, data must be extrapolated from
cancer incidence rates in individuals who
received extremely high exposures, such as the
victims of nuclear weapons, accidents, or
experimental medical procedures. An additional
problem in making an accurate assessment is the
factor of age at the time of exposure. The time
of onset can be delayed for 30 years or more
after the exposure (latent period). To estimate
the possible risks to us as users of radiation,
information is needed about the properties of
radionuclides, the measurement of radiation
exposure, and the other topics presented in this
Manual.
C. GENETIC DAMAGE
Genetic effects
occur when there is radiation damage to the germ
cells carried by the parents, due to radiation
exposure of either parent. These effects may show
up as birth or other defects in the children of
the exposed parents or in succeeding generations.
From animal studies it is estimated that the risk
of producing serious genetic effects is about one-third
the risk of producing cancer. However, it is
difficult to apply animal data to humans. Damage
to germ cells should not be confused with damage
to the cells of an embryo/fetus from in utero
irradiation.
D. EXPOSURE OF UNBORN
CHILDREN
While the risks of
cancer or genetic damage are barely significant
for a prudent worker, the unborn child is at a
higher risk. The more rapidly dividing cells of
the embryo/fetus are more sensitive to the
effects of radiation than slowly dividing cells
such as brain or bone cells. Cells in the unborn
child are dividing very rapidly. Furthermore, the
child has its whole life ahead during which
delayed effects might occur.
Women who work
with radioactivity and are considering pregnancy
should carefully read the material presented in
Chapter 3, Section E of this manual. Supervisors
and co-workers of fertile women should also be
familiar with this material to be sure that
situations that might put the embryo/fetus at
risk are avoided.
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