4.2 ACUTE AND CHRONIC RADIATION DOSE
Potential biological effects depend on how much and how fast a radiation dose is received. Radiation
doses can be grouped into two categories, acute and chronic dose.
4.2.1 Acute dose
An acute radiation dose is defined as a large dose (10 rad or greater, to the whole body)
delivered during a short period of time (on the order of a few days at the most). If large enough,
it may result in effects which are observable within a period of hours to weeks.
Acute doses can cause a pattern of clearly identifiable symptoms (syndromes). These conditions are
referred to in general as Acute Radiation Syndrome. Radiation sickness symptoms are apparent
following acute doses >100 rad. Acute whole body doses of >450 rad may result in a statistical
expectation that 50% of the population exposed will die within 60 days without medical attention.
As in most illnesses, the specific symptoms, the therapy that a doctor might prescribe, and the
prospects for recovery vary from one person to another and are generally dependent on the age and
general health of the individual.
Blood-forming organ (Bone marrow) syndrome (>100 rad) is characterized by damage to cells
that divide at the most rapid pace (such as bone marrow, the spleen and lymphatic tissue).
Symptoms include internal bleeding, fatigue, bacterial infections, and fever.
Gastrointestinal tract syndrome (>1000 rad) is characterized by damage to cells that divide less
rapidly (such as the linings of the stomach and intestines). Symptoms include nausea, vomiting,
diarrhea, dehydration, electrolytic imbalance, loss of digestion ability, bleeding ulcers, and the
symptoms of blood-forming organ syndrome.
Central nervous system syndrome (>5000 rad) is characterized by damage to cells that do not
reproduce such as nerve cells. Symptoms include loss of coordination, confusion, coma,
convulsions, shock, and the symptoms of the blood forming organ and gastrointestinal tract
syndromes. Scientists now have evidence that death under these conditions is not caused by actual
radiation damage to the nervous system, but rather from complications caused by internal bleeding, and fluid and pressure build-up on the brain
Other effects from an acute dose include:
200 to 300 rad to the skin can result in the reddening of the skin (erythema), similar to a mild
sunburn and may result in hair loss due to damage to hair follicles.
125 to 200 rad to the ovaries can result in prolonged or permanent suppression of menstruation in
about fifty percent (50%) of women.
600 rad to the ovaries or testicles can result in permanent sterilization.
50 rad to the thyroid gland can result in benign (non cancerous) tumors.
As a group, the effects caused by acute doses are called deterministic. Broadly speaking, this means
that severity of the effect is determined by the amount of dose received. Deterministic effects
usually have some threshold level - below which, the effect will probably not occur, but above which
the effect is expected. When the dose is above the threshold, the severity of the effect increases as
the dose increases.
4.2.2 Chronic dose
A chronic dose is a relatively small amount of radiation received over a long period of time.
The body is better equipped to tolerate a chronic dose than an acute dose. The body has time to
repair damage because a smaller percentage of the cells need repair at any given time. The body also
has time to replace dead or non-functioning cells with new, healthy cells. This is the type of dose
received as occupational exposure.
The biological effects of high levels of radiation exposure are fairly well known, but the effects of
low levels of radiation are more difficult to determine because the deterministic effects described
above do not occur at these levels.
Since deterministic effects do not generally occur with chronic dose, in order to assess the risk of this
exposure, we must look to other types of effects. Studies of people who have received high doses
have shown a link between radiation dose and some delayed, or latent effects. These effects include
some forms of cancer and genetic effects.
The risks for these effects are not directly measurable in populations of exposed workers, therefore
the risk values at occupational levels are estimates based on risk factors measured at high doses.
To make these estimates, we must use a relationship between the occurrence of cancer at high doses
and the potential for cancer at low doses. Since the probability for cancer at high doses increases
with increasing dose, this relationship is assumed to hold true with low doses. This type of risk
model is called stochastic.
Using this model and knowledge of high-dose cancer risks, we can calculate the probability of
cancer occurrence at a given dose. In this way, the rem can be used as a unit of potential harm. For
instance, the relatively well known cancer risk from doses in the range of hundreds of rem can be
'scaled down' to assess the potential risk from a dose of 100 mrem (0.1 rem). This scaling, or
extrapolation is generally considered to be a conservative approach (may over-estimate the risk) to
estimating low-dose risks.
We will use such estimates in a moment to help put the risks from exposure into perspective.
4.3 SOMATIC VS GENETIC EFFECTS
Somatic effects appear in the exposed person. Somatic effects may be divided into two classes based
on the rate at which the dose was received.
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Prompt somatic effects are those that occur soon after an acute dose (typically 10 rad or greater to
the whole body in a short period of time). One example of a prompt effect is the temporary hair loss
which occurs about three weeks after a dose of 400 rad to the scalp. New hair is expected to grow
within two months after the dose, although the color and texture may be different.
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Delayed somatic effects are those that may occur years after radiation doses are received. Among
the delayed effects thus far observed have been an increased potential for the development of cancer
and cataracts. Since some forms of cancer are among the most probable delayed effects, the
established dose limits were formulated with this risk in mind. These limits are set such that the
calculated risk of cancer in radiation workers is an increase of a very small fraction of the normal
cancer risk. (More on risk in a moment)
Genetic, or heritable effects appear in the future generations of the exposed person as a result of
radiation damage to the reproductive cells. Genetic effects are abnormalities that may occur in the
future generations of exposed individuals. They have been extensively studied in plants and animals, but
risks for genetic effects in humans are seen to be considerably smaller than the risks for somatic effects.
Therefore, the limits used to protect the exposed person from harm are equally effective to protect future
generations from harm.
Did you know? If you don't smoke, your overall risk for death from cancer - not counting occupational
radiation exposure - is about 20% - that is, about one in five Americans die from cancer.
4.4 POSSIBLE CONSEQUENCES OF RADIATION DOSE
The table below places the possible effects from acute and chronic dose into risk categories. We will
look at a comparison of the amount of risk involved in a moment.
