Injury to living tissue results from the transfer of energy to atoms
and molecules in the cellular structure. Ionizing radiation causes atoms
and molecules to become ionized or excited. These excitations and ionizations
Produce free radicals.
Break chemical bonds.
Produce new chemical bonds and cross-linkage between macromolecules.
Damage molecules that regulate vital cell processes (e.g. DNA, RNA, proteins).
The cell can repair certain levels of cell damage. At low doses, such as
that received every day from background radiation, cellular damage is rapidly
At higher levels, cell death results. At extremely high doses,
cells cannot be replaced quickly enough, and tissues fail to function.
In general, the radiation sensitivity of a tissue is:
proportional to the rate of proliferation of its cells
inversely proportional to the degree of cell differentiation
For example, the following tissues and organs are listed from most radiosensitive
to least radiosensitive:
Bone and teeth
Least sensitive: Nervous system
This also means that a developing embryo is most sensitive to radiation
during the early stages of differentiation, and an embryo/fetus is more
sensitive to radiation exposure in the first trimester than in later
Radiation effects can be categorized by when they appear.
Prompt effects: effects, including radiation sickness and radiation
burns, seen immediately after large doses of radiation delivered over short
periods of time.
Delayed effects: effects such as cataract formation and cancer induction
that may appear months or years after a radiation exposure
High doses delivered to the whole body of healthy adults within short
periods of time can produce effects such as blood component changes, fatigue,
diarrhea, nausea and death. These effects will develop within hours, days
or weeks, depending on the size of the dose. The larger the dose, the sooner
a given effect will occur.
(Adapted from NCRP Report No. 98 "Guidance on Radiation
Received in Space Activities, NCRP, Bethesda, MD (1989))
|Blood count changes
|LD50/60* (with minimal
||320 – 360 rem
|LD50/60 (with supportive medical
||480 – 540 rem
|100% mortality (with best available treatment)
* The LD50/60 is that dose at which 50%of the
exposed population will die within 60 days.
compare these dose levels with federal dose limits and University investigational
These acute effects apply only when the whole body is relatively uniformly
irradiated. The effects can be significantly different when only portions
of the body or an individual organ system are irradiated, such as might
occur during the use of radiation for medical treatment. For example, a
dose of 500 rem delivered uniformly to the whole body may cause death while
a dose of 500 rem delivered to the skin will only cause hair loss and skin
further information about how specific organ systems respond to acute exposure
Cataracts are induced when a dose exceeding approximately 200-300 rem is
delivered to the lens of the eye. Radiation-induced cataracts may
take many months to years to appear.
Studies of people exposed to high doses of radiation have shown that there
is a risk of cancer induction associated with high doses.
The specific types of cancers associated with radiation exposure include
leukemia, multiple myeloma, breast cancer, lung cancer, and skin cancer.
Radiation-induced cancers may take 10 - 15 years or more to appear.
There may be a risk of cancer at low doses as well. The following
frames discuss the risk of cancer at lower doses
Why cancer risks at low doses are uncertain
It has been difficult to estimate cancer induction risks, because most
of the radiation exposures that humans receive are very close to background
levels. At low dose levels of millirem to tens of rem, the risk of radiation-induced
cancers is so low, that if the risk exists, it is not readily distinguishable
from normal levels of cancer occurrence. In addition, leukemia or solid
tumors induced by radiation are indistinguishable from those that result
from other causes.
further information about radiation-induced cancer risk studies
Using the linear no-threshold risk model, the 1990 BEIR* V report provided
the following estimate:
The average lifetime risk of death from cancer following an acute dose
equivalent to all body organs of 0.1 Sv (10 rem) is estimated to be 0.8%.
This increase in lifetime risk is about 4% of the current baseline
risk of death due to cancer in the United States. The current baseline
risk of cancer induction in the United States is approximately 25%.
Another way of stating this risk:
A dose of 10 mrem creates a risk of death from cancer of approximately
* The National Academy of Sciences Committee on the Biological Effects
of Ionizing Radiation
(the BEIR Committee)
a more detailed excerpt from the BEIR V report
One way of considering the level of a risk
is to look at the number of "days lost" out of a population due to early
death from a given cause, then distributing those days lost over the population
to get an "average life expectancy lost" due to that cause. The following
table provides an estimate of life expectancy lost due to several causes:
Estimated Life Expectancy Lost
|Smoking 20 cigarettes a day
|Overweight by 15%
|Alcohol (US average)
|All natural hazards
|Occupational dose of 300 mrem/year
Source: these estimates are taken from NRC Draft Guide DG-8012 and were
adapted from B. L. Cohen and L. S. Lee, "Catalogue of Risks Extended and
Updates," Health Physics, Vol. 61, September 1991.
You can also look at risk by considering the Relative Risk of a 1
in a million chance of death from activities common to our society:
Smoking 1.4 cigarettes in a lifetime (lung cancer)
Eating 40 tablespoons of peanut butter (aflatoxin)
Spending two days in New York City (air pollution)
Driving 40 miles in a car (accident)
Flying 2500 miles in a jet (accident)
Canoeing for 6 minutes (drowning)
Receiving a dose of 10 mrem of radiation (cancer)
(Adapted from DOE Radiation Worker Training based on work by B.L. Cohen,
||There is no direct evidence of radiation-induced
genetic effects in humans, even at high doses. Various analyses indicate
that the rate of genetic disorders produced in humans is expected to be
extremely low, on the order of a few disorders per million liveborn per
rem of parental exposure.
Rapidly proliferating and differentiating tissues are most sensitive
to radiation damage. Consequently, radiation exposure can produce developmental
problems, particularly in the developing brain, when an embryo/fetus is
The developmental conditions most commonly associated with prenatal
radiation exposure include low birth weight, microcephaly, mental retardation,
and other neurological problems. These effects are related to the developmental
stage at which the exposure occurs.
The threshold dose for developmental effects is approximately 10
The evidence that the developing embryo/fetus is more sensitive to radiation-induced
cancer is inconclusive. But it is prudent to assume that there is some
more information about the University of Washington program to control prenatal
radiation exposures (the Declared Pregnant Worker Program)
This is the end of the Biological Effects Module, which is the third
of the seven Sealed Source Radiation Basics modules. The next module
is the Government Regulations Module.
Regulations (Module 4)
to the Sealed Source Training Introduction Page
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