United States Air and
Environmental Protection Radiation
Agency (6601J)
EPA402-F-98-010
May 1998
\°/EPA Ionizing Radiation Series No. 2
Health Effects From Exposure
To Ionizing Radiation
Ionizing radiation can cause changes in the
chemical balance of cells. Some of those
changes can result in cancer. In addition, by
damaging the genetic material (DNA)
contained in all cells of the body, ionizing
radiation can cause harmful genetic mutations
that can be passed on to future generations.
Exposure to large amounts of radiation, a rare
occurrence, can cause sickness in a few hours
or days and death within 60 days of exposure.
In extreme cases, it can cause death within a
few hours of exposure.
Sources of Exposure
The ionizing radiations of primary concern are
alpha and beta particles, gamma rays, and x
rays. Alpha and beta particles and gamma rays
can come from natural sources or can be
technologically produced. Most of the x-ray
exposure people receive is technologically
produced. Natural radiation comes from
cosmic rays, naturally occurring radioactive
elements found in the earth's crust (uranium,
thorium, etc.), and radioactive decay products
such as radon and its subsequent decay
products. The latter group represents the
majority of the radiation exposure of the
general public.
In additon to these natural sources, radiation
can come from such wide-ranging sources as
hospitals, research institutions, nuclear reactors
and their support facilities, certain
manufacturing processes, and Federal facilities
involved in nuclear weapons production. The
figure on page 2 shows the percentage
contribution that various radiation sources
make toward the yearly average effective dose
received by the U.S. population (NCRP Report
No. 93).
Any release of radioactive material is a
potential source of radiation exposure to
the population. In addition to exposure
from external sources, radiation exposure
can occur internally by ingesting, inhaling,
injecting, or absorbing radioactive materials.
Both external and internal sources may
irradiate the whole body or a portion of the
body. The amount of radiation exposure is
usually expressed in a unit called millirem
(mrem). In the United States, the average
person is exposed to an effective dose
equivalent of approximately 360 mrem
(whole-body exposure) per year from all
sources (NCRP Report No. 93).
Results of Exposure
Ionizing radiation affects people by
depositing energy in body tissue, which can
cause cell damage or cell death. In some
cases there may be no effect. In other
cases, the cell may survive but become
abnormal, either temporarily or
permanently, or an abnormal cell may
become malignant. Large doses of
radiation can cause extensive cellular
damage and result in death. With smaller
doses, the person or particular irradiated
organ(s) may survive, but the cells are
damaged, increasing the chance of cancer.
The extent of the damage depends upon
the total amount of energy absorbed, the
time period and dose rate of exposure, and
the particular organ(s) exposed.
Evidence of injury from low or moderate
doses of radiation may not show up for
months or even years. For leukemia, the
minimum time period between the
Printed on Recycled Paper
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Medical xrays 11%
Nuclear Medicine 4%
Consumer Products 3%
OTHER <1%
Occupational 0.3%
Fallout <0.3%
Nuclear Fuel Cycle 0.1%
Miscellaneous 0.1%
Sources of Radiation Exposure
(NCRP Report No. 93)
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radiation exposure and the appearance of disease
(latency period) is 2 years. For solid tumors, the
latency period is more than 5 years. The types of
effects and their probability of occurrence can
depend on whether the exposure occurs over a large
part of a person's lifespan (chronic) or during a very
short portion of the lifespan (acute). It should be
noted that all of the health effects of exposure to
radiation can also occur in unexposed people due to
other causes. Also, there is no detectable difference
in appearance between radiation induced cancers
and genetic effects and those due to other causes.
Chronic Exposure
Chronic exposure is continuous or intermittent
exposure to low levels of radiation over a long
period of time. Chronic exposure is considered to
produce only effects that can be observed some
time following initial exposure. These include
genetic effects and other effects such as cancer,
precancerous lesions, benign tumors, cataracts, skin
changes, and .congenital defects.
Acute Exposure
Acute exposure is exposure to a large, single dose
of radiation, or a series of doses, for a short period
of time. Large acute doses can result from
accidental or emergency exposures or from special
medical procedures (radiation therapy). In most
cases, a large acute exposure to radiation can cause
both immediate and delayed effects. For humans
and other mammals, acute exposure, if large
enough, can cause rapid development of radiation
sickness, evidenced by gastrointestinal disorders,
bacterial infections, hemorrhaging, anemia, loss of
body fluids, and electrolyte imbalance. Delayed
biological effects can include cataracts, temporary
sterility, cancer, and genetic effects. Extremely high
levels of acute radiation exposure can result in
death within a few hours, days or weeks.
Risks of Health Effects
All people are chronically exposed to background
levels of radiation present in the environment.
Many people also receive additional chronic
exposures and/or relatively small acute exposures.
For populations receiving such exposures, the
primary concern is that radiation could increase the
risk of cancers or harmful genetic effects.
The probability of a radiation-caused cancer or
genetic effect is related to the total amount of
radiation accumulated by an individual. Based on
current scientific evidence, any exposure to radiation
can be harmful (or can increase the risk of cancer);
however, at very low exposures, the estimated
increases in risk are very small. For this reason,
cancer rates in populations receiving very low doses
of radiation may not show increases over the rates
for unexposed populations.
For information on effects at high levels of
exposure, scientists largely depend on
epidemiological data on survivors of the Japanese
atomic bomb explosions and on people receiving
large doses of radiation medically. These data
demonstrate a higher incidence of cancer among
exposed individuals and a greater probability of
cancer as the level of exposure increases. In the
absence of more direct information, that data is also
used to estimate what the effects could be at lower
exposures. Where questions arise, scientists try to
extrapolate based on information obtained from
laboratory experiments, but these extrapolations are
acknowledged to be only estimates. For radon,
scientists largely depend on data collected on
underground miners. Professionals in the radiation
protection field prudently assume that the chance of
a fatal cancer from radiation exposure increases hi
proportion to the magnitude of the exposure and
that the risk is as high for chronic exposure as it is
for acute exposure. In other words, it is assumed
that no radiation exposure is completely risk free.
Suggested Reading
The following books are possible sources for more
in-depth information on the health effects of
radiation exposure.
Cancer - Volume I. Frederick F. Becker, Editor,
Plenum Press - 1975.
Radiation Protection - Second Edition. Jacob
Shapiro, Harvard University Press - 1981.
Atoms. Radiation, and Radiation Protection. James
E. Turner, Pergamon Press 1986.
Ionizing Radiation Exposure of the Population of
the United States- (NCRP Report No. 93).
National Council on Radiation Protection and
Measurements (NCRP)- 1987.
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