Radioactive materials that decay spontaneously produce
ionizing
radiation, which has sufficient energy to strip away
electrons from atoms (creating two charged ions)
or to break some chemical bonds. Any living tissue in the
human body can be damaged by ionizing radiation. The body
attempts to repair the damage, but sometimes the damage
is too severe or widespread, or mistakes are made in the
natural repair process. The most common forms of ionizing
radiation are alpha
and beta particles,
or gamma and X-rays.
What kinds of health effects occur from exposure to radionuclides?
In general, the amount and duration of radiation exposure
affects the severity or type of health effect. There are
two broad categories of health effects: stochastic and non-stochastic.
Stochastic Health Effects
Stochastic effects are associated with long-term, low-level
(chronic) exposure to radiation. ("Stochastic"
refers to the likelihood that something will happen.) Increased
levels of exposure make these health effects more likely
to occur, but do not influence the type or severity of the
effect.
Cancer is considered by most people the primary health
effect from radiation exposure. Simply put, cancer is the
uncontrolled growth of cells. Ordinarily, natural processes
control the rate at which cells grow and replace themselves.
They also control the body's processes for repairing or
replacing damaged tissue. Damage occurring at the cellular
or molecular level, can disrupt the control processes, permitting
the uncontrolled growth of cells--cancer. This is why ionizing
radiation's ability to break chemical bonds in atoms and
molecules makes it such a potent carcinogen.
Other stochastic effects also occur. Radiation can cause
changes in DNA, the "blueprints" that ensure cell
repair and replacement produces a perfect copy of the original
cell. Changes in DNA are called mutations.
Sometimes the body fails to repair these mutations or even
creates mutations during repair. The mutations can be teratogenic
or genetic. Teratogenic mutations affect only the individual
who was exposed. Genetic mutations are passed on to
offspring.
Non-Stochastic Health Effects
Non-stochastic effects appear in cases of exposure to high
levels of radiation, and become more severe as the exposure
increases. Short-term, high-level exposure is referred
to as 'acute' exposure.
Many non-cancerous health effects of radiation are non-stochastic.
Unlike cancer, health effects from 'acute' exposure to radiation
usually appear quickly. Acute health effects include burns
and radiation sickness. Radiation sickness is also called
'radiation poisoning.' It can cause premature aging
or even death. If the dose is fatal, death usually occurs
within two months. The symptoms of radiation sickness include:
nausea, weakness, hair loss, skin burns or diminished organ
function.
Medical patients receiving radiation treatments often experience
acute effects, because they are receiving relatively high
"bursts" of radiation during treatment.
There is no firm basis for setting a "safe" level
of exposure above background for stochastic effects. Many
sources emit radiation that is well below natural background
levels. This makes it extremely difficult to isolate its
stochastic effects.
Some scientists assert that low levels of radiation are
beneficial to health (this idea is known as hormesis).
However, there do appear to be threshold exposures for
the various non-stochastic effects. (Please note that the
acute affects in the following table are cumulative. For
example, a dose that produces damage to bone marrow will
have produced changes in blood chemistry and be accompanied
by nausea.)
Exposure
(rem)
Health Effect
Time to Onset
radiation
burns;
more severe as exposure increases.
Basically, we have learned through observation. When people
first began working with radioactive materials, scientists
didn't understand radioactive decay, and reports of illness
were scattered.
As the use of radioactive materials and reports of illness
became more frequent, scientists began to notice patterns
in the illnesses. People working with radioactive materials
and x-rays developed particular types of uncommon medical
conditions. For example, scientists recognized as early
at 1910 that radiation caused skin cancer. Scientists began
to keep track of the health effects, and soon set up careful
scientific studies of groups of people who had been exposed.
Among the best known long-term studies are those of Japanese
atomic bomb blast survivors, other populations exposed to
nuclear testing fallout (for example, natives of the Marshall
Islands), and uranium miners.
Aren’t children more sensitive to radiation than adults?
Yes, because children are growing more rapidly, there are
more cells dividing and a greater opportunity for radiation
to disrupt the process. EPA's radiation protection standards
take into account the differences in the sensitivity due
to age and gender.
Fetuses are also highly sensitive to radiation. However,
the period during which they may be exposed is short.
Both the type of radiation to which the person is exposed
and the pathway by which they are exposed influence health
effects. Different types of radiation vary in their ability
to damage different kinds of tissue.
Radiation and radiation emitters (radionuclides) can expose
the whole body (direct exposure) or expose tissues inside
the body when inhaled or ingested. All kinds of ionizing
radiation can cause cancer and other health effects. The
main difference in the ability of alpha and beta particles
and gamma and x-rays to cause health effects is the amount
of energy they have. Their energy determines how far they
can penetrate into tissue. It also determines how much energy
they are able to transmit directly or indirectly to tissues
and the resulting damage.
Radioactive elements and compounds behave chemically exactly
like their non-radioactive forms. For example, radioactive
lead has the same chemical properties as non-radioactive
lead. The public health protection question that EPA's scientists
must answer is, "How do we best manage all the hazards
a pollutant presents?"
Do chemical properties of radionuclides contribute to radiation
health effects?
The chemical properties of a radionuclide can determine
where health effects occur. To function properly many organs
require certain elements. They cannot distinguish between radioactive
and non-radioactive forms of the element and accumulate one
as quickly as the other.
Radioactive iodine
concentrates in the thyroid. The thyroid needs iodine to
function normally, and cannot tell the difference between
stable and radioactive isotopes. As a result, radioactive
iodine contributes to thyroid cancer more than other types
of cancer.
Calcium, strontium-90,
and radium-226
have similar chemical properties. The result is that strontium
and radium in the body tend to collect in calcium rich areas,
such as bones and teeth. They contribute to bone cancer.return to: [top] [previous
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Estimating Health Effects
What is the cancer risk from radiation? How does it compare
to the risk of cancer from other sources?
Each radionuclide represents a somewhat different health
risk. However, health physicists currently estimate
that overall, if each person in a group of 10,000 people
exposed to 1 rem of ionizing radiation, in small doses over
a life time, we would expect 5 or 6 more people to die of
cancer than would otherwise.
In this group of 10,000 people, we can expect about 2,000
to die of cancer from all non-radiation causes. The accumulated
exposure to 1 rem of radiation, would increase that number
to about 2005 or 2006.
To give you an idea of the usual rate of exposure, most
people receive about 3 tenths of a rem (300 mrem) every
year from natural background sources of radiation (mostly
radon).
What are the risks of other long-term health effects?
Other than cancer, the most prominent long-term
health effects are teratogenic and genetic mutations.
Teratogenic mutations result from the exposure of fetuses
(unborn children) to radiation. They can include smaller
head or brain size, poorly formed eyes, abnormally slow
growth, and mental retardation. Studies indicate that fetuses
are most sensitive between about eight to fifteen
weeks after conception. They remain somewhat less sensitive
between six and twenty-five weeks old.
The relationship between dose and mental retardation is
not known exactly. However, scientists estimate that if
1,000 fetuses that were between eight and fifteen weeks
old were exposed to one rem, four fetuses would become mentally
retarded. If the fetuses were between sixteen and twenty-five
weeks old, it is estimated that one of them would be mentally
retarded.
Genetic effects are those that can be passed from parent
to child. Health physicists estimate that about fifty severe
hereditary effects will occur in a group of one million
live-born children whose parents were both exposed to one
rem. About one hundred twenty severe hereditary effects
would occur in all descendants.
In comparison, all other causes of genetic effects result
in as many as 100,000 severe hereditary effects in one
million live-born children. These genetic effects include
those that occur spontaneously ("just happen")
as well as those that have non-radioactive causes.
What limits does EPA set on exposure to radiation?
Health physicists generally agree on limiting a person's
exposure beyond background radiation to about 100 mrem per
year from all sources. Exceptions are occupational, medical
or accidental exposures. (Medical X-rays generally deliver
less than 10 mrem). EPA and other regulatory agencies
generally limit exposures from specific source to the public
to levels well under 100 mrem. This is far below
the exposure levels that cause acute
health effects.
How does EPA protect against radionuclides that are also toxic?
In most cases, the radiation hazard is much greater
than the chemical (toxic) hazard. Radiation protection limits
are lower than the chemical hazard protection limits would be.
By issuing radiation protection regulations, EPA can protect
people from both the radiation and the chemical hazard. However,
deciding which hazard is greater is not always straightforward.
Several factors can tip the balance:
toxicity of the radionuclide
strength of the ionizing radiation
how quickly the radionuclide emits radiation (half-life)
relative abundance of the radioactive and non-radioactive
forms
For example:
Uranium-238 radioactive and very toxic. Its half-life
of 4.5 billion years means that only a few atoms emit radiation
at a time. A sample containing enough atoms to pose a radiation
hazard contains enough atoms to pose a chemical hazard.
As a result, EPA regulates uranium-238 as both a chemical
and a radiation hazard.
Radioactive isotopes of lead are both radioactive and
toxic. In spite of the severe effects of lead on the brain
and the nervous system, the radiation hazard is greater.
However, the radioactive forms of lead are so uncommon that
paint or other lead containing products do not contain enough
radioactive lead to present a radiation hazard. As
a result, EPA regulates lead as a chemical hazard.
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Radiation Protection
Basics 
