1.1 What is thorium? |
1.2 How might I be exposed to thorium? |
1.3 How can thorium enter and leave my
body? |
1.4 How can thorium affect my health? |
1.5 What levels of exposure have resulted
in harmful health effects? |
1.6 Is there a medical test to determine
whether I have been exposed to thorium? |
1.7 What recommendations has the federal
government made to protect human health? |
1.8 Where can I get more information? |
References |
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October 1990 |
Public Health Statement |
for |
Thorium |
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This Public Health Statement is the
summary chapter from the Toxicological
Profile for thorium. It is one in a series of Public Health
Statements about hazardous substances and their health effects.
A shorter version, the ToxFAQs™,
is also available. This information is important because this
substance may harm you. The effects of exposure to any hazardous
substance depend on the dose, the duration, how you are exposed,
personal traits and habits, and whether other chemicals are
present. For more information, call the ATSDR Information
Center at 1-888-422-8737.
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This Statement was prepared to give you
information about thorium and to emphasize the human health
effects that may result from exposure. At this time, thorium
has been found above background levels at 16 out of 1177 National
Priorities List (NPL) hazardous waste sites. We do not know
how many of the 1,177 NPL sites have been evaluated for thorium.
As EPA evaluates more sites, the number of sites at which
thorium is found at above background levels may change. Because
these sites are potential or actual sources of human exposure
to thorium and because thorium may cause harmful health effects,
this information is important for you to know.
When a radioactive chemical is released
from a large area, such as an industrial plant, or from a
container, such as a drum or bottle, it enters the environment
as a radioactive chemical emission. This emission, which is
also called a release, does not always lead to exposure. You
are exposed only when you come into contact with the radioactive
chemical. You can come into contact with it in the environment
through breathing air, eating, drinking or smoking substances
containing the radioactive chemical. Exposure may also result
from skin contact with the radioactive chemical alone, or
with a substance containing it. Exposure can also occur by
being near radioactive chemicals at concentrations that may
be found at hazardous waste sites or industrial accidents.
If you are exposed to a hazardous chemical,
several factors determine whether harmful effects will occur
and the type and severity of those health effects. These factors
include the dose (how much), the duration (how long), the
pathway by which you are exposed (breathing, eating, drinking,
or skin contact), the other chemicals to which you are exposed,
and your individual characteristics such as age, sex, eating
habits, family traits, life style, and state of health.
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1.1
What is thorium? |
Thorium is a naturally-occurring, radioactive
metal. Small amounts of thorium are present in all rocks,
soil, above-ground and underground water, plants, and animals.
These small amounts of thorium contribute to the weak background
radiation for such substances. Soil commonly contains an average
of about 6 parts of thorium per million parts (ppm) of soil.
Rocks in some underground mines may also contain thorium in
a more concentrated form. After these rocks are mined, thorium
is usually concentrated and changed into thorium dioxide or
other chemical forms. Thorium-bearing rock that has had most
of the thorium removed from it is called "depleted" ore or
tailings.
More than 99% of natural thorium exists
in the form (isotope) thorium-232. Besides this natural thorium
isotope, there are more than 10 other different isotopes that
can be artificially produced. In the environment, thorium-232
exists in various combinations with other minerals, such as
silica. Most thorium compounds commonly found in the
environment do not dissolve easily in water and do not evaporate
from soil or water into the air.
The thorium isotope-232 is not stable.
It breaks down into two parts. This process of breaking down
is called decay. The decay of thorium-232 produces a small
part called "alpha" radiation and a large part called the
decay product. The decay product of thorium-232 also is not
stable. Like thorium-232, it in turn breaks down to an unstable
isotope and the process continues until a stable product is
formed. During these decay processes, the parent thorium-232,
its decay products, and their next decay products produce
a series of new substances (including radium and radon), alpha
and beta particles, and gamma radiation. The alpha particles
can travel only very short distances through most materials
and cannot go through human skin. The gamma radiation can
travel farther and can easily go through human skin. The decay
of thorium-232 into its decay products happens very slowly.
In fact, it takes about 14 billion years for half the thorium-232
to change into new forms. Fourteen billion years is called
the radioactive half-life of thorium-232.
Due to the extremely slow rate of decay,
the total amount of natural thorium in the earth remains almost
the same, but it can be moved from place to place by nature
and people. For example, when rocks are broken up by wind
and water, thorium or its compounds becomes a part of the
soil. When it rains, the thorium-containing soil can be washed
into rivers and lakes. Also, activities such as burning coal
that contains small amounts of thorium, mining or milling
thorium, or making products that contain thorium also release
thorium into the environment. Smaller amounts of other isotopes
of thorium are produced usually as decay products of uranium-238,
uranium-235, and thorium-232, and as unwanted products of
nuclear reactions.
Thorium is used to make ceramics, lantern
mantles, and metals used in the aerospace industry and in
nuclear reactions. Thorium can also be used as a fuel
for generating nuclear energy. More than 30 years ago thorium
oxides were used in hospitals to make certain kinds of diagnostic
X-ray photographs.
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1.2
How might I be exposed to thorium? |
Since thorium is found almost everywhere,
you will be exposed to small amounts of it in the air you
breathe and in the food and water you eat and drink. Scientists
know, roughly, the average amounts of thorium in food and
drinking water. Most people in the United States eat some
thorium with their food every day. Normally, very little of
the thorium in lakes, rivers, and oceans gets into the fish
or seafood we eat. The amounts in the air are usually so small
that they can be ignored.
There may be more thorium than normal
near an uncontrolled hazardous waste site in which thorium
has not been disposed of properly. Consequently, you may be
exposed to slightly more thorium if you live near one of these
sites because you could breathe windblown dust containing
thorium or eat food grown in soil contaminated with thorium.
Children playing near a waste site could get thorium into
their bodies if they eat contaminated soil. You could also
be exposed to more thorium than normal if you work in an industry
that mines, mills, or manufactures products containing thorium,
or work in a research laboratory performing experiments with
thorium. Larger-than-normal amounts of thorium might also
enter the environment through accidental releases from thorium
processing plants.
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1.3
How can thorium enter and leave my body? |
Only a small amount of the thorium that
you breathe or swallow in food, water, or soil enters your
blood. One animal study has shown that thorium can enter the
body if it is placed on the skin. After breathing thorium,
you will usually sneeze, cough, or breathe out some of it
within minutes. Some forms of thorium can stay in your lungs
for long periods of time. However, in most cases, the small
amount of thorium left in your lungs will leave your body
in the feces and urine within days. After you eat or drink
thorium, almost all of it leaves your body in the feces. The
small amount of thorium left in your body may enter your bones
from the blood and stay there for many years. The main way
thorium will enter your body is by breathing dust contaminated
with thorium.
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1.4
How can thorium affect my health? |
Studies of thorium workers have shown
that breathing thorium dust may cause an increased chance
of developing lung disease and cancer of the lung or pancreas
many years after being exposed. Changes in the genetic material
of body cells have also been shown to occur in workers who
breathed thorium dust. Liver diseases and effects on the blood
have been found in people injected with thorium in order to
take special X-rays.
Many types of cancer have also been shown
to occur in these people many years after thorium was injected into their bodies. Since
thorium is radioactive and may be stored in bone for a long
time, bone cancer is also a potential concern for people exposed
to thorium.
Animal studies have shown that breathing
in thorium may result in lung damage. Other studies in animals
suggest drinking massive amounts of thorium can cause death
from metal poisoning. The presence of large amounts of thorium
in your environment could result in exposure to more hazardous
radioactive decay products of thorium, such as radium and
thoron, which is an isotope of radon. Thorium is not known
to cause birth defects or to affect the ability to have children.
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1.5
What levels of exposure have resulted in harmful health effects? |
Thorium is odorless and tasteless, so
you cannot tell if you are being exposed to thorium. We know
very little about specific exposure levels of thorium that
result in harmful effects in people or animals. High levels
of exposure have been shown to cause death in animals, but
no direct cause of death could be determined and no other
health effects have been reported.
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1.6
Is there a medical test to determine whether I have been exposed
to thorium? |
Special tests that measure the level
of radioactivity from thorium or thorium isotopes in your
urine, feces, and air you breathe out can determine if you
have been exposed to thorium. These tests are useful only
if run within several days to a week after exposure. The tests
cannot, however, tell you if your health will be affected
by the exposure. The tests can be run only with special equipment
and are probably not available at your local clinic or hospital.
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1.7
What recommendations has the federal government made to protect
human health? |
The EPA has set a drinking water limit
of 15 picocuries per liter (15 pCi/L) of water for gross alpha
particle activity and 4 millirems per year for beta particles
and photon activity (for example, gamma radiation and X-rays).
The federal recommendations have been
updated as of July 1999.
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1.8 Where can I get more information? |
If you have any more questions or concerns, please contact
your community or state health or environmental quality department or:
Agency for Toxic Substances and Disease Registry
Division of Toxicology
1600 Clifton Road NE, Mailstop F-32
Atlanta, GA 30333
Information line and technical assistance:
Phone: 888-422-8737
FAX: (770)-488-4178
ATSDR can also tell you the location of occupational and environmental health
clinics. These clinics specialize in recognizing, evaluating, and treating illnesses
resulting from exposure to hazardous substances.
To order toxicological profiles, contact:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Phone: 800-553-6847 or 703-605-6000
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References |
Agency for Toxic Substances and Disease
Registry (ATSDR). 1990. Toxicological
profile for thorium. Atlanta, GA: U.S. Department of Health
and Human Services, Public Health Service.
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