EPA Facts about Thorium
What is thorium?
Thorium is a naturally occurring radioactive
metal that is found at low levels in soil, rocks,
water, plants, and animals. Almost all naturally
occurring thorium exists in the form of either
radioactive isotope thorium-232, thorium-230,
and thorium-228. There are more than 10 other
thorium isotopes that can be artificially
produced. Smaller amounts of these isotopes
are usually produced as decay products of other
radionuclides and as unwanted products of
nuclear reactions.
What are the uses of thorium?
Thorium is used to make ceramics, lantern
mantles, welding rods, camera and telescope
lenses, and metals used in the aerospace
industry.
How does thorium change in the
environment?
Thorium-232 is not a stable isotope. As thorium-
232 decays, it releases radiation and forms
decay products that include radium-228 and
thorium-228. The decay process continues until
a stable, nonradioactive decay product is
formed. In addition to thorium-232, thorium-
228 is present naturally in background. Thorium-
228 is a decay product of radium-228, and
thorium-228 decays into radium-224.
The radiation from the decay of thorium and its
decay products is in the form of alpha and beta
particles and gamma radiation. Alpha particles
can travel only short distances and cannot
penetrate human skin. Beta particles are
generally absorbed in the skin and do not pass
through the entire body. Gamma radiation,
however, can penetrate the body.
The time required for a radioactive substance to
lose 50 percent of its radioactivity by decay is
known as the half-life. The half-life of thorium-
232 is very long at about 14 billion years. As a
result of the extremely slow rate of decay, the
total amount of natural thorium in the earth
remains fairly constant, but it can be moved
from place to place by natural processes and
human activities.
How are people exposed to thorium?
Since thorium is present at very low levels
almost everywhere in the natural environment,
everyone is exposed to it in air, food, and water.
Normally, very little of the thorium in lakes,
rivers, and oceans is absorbed by the fish or
seafood that a person eats. The amounts in the
air are usually small and do not constitute a
health hazard.
Exposure to higher levels of thorium may occur
if a person lives near an industrial facility that
mines, mills, or manufactures products with
thorium.
Thorium-232 on the ground is of a health risk
because of the rapid build-up of radium-228 and
its associated gamma radiation. Thorium-232 is
typically present with its decay product radium-
224, which will produce radon-220 gas, also
known as thoron, and its decay products that
result in lung exposure. Thorium-230 is part of
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the uranium-238 decay series. Thorium- 230 is
typically present with its decay product radium-
226, and it is therefore a health risk from
gamma radiation from radium-226 decay
products, lung exposure from radon-222 gas and
its decay products, and inhalation and ingestion
exposure.
How does thorium get into the body?
Thorium can enter the body when it is inhaled or
swallowed. In addition, radium can come from
thorium deposited in the body. Thorium enters
the body mainly through inhalation of
contaminated dust. If a person inhales thorium
into the lungs, some may remain there for long
periods of time. In most cases, the small amount
of thorium left in the lungs will leave the body in
the feces and urine within days.
If thorium is swallowed in water or with food,
most of it will promptly leave the body in the
feces. The small amount of thorium left in the
body will enter the bloodstream and be
deposited in the bones, where it may remain for
many years.
Is there a medical test to determine
exposure to thorium?
Special tests that measure the level of
radioactivity from thorium or thorium isotopes
in the urine, feces, and exhaled air can
determine if a person has been exposed to
thorium. These tests are useful only if
administered within a short period of time after
exposure. They require special equipment and
cannot be done in a doctor's office.
How can thorium affect people's health?
Studies of workers have shown that inhaling
thorium dust will cause an increased risk of
developing lung disease, including lung cancer,
or pancreatic cancer. Liver disease and some
types of cancer have been found in people
injected in the past with thorium to take special
X-rays. Bone cancer is also a potential health
effect through the storage of thorium in the
bone.
What recommendations has the U.S.
Environmental Protection Agency made
to protect human health?
Please note that the information in this section
is limited to recommendations EPA has made to
protect human health from exposure to
thorium. General recommendations EPA has
made to protect human health at Superfund
sites (the 10~4 to 10"6 cancer risk range), which
cover all radionuclides including thorium, are
summarized in the fact sheet "Primer on
Radionuclides Commonly Found at Superfund
Sites."
For uranium mill tailing sites, EPA has
established 5 picoCuries per gram (pCi/g) of
radium as a protective health based level for
cleanup of the top 15 centimeters of soil. Since
thorium decays into radium, these regulations
for radium under 40 Code of Federal Regulations
(CFR) Part 192.12 have often been used as
Applicable or Relevant and Appropriate
Requirements (ARARs) at Superfund sites for
thorium-contaminated soil. The EPA document
"Use of Soil Cleanup Criteria in 40 CFR Part 192
as Remediation Goals for CERCLA Sites" provides
guidance to EPA staff regarding when 5 pCi/g of

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thorium is an ARAR or otherwise recommended
cleanup level for any 15 centimeters of
subsurface soil contaminated by thorium other
than the first 15 centimeters. This document is
available online at:
http://www.epa.gov/superfund/health/contami
nants/radiation/pdfs/u mtrcagu.pdf.
If regulations under 40 CFR Part 192.12 are an
ARAR for radium in soil at a Superfund site, then
Nuclear Regulatory Commission (NRC)
regulations for uranium mill tailing sites under
10 CFR Part 40 Appendix A, I, Criterion 6(6), may
be an ARAR at the same site.
Criterion 6(6) requires that the level of radiation,
called a "benchmark dose," that an individual
would receive be estimated after that site was
cleaned up to the radium soil regulations under
40 CFR Part 192.12. This benchmark dose then
becomes the maximum level of radiation that an
individual may be exposed to from all
radionuclides, except radon, in both the soil and
buildings at the site. The EPA document
"Remediating Goals for Radioactively
Contaminated CERCLA Sites Using the
Benchmark Dose Cleanup Criterion 10 CFR Part
40 Appendix A, I, Criterion 6(6)" provides
guidance regarding how Criterion 6(6) should be
implemented as an ARAR at Superfund sites,
including using a radium soil cleanup level of 5
pCi/g in both the surface and subsurface when
estimating a benchmark dose. This document is
available online at:
http://www.epa.gov/superfund/health/contami
nants/radiation/pdfs/part40.pdf.
EPA has established a Maximum Contaminant
Level (MCL) of 15 picoCuries per liter (pCi/L) for
alpha particle activity, excluding radon and
uranium, in drinking water. Thorium is covered
under this MCL.
For more information about how EPA addresses
thorium at Superfund sites
Contact Stuart Walker of EPA:
(703) 603-8748 or walker.stuart@epa.gov,
or visit EPA's Superfund Radiation Webpage:
http://www.epa.gov/superfund/resources/radiation/

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