Thorium (chemical symbol Th) is a naturally-occurring radioactive
metal found at very low levels in soil, rocks, and water. It has
several different isotopes, both natural and man-made, all of
which are radioactive. The most common form of thorium is thorium-232,
found naturally.
Who discovered thorium?
Thorium was discovered in 1828 by the Swedish chemist Jons Jakob
Berzelius. After determining that it was a new element, Berzelius
named his discovery after the Norse god of thunder and weather,
Thor. Thorium was discovered to be radioactive independently in
1898 by Gerhard Carl Schmidt and by Marie Curie.
Where does thorium come from?
Almost all thorium is natural, but, thorium isotopes can be artificially
produced. Thorium occurs at very low levels in virtually all rock,
soil, and water, and therefore is found in plants and animals
as well. Minerals such as monazite, thorite and thorianite are
rich in thorium and may be mined for the metal. Generally, artificial
isotopes come from decay of other man-made radionuclides, or absorption
in nuclear reactions.
What are the properties of thorium?
Thorium is a soft, silvery white metal. Pure thorium will
remain shiny for months in air, but if it contains impurities it
tarnishes to black when exposed to air. When heated, thorium oxide
glows bright white, a property that makes it useful in lantern mantles.
It dissolves slowly in water. Thorium-232 has a half-life of 14
billion (14x109) years, and decays by alpha emission, with accompanying
gamma radiation. Thorium-232 is the top of a long decay series that
contains key radionuclides such as radium-228, its direct decay
product, and radon-220. Two other isotopes of thorium, which can
be significant in the environment, are thorium-230 and thorium-228.
Both belong to other decay series. They also decay by alpha emission,
with accompanying gamma radiation, and have half-lives of 75,400
years and 1.9 years, respectively.
What is thorium used for?
Thorium has coloring properties that has made it useful in ceramic
glazes. But, it has been most widely used in lantern mantles for
the brightness it imparts (though alternatives are replacing it),
and in welding rods, which burn better with small amounts of added
thorium. Thorium improves the properties of ophthalmic lenses,
and is an alloying agent in certain metals used in the aerospace
industry. More than 30 years ago, thorium oxides were used in
hospitals to make certain kinds of diagnostic X-ray photographs.
But, this practice has been discontinued.
Natural thorium is present in very small quantities in virtually
all rock, soil, water, plants and animals. Where high concentrations
occur in rock, thorium may be mined and refined, producing waste
products such as mill tailings. If not properly controlled, wind
and water can introduce the tailings into the wider environment.
Commercial and federal facilities that have processed thorium
may also have released thorium to the air, water, or soil. Man-made
thorium isotopes are rare, and almost never enter the environment.
How does thorium change in the environment?
As thorium-232 undergoes radioactive decay, it emits an alpha
particle, with accompanying gamma radiation, and forms radium-228.
This process of releasing radiation and forming a new radionuclide
continues until stable lead-208 is formed. The half-life of thorium-232
is about 14 billion years. Two other isotopes of thorium, which
can be significant in the environment, are thorium-230 and thorium-228.
Both decay by alpha emission, with accompanying gamma radiation,
in 75,400 years and 1.9 years, respectively.
How do people come in contact with thorium?
Since thorium is naturally present in the environment, people
are exposed to tiny amounts in air, food and water. The amounts
are usually very small and pose little health hazard. Thorium
is also present in many consumer products such as ceramic glazes,
lantern mantles, and welding rods.
People who live near a facility that mines or mills thorium,
or manufactures products with thorium, may receive higher exposures.
Also, people who work with thorium in various industries may receive
higher exposures.
How does thorium get into the body?
People may inhale contaminated dust, or swallow thorium with
food or water. Living near a thorium contaminated site, or working
in an industry where thorium is used, increases your chance of
exposure to thorium.
What does thorium do once it gets into the body?
If inhaled as dust, some thorium may remain in the lungs for
long periods of time, depending on the chemical form. If ingested,
thorium typically leaves the body through feces and urine within
several days. 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. There is some evidence that the body may
absorb thorium through the skin, but that would not likely be
the primary means of entry.
The principal concern from low to moderate level exposure to
ionizing radiation is increased risk of cancer. Studies have shown
that inhaling thorium dust causes an increased risk of developing
lung cancer, and cancer of the pancreas. Bone cancer risk is also
increased because thorium may be stored in bone.
Is there a medical test to determine exposure to thorium?
There are special tests that measure the level of thorium in
the urine, feces, and also via exhaled air that can determine
if a person has been exposed to thorium. These tests are useful
only if taken within a week after exposure. You need special equipment
to detect thorium not available in doctors offices or most hospitals.
Some federal facilities and specialized laboratories have this
capability.
What can I do to protect myself and my family from thorium?
Most people are not exposed to dangerous levels of thorium. However,
people who live near thorium mining areas, or near certain government
or industrial facilities may have increased exposure to thorium,
especially if their water is from a private well. Analytical laboratories
can test water for thorium content. Occasionally, household items
may be found with thorium in them, such as some older ceramic
wares in which uranium was used in the glaze, or gas lantern mantles.
These generally do not pose serious health risks, but may nevertheless
be retired from use as a prudent avoidance measure. A radiation
counter is required to confirm if ceramics contain thorium.
How do I know thorium if I'm near thorium?
You need special equipment to detect thorium, and special training.
Health physicists and radiation safety officers are trained to
measure thorium.
What is EPA doing about thorium?
EPA protects people and the environment from thorium by establishing
standards for the clean-up of contaminated sites, and by setting
limits on the amount of thorium (and other radionuclides) that
may be released to the air from specific sources, or found in
public drinking water.
The standards for the clean-up of existing contaminated sites
generally fall under the Comprehensive
Environmental Response, Compensation, and Liability Act, commonly
called Superfund. Clean ups must meet all requirements that are
relevant or applicable, such as state regulations and regulations
issued in connection with other environmental laws. When these
types of regulations are not applicable, or not protective enough,
EPA sets site-specific cleanup levels that limit the chance of
developing cancer due to exposure to a site-related carcinogen
(such as thorium) to between one in 10,000 and one in 1,000,000.
EPA issued special regulations for cleaning up uranium and thorium
mill tailing sites under the "Uranium Mill Tailings Radiation
Control Act" (federal regulations are found in 40CFR192,
"Health and Environmental Protection Standards for Uranium
and Thorium Mill Tailings"). These mills are found mostly
in the western states of Colorado, Utah, Arizona and New Mexico.
EPA's Superfund Hotline:
1-800-424-9346 or 1-800-535-0202
EPA uses its Clean Air Act
authority to set limits on the emissions of hazardous air pollutants
from specific sources. Hazardous air pollutants include both chemicals
and radionuclides that are known or suspected to cause serious
health problems. While no air emissions standards list thorium
specifically, radionuclides are limited as a group.
EPA uses its Safe Drinking Water Act authority to establish
maximum contaminant levels (MCLs) for beta emitters such as thorium
in public drinking water. For example the MCL for beta emitters
is 4 millirem per year or 8 picoCuries per liter of water.
