Iodine (chemical symbol I) is a nonmetallic solid element. There
are both radioactive and non-radioactive isotopes of iodine. Iodine-129
and -131 are the most important radioactive isotopes in the environment.
Some isotopes of iodine, such as I-123 and I-124 are used in medical
imaging and treatment, but are generally not a problem in the
environment because they have very short half-lives.
Who discovered iodine?
In 1811, Bernard Courtois discovered natural iodine in water
that was used to dissolve certain parts of seaweed ash for use.
Radioactive iodine-131 was discovered by Glenn T. Seaborg and
John Livingood at the University of California - Berkeley in the
late 1930's.
Where do iodine-129 and iodine-131 come from?
Both iodine-129 and iodine-131 are produced by the fission of
uranium atoms during operation of nuclear reactors and by plutonium
(or uranium) in the detonation of nuclear weapons.
What are the properties of iodine-129 and iodine-131?
Radioactive iodines have the same physical properties as stable
iodine. However, radioactive iodines decay with time
Iodine is a nonmetallic, purplish-black crystalline solid. It
has the unusual property of sublimation,' which means that
it can go directly from a solid to a gas, without first becoming
liquid. It sublimes to a deep violet vapor at room temperature.
This vapor is irritating to the eyes, nose and throat. Iodine
dissolves in alcohol and in water. It melts at 236 °F.
Iodine reacts easily with other chemicals, and isotopes of iodine
are found as compounds rather than as a pure elemental nuclide.
Thus, iodine-129 and -131 found in nuclear facilities and waste
treatment plants quickly form compounds with the mixture of chemicals
present. However, iodine released to the environment from nuclear
power plants is usually a gas.
Iodine-129 has a half-life of 15.7 million years; iodine-131
has a half-life of about 8 days. Both emit beta particles upon
radioactive decay.
What are iodine radioisotopes used for?
Iodines are among the most widely used radionuclides, mostly
in the medical field. Because of its short half-life and useful
beta emission, iodine-131 is used extensively in nuclear medicine.
Its tendency to collect in the thyroid gland makes iodine
especially useful for diagnosing and treating thyroid problems.
Iodine-123 is widely used in medical imaging, and I-124 is useful
in immunotherapy.
Iodine's chemical properties make it easy to attach to molecules
for imaging studies. It is useful in tracking the metabolism
of drugs or compounds, or for viewing structural defects in
various organs, such as the heart.
A less common isotope, iodine-125, is sometimes used to treat
cancerous tissue.
Iodine-129 has little practical use, but may be used to check
some radioactivity counters in diagnostic testing laboratories.
How do iodine-129 and iodine-131 get into the environment?
Iodine-129 and iodine-131 are gaseous fission products that form
within fuel rods as they fission. Unless reactor chemistry is
carefully controlled, they can build up too fast, increasing pressure
and causing corrosion in the rods. As the rods age, cracks or
wholes may breach the rods.
Cracked rods can release radioactive iodine into the water that
surrounds and cools the fuel rods. There, it circulates with the
cooling water throughout the system, ending up in the airborne,
liquid, and solid wastes from the reactor. From time to time,
reactor gas capture systems release gases, including iodine, to
the environment under applicable regulations.
Anywhere spent nuclear fuel is handled, there is a chance that
iodine-129 and iodine-131 will escape into the environment. Nuclear
fuel reprocessing plants dissolve the spent fuel rods in strong
acids to recover plutonium and other valuable materials. In the
process, they also release iodine-129 and -131 into the airborne,
liquid, and solid waste processing systems. In the U.S., spent
nuclear fuel is no longer reprocessed, because of concerns about
nuclear weapons proliferation.
Currently, spent nuclear fuel remains in temporary storage at
nuclear power plants around the country. If the nuclear waste
repository at Yucca
Mountain opens, it will provide permanent disposal for spent
nuclear fuel and other high-level radioactive wastes. Wherever
spent nuclear fuel is stored, the short-lived iodine-131 it contains
will decay away quickly and completely. However, the long-lived
iodine-129 will remain for millions of years. Keeping it from
leaking into the environment, requires carefully designed, long-term
safeguards.
The detonation of nuclear weapons also releases iodine-129 into
the environment. Atmospheric testing in the 1950's and 60's released
radioactive iodine to the atmosphere which has disseminated around
the world, and is now found at very low levels in the environment.
Most I-129 in the environment came from weapons testing.
How do iodine-129 and iodine-131 change in the environment?
Radioactive iodine can disperse rapidly in air and water, under
the right conditions. However, it combines easily with organic
materials in soil. This is known as organic fixation' and
slows iodine's movement in the environment. Some soil minerals
also attach to, or adsorb, iodine, which also slows its movement.
The long half-life of iodine-129, 15.7 million years, means that
it remains in the environment. However, iodine-131's short half-life
of 8 days means that it will decay away completely in the environment
in a matter of months. Both decay with the emission of a beta
particle, accompanied by weak gamma radiation.
How do people come in contact with iodine-129 and iodine-131?
Radioactive iodine can be inhaled as a gas or ingested in food
or water. It dissolves in water so it moves easily from the atmosphere
into humans and other living organisms. People are exposed to
I-129 from the past testing of nuclear weapons, and I-131 from
nuclear power plant emissions. Some industrial facilities also
emit radioactive iodine to the environment, as well as medical
institutions. Radioactive iodine is usually emitted as a gas,
but may contaminate liquids or solid materials as well. If a family
member has been treated with I-131, you may have increased exposure
to it through their body fluids.
How do iodine-129 and iodine-131 get into the body?
Radioactive iodine can enter the body by ingestion or inhalation.
It dissolves in water so it moves easily from the atmosphere into
humans and other living organisms. For example, I-129 and -131
can settle on grass where cows can eat it and pass it to humans
through their milk. It may settle on leafy vegetables and be ingested
by humans. Iodine isotopes also concentrate in marine and freshwater
fish, which people may then eat.
Also, doctors may give thyroid patients radioactive iodine, usually
iodine-131, to treat or help diagnose certain thyroid problems.
The tendency of iodine to collect in the thyroid makes it very
useful for highlighting parts of its structure in diagnostic images.
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What do iodine-129 and iodine-131 do once they get into the body?
When I-129 or I-131 is ingested, some of it concentrates in the
thyroid gland. The rest passes from the body in urine.
Airborne I-129 and I-131 can be inhaled. In the lung, radioactive
iodine is absorbed, passes into the blood stream, and collects
in the thyroid. Any remaining iodine passes from the body with
urine.
In the body, iodine has a biological
half-life of about 100 days for the body as a whole. It has
different biological half-lives for various organs: thyroid
- 100 days, bone - 14 days, and kidney, spleen, and reproductive
organs - 7 days.
Health Effects of Iodine-129 and Iodine-131
How can iodine-129 and iodine-131affect people's health?
Radioactive iodine can cause thyroid problems, and help diagnose
and treat thyroid problems. Long-term (chronic) exposure to radioactive
iodine can cause nodules, or cancer of the thyroid. However, once
thyroid cancer occurs, treatment with high doses of I-131 may
be used to treat it. Doctors also use lower doses of I-131 to
treat overactive thyroids.
Low doses can reduce activity of the thyroid gland, lowering
hormone production in the gland. Doctors must maintain the fine
balance between the risks and benefits of using radioactive iodine.
On one hand, this small, additional exposure may tip the balance
in favor of cancer formation. On the other, this small additional
exposure can restore health by slowing an overactive thyroid and
improve health conditions.
Is there a medical test to determine exposure to iodine-129 and
iodine-131?
Since iodine is concentrated in the thyroid gland, a radioassay
of the thyroid can determine the level of exposure to many of
its isotopes. However, I-129 has very low activity and emits extremely
low energy beta particles, making a radioassay much more difficult.
Tests for I-131 in the body should be available through most major
medical centers.
What can I do to protect myself and my family from iodine?
The thyroid cannot tell the difference between radioactive and
non-radioactive iodine. It will take up radioactive iodine in
whatever proportion it is available in the environment.
If large amounts of radioactive iodine are released during an
nuclear accident, large doses of stable iodine may be distributed
by government agencies to keep your thyroid gland from absorbing
too much radioactive iodine: Raising the concentration of stable
iodine in the blood, increases the likelihood that the thyroid
will absorb it instead of radioactive iodine. (Note: Large doses
of stable iodine can be a health hazard and should not be taken
except in an emergency. However iodized table salt is an important
means of acquiring essential non-radioactive iodine to maintain
health.
How do I know if I'm near radioactive iodine?
Living near a nuclear power plant may slightly increase your
annual exposure to I-131. Detecting radioactive iodine in the
environment requires specialized equipment. Most major medical
centers can test for isotopes of iodine in your body.
What is EPA doing about iodine-129 and iodine-131?
EPA has issued a variety of regulations that limit the release
of radionuclides, including I-129 and I-131, to the environment.
These regulations address airborne and liquid releases from nuclear
reactors, airborne emissions from a variety of industrial and
governmental facilities, and allowable radioactive releases from
radioactive waste disposal systems.
EPA has established Maximum Contaminant Levels that limit the
concentration of radioactive iodine and other radionuclides in
drinking water from public water suppliers.
Recently, EPA issued its environmental standards for the potential
waste repository at Yucca Mountain, Nevada. Iodine-129 is one
of the more important radionuclides of concern in the large inventory
of spent reactor fuel and defense high-level waste. This standard
limits the radiation exposure of individuals, and radionuclide
concentrations in ground water from the release of I-129 and other
radionuclides in the vicinity of Yucca Mountain.