2011 Nuclear Power Plant Sourcebook: Iodine-131 Radioisotope, Radiation Health Effects and Toxicological Profile, Medical Treatment with Potassium Iodide, Fukushima Accident Radioactive Release
U.S. Government, Nuclear Regulatory Commission (NRC), Environmental Protection Agency (EPA), Centers for Disease Control (CDC), Agency for Toxic Substances and Disease Registry (ATSDR)
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Copyright 2011 Progressive Management
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Radioisotope Brief: Iodine-131 (I-131)
Half-life: 8.06 days
Mode of decay: Beta particles and gamma radiation
Chemical properties: I-131 can change directly from a solid into a gas, skipping the liquid phase, in a process called sublimation. I-131 dissolves easily in water or alcohol. I-131 readily combines with other elements and does not stay in its pure form once released into the environment.
What is it used for?
I-131 is used in medicine to diagnose and treat cancers of the thyroid gland.
Where does it come from?
I-131 is produced commercially for medical and industrial uses through nuclear fission. It also is a byproduct of nuclear fission processes in nuclear reactors and weapons testing.
What form is it in?
In medicine, I-131 is supplied in capsules or liquid of a specific activity designed to be swallowed by patients. As a product of nuclear fission, it is a dark purple gas that can be inhaled, or absorbed through the skin. I-131 in fallout from nuclear weapons or reactor accidents can occur in particle form, which can be ingested in food or water.
What does it look like?
Pure I-131 is a non-metallic, purplish-black crystalline solid. However, because it readily binds with other elements, I-131 usually is found as a compound rather than in its pure form. For medical purposes, the I-131 capsules contain small granules of I-131 sodium iodide that are designed to be swallowed by patients. Liquid I-131 sodium iodide used to diagnose and treat thyroid disease is a clear liquid.
How can it hurt me?
External exposure to large amounts of I-131 can cause burns to the eyes and on the skin. Internal exposure can affect the thyroid gland, a small organ located in the neck near the Adam’s apple. The thyroid gland uses iodine to produce thyroid hormones and cannot distinguish between radioactive iodine and stable (nonradioactive) iodine. If I-131 were released into the atmosphere, people could ingest it in food products or water, or breathe it in. In addition, if dairy animals consume grass contaminated with I-131, the radioactive iodine will be incorporated into their milk. Consequently, people can receive internal exposure from drinking the milk or eating dairy products made from contaminated milk. Once inside the body, I-131 will be absorbed by the thyroid gland exposing it to radiation and potentially increasing the risk for thyroid cancer or other thyroid problems.
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Iodine
CAS ID #: 7553-56-2
Affected Organ Systems: Endocrine (Glands and Hormones)
Cancer Effects: None
Chemical Classification: Inorganic substances, Metals/Elements (the simplest forms of matter), Radionuclides (radioactive materials)
Summary: Iodine is a naturally occurring element found in sea water and in certain rocks and sediments. There are non radioactive and radioactive forms of iodine. Iodine is used as a disinfectant for cleaning surfaces and storage containers and is used in skin soaps and bandages, and for purifying water. Iodine is also added to some table salt to ensure that all people in the United States have enough iodine in their diet. Most radioactive iodine is manmade. It is used in medical tests and to treat certain diseases. Most radioactive forms of iodine change very quickly (seconds to days) to stable elements that are not radioactive. However, 129I (read as iodine 129) changes very slowly (over millions of years).
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This document answers the most frequently asked health questions about iodine. For more information, you may call the ATSDR Information Center at 1-888-422-8737. This fact sheet is one in a series of summaries about hazardous substances and their health effects. 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.
HIGHLIGHTS: Iodine is a naturally occurring element that is required for good health. Exposure to high levels of stable or radioactive iodine can cause damage to the thyroid. This chemical has been found in at least 9 of the 1,636 National Priorities List sites identified by the Environmental Protection Agency (EPA).
What is iodine?
Iodine is a naturally occurring element found in sea water and in certain rocks and sediments. There are non radioactive and radioactive forms of iodine.
Iodine is used as a disinfectant for cleaning surfaces and storage containers and is used in skin soaps and bandages, and for purifying water. Iodine is also added to some table salt to ensure that all people in the United States have enough iodine in their diet.
Most radioactive iodine is manmade. It is used in medical tests and to treat certain diseases. Most radioactive forms of iodine change very quickly (seconds to days) to stable elements that are not radioactive. However, 129I (read as iodine 129) changes very slowly (over millions of years).
What happens to iodine when it enters the environment?
The primary source of nonradioactive iodine is the ocean. It enters the air from sea spray or as iodine gas. Once in the air, iodine can combine with water or with particles in air and can enter the soil and surface water, or land on vegetation when these particles fall to the ground or when it rains. Iodine can remain in soil for a long time. It can also be taken up by some plants that grow in the soil, but plants are considered a poor source of dietary iodine.
Small amounts of radioactive iodine are produced from the operation of nuclear power plants, which can release minor amounts to air and water. Large amounts have been released during rare power plant accidents. Iodine is also released from atomic bomb explosions. Very large amounts are made in nuclear power plants for medical use. Once given to the patient, most iodine decays in the body. The rest is exhaled or excreted in the urine and decays naturally in the environment.
How might I be exposed to iodine?
The general population is exposed to low levels of iodine in air, some food, and some beverages. Food (iodized salt, salt water, bread, and dairy products) is the largest source of exposure to iodine.
The general population is rarely exposed to radioactive iodine, unless they undergo certain medical tests or are given it for the treatment of thyroid disease.
People who work at facilities using radioactive iodine may be exposed to higher than normal levels.
How can iodine affect my health?
Iodine has both beneficial and harmful effects on human health. Iodine is needed by your thyroid gland to produce thyroid hormones. However, exposure to unnecessarily high levels of nonradioactive and radioactive iodine can damage the thyroid. Damage to the thyroid gland can result in effects in other parts of your body, such as your skin, lung, and reproductive organs.
Radioactive iodine can be used by doctors to check your
thyroid for medical problems, and to cure thyroid cancer.
How likely is iodine to cause cancer?
Some human studies have found an increased risk of thyroid cancer in certain populations, particularly populations with iodine deficient diets receiving iodine supplements. Other human studies have not found an association between exposure to high levels of iodine and cancer risk. Exposure to high levels of radioactive iodine may also increase the risk of thyroid cancer. However, the evidence is inconclusive for exposures in the United States.
How can iodine affect children?
Iodine is essential for the growth and development of children. However, children are more sensitive to the harmful effects of excessively high levels of stable and radioactive iodine than adults because their thyroid glands are still growing. If babies and children receive too much iodine, they can develop an enlarged thyroid gland (called a goiter), which does not produce enough thyroid hormone for normal growth.
Radioactive iodine in food can be more harmful to babies and children than to adults. Because a child’s thyroid gland is smaller than that of an adult, a child’s thyroid gland will receive a higher radiation dose than the adult exposed to the same amount of iodine.
How can families reduce the risk of exposure to iodine?
We need iodine to maintain growth and health, but we want to prevent exposure to too much iodine. Foods are not normally expected to have enough iodine to harm your health. Unless you are exposed to radioactive waste or emissions, you generally do not have to worry about excessive exposure.
Is there a medical test to show whether I've been exposed to iodine?
There are reliable tests that can measure iodine in the blood, urine, and saliva. These tests are not available at your doctor’s office, but your doctor can send the samples to a laboratory that can perform the tests. However, these tests cannot predict whether you will experience any health effects.
Two types of tests are available for radioactive iodine. One is to see if you have been exposed to a large dose of radiation, and the other is to see if iodine is in your body. The first looks for changes in blood cell counts or in your chromosomes that occur at 3 to 5 times the annual occupational dose limit. It cannot tell if the radiation came from radioactive iodine. The second type of test involves examining your blood, feces, saliva, urine, and even your entire body. It is to see if iodine is being excreted from or remains inside your body. Either the doctor’s office collects and sends the samples to a special lab for testing, or you must go to the lab for testing.
Has the federal government made recommendations to protect human health?
The National Research Council has established a recommended dietary allowance (RDA) for iodine of 150 micrograms per day (150 µg/day), with additional allowances of 25 µg/day during pregnancy and 50 µg/day during nursing. These dietary intake levels are sufficient to satisfy the metabolic needs of the body.
The Nuclear Regulatory Commission (NRC) has set limits for radioactive iodine in workplace air of 2x10-8 microcurie per milliliter (µCi/mL) for 131I. EPA has set an average annual drinking water limit of 3 pCi/L for 131I so the public radiation dose will not exceed 4 millirem.
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Public Health Statement for Iodine
(Iodo)
April 2004
This Public Health Statement is the summary chapter from the Toxicological Profile for iodine. 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.
This public health statement tells you about iodine and the effects of exposure.
The Environmental Protection Agency (EPA) identifies the most serious hazardous waste sites in the nation. These sites make up the National Priorities List (NPL) and are the sites targeted for long-term federal cleanup activities. Iodine has been found in at least 8 sites. Radioactive iodine has been found at 9 sites, including iodine-129 (129I) in at least 3 sites, and iodine-131 (131I) in at least 6 sites of the 1,636 current or former NPL sites. However, the total number of NPL sites evaluated for iodine is not known. As more sites are evaluated, the sites at which iodine is found may increase. This information is important because exposure to iodine may harm you and because these sites may be sources of exposure.
When a substance 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. This release does not always lead to exposure. You are exposed to a substance only when you come in contact with it. You may be exposed by breathing, eating, or drinking the substance, or by skin contact.
External exposure to radiation may occur from natural or man-made sources. Naturally occurring sources of radiation are cosmic radiation from space or radioactive materials in soil or building materials. Man-made sources of radioactive materials are found in consumer products, industrial equipment, atom bomb fallout, and to a smaller extent from hospital waste and nuclear reactors.
If you are exposed to either radioactive or stable iodine, many factors determine whether you'll be harmed. These factors include the dose (how much), the duration (how long), and how you come in contact with it. You must also consider the other chemicals you're exposed to and your age, sex, diet, family traits, lifestyle, and state of health.
1.1 What is iodine?
Iodine is a naturally occurring element that is essential for the good health of people and animals. Iodine is found in small amounts in sea water and in certain rocks and sediments. Iodine occurs in many different forms that can be blue, brown, yellow, red, white, or colorless. Most forms of iodine easily dissolve in water or alcohol. Iodine has many uses. Its most important use is as a disinfectant for cleaning surfaces and storage containers. Iodine is also used in skin soaps and bandages, and for purifying water. Iodine is used in medicines. Iodine is added to food, such as table salt, to ensure that all people in the United States have enough iodine in their bodies to form essential thyroid hormones. Iodine is put into animal feeds for the same reason. Iodine is used in the chemical industry for making inks and coloring agents, chemicals used in photography, and in making batteries, fuels, and lubricants. Radioactive iodine also occurs naturally. Radioactive iodine is used in medical tests and to treat certain diseases, such as over-activity or cancer of the thyroid gland.
1.2 What happens to iodine when it enters the environment?
The oceans are the most important source of natural iodine in the air, water, and soil. Iodine in the oceans enters the air from sea spray or as iodine gases. Once in the air, iodine can combine with water or with particles in the air and can enter the soil and surface water, or land on vegetation when these particles fall to the ground or when it rains. Iodine can remain in soil for a long time because it combines with organic material in the soil. It can also be taken up by plants that grow in the soil. Cows or other animals that eat these plants will take up the iodine in the plants. Iodine that enters surface water can reenter the air as iodine gases. Iodine can enter the air when coal or fuel oil is burned for energy; however, the amount of iodine that enters the air from these activities is very small compared to the amount that comes from the oceans.
Radioactive iodine also forms naturally from chemical reactions high in the atmosphere. Most radioactive forms of iodine change very quickly (seconds to days) to stable elements that are not radioactive. However, one form, 129I, changes very slowly (millions of years), and its levels build up in the environment. Small amounts of radioactive iodine, including 129I and 131I, can also enter the air from nuclear power plants, which form radioiodine from uranium and plutonium. Larger amounts of radioactive iodine have been released to the air from accidents at nuclear power plants and from explosions of nuclear bombs.
1.3 How might I be exposed to iodine?
Iodine is a natural and necessary part of the food that you eat and the water that you drink. In the United States, most table salt contains iodine. Iodine is put into table salt to make sure that everyone has enough iodine in their bodies to form essential thyroid hormones. In the past, people in some areas of the United States did not get enough iodine in their diets. Iodine is in some breads because it is added to flour to condition bread dough for baking. Iodine is also in cow and goat milk. Iodine gets into milk when cows or goats eat iodine that is in their food and water. Iodine can also get into milk when iodine is used to clean milking machines and milk storage containers, and to clean the animals' udders at dairy farms and dairies. Iodine is in ocean fish, shellfish, and certain plants that grow in the ocean (kelp). This is because there is iodine in sea water, and some sea plants and animals concentrate iodine in their tissues. Iodine can also be in the air. Iodine is in sea spray and mist, which are tiny drops of sea water. Iodine is in cleansers and medicines that are used to clean and bandage skin wounds (tincture of iodine). You can be exposed to these if they are placed on your skin. Some medicines have iodine in them. Iodine is used to treat water to make it safe for drinking. You can buy iodine water purifying tablets that you add directly to water. You can also buy water treatment cartridges for your home that have iodine in them. Some iodine will get into the water that you drink if you use these tablets or cartridges.
People are almost never exposed to radioactive iodine, unless they work in a place where radioactive iodine is used or if they are given radioactive iodine by their doctors. Radioactive iodine is used in certain medical tests and treatments. You might have these tests if your doctor needs to look for problems in your thyroid gland or if your doctor needs to treat you for a problem with your thyroid gland. In the past, people were exposed to radioactive iodine released from nuclear bomb tests, after accidental explosions and fires at nuclear power plants, or from facilities that processed or used nuclear fuel for power plants.
1.4 How can iodine enter and leave my body?
Most of the iodine that enters your body comes from the food that you eat. A smaller amount comes from the water that you drink. Iodine will enter your body if it is in the air that you breathe. Some forms of iodine can enter your body when placed on the skin. Iodine can also be injected into your blood by your doctor for special medical tests or treatments. Iodine that enters your body quickly goes into your thyroid gland, a small important organ in your neck. Iodine is used in the thyroid gland to make hormones that are needed for growth and health. Almost all of the iodine in your body is in your thyroid gland. Iodine that does not go into your thyroid gland leaves the body in your urine in a few weeks to months. Small amounts of iodine can also leave your body in sweat or in breast milk. Iodine that leaves your body each day is usually replaced by the iodine that you eat in your food, so that the amount of iodine in your body is just enough to keep you healthy.
1.5 How can iodine affect my health?
Iodine is needed for your thyroid gland to produce thyroid hormones. You and your thyroid gland are healthy when there is just enough iodine in your body, about 10-15 milligrams, so that just the right amount of thyroid hormones are produced. This amount would look like much less than a pinch of table salt if placed in your hand. This amount of iodine is in most people when they eat the foods that people normally eat in the United States. Your thyroid gland can become unhealthy if more or less than this amount of iodine is in your body. An unhealthy thyroid gland can affect your entire body. If the thyroid gland cannot make enough hormone, then you would have to be given thyroid hormone in pills. If your thyroid gland makes too much hormone, then you would have to be given drugs to make your thyroid make less hormone. Radioactive iodine can also be unhealthy for your thyroid gland. If too much radioactive iodine enters your body, the radioactive iodine will destroy your thyroid gland so that the gland will stop making hormones. Too much radioactive iodine in your body can also cause thyroid nodules or cancer.
To protect the public from the harmful effects of toxic chemicals and to find ways to treat people who have been harmed, scientists use many tests.
One way to see if a chemical will harm people is to learn how the chemical is absorbed, used, and released by the body. In the case of a radioactive chemical, it is also important to gather information concerning the radiation dose and the dose rate to the body. For some chemicals, animal testing may be necessary. Animal testing may also be used to identify health effects such as cancer or birth defects. Without laboratory animals, scientists would lose a basic method to get information needed to make wise decisions to protect public health. Scientists have the responsibility to treat research animals with care and compassion. Laws today protect the welfare of research animals, and scientists must comply with strict animal care guidelines.
1.6 How can iodine affect children?
This section discusses potential health effects from exposures during the period from conception to maturity at 18 years of age in humans.
Babies and children need iodine to form thyroid hormones, which are important for growth and health. If infants and children do not have enough iodine in their bodies, their thyroid glands will not produce enough thyroid hormone and they will not grow normally. If they have too much iodine in their bodies, they may develop an enlarged thyroid gland (goiter), which may not produce enough thyroid hormone for normal growth. We also need just the right amount of iodine from our mothers before we are born. Too much iodine from the mother can cause a baby's thyroid gland to be so large that it makes breathing difficult or impossible. Not enough iodine from the mother can cause a baby to not produce enough thyroid hormone, which can affect growth and mental development of the baby. Radioactive iodine in food can be more harmful to babies and children than to adults. When radioactive iodine is in the air, it can get onto the grass and water that the cows eat and drink. Infants and children drink a lot more milk than most adults. If there is radioactive iodine in the milk that a child or infant drinks, more iodine will enter the thyroid gland of the child than of an adult who drinks less milk. In addition, because the thyroid gland of a child or infant is smaller than that of an adult, a child's thyroid gland will receive a higher radiation dose than an adult. Children are more sensitive to the harmful toxic effects of iodine and radioactive iodine than adults because their thyroid glands are still growing and the thyroid gland tissues are more easily harmed by radioactive iodine, and because children need a healthy thyroid gland for normal growth.
1.7 How can families reduce the risk of exposure to iodine?
We all are exposed to iodine in the food that we eat and in the water that we drink. Iodine is needed for your good health. We do not want to prevent exposure to iodine, but we do want to try to prevent exposure to too much iodine. This is not likely to happen from eating a normal diet in the United States or from drinking water or breathing air. It could happen if you were careless about storing soaps or cleansers or medicines that have iodine in them. For example, a child could swallow medicines that contain iodine. People are rarely exposed to radioactive iodine, unless they work in a place where radioactive iodine is used or if they are given radioactive iodine by their doctors for certain medical tests or treatments.
If your doctor finds that you have been exposed to significant amounts of iodine, ask whether your children might also be exposed. Your doctor might need to ask your state health department to investigate.
1.8 Is there a medical test to determine whether I have been exposed to iodine?
Most physicians do not test for iodine in their offices, but can collect samples and send them to special laboratories. They can also feel the thyroid for lumps that may have been caused by disease or past exposure to radioactive iodine, but the results do not tell the cause. Every person's body contains a small amount of iodine, but normally not radioactive iodine (such as 131I). Iodine can be measured in the blood, urine, and saliva. The amount is normally measured by its mass (in grams). If the iodine is radioactive, it can be measured by its mass or by its radiation emissions. These emissions are used to tell the amount of radioactive iodine (in curies or becquerels) and the radiation dose it gives to your body (in sieverts or rem).
Radiation detectors can measure radioactive iodine inside your body using the radiation coming from the thyroid gland in your neck. This is useful only if you recently inhaled or ingested some, or if your physician recently gave you some for medical purposes. Your body quickly eliminates iodine and radioactive iodine, so tests should be done shortly after exposure.
1.9 What recommendations has the federal government made to protect human health?
The federal government develops regulations and recommendations to protect public health. Regulations can be enforced by law. Federal agencies that develop regulations for toxic substances include the Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA), the Food and Drug Administration (FDA), the Department of Energy (DOE), and the U.S. Nuclear Regulatory Commission (USNRC).
Recommendations provide valuable guidelines to protect public health but cannot be enforced by law. Federal organizations that develop recommendations for toxic substances include the Agency for Toxic Substances and Disease Registry (ATSDR), the National Institute for Occupational Safety and Health (NIOSH), and the FDA.
Regulations and recommendations can be expressed in not-to-exceed levels in air, water, soil, or food that are usually based on levels that affect animals; then they are adjusted to help protect people. Sometimes these not-to-exceed levels differ among federal organizations because of different exposure times (an 8-hour workday or a 24-hour day), the use of different animal studies, or other factors.
Recommendations and regulations are also periodically updated as more information becomes available. For the most current information, check with the federal agency or organization that provides it. Some regulations and recommendations for iodine include the following:
The National Research Council has established a Recommended Dietary Allowance for iodine of 150 micrograms per day (µg/day), with additional allowances of 25 µg/day during pregnancy and 50 µg/day during nursing. The EPA has established regulations that limit releases of certain forms of radioactive iodine to the environment and require that industries report releases of certain forms of radioactive iodine. NIOSH has established recommendations for limits of worker exposures to iodine and radioactive iodine. The Nuclear Regulatory Commission, the National Council of Radiation Protection and Measurements (NRCP) and the International Commission of Radiological Protection (ICRP) have established recommended limits for worker exposures to radioactive iodine and for releases of radioactive iodine to the environment.
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TOXICOLOGICAL PROFILE FOR IODINE
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES * Public Health Service * Agency for Toxic Substances and Disease Registry * April 2004
A Toxicological Profile for Iodine, Draft for Public Comment was released in September 2001. This edition supersedes any previously released draft or final profile.
Toxicological profiles are revised and republished as necessary. For information regarding the update status of previously released profiles, contact ATSDR at:
Agency for Toxic Substances and Disease Registry * Division of Toxicology/Toxicology Information Branch * 1600 Clifton Road NE, * Mailstop F-32 * Atlanta, Georgia 30333
Background Information
The toxicological profiles are developed by ATSDR pursuant to Section 104(i) (3) and (5) of the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA or Superfund) for hazardous substances found at Department of Energy (DOE) waste sites. CERCLA directs ATSDR to prepare toxicological profiles for hazardous substances most commonly found at facilities on the CERCLA National Priorities List (NPL) and that pose the most significant potential threat to human health, as determined by ATSDR and the EPA. ATSDR and DOE entered into a Memorandum of Understanding on November 4, 1992 which provided that ATSDR would prepare toxicological profiles for hazardous substances based upon ATSDR=s or DOE=s identification of need. The current ATSDR priority list of hazardous substances at DOE NPL sites was announced in the Federal Register on July 24, 1996 (61 FR 38451).
1. PUBLIC HEALTH STATEMENT
This public health statement tells you about iodine and the effects of exposure.
The Environmental Protection Agency (EPA) identifies the most serious hazardous waste sites in the nation. These sites make up the National Priorities List (NPL) and are the sites targeted for long-term federal cleanup activities. Iodine has been found in at least 8 sites. Radioactive iodine has been found at 9 sites, including iodine-129 (129I) in at least 3 sites, and iodine-131 (131I) in at least 6 sites of the 1,636 current or former NPL sites. However, the total number of NPL sites evaluated for iodine is not known. As more sites are evaluated, the sites at which iodine is found may increase. This information is important because exposure to iodine may harm you and because these sites may be sources of exposure.
When a substance 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. This release does not always lead to exposure. You are exposed to a substance only when you come in contact with it. You may be exposed by breathing, eating, or drinking the substance, or by skin contact. External exposure to radiation may occur from natural or man-made sources. Naturally occurring sources of radiation are cosmic radiation from space or radioactive materials in soil or building materials. Man-made sources of radioactive materials are found in consumer products, industrial equipment, atom bomb fallout, and to a smaller extent from hospital waste and nuclear reactors.
If you are exposed to either radioactive or stable iodine, many factors determine whether you’ll be harmed. These factors include the dose (how much), the duration (how long), and how you come in contact with it. You must also consider the other chemicals you’re exposed to and your age, sex, diet, family traits, lifestyle, and state of health.
1. PUBLIC HEALTH STATEMENT
1.1 WHAT IS IODINE?
Iodine is a naturally occurring element that is essential for the good health of people and animals. Iodine is found in small amounts in sea water and in certain rocks and sediments. Iodine occurs in many different forms that can be blue, brown, yellow, red, white, or colorless. Most forms of iodine easily dissolve in water or alcohol. Iodine has many uses. Its most important use is as a disinfectant for cleaning surfaces and storage containers. Iodine is also used in skin soaps and bandages, and for purifying water. Iodine is used in medicines. Iodine is added to food, such as table salt, to ensure that all people in the United States have enough iodine in their bodies to form essential thyroid hormones. Iodine is put into animal feeds for the same reason. Iodine is used in the chemical industry for making inks and coloring agents, chemicals used in photography, and in making batteries, fuels, and lubricants. Radioactive iodine also occurs naturally. Radioactive iodine is used in medical tests and to treat certain diseases, such as over-activity or cancer of the thyroid gland.
1.2 WHAT HAPPENS TO IODINE WHEN IT ENTERS THE ENVIRONMENT?
The oceans are the most important source of natural iodine in the air, water, and soil. Iodine in the oceans enters the air from sea spray or as iodine gases. Once in the air, iodine can combine with water or with particles in the air and can enter the soil and surface water, or land on vegetation when these particles fall to the ground or when it rains. Iodine can remain in soil for a long time because it combines with organic material in the soil. It can also be taken up by plants that grow in the soil. Cows or other animals that eat these plants will take up the iodine in the plants. Iodine that enters surface water can reenter the air as iodine gases. Iodine can enter the air when coal or fuel oil is burned for energy; however, the amount of iodine that enters the air from these activities is very small compared to the amount that comes from the oceans.
Radioactive iodine also forms naturally from chemical reactions high in the atmosphere. Most radioactive forms of iodine change very quickly (seconds to days) to stable elements that are not radioactive. However, one form, 129I, changes very slowly (millions of years), and its levels build up in the environment. Small amounts of radioactive iodine, including 129I and 131I, can also enter the air from nuclear power plants, which form radioiodine from uranium and plutonium. Larger amounts of radioactive iodine have been released to the air from accidents at nuclear power plants and from explosions of nuclear bombs.
1.3 HOW MIGHT I BE EXPOSED TO IODINE?
Iodine is a natural and necessary part of the food that you eat and the water that you drink. In the United States, most table salt contains iodine. Iodine is put into table salt to make sure that everyone has enough iodine in their bodies to form essential thyroid hormones. In the past, people in some areas of the United States did not get enough iodine in their diets. Iodine is in some breads because it is added to flour to condition bread dough for baking. Iodine is also in cow and goat milk. Iodine gets into milk when cows or goats eat iodine that is in their food and water. Iodine can also get into milk when iodine is used to clean milking machines and milk storage containers, and to clean the animals’ udders at dairy farms and dairies. Iodine is in ocean fish, shellfish, and certain plants that grow in the ocean (kelp). This is because there is iodine in sea water, and some sea plants and animals concentrate iodine in their tissues. Iodine can also be in the air. Iodineis in sea spray and mist, which are tiny drops of sea water. Iodine is in cleansers and medicines that are used to clean and bandage skin wounds (tincture of iodine). You can be exposed to these if they are placed on your skin. Some medicines have iodine in them. Iodine is used to treat water to make it safe for drinking. You can buy iodine water purifying tablets that you add directly to water. You can also buy water treatment cartridges for your home that have iodine in them. Some iodine will get into the water that you drink if you use these tablets or cartridges.
People are almost never exposed to radioactive iodine, unless they work in a place where radioactive iodine is used or if they are given radioactive iodine by their doctors. Radioactive iodine is used in certain medical tests and treatments. You might have these tests if your doctor needs to look for problems in your thyroid gland or if your doctor needs to treat you for a problem with your thyroid gland. In the past, people were exposed to radioactive iodine released from nuclear bomb tests, after accidental explosions and fires at nuclear power plants, or from facilities that processed or used nuclear fuel for power plants.
1.4 HOW CAN IODINE ENTER AND LEAVE MY BODY?
Most of the iodine that enters your body comes from the food that you eat. A smaller amount comes from the water that you drink. Iodine will enter your body if it is in the air that you breathe. Some forms of iodine can enter your body when placed on the skin. Iodine can also be injected into your blood by your doctor for special medical tests or treatments. Iodine that enters your body quickly goes into your thyroid gland, a small important organ in your neck. Iodine is used in the thyroid gland to make hormones that are needed for growth and health. Almost all of the iodine in your body is in your thyroid gland. Iodine that does not go into your thyroid gland leaves the body in your urine in a few weeks to months. Small amounts of iodine can also leave your body in sweat or in breast milk. Iodine that leaves your body each day is usually replaced by the iodine that you eat in your food, so that the amount of iodine in your body is just enough to keep you healthy.
1.5 HOW CAN IODINE AFFECT MY HEALTH?
Iodine is needed for your thyroid gland to produce thyroid hormones. You and your thyroid gland are healthy when there is just enough iodine in your body, about 10–15 milligrams, so that just the right amount of thyroid hormones are produced. This amount would look like much less than a pinch of table salt if placed in your hand. This amount of iodine is in most people when they eat the foods that people normally eat in the United States. Your thyroid gland can become unhealthy if more or less than this amount of iodine is in your body. An unhealthy thyroid gland can affect your entire body. If the thyroid gland cannot make enough hormone, then you would have to be given thyroid hormone in pills. If your thyroid gland makes too much hormone, then you would have to be given drugs to make your thyroid make less hormone. Radioactive iodine can also be unhealthy for your thyroid gland. If too much radioactive iodine enters your body, the radioactive iodine will destroy your thyroid gland so that the gland will stop making hormones. Too much radioactive iodine in your body can also cause thyroid nodules or cancer.
To protect the public from the harmful effects of toxic chemicals and to find ways to treat people who have been harmed, scientists use many tests.
One way to see if a chemical will harm people is to learn how the chemical is absorbed, used, and released by the body. In the case of a radioactive chemical, it is also important to gather information concerning the radiation dose and the dose rate to the body. For some chemicals, animal testing may be necessary. Animal testing may also be used to identify health effects such as cancer or birth defects. Without laboratory animals, scientists would lose a basic method to get information needed to make wise decisions to protect public health. Scientists have the responsibility to treat research animals with care and compassion. Laws today protect the welfare of research animals, and scientists must comply with strict animal care guidelines.
1.6 HOW CAN IODINE AFFECT CHILDREN?
This section discusses potential health effects from exposures during the period from conception to maturity at 18 years of age in humans.
Babies and children need iodine to form thyroid hormones, which are important for growth and health. If infants and children do not have enough iodine in their bodies, their thyroid glands will not produce enough thyroid hormone and they will not grow normally. If they have too much iodine in their bodies, they may develop an enlarged thyroid gland (goiter), which may not produce enough thyroid hormone for normal growth. We also need just the right amount of iodine from our mothers before we are born. Too much iodine from the mother can cause a baby’s thyroid gland to be so large that it makes breathing difficult or impossible. Not enough iodine from the mother can cause a baby to not produce enough thyroid hormone, which can affect growth and mental development of the baby. Radioactive iodine in food can be more harmful to babies and children than to adults. When radioactive iodine is in the air, it can get onto the grass and water that the cows eat and drink. Infants and children drink a lot more milk than most adults. If there is radioactive iodine in the milk that a child or infant drinks, more iodine will enter the thyroid gland of the child than of an adult who drinks less milk. In addition, because the thyroid gland of a child or infant is smaller than that of an adult, a child’s thyroid gland will receive a higher radiation dose than the an adult. Children are more sensitive to the harmful toxic effects of iodine and radioactive iodine than adults because their thyroid glands are still growing and the thyroid gland tissues are more easily harmed by radioactive iodine, and because children need a healthy thyroid gland for normal growth.
1.7 HOW CAN FAMILIES REDUCE THE RISK OF EXPOSURE TO IODINE?
We all are exposed to iodine in the food that we eat and in the water that we drink. Iodine is needed for your good health. We do not want to prevent exposure to iodine, but we do want to try to prevent exposure to too much iodine. This is not likely to happen from eating a normal diet in the United States or from drinking water or breathing air. It could happen if you were careless about storing soaps or cleansers or medicines that have iodine in them. For example, a child could swallow medicines that contain iodine. People are rarely exposed to radioactive iodine, unless they work in a place where radioactive iodine is used or if they are given radioactive iodine by their doctors for certain medical tests or treatments.
If your doctor finds that you have been exposed to significant amounts of iodine, ask whether your children might also be exposed. Your doctor might need to ask your state health department to investigate.
1.8 IS THERE A MEDICAL TEST TO DETERMINE WHETHER I HAVE BEEN EXPOSED TO IODINE?
Most physicians do not test for iodine in their offices, but can collect samples and send them to special laboratories. They can also feel the thyroid for lumps that may have been caused by disease or past exposure to radioactive iodine, but the results do not tell the cause. Every person’s body contains a small amount of iodine, but normally not radioactive iodine (such as 131I). Iodine can be measured in the blood, urine, and saliva. The amount is normally measured by its mass (in grams). If the iodine is radioactive, it can be measured by its mass or by its radiation emissions. These emissions are used to tell the amount of radioactive iodine (in curies or becquerels) and the radiation dose it gives to your body (in sieverts or rem).
Radiation detectors can measure radioactive iodine inside your body using the radiation coming from the thyroid gland in your neck. This is useful only if you recently inhaled or ingested some, or if your physician recently gave you some for medical purposes. Your body quickly eliminates iodine and radioactive iodine, so tests should be done shortly after exposure.
1.9 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO PROTECT HUMAN HEALTH?
The federal government develops regulations and recommendations to protect public health. Regulations can be enforced by law. Federal agencies that develop regulations for toxic substances include the Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA), the Food and Drug Administration (FDA), the Department of Energy (DOE), and the U.S. Nuclear Regulatory Commission (USNRC).
Recommendations provide valuable guidelines to protect public health but cannot be enforced by law. Federal organizations that develop recommendations for toxic substances include the Agency for Toxic Substances and Disease Registry (ATSDR), the National Institute for Occupational Safety and Health (NIOSH), and the FDA.
Regulations and recommendations can be expressed in not-to-exceed levels in air, water, soil, or food that are usually based on levels that affect animals; then they are adjusted to help protect people. Sometimes these not-to-exceed levels differ among federal organizations because of different exposure times (an 8-hour workday or a 24-hour day), the use of different animal studies, or other factors.
Recommendations and regulations are also periodically updated as more information becomes available. For the most current information, check with the federal agency or organization that provides it. Some regulations and recommendations for iodine include the following:
The National Research Council has established a Recommended Dietary Allowance for iodine of 150 micrograms per day (µg/day), with additional allowances of 25 µg/day during pregnancy and 50 µg/day during nursing. The EPA has established regulations that limit releases of certain forms of radioactive iodine to the environment and require that industries report releases of certain forms of radioactive iodine. NIOSH has established recommendations for limits of worker exposures to iodine and radioactive iodine. The Nuclear Regulatory Commission, the National Council of Radiation Protection and Measurements (NRCP) and the International Commission of Radiological Protection (ICRP) have established recommended limits for worker exposures to radioactive iodine and for releases of radioactive iodine to the environment.
1.10 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, your regional Nuclear Regulatory Commission office, or contact ATSDR at the address and phone number below.
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.
Toxicological profiles are also available on-line at www.atsdr.cdc.gov and on CD-ROM. You may request a copy of the ATSDR ToxProfiles CD-ROM by calling the information and technical assistance toll-free number at 1-888-42ATSDR (1-888-422-8737), by email at atsdric@cdc.gov, or by writing to:
Agency for Toxic Substances and Disease Registry * Division of Toxicology * 1600 Clifton Road NE * Mailstop F-32 * Atlanta, GA 30333 * Fax: 1-770-488-4178 *
For-profit organizations may request a copy of final profiles from the following:
National Technical Information Service (NTIS) * 5285 Port Royal Road * Springfield, VA 22161 * Phone: 1-800-553-6847 or 1-703-605-6000 * Web site: http://www.ntis.gov/ *
2.1 BACKGROUND AND ENVIRONMENTAL EXPOSURES TO IODINE IN THE UNITED STATES
Iodine is an essential nutrient. An adequate intake of iodine is required for the production of thyroid hormones. The term iodine excess is used in this profile to refer to increases in intake relative to estimated physiological requirements. As a reference point, the chronic dietary intake of iodine in U.S. populations has been estimated to range from approximately 150 to 950 µg/day. Estimates for various populations have ranged from <50 µg/day in iodine-deficient regions to >10 mg/day in populations that regularly ingest seaweeds containing a high iodine content. The National Research Council (NRC) Recommended Dietary Allowance (RDA) for iodine is 150 µg/day (2.1 µg/kg/day for a 70-kg adult), with additional allowances of 25 and 50 µg/day during pregnancy and lactation, respectively.
The diet is the major source of iodine intake in the U.S. population. Iodine enters the human diet from a variety of natural sources, including mineral dissolution and atmospheric transport and deposition of seawater aerosols to surface water, vegetation, and soil. Major food categories that contribute to dietary iodine include marine produce (e.g., fish and shellfish) and milk. Cows and goats absorb iodine from ingested vegetation and water, when iodine is either deposited on the vegetation or in water or when the iodine is taken up by vegetation grown in soils containing iodine. The absorbed iodine is excreted into their milk; goat milk typically has higher concentrations of iodine than cow milk for equal deposition on feed. Additional sources of iodine in milk derive from the use of iodine disinfectants on cows, milking machines, and other milk processing equipment, as well as from supplementation of dairy feed with iodine-containing compounds. Breast milk is the primary source of iodine intake in nursing infants. Commercial infant formula preparations are fortified with sufficient iodine to support infant health, growth, and development. Cow milk is a significant source of iodine intake in children. Iodine is also intentionally added to the U.S. diet as iodized table salt and as iodine-containing bread dough oxidizers. Other sources of intake derive from the use of iodine-containing topical disinfectants (e.g., povidone-iodine), iodine-containing diagnostic and therapeutic agents, dietary supplements, and water purifiers containing iodine.
Thirty-five isotopes of iodine are recognized (108I through 142I). Only one isotope is stable (127I); the remaining are radioactive. Most of these have radioactive half-lives of minutes or less. Twelve have half-lives that exceed 1 hour, and six have half-lives that exceed 12 hours (123I, 124I, 125I, 126I, 129I, and 131I). Four isotopes (123I, 125I, 129I, and 131I) are of particular interest with respect to human exposures because 123I and 131I are used medically and all four are sufficiently long-lived to be transported to human receptors after their release into the environment. The U.S. population has been exposed to radioiodine in the general environment as a result of atmospheric fallout of radioiodine released from uncontained and/or uncontrolled nuclear reactions. Historically, this has resulted from surface or atmospheric detonation of nuclear bombs, from routine and accidental releases from nuclear power plants and nuclear fuel reprocessing facilities, and from hospitals and medical research facilities. Estimates have been made of radiation doses to the U.S. population attributable to nuclear bomb tests conducted during the 1950s and 1960s at the Nevada Test Site; however, dose estimates for global fallout have not been completed. Geographic-specific geometric mean lifetime doses are estimated to have ranged from 0.19 to 43 cGy (rad) for a hypothetical individual born on January 1, 1952 who consumed milk only from commercial retail sources, 0.7–55 cGy (rad) for people who consumed milk only from home-reared cows, and 6.4– 330 cGy (rad) for people who consumed milk only from home-reared goats. Additional information is available on global doses from nuclear bomb tests and doses from nuclear fuel processing and medical uses can be found in United Nations Scientific Committee on the Effects of Atomic Radiations.
Individuals in the United States can also be exposed to radioiodine, primarily 123I and 131I, as a result of clinical procedures in which radioiodine compounds are administered to detect abnormalities of the thyroid gland or to destroy the thyroid gland to treat thyrotoxicosis or thyroid gland tumors. Diagnostic uses of radioiodine typically involve administration, by the oral or intravenous routes, of 0.1–0.4 mCi (4– 15 MBq) of 123I or 0.005–0.01 mCi (0.2–0.4 MBq) of 131I. These correspond to approximate thyroid radiation doses of 1–5 rad (cGy) and 6–13 rad (cGy) for 123I and 131I, respectively. Cytotoxic doses of 131I are delivered for ablative treatment of hyperthyroidism or thyrotoxicosis; administered activities typically range from 10 to 30 mCi (370–1,110 MBq). Higher activities are administered if complete ablation of the thyroid is the objective; this usually requires 100–250 mCi (3,700-9,250 MBq). Thyroid gland doses of approximately 10,000–30,000 rad (300 Gy) can completely ablate the thyroid gland. An administered activity of 5–15 mCi (185–555 MBq) yields a radiation dose to the thyroid gland of approximately 5,000– 10,000 rad (50–100 Gy).
2.2 SUMMARY OF HEALTH EFFECTS
An extensive amount of literature is available on the effects of iodine on human physiology and health. The intense interest in iodine derives from early recognition of the necessity of appropriate amounts of iodine for maintenance of normal function of the thyroid gland and of awareness of diseases of the thyroid gland that are caused or affected by iodine intake. The prevalence of thyrotoxicosis (the clinical outcome of uncontrolled hyperthyroidism) has been estimated to be approximately 0.5%, and that of hypothyroidism is of a greater magnitude. Research directed at understanding the epidemiology, pathophysiology, and therapeutic strategies for these relatively common diseases have given way to a fairly comprehensive, although not complete, understanding of the role of iodine in thyroid gland physiology and the related health consequences and risks associated with excessive or inadequate iodine intake. The use of radioactive iodine (131I) for treating thyrotoxicosis, as well as studies of the thyroid gland as a target for internal exposures to atmospheric 131I fallout, have further complemented our understanding of iodine toxicity as it relates to exposures to radioactive isotopes of iodine.
This profile does not attempt to summarize in detail all of the studies relevant to the adverse effects of iodine on the thyroid, as to do so would require several volumes. Instead, the focus is on literature that identifies the lowest observable iodine exposure levels associated with adverse effects in humans. Where applicable, relevant studies in animals are summarized, particularly when such studies have identified potential targets of toxicity not already documented in humans or for which adequate dose-response information does not exist for humans. This strategy leads to a focus on the thyroid gland as the primary and most sensitive target of iodine for both chemical and radiologic toxicity. This is not surprising given that avid uptake of absorbed iodine by the thyroid gland results in approximately 90% of the body iodine content residing in the thyroid gland (see Section 3.4, Toxicokinetics). Adverse effects on a wide variety of other organ systems can result from disorders of the thyroid gland, including disturbances of the skin, cardiovascular system, pulmonary system, kidneys, gastrointestinal tract, liver, blood, neuromuscular system, central nervous system, skeleton, male and female reproductive systems, and numerous endocrine organs, including the pituitary and adrenal glands. Although these secondary effects are noted in the profile, they are not discussed in detail and the reader is referred to authoritative references on these subjects for further information.
An important consideration in interpreting the iodine toxicology literature is that the effect of an increase in iodine intake will depend, in part, on the preexisting background dietary intake and the associated physiological adaptations to background intake. The response to an upward increase in intake may be quite different in individuals who have adapted to either low dietary or high dietary intake. Examples of this are described in appropriate sections of this report (e.g., Section 3.2.2.2). In this profile, the term molecular iodine is used to refer to I2; the term iodide is used to refer to the anion, I-, the term iodate is used to refer to the anion IO3-, and the term iodine is used to refer to the element in any form, usually when the form was not specified in the literature being summarized or when the form is not relevant to the discussion. From a physiological perspective, regardless of the form of iodine that is absorbed after exposure, iodide is the form of iodine that is taken up into the thyroid gland, and effects from exposures to iodine ultimately derive from exposure of the thyroid gland to iodide. A more important toxicological distinction is that, unlike iodide, molecular iodine (I2) is a relatively strong oxidizing agent and has the potential to produce injuries related to redox reactions with proteins. This is the primary basis for the use of I2 as a topical antiseptic and antimicrobial disinfectant for drinking water.
The health effects of exposure to radioiodine derive from the emission of beta and gamma radiation. Radioiodine that is absorbed into the body quickly distributes to the thyroid gland and, as a result, the tissues that receive the highest radiation doses are the thyroid gland and surrounding tissues (e.g., parathyroid gland). Tissues other than the thyroid gland can accumulate radioiodine, including salivary glands, gastric mucosa, choroid plexus, mammary glands, placenta, and sweat gland. Although these tissues may also receive a radiation dose from internal radioiodine, the thyroid gland receives a far higher radiation dose. The radiation dose to the thyroid gland from absorbed radioiodine varies with isotope and its radiation emission properties (e.g., type of radiation, energy of emission, effective radioactive half-life). A comparison of the doses delivered to the thyroid gland from a few of the isotopes of iodine is in Table 2-1. The highest total doses are achieved with 131I, whereas the highest dose rates (rad/hour) are delivered from 132I.
Endocrine Effects. The principal direct effects of excessive iodine ingestion on the endocrine system are on the thyroid gland and regulation of thyroid hormone production and secretion. Effects of excess iodine on the thyroid gland can be classified into three types: hypothyroidism, hyperthyroidism, and thyroiditis. Hypothyroidism refers to the diminished production of thyroid hormones leading to clinical manifestations of thyroid hormone insufficiency. This can occur with or without goiter, an enlargement of the gland that occurs in response to elevated circulating levels of the pituitary hormone, thyroid stimulating hormone (TSH), during periods of suppressed thyroid hormone production. A typical biomarker of hypothyroidism is a decrease in the circulating levels of thyroxine (T4) and, when thyroid failure is far advanced, triiodothyronine (T3). This is always accompanied by an elevation of TSH (also known as thyrotropin) above the normal range, unless the cause of the hypothyroidism resides in the pituitary-hypothalmus. Hyperthyroidism is an excessive production and/or secretion of thyroid hormones. The clinical manifestation of abnormally elevated circulating levels of T4 and/or T3 is thyrotoxicosis. Thyroiditis refers to an inflammation of the gland, which is often secondary to thyroid gland autoimmunity. The above three types of adverse effects of excess iodine can occur in children and adults, in fetuses exposed in utero, or in infants exposed during lactation. The primary effect of iodide excess in the fetus is goiter formation secondary to a suppression of thyroid hormone production and an elevation in TSH levels.