Agency for Toxic Substances and Disease Registry
Radiation Exposure from Iodine 131
Exposure Pathways

Exposure Pathways

"If the levels of environmental I-131 ... were released now instead of in the late 1940s-1960s, contaminated food quarantine, and emergency evacuation of many parts of the country would occur. The population would be outraged. A national health emergency would be declared. Congress would pour monies into health care and other help for those exposed. Ours may be exposures of the past, but those of us exposed as children, when we were most vulnerable to radioactive harm, are still alive and some of us have developed exposure health outcomes. We must not be discounted...."

Testimony of a Hanford community member exposed to I-131 (Hanford, Washington).

During the Cold War, national security policies prevented government authorities from disclosing the risks and health hazards associated with living near or working at weapon production facilities. These facilities released harmful levels of radiation into the environment.

Many people in the United States, especially those living near or working at weapon production facilities, such as the Hanford Nuclear Reservation, were unknowingly exposed to multiple sources of I-131, including fallout. The Nevada Test Site (NTS), which was used to test nuclear weapons, produced considerable amounts of fallout, which exposed most of the American population. The existing national security policies kept that information from reaching the American public.

This case study focuses on iodine 131 (I-131) because large amounts of this isotope were released during the production and testing of nuclear weapons. Iodine 127 (I-127) is the only naturally occurring iodine isotope, and it is the only nonradioactive (stable) iodine isotope. All other iodine isotopes (I-123, I-125, I-129, I-131, and I-135) are radioactive. Only I-131 and I-135 are associated with medical administration.

• I-131 is radioactive, has an 8.03 day half-life, and emits beta and gamma radiation.
• I-131 is normally present at low levels in hospital nuclear medicine departments, in patients administered radioactive iodine in the last 3 months, and in releases from nuclear power plants.
• I-131 is produced during nuclear fission, which occurs during the operation of nuclear reactors or detonation of a nuclear bomb. When uranium or plutonium atoms undergo fission, about 1.5%-2.0% of the fission products become I-131.
  

The Legacy of I-131 in the Environment

• The main sources of I-131 in the environment have come from nuclear power plant releases and from the production and testing of nuclear weapons.
• The highest levels of combined I-131 releases occurred between the early 1940s and mid-1960s.

In the United States, past releases of I-131 have occurred at fuel reprocessing plants and some weapon production facilities of the Department of Energy (DOE). Since 1944, when the first production atomic reactor came into service, large amounts of I-131 have been periodically released into the atmosphere. I-131 was released to the atmosphere as a gas during nuclear weapons production (1945-1980s), aboveground nuclear tests (1951-1962), medical isotope production, medical administrations to patients, and unintentional releases. Multiple releases over time could have maintained constant or repetitive high levels of radioactivity, particularly around weapon production facilities. The highest levels of combined I 131 releases occurred from the early 1940s through the mid-1960s.

The annual dose of background radiation received by an average person in the United States comes from the following sources: radon gas, 55%; internal radiation, 11%; cosmic rays, 8%; terrestrial radiation, 8%; and man-made products, 18%. Less than 1% of the radiation from man-made products comes from nuclear power plant releases and fallout. Typically, little of this dose is from I-131 because of the short half-life of the element: it decays (loses its level of radioactivity) rapidly and rarely exists at any meaningful level in the environment. However, this changes if a major nuclear release occurs. When a nuclear bomb detonates or nuclear power plant fuel melts and causes an explosion, the volatile I-131 produced is forced up to various elevations (potentially exceeding 10 kilometers [6.2 miles]) by the intense heat, and is subsequently swept by the winds. I-131 can be deposited on the ground as dry deposition (I-131 adsorbs to particulates in the air and drops to the ground) or as wet deposition (I-131 dissolves in atmosphere moisture, some of which becomes rainwater and falls to the ground). The initial quantity released determines the significance of the fallout. Persons living in the direction in which the wind blows are referred to as "downwinders." Many of the persons living downwind from the Hanford Nuclear Reactor could have received multiple exposures over time.

Total releases of I-131 worldwide equal 24,000,000,000 curies. The curie, or Ci, is the measurement for the rate of radioactive decay. If you would like more information on general ionizing radiation principles, please see ATSDR's Case Studies in Environmental Medicine: Ionizing Radiation (ATSDR 1993a).

Worldwide, major significant I-131 releases occurred at the following locations

The peak years for the releases at the Hanford Nuclear Reservation were 1944-1947 (92%), with minimal releases after 1947, except for two peaks in December 1949 (the Green Run) and May 1951 (filters removed). The largest I-131 releases from the Oak Ridge National Laboratory occurred between 1952 and 1956. An April 29, 1954, accident released 105 to 500 Ci over 2½ hours, accounting for about 6.5% of the total release for 1954. The Nevada Test Site had 90 nuclear tests that released almost 99% of the total I-131 released into the atmosphere from 1952 through 1957. The Windscale release in the United Kingdom in 1957 was caused by a fire in the graphite moderator of an air-cooled plutonium production reactor. The Three Mile Island release in Harrisburg, Pennsylvania, in 1979, released 15-21 Ci of I-131 into the atmosphere. During and after the explosion and fire at the Chernobyl nuclear plant, large amounts of radioactive materials were released over a 10-day period, with 25% of the total amount released in the first day. These materials were subsequently spread over parts of Europe and the rest of the world by wind.

Exposure Route

• The activity of I-131 (quantity of radioactive material present), the exposure route, and the individual's age are factors that determine the exposure dose from radiation.
• Infants' and children's increased rate of growth and development make them more vulnerable to radiation exposures.
• Ingestion of contaminated milk has been the major I-131 exposure pathway for humans.
• The concentration of I-131 in milk from goats and sheep is 10 times higher than the concentration in cow's milk.

The amount of I-131 available to expose a person after a release depends on the

• amount released
• distance between the populated area and the place of the release
• height of the release, and
• meteorologic conditions at and after the time of the release.

The exposure pathway of greatest public health significance is the deposition of I-131 on pasture grasses, followed by the ingestion by cows or goats and the subsequent consumption of contaminated milk and fresh dairy products by humans.

Exposure begins immediately for persons in the immediate vicinity of a nuclear release who are in the plume (the visible or invisible cloud of contamination). Internal exposure by inhalation occurs for persons inside the plume. External exposure occurs while the person is in the plume or on land left contaminated by fallout from the plume. Internal exposure by ingestion occurs when persons eat food that is contaminated with the fallout. The oral pathway is the main route of internal I-131 exposure for people. Milk is the major source of internal exposure.

Dietary intake of iodine before exposure is important because a relative iodine deficiency increases the thyroid uptake of I-131. After exposure, the most critical dietary information needed is the amount and type of milk and milk products consumed, their I-131 concentrations, and the time they were consumed relative to the time of the release.

Goat's milk and sheep's milk contain approximately 10 times the concentration of radioiodine found in cow's milk. Inhalation, especially near releases of I-131 in the absence of rain, is another route of internal exposure. However, doses to humans from inhalation and from ingestion of plants, animals, or water are usually small in comparison. Figure 1 shows the exposure pathways of I-131 from the environment to humans.

Figure 1. Exposure Pathways of I-131 from Environment to Humans

Acute Exposure to I-131

• Acute exposure to I-131 today could occur from unintentional or intentional releases.
• Public exposure to I-131 or contamination of soil, food, or water by I-131 engenders intense fear. The emotional and psychologic stresses resulting from exposure should be recognized and addressed early in a radiation incident.

Explosion of a nuclear bomb produces a small amount of local I-131 fallout; the remainder distributes over large distances, with only 10% making its way to the surface before transforming to stable xenon 131 (UNSCEAE 2000a, 2000b). The less intense heat from a nuclear reactor release allows higher local I-131 fallout.

The current main sources of I-131 exposure would be a localized hospital accident, a major nuclear power plant release involving melted fuel, or an aboveground atomic bomb detonation. The resulting iodine levels along the plume path would vanish over a period of a few days to months depending on dilution and radioactive decay.

Doses of I-131 that result from medical procedures, including therapeutic thyroid ablations, release low levels of radiation in hospital nuclear medicine departments. Therapeutic thyroid ablations have a mean thyroid dose of 10-100 Gray (Gy) to the patient, which is equivalent to a radiation absorbed dose (rad) of 1,000-10,000. These ablations significantly exceed an entire year's worth of background radiation. Patients undergoing this procedure release low levels of radiation for about 3 months.

Challenge Questions

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