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 manmade products,
18%. Less than 1% of the radiation from manmade 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
|
|
|
|
|
|
Total
Estimated Amount
of I-131 Released From the Site (in curies) |
|
Site
|
|
Time
Period |
|
|
|
|
|
150,000,000
Ci |
|
Nevada
Test Site, Nevada |
|
1952–1970 |
50,000,000
Ci |
|
Chernobyl
(former Soviet Union) |
|
1986 |
740,000
Ci |
|
Hanford
Reservation, Washington |
|
1944–1972 |
60,000
Ci |
|
Savannah
River Site, South Carolina |
|
1955–1990 |
8,000–42,000
Ci |
|
Oak
Ridge National Laboratory, Tennessee |
|
1944–1956 |
20,000
Ci |
|
Windscale,
United Kingdom |
|
1957 |
15–21
Ci |
|
Three
Mile Island, Pennsylvania |
|
1979 |
|
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.
Challenge
question
2. Why is dietary
intake information important for assessing the patient's exposure
to I-131?
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
question
3. Which age groups
are the most sensitive to I-131 exposure?
For a printer-friendly
version of the Radiation Exposure From Iodine 131 case study,
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