REPORT NO. 5
background material
for the development of
radiation protection
standards
July 1964
Staff Report of the
FEDERAL RADIATION COUNCIL
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REPORT NO. 5
background material
for the development of
radiation protection
standards
July 1964
Staff Report of the
FEDERAL RADIATION COUNCIL
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C., 20402 - Price 20 cents
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CONTENTS
Section I
Introduction .
Page No.
1
Section II Origin and Distribution of Radioactive
Contamination
Section III Protective Actions and Their Applications.
Section IV Guidance Applicable to Iodine-131.
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SECTION I
INTRODUCTION
This report contains background material used in the development of
guidance for Federal agencies in respect to: (1) planning protective
actions to reduce potential doses to the population from radioactive
fission products which may gain access to food, and (2) doses at
which implementation of protective actions may be appropriate.
The material is directed to guidance concerned with protective ac-
tions affecting the normal production, processing, distribution, and
use of food for human consumption when the action is taken because
of the radioactive content of the food.
The first two reports of the Federal Radiation Council contained
background material used in the development of guidance given in the
Memorandums approved by the President on May 13, 1960, and September
13, 1961. These reports provided a general philosophy of radiation
protection and general principles of control based on the annual in-
take of radioactive materials. The recommendations contained there-
in were intended to provide the basis for the control and regulation
of normal peacetime operations in which exposure to radiation is a
factor. Numerical values for the Radiation Protection Guides de-
signed to limit the exposure of the whole body and certain organs as
the result of normal peacetime operations were provided.
During the period of atmospheric testing of nuclear weapons in
1961 and 1962 the question arose as to the possible need for protec-
tive actions and the use of existing Radiation Protection Guides for
determining the conditions under which the production, processing,
distribution, and use of food, particularly fresh fluid milk, should
be altered to reduce human intake of radioactive materials from
fallout.
In September 1962 the Federal Radiation Council stated, in effect,
that the Radiation Protection Guides provided for the conduct of
normal peacetime operations are not intended to set a limit at which
protective action affecting the normal production, processing, dis-
tribution, and use of food should be taken, nor to indicate what
kind of action should be taken. In the 1963 hearings, "Fallout,
Radiation Standards, and Countermeasures," conducted by the Joint
Committee on Atomic Energy, the Council reiterated that position and
noted that it would recommend to the President guidance for the ap-
propriate Federal agencies applicable to a determination of the need
for protective actions.
Limiting the exposure of members of the population to man-made
radioactive material can be accomplished by controlling the release
of such material from its place of origin or use, or by protecting
the population after the material is released to the environment.
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This report is directed to guidance for protecting the population
from radioactive material after it has been released to the environ-
ment in concentrations which justify action.
Situations justifying protective action could occur from such
events as: (1) an industrial accident, possibly involving a nuclear
reactor or a nuclear fuel processing plant, and (2) release of
radioactive materials from the detonation of nuclear weapons or
other nuclear devices.
The guidance concerns protective actions which might be applied to
the production, processing, distribution, or use of food to reduce
the potential human intake of such radioactive material. This guid-
ance is confined in application to those conditions under which the
hazard of concern is that associated with the ingestion of radio-
active materials. Conditions requiring protection from external
gamma radiation or protection when inhalation may also be a signif-
icant mode of entry for radioactive material into the human body in-
volve different considerations.
This report includes guidance as to the general principles con-
cerned with protective actions and specific guidance applicable to
iodine- 131.
Preparation of the Staff Report
In the development of this report, the Staff has reviewed the
literature on the origin, distribution, mechanisms of transmission
through the environment, and potential biological effects of radio-
active materials. In particular, the Staff has studied the trans-
scripts of the hearings conducted by the Joint Committee on Atomic
Energy; the reports by the National Committee on Radiation Protec-
tion and Measurements, the World Health Organization and Food and
Agricultural Organization of the United Nations, the United Kingdom
Medical Research Council's Committee on Protection against Ionizing
Radiations, the National Advisory Committee on Radiation — an ad-
visory committee to the Surgeon General, United States Public Health
Service; and the "Proceedings of the Hanford Symposium on the Biol-
ogy of Radioiodine ."
The Staff also has had considerable assistance from many individ-
ual scientists and technical experts.
Definitions
The absorbed dose is the energy imparted to a volume of irradiated
material per unit mass of that volume.
The rad is a unit of absorbed dose equal to 100 ergs per gram.
The projected dose is the dose that would be received in the
future by individuals in the population group from the contaminating
event if no protective action were taken.
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The Protective Action Guide (PAG) is the projected absorbed dose
to individuals in the general population which warrants protective
action following a contaminating event.
The curie is a unit of radioactivity defined as 3.7 x 1010 trans-
formations per second. Commonly used multiples of the curie are the
following:
1 millicurie = 1 x 10 3 curie
1 microcurie = 1 x 10s curie
1 nanocurie = 1 x 10 9 curie
1 picocurie = 1 micromicrocurie = 1 x 10 12 curie
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SECTION II
ORIGIN AND DISTRIBUTION OF RADIOACTIVE CONTAMINATION
The origin and distribution of radioactive material injected into
the atmosphere and its transport mechanisms through the environment
to man have been studied intensively both nationally and inter-
nationally for the past decade in connection with the atmospheric
testing of nuclear weapons. The past and anticipated concentrations
of radioactive materials in the environment from weapons testing
through 1962 have been studied and evaluated by the Council in its
Reports No. 3 and No. 4.
Material injected into the stratosphere by nuclear weapons tests
eventually descends to the troposphere from which it is deposited on
the earth's surface. During storage in the stratosphere, short-
lived radionuclides decay essentially to zero. Long-lived radionu-
clides which find their way to the troposphere deposit relatively
uniformly on a regional basis, although the quantities vary with
latitude and with rainfall. A somewhat similar distribution pattern
of short-lived radioactive material such as iodine-131 has been ob-
served in the U.S. for the tropospheric distribution of debris from
tests conducted outside the U.S.
When radioactive material is released to the atmosphere at ground
level, as would generally be the case in an industrial accident,
diffusion in the troposphere is limited and the passage of the
radioactive cloud over an area takes a relatively short time. How-
ever, the concentrations in the cloud can be high. The deposition
of radioactive materials in this case can lead to possible radiation
doses that warrant protective action.
Radioactive Nuclides of Interest
Although nuclear fission results in many nuclides, most of which
are radioactive, their chemical and physical properties are such
that few of them are of interest as potential radioactive contami-
nants of food. Some of these radionuclides have such short radio-
active half-lives that their radioactive decay to stable nuclides is
complete before the food is consumed. Those of principal interest
are isotopes of chemical elements readily utilized by vegetation or
animals, and of sufficiently long radioactive half-lives that much of
their radioactivity will not have disappeared before they have
reached the human diet.
The relative importance of different radionuclides may depend on
many factors such as the time that elapses between fission and the
release of fission products to the environment, chemical or physical
separation or fractionation, conditions of release, and season of
of year. For example, in unseparated fission products only a few
days of age, the properties of iodine make it the critical radio-
nuclide, while a few weeks later the disappearance of iodine-131
will leave the longer-lived strontium-89, strontium-90, and
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cesium-137 as the nuclides of interest. Many studies of possible
types of release important to this report have led to the conclusion
that events requiring protective actions are most likely to involve
iodine-131 as the critical nuclide.
The Transmission Chain
The path of radioactive material from the atmosphere through the
food chain to man is shown in Figure I. The material is scavenged
from the atmosphere by meteorological processes, particularly rain.
If the air mass containing the radioactive material is at ground
level the radionuclides may be directly removed from the air by
vegetation. Following the initial deposition, the radioactive con-
tamination tends to be removed by various processes, such as being
washed off by subsequent rainfall or being blown off by the wind.
The extent to which such removal occurs varies widely depending on
the particle size and chemical properties of the material deposited.
Although radionuclides may be incorporated subsequently into plants
through absorption by the root system, their concentration on the
surfaces of vegetation will be the dominant factor leading to a
possible need for protective action.
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FIGURE I
IMPORTANT STEPS IN THE TRANSMISSION OF RADIOACTIVE
MATERIAL THROUGH THE FOOD CHAIN TO MAN
food crops
meat and
meat products
4
MAN
atmosphere
I
pasturage
animals
fresh fluid milk
processed
milk products
i
feed crops
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SECTION III
PROTECTIVE ACTIONS AND THEIR APPLICATIONS
A protective action, within the scope of this report, is an action
or measure taken to avoid most of the exposure to radiation that
would occur from future ingestion of foods contaminated with radio-
active materials.
Since a protective action is taken to abate such an exposure risk
after the radioactive material has been released, in the development
of guidance for taking such action it is necessary to consider:
1. the possible risk to health associated with the projected
dose to the population from fission products.
2. the amount by which the projected doses can be reduced by
taking certain protective actions.
3. the total impact, including risks to health associated
with these protective actions, and
4. the feasibility of taking the actions.
Protective actions are appropriate when the health benefit asso-
ciated with the reduction in dose that can be achieved is considered
sufficient to offset the undesirable factors associated with the
action.
The value of a protective action depends on how much the projected
dose per individual can be reduced by the action and the number of
people affected. Protective actions will yield a greater return in
relation to their disadvantages if projected doses are high rather
than low. Since high levels of contamination probably will be
limited to small areas, protective actions are more likely to be re-
quired in such areas rather than over large regions.
It is possible to estimate the projected dose that will result
from measured concentrations of radionuclides in the environment.
This estimate is usually related to a daily or total intake of
radioactive materials and can be based either on the assumption that
more materials will not be added to the food chain during the period
of concern, or that potential additions can be quantitatively esti-
mated. A quantitative estimate of the projected dose is necessary
for determining whether or not protective action should be taken.
Impact of Protective Actions
A decision to implement a protective action involves a comparison
of the risk due to radiation exposure with the undesirable features
of the contemplated action.
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The impact of the protective action will involve such factors as
the degree of departure from the usual practice, the length of time
over which the action is applied, the relative ease with which the
action can be executed, and possible health risks associated with
the action.
One of the well-established actions to reduce the intake of con-
taminants on foods is based on the fact that they are on the sur-
face of fresh fruits and vegetables. A large proportion of these
contaminants, including radioactive material, can be removed by ap-
plying the usual practices of food preparation. The only departures
from these usual practices are that: (1) more attention is given to
insuring that the surfaces are washed, (2) the outer leaves of leafy
vegetables are removed, and (3) more than normal preference is given
to peeling.
The impact of these actions is small because they are already ac-
cepted practice and no innovations are involved. If it were required
that all fruits be peeled before eating, the impact would be greater:
and hence the risk would have had to be correspondingly greater to
warrant this degree of intervention in accustomed activities.
Some actions, such as discarding a food item, involve a marked de-
parture from usual practice. They may, nevertheless, be of low im-
pact if limited to a small quantity of produce or to a small area,
or if applied infrequently and only for short periods.
The feasibility of executing a protective action depends on the
ease with which it can be applied by diversion of available resource
factors such as the facilities, equipment, personnel, and alterna-
tive supplies of animal feeds or agricultural produce needed to im-
plement the action. Diversion of a small fraction of readily avail-
able resources is usually easy, but diversion of a large proportion
increases the impact and decreases the feasibility very rapidly.
There will be very few factors, possibly only one, that are limiting
in any complex of resources. These will not necessarily be the same
at each place or time the protective action may be needed.
Types of Protective Actions
The types of actions to which guidance in this report may be
related are:
1. Altering production, processing, or distribution practices
affecting the movement of radioactive contamination through the
food chain and into the human body. This action includes a
storage of food and animal feed supplies to allow for the radio-
active decay of short-lived nuclides.
2. Diverting affected products to uses other than human con-
sumption.
3. Condemning foods.
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Other possible types of action currently are judged to be less de-
sirable for reasons of effectiveness, safety, or practicality. The
use of additives in cattle rations, soil treatment, and the chemical
removal of radionuclides from milk are not included among the types
of actions listed.
Protective actions to reduce the intake of radioactive materials
by special alterations of the normal diet are accomplished best on
an individual basis under the supervision of medical authorities.
Application of Protective Actions
In providing guidance for protective actions applicable to radio-
active contamination of the environment, the Council is concerned
with a balance between the risk of radiation exposure and the impact
on public well-being associated with the alteration of the normal
production, processing, distribution, or use of food.
It is recommended that the term "Protective Action Guide" (PAG)
be used to indicate the projected dose at which the above balance
is judged to occur for the general types of protective actions con-
sidered in this section. Thus, the Protective Action Guide serves
as a basis for deciding when such protective actions are indicated.
In the application of the Protective Action Guides the following
guidance is provided:
1. If the projected dose exceeds the PAG, protective action is
indicated.
2. The amount of effort that properly may be given to protec-
tive action will increase as the projected dose increases.
3. The objective of any action is to achieve a substantial re-
duction of dose that would otherwise occur — not to limit it
to some prespecified value.
4. Proposed protective actions must be weighed against their
total impact. Each situation should be evaluated individually.
As the projected doses become less, the value of protective
actions becomes correspondingly less.
5. The Protective Action Guide is based on the assumption that
the occurrence, in a particular area, of environmental contami-
nation that would require protective action is an unlikely event.
Circumstances that involve either repetitive occurrence or in
which there appears a substantial probability of recurrence with-
in a period of one or two years would require special considera-
tion. In such a case the total projected dose from the several
events and the total impact of the protective actions that might
be taken to avoid the dose from one or more of these events must
be considered. In contemplating the possibility of a future
event it is necessary to consider not only the possible magni-
tude but also the probability that the event will occur.
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6. Federal agencies should plan protective action programs
designed to reduce the projected dose to individuals in the
general public by modifying the normal production, processing,
distribution, or use of food products or animal feeds. The
need for implementing such plans should be determined on the
basis of the estimated projected dose and the appropriate PAG.
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SECTION IV
GUIDANCE APPLICABLE TO IODINE- 131
The Environmental Pathway
The physical and biochemical characteristics of iodine-131 make it
the radionuclide most likely to warrant rapid application of protec-
tive actions. This is especially true if radioactive contamination
occurs before appreciable radioactive decay has taken place.
The important pathway for iodine-131 from the source to the body,
and the one considered applicable to protective action criteria, is
through pasture to the cow, milk, and into the human body.
The shortest time-span from source to individual occurs when fresh
milk has not gone through processes of pasteurization and distribu-
tion. Iodine-131 may appear in milk a few hours after deposition on
pasturage. From a single deposition it can reach a maximum concen-
tration in milk as early as two to four days after deposition; it
then decreases by half about every five days due to a combination of
radioactive decay and weathering losses from grass.
Deposition of iodine-131 can vary greatly within a relatively
small geographical area. As a result, there can be large differen-
ces between the iodine-131 concentrations in milk produced on farms
only a few miles apart. Because of variations in deposition and in
animal feeding habits, and inadequate data for evaluating the ef-
fects of these variations, it is not yet possible to predict relia-
bly the maximum concentration of iodine-131 in milk from deposition
data.
Development of the Protective Action Guide Against Iodine-131
Factors affecting the relationships between exposures of humans to
iodine-131 and subsequent biological effects have been discussed in
FRC Reports No. 1 and No. 2, in a report* prepared for the FRC by
the National Academy of Sciences Committees on the Biological Effects
of Atomic Radiation, and in the "Proceedings of the Hanford Sympos-
ium on the Biology of Radioiodine."
According to these reports, the uptake of iodine-131 in the thy-
roids of children and adults is approximately the same. Thus a
given intake would result in a ten times larger dose to the thyroid
of a one year old child (thyroid weight 2 grams) than to an adult
(thyroid weight 20 grams). Children, one year of age are assumed to
be the critical segment of the population.
*Pathological Effects of Thyroid Irradiation - A report of a panel
of experts from the Committees on the Biological Effects of Atomic
Radiation; National Academy of Sciences; National Research Council,
published by the Federal Radiation Council, July 1962.
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Young children treated with X-rays in the neck region for enlarged
thymus or for other benign head and neck conditions have had a sig-
nificantly higher incidence of tumors, including thyroid carcinoma,
than have children in control groups. Radiation doses to the thy-
roid found to be associated with thyroid carcinoma under these con-
ditions range upward from about 150 rads. Experience with exposure
of the thyroid to large doses of radiation from iodine-131 for thera-
peutic reasons is extensive but is almost entirely confined to
adults. The report of the panel of experts of the NAS-NRC Commit-
tees states that, although therapeutic doses from iodine-131 to the
thyroid have been in the range of a few thousand rads upward,
iodine-131 has not been identified in a causative way with the de-
velopment of thyroid cancer in humans, except in one doubtful case.
X—ray doses to the thyroid appear to be from 5 to 15 times as effec-
tive in producing biological changes as iodine-131.
The initiation of protective action against contamination in the
environment resulting from a single event (i.e., an event isolated
in time from any other event that might affect the same area) in-
volves undesirable features that may be expected to vary in impor-
tance from one circumstance to another. Of various actions that
might be effective in averting the major part of the projected dose,
two appear to provide the most acceptable combination of maximum ef-
fectiveness and minimum undesirable consequences. One of these is
the diversion of contaminated milk to the production of dairy prod-
ucts that may be stored conveniently until the iodine-131 essen-
tially has decayed, a matter of a few weeks. The other is the sub-
stitution of stored feed for pasturage until most of the iodine-131
has decayed. The choice may depend upon many factors.
Considering existing information on the biological risks asso-
ciated with doses from iodine-131 and the kinds of protective action
available to avert the dose from iodine-131 that has been deposited
on pastures used by dairy cows, the Council has concluded that such
protective action as the diversion of milk or the substitution of
stored feed for pasturage to avert individual doses less than 30
rads would not usually be justifiable under the conditions consid-
ered most likely to occur. This dose is recommended as the Protec-
tive Action Guide for iodine-131.
Generally it will not be practical to estimate individual doses.
In such cases decisions to take an action will be based on average
values. As noted in FRC Report No. 1, paragraph 5.4, it is assumed
that the majority of the individuals do not vary from the average by
a factor greater than three. As an operational technique, it is
considered that the PAG will not be exceeded if the average projected
doses to the thyroids of a suitable sample of the population do not
exceed 10 rads. A suitable sample is considered to consist of
children of approximately one year of age using milk from a reason-
ably homogeneous supply.
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The PAG is stated in terms of a projected dose; i.e., the dose
that might otherwise be received if the protective action were not
initiated. However, since the value of the contemplated action and,
consequently, the justification of the action, depends on the dose
averted, it is valid to use the projected dose as a basis for imple-
mentation of a proposed protective action only if it is expected
that most of the projected dose will be averted.
Because of the differences that may exist in various circumstances
it is necessary to evaluate each situation individually. It is not
feasible to provide detailed criteria for taking into account dif-
ferences that may occur. In general, the PAG represents the Coun-
cil's judgment regarding the benefit-impact balance for the two pro-
tective actions considered acceptable and for the conditions con-
sidered most likely to occur. If in a particular situation there is
available an effective action with low total impact, initiation of
such action at a projected dose lower than the PAG may be justifi-
able. If only high impact protective action would be effective,
initiation of such action at a projected dose higher than the PAG
may be justifiable. For example, diversion of milk from fresh milk
channels to processed products may be less difficult in a locality
where surplus production makes allocation between the two uses a
normal practice. The action would be more difficult and expensive,
and the total impact would be high if the milk had to be transported
large distances to a processing center, or if there were adverse ef-
fects on the quality or quantity of milk available to the consumer
or adverse effects on the dietary habits of individuals in the
population.
In considering the net benefit of a protective action, assuming
the protective action is initiated at progressively lower values of
projected dose, as the projected dose becomes less, the net benefit
to public well-being from reduction of exposure becomes less.
Application of Protective Actions Against Iodine-131
The benefit of avoiding a potential dose from iodine-131 must be
evaluated against the feasibility and the disadvantages of any pro-
tective action under consideration. A selected action must be feas-
ible for the particular situation. It must not be subject to limi-
tations such as lack of communications or transportation which would
nullify its effectiveness. If warranted under certain unusual con-
ditions, the application of protective actions in consecutive or
concurrent operation may be considered. Local conditions must also
be considered. For example, the proportion of fresh milk use to
processed milk use in the area, or the seasonal feeding pattern for
cattle may affect the efficacy of a specific protective action.
Iodine-131 concentrations may vary widely within a given milkshed.
Therefore concentrations at the point of milk production must be
known if efficient protective actions are to be taken. On the other
hand, concentrations at the point of consumption must be known in
order to evaluate the projected doses received by the population
group.
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Protective actions cannot attain maximum effectiveness without
adequate communications. Information regarding deposition patterns
and concentrations of radioiodine in milk must be obtained promptly
for those groups responsible for taking protective action. The ac-
ceptance of and participation in protective actions by milk produc-
ers, processors, distributors, and consumers must be achieved. Such
acceptance and participation will tend to avoid unnecessary rejec-
tion of acceptable fresh milk supplies by the public.
The specific protective actions considered in selecting the Pro-
tective Action Guide are:
1. The change of cattle from pasture to stored feed.
2. The substitution of unaffected fresh milk for affected
fresh milk by alteration of processing or distributing prac-
tices.
Preference for the second action may depend upon the practica-
bility of diverting the affected milk to the production of dairy
products which may be stored for several weeks before use. This
does not influence the effectiveness of the action in averting a
dose from iodine-131. In a sufficiently severe situation in which a
more desirable alternative did not exist, it might be appropriate to
substitute unaffected milk for current use with no utilization of
the affected milk.
The projected future intake at any time after the maximum concen-
tration has been reached is approximately seven times the estimated
daily intake at that time, provided that additional iodine-131 is
not being deposited on the pasture. If the concentration of iodine-
131 in milk has passed its maximum value and is decreasing by half
every five days, the relationship between the daily rate of intake
at any time and the total projected subsequent intake is indicated
in columns 1 and 2 of Table I.
Using the projected total intake, a projected dose for children
approximately one year of age may be calculated by assuming that 30
percent of the ingested iodine is retained in a 2 gram thyroid, and
by selecting an estimated value of 1 liter as the daily consumption
of milk. The relationship between the total intake of iodine-131
and the projected dose is indicated in columns 2 and 3 of Table I.
A total intake of iodine-131 of 600 nanocuries would result in a
dose of about 10 rads to a 2 gram thyroid. In a single event, about
20 to 25 percent of the total intake may result from the use of that
portion of the milk produced before the maximum concentration was
reached. Under these conditions, and if no protective action were
taken, an estimated maximum concentration in milk of 60 to 70 nano-
curies per liter would result in a total intake of about 600 nano-
curies and a dose to the thyroid of 10 rads.
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TABLE I
INTAKE OF IODINE-13IFOR DIFFERENT RADIATION DOSES
TO A 2 GRAM THYROID FOLLOWING A SINGLE DEPOSITION
OF IODINE-131 ON PASTURAGE "
Estimated intake
of iodine- 131 in
one day at the
time of the max-
imum concentra-
tion or later
(nanocuries)
4.2
8.4
25
42
84
250
1250
2500
Projected intake
of iodine- 131
(based on column 1)
(nanocuries)
29
58
175
290
580
1750
8750
17500
Projected
thyroid dose
(based on column 2)
(rads)
.5
1
3
5
10
30
150
300
This table is illustrative and does not indicate specific intake
values at which protective actions should be initiated or discon-
tinued.
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The effectiveness of a protective action, if taken, will be highly
dependent upon the promptness with which it is initiated. If milk
from an unaffected area is to be substituted for the contaminated
milk, delay of initiation of the action by as much as 10 days after
the deposition occurs will reduce the total exposure that can be
avoided to substantially less than half of the total exposure that
would result if no action were taken. In the case of substitution
of stored feed for pasturage, the same delay would reduce even fur-
ther the benefit of the action because of the time required for the
iodine-131 in the cow and, consequently, the concentration in the
milk to decrease to negligible levels.
The exposure avoided will also depend upon the length of time the
protective action is maintained. In the case of substitution of un-
affected milk for contaminated milk, the reduction in the dose that
would result from subsequent intake would be about 90 percent if the
action were maintained for 15 to 20 days, and about 99 percent if
maintained for 30 to 40 days. In the case of substitution of stored
feed for pasturage, reductions would be less.
Summary
Following release of fresh fission products iodine-131 is the
radionuclide considered most likely to reach concentrations in foods
which warrant protective action to reduce the projected dose. The
important mode of transmission to humans is through the consumption
of fresh fluid milk.
Iodine-131 can appear in milk within a few hours after its depo-
sition on pasture. The concentration in milk may reach a maximum in
two to four days, after which the concentration diminishes by half
about every five days.
Removal of dairy cattle from contaminated pastures or the diver-
sion of contaminated milk to processed dairy products are recom-
mended as protective actions to reduce human exposure from iodine-
131.
The Federal Radiation Council has developed the concept of the
Protective Action Guide. The PAG is defined as the projected ab-
sorbed dose to individuals in the general population which warrants
protective action following a contaminating event.
A projected dose of 30 rads to the thyroid of individuals in the
general population has been recommended as the Protective Action
Guide for iodine-131. As an operational technique it is assumed
that this condition will be met effectively if the average projected
dose to a suitable sample of the population does not exceed 10 rads.
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