REPORT NO. 2
    background material
  for the development of
radiation protection
           standards
                September 1961
             Staff Report of the
   FEDERAL RADIATION COUNCIL

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REPORT NO. 2
background material
for the development of
radiation protection
standards
September 1961
Staff Report of the
FEDERAL RADIATION COUNCIL
Reprinted by the
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Washington 25, D.C

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    Federal  Radiation  Council




Secretary   of  Agriculture




Chairman,  Atomic Energy Commission




Secretary   of  Commerce




Secretary  of  Defense




Secretary  of Health, Education & Welfare  (Chairman)




Secretary  of  Labor
             Reprinted June  1963

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                              CONTENTS

                                                                   Page
 I.-Introduction	   1


 II- Thyroid Gland and Iodine-131	   8


III.- Bone  and Radium-226	  11


IV.-Bone  Marrow, Bone  and Radioisotopes of Strontium	  14
                               in

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                                 SECTION I.-INTRODUCTION
    1.1  Report No.  1  of the Federal Radiation Council provided a general philosphy or radiation
protection  for  Federal  agencies.  It introduced and defined the term  "Radiation Protection
Guide (RPG)."  It provided numerical values  for Radiation Protection Guides for the whole body
and certain organs  of radiation workers and for  the whole body of individuals  in the general
population, as well as an average population gonadal dose.   It introduced as an operational technique
where individual whole body doses are not  known, the use of a "suitable  sample" of the exposed
population in  which the guide for the average exposure of the  sample should be one-third the
RPG for individual members of the group.   It emphasized that this operational technique  should
be  modified  to meet  special situations.  In  selecting a suitable  sample,  particular care should
be taken to assure that a disproportionate fraction of the average dose is  not received  by the
most sensitive population elements.   The observations,  assumptions, and  comments set out  in
the memorandum  published  in the Federal Register  on May  18,  1960,  are equally applicable to
this   report.

    1.2  This  report is concerned with the  problem of providing guidance for Federal agencies
in  activities  designed  to  limit exposure of  members  of population groups to  radiation from
radioactive materials  deposited in the body  as a result of their occurrence in the environment.
Included are  the following:  (1) Radiation Protection Guides  for certain  organs of individuals in
the general population,  as well as averages over suitable samples of exposed groups, (2)  guidance
on  general principles  of control  applicable to  all  radionuclides  occurring  in  the environment,
(3)   some  general  principles   by   which   Federal   agencies  may   establish  appropriate
concentration values,  and (4) specific guidance in connection with exposure  of population  groups to
radium-226, iodine -131, strontium -90,  and strontium-89.

    1.3  In Report No.  1, the  RPG's for radiation workers  apply to individuals.   Similarly, the
whole body RPG for the general  population  of 0.5 rem per  year  applies to individual members
of the  general  population.  As this  report  is concerned with  radioactive materials  in the
environment, individual whole body or organ doses will usually not be known.  Therefore,  this
report provides Radiation Protection Guides not only for individuals in the general population, but
also,  using the operational  technique  referred to  in paragraph  1.1, for the average of suitable
samples of exposed population groups.  In the  development of the guidance on intake, the Radiation
Protection  Guides for averages have  been used.

    1.4   For   radionuclides not   considered  in  this  report,   Federal  agencies  should  use
concentration  values  in air, water,  or  items  of  food which are  consistent with recommended
Radiation Protection Guides and the general guidance on intake.
    1.5   In the future,  the Council  will  direct attention  to the development  of appropriate
radiation  protection  guidance  for  those radionuclides  for  which  such  consideration  appears
appropriate or necessary.  In particular, the Council will study  any radionuclides for which useful
applications of radiation or nuclear energy require release to the environment of significant
amounts of these  nuclides.  Federal agencies  are  urged to inform  the Council of such situations.

    1.6  Radiation Protection Guide has been defined in FRC  Report No.  1 (see paragraph 1.18).
For convenience,  it is repeated here.

        "Radiation Protection Guide (RPG) is  the radiation dose which should not be exceeded
        without careful consideration  of the reasons fordoing so;  every  effort should be  made
        to  encourage  the maintenance  of radiation  doses  as far below this guide as practicable."

    1.7  Report No.  1  also  introduced and defined the  term "Radioactivity Concentration Guide."
This  term  is not used  in this  report.  The  guidance in this  report is concerned with total daily


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intake from all  sources of radionuclides  rather  than concentration values  in  air,  water,  or
 individual items  of food.  Agencies, however,  may find the term "Radioactivity Concentration
Guide" useful in some  of their programs.

Preparation of the  Staff Report

    1.8  The preparation of this report followed a pattern similar to that for Report No.  1.   The
Federal Radiation Council has received written comments from, and consulted with:   (1) members
of  various  Federal agencies responsible  for the administration  of radiation  protection  programs,
(2)  governmental  and  non-governmental   scientists  in   many  related  disciplines,  and   (3)
other individuals and groups who are interested in the subject.

    1.9 In developing the recommendations  given in this report, the staff of the Council considered
the  extensive  studies   made   by  the  National  Committee   on  Radiation  Protection   and
Measurements (NCRP) and the International  Commission on Radiological Protection (ICRP)  of the
behavior and effects  of the radionuclides under discussion.   The Council staff consulted scientists
from  the  many  disciplines  involved  in the  studies  such  as physicians,   radiobiologists,
health physicists,  radiochemists,  and physicists.   Many  of the  scientists  consulted  were,  or had
been,  affiliated  with NCRP,  ICRP,  the National Academy of Sciences  (NAS),  the  American
Standards Association (ASA), and other scientific groups.  The staff also studied available  literature
and publications  of the  above groups as  well as  those  of the Medical Research Council and the
Agricultural  Research  Council  of the  United  Kingdom  and  the  United  Nations  Scientific
Com mittee on the  Effects of Atomic Radiation.  In some consultations the Council staff had the
opportunity to review current unpublished biological data.

    1.10   In order  to consider  as completely as  possible the  many factors involved in establishing
 radiation  protection  standards for the general population,  the  Council  solicited  comments  from
 interested  organizations and  individuals.  For  this  purpose, the Council, prepared and transmitted
 widely a paper  stating  major  policy  issues involved  in the  development of radiation protection
 guidance  in connection with the radionuclides  considered in  this  report.   Among these  policy
 issues is  the question  as to the appropriateness of specific  radiation protection standards  from
 the  point  of view of their  social and  economic  impact.  Questions of this sort do not  lend
themselves to exact quantitative treatment.  They are  matters of judgment on which the  best available
 information  is brought  to bear.

 Radiation  Protection Guides

    1.11   It has  been emphasized in Report  No.  1  of the  Federal Radiation  Council  that the
 establishment of radiation protection standards involves  a balancing of the benefits to be derived
 from the controlled  use of radiation and atomic energy against the  risk  of radiation  exposure.
 This  principle is  based upon  the position  adopted  by the Federal  Radiation Council that  any
radiation exposure  of the population  involves some risk;  the magnitude of which  increases with
the exposure.  As stated  in "Radiation  Protection Guidance for Federal Agencies," approved by
the President, May 13, 1960,  "There should not be any man-made  radiation  exposure withoutthe
 expectation  of benefit  resulting from   such  exposure."   In  recommending  use  of the  term,
 "Radiation Protection Guide" it was stated that "This term is defined as the radiation dose which
 should not be exceeded without  careful  consideration  of the reasons for doing so; every effort
 should be  made  to  encourage  the  maintenance of radiation doses  as  far below this  guide as
practicable."  Consistent with these principles,  no exposure to  radiation should be  permitted unless
 it  satisfies two  criteria:
         (1) The various benefits to be  expected as  a  result of the exposure,  as  evaluated by the
 appropriate responsible  group,  must outweigh the  potential hazard or risk, and
        (2)  the  reasons for accepting or permitting  a particular level  of exposure rather  than
 reducing the exposure to a  lower level must outweigh the decrease in risk to be  expected  from
 reducing  the exposure.

    1. 12   In view  of the  considerations  discussed  above,  ideally, an individual radiation protection
guide  should be   developed for each activity  or set of  circumstances  involving exposure to
radiation.  Recognizing  the impracticability  of establishing an individual guide for  each  application,
the Council, in its  Report No.  1, pointed out  the need for a compromise between this ideal and

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 the application of a single guide to widely differing sets of conditions. The following is taken
 from the Council's  recommendations approved by the  President:

          "There can be no  single  permissible  or acceptable  level of exposure without regard to
          the  reasons for  permitting the exposure  ...  It is  basic that exposure to radiation
          should result  from  a  real determination of its  necessity.

          There can  be different Radiation Protection  Guides with  different  numerical values,
          depending  upon the circumstances.   The guides  recommended  herein  are  appropriate
          for  normal peacetime  operations."

     1.13   On the basis of extensive consultation, the Council has recommended to the President
 a  set  of Radiation  Protection Guides  which  represent  a generalized   balance  between  the
 considerations discussed above.   Despite wide differences in the assignment of relative values to
 the various  factors involved, the Council believes that the  overall benefits from  useful activities
 involving exposures  to  radiation  at  levels  within  those  specified  in these  guides  will
 outweigh the risks associated with such exposures.  There is also sufficient experience  in limiting
 ing radiation  exposures to levels  similar to these to  demonstrate  the general feasibility,  with
 few  exceptions,  of  operating  at  or below the  levels  specified  in  these  guides  in normal
 peace-time operations.

    1.14  The Federal agencies, when applying these Radiation Protection Guides should recognize
 that  they represent  generalized  and not specific guidance.   Because the reasons  for  accepting
 or permitting  exposure to  radiation vary  so widely  from  one  situation to another, the  guides
 cannot represent  the most  appropriate ones  for  some situations without being inappropriately
 high or low for others.   Each agency  should determine, in any specific  application, the extent
 to which the generalized guides  apply in  the specific situation.   For example, certain  applications
 may be able to be  conducted at a guide  specifying a lower dose than  the  RPG   recommended
 by the Council.  On the  other  hand, some applications which are  not practicable under existing
 guides  and  for which  the needs are very  great  may  require  a guide  specifying a higher
 dose.   The  possibility  of certain  situations, such as  accidents, may  require the  development of
 guides  to be used when  an  agency considers such drastic  actions as  exclusion of persons from
 a  specified area, evacuation, or  condemnation  of supplies  of food.
    1.15   "Radiation Protection  Guidance for  Federal  Agencies"  published  in  the  Federal
 Register May  18,  1960, recognized that in certain instances the balance of benefit versus risk
 might necessitate  an  RPG  higher than  specified for  normal peacetime  operations.   This  was
 expressed  in  the following language:

          "The guides may be  exceeded  only after the  Federal agency having  jurisdiction  over
         the matter has  carefully considered the  reason for doing  so in light of the recommendations
         in this paper."

 Arrangements have  been made for the Council  to follow the activities of the Federal agencies
 in  this  area and  to  promote  the  necessary  coordination to achieve  an  effective Federal
 program.   These have been described in a memorandum  from the  Chairman of the Council to the
 President,  made public  on October  13,  1960.

 Control  of Environmental Radioactivity

    1.16   The  objective of the control  of population exposure from  radionuclides occurring in
the environment is  to assure that appropriate  RPG's are not exceeded.  This control is  accomplished
in  general either by  restrictions   on  the entry  of radioactive  materials  into  the  environment
or  through measures  designed  to  limit  the  intake  of such  materials   by  members of  the
population.   The most  direct means  of evaluating the  effectiveness of control measures is the
determination  of the amount  of radioactive material in the bodies of the  members  of exposed
population groups.   Although the determination of such  body burdens  may at  times  be indicated,
in  routine practice potential  exposures  will generally  be assessed on  the basis  of either  one  or
a  combination  of two general  approaches:    (1)  calculations based  upon known   amounts  of
radioactive material  released to  the  environment,  and  assumptions as  to  the fraction  of this
material reaching exposed population groups,  or (2) environmental measurements of the amount of
radioactive  material  in  various  environmental  media.

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   1.17  Both  of these general approaches  involve  the  calculation or determination  of actual or
potential  concentrations  of radioactive  material  in  air,  water,  or food.   As  stated above,
controls should be based upon an evaluation  of population exposure with  respect to the  RPG.   For
this  purpose,  the average total daily intake of radioactive materials by exposed population
groups, averaged over periods of the order  of a year,  constitutes an  appropriate  criterion.
   1.18  There  is for any radioactive material  a daily  intake which is calculated to  result,
under  specified conditions, in  whole body or organ doses equal to a Radiation Protection Guide.
The  resulting value represents  either the continuous  or the  average daily  intake which would
meet an   RPG stated in terms of an annual dose.  It is evident that the daily intake of radioactive
material  might  fluctuate  very  widely around the  average  and  still  result  in  an  annual  dose
which  would not exceed the associated  RPG.
   1.19   The control  of the intake of radioactive materials  from  the environment can involve
many  different actions.   The  character and import  of these  actions vary widely  from those
which  entail little  interference with usual  activities,  such  as monitoring  and surveillance,  to
those which involve a  major disruption,  such as condemnation of food  supplies.  Some control
actions would  require prolonged  lead  times before  becoming effective, e.g.,  major  changes  in
water  supplies.   For these reasons,  control programs developed by the agencies  should be
based  upon appropriate  actions taken  at different levels  of intake.  In order to provide guidance
to the agencies  in  developing appropriate  programs, this report describes a graded  approach
for  the  radionuclides  considered,  involving three  ranges  of transient rates  of daily intake
applicable to different degrees  or kinds of action.
   1.20  The  objective of the graded  scale of actions is  to  limit intake of radioactive materials
so that specified RPG's will not be exceeded.   Daily intakes varying within the total  extent of all
three ranges  of intake might result in  annual  doses  not exceeding a single  RPG.   However, in
instances  in which the daily intake is  fluctuating above the average which  would meet the RPG,
it may not be possible to be assured that this will be the case.   The actions  outlined below would
be appropriate, not only  when intakes  are fluctuating  so as not to exceed a given RPG, but also
in those  situations in  which valid reasons  exist  for  the  responsible agency to  permit  the
possibility of doses which would exceed the RPG.
   1.21   A suggested graded  system of actions is outlined below.   For each of the  three ranges
of transient rates of daily intake,  specific values for which  are given  in the  sections devoted
to the specific  radionuclides,  the general type  of action appropriate for the  range  is  outlined.

                                          RANGE I

       Intakes  falling into this  range would not under normal conditions be expected to result
   in  any appreciable number of individuals  in the population reaching a  large fraction of the
   RPG.  Therefore, if calculations based upon a knowledge of the  sources of release of radioactive
   materials  to  the  environment  indicate  that  intakes of the  population  are in  this  range.
   the only  action required is surveillance adequate  to  provide  reasonable confirmation of
   calculations.

                                         RANGE II

       Intakes  falling  into this  range would be  expected to  result in average  exposures  to
   population groups not exceeding the RPG.  Therefore such intakes call for  active surveillance
   and routine  control.

   Surveillance

       Surveillance must  be  adequate to provide  reasonable  assurance  that efforts being made
   to  limit  the release of radioactive  materials to  the  environment are effective.   Surveillance
   must be adequate to provide estimates of the probable variation in  average daily intake in
   time and  location.   Detection of sharply rising trend6  is  very important.   In  some cases,
   because of the complexities of the  environment,  surveillance data may have to be sufficiently
   reliable to be used as a rough check on whether  radioactive materials  in  the  environment
   are behaving  as  expected. Not only  the  radioactive  material in  question, but  also  the

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   environment  must be  studied.   Appropriate  efforts might be  made to obtain  measurements
   in man as  well as to  study physical,  chemical,  and metabolic factors  affecting  uptake.
   Appropriate consideration should be  given to other independent sources of exposure to the
   body (the same organs or different ones)  to avoid  exceeding  RPG's.
      Routine control  of useful applications of radiation and atomic  energy should he such that
   expected average  exposures of  suitable samples  of an  exposed population  group will not
   exceed the upper value of Range II.  The sample should be taken with due  regard for the most
   sensitive population elements.  Control  actions for intakes in Range II  would give primary
   emphasis to three things:   (1) assuring by  actions primarily directed at  any  trend sharply
   upward that average levels  do  not rise  above Range II,  (2) assuring by  actions primarily
   directed either  at specific causes  of the environmental exposure  levels  encountered  or at
   the environment that  a  limit is placed  on any tendencies of specific population segments to
   rise above  the RPG,  and (3) reducing the levels of exposure to segments  of the population
   furthest above  the average or tending to exceed Range  II.

                                         RANGE III

      Intakes within this range would be presumed to result in exposures exceeding the   RPG
   if  continued for a sufficient period of time.   However, transient rates of intake within this
   range could  occur without the population group  exceeding the  RPG  if the circumstances
   were such that  the annual  average intake  fell within  Range  II or  lower.   Therefore,  any
   intake within this range must be evaluated from the point of view of the  RPG and if necessary,
   appropriate  positive  control  measures  institued.

   Surveillance

      The   surveillance described for intakes in Range II  should  be  adequate to  define clearly
   with a minimum of delay the extent of the  exposure (level of intake, size of population group)
   within Range  III.  Surveillance would  need to  provide adequate data  to give prompt and
   reliable formation concerning the effectiveness of control  actions.

   Control

      Control actions  would  be designed  to reduce  the  levels to  Range  II  or  lower and to
   provide  stability at  lower levels.  These  actions can be  directed toward  further restriction of
   the entry of radioactive materials  into the environment or the control of radioactive  materials
   after entry  into the  environment  in  order  to  limit  intake  by  humans.   Sharply   rising
   trend  Range  III would suggest strong and prompt action.

   1.22 The remaining  sections of this  report provide recommended  values  for the three
ranges of transient  rates  of  daily  intake for  iodine-   131,   radium-226,  strontium-89  and
strontium-90.  The  guidance is given in terms of transient rates of intake of radioactive material
in micromicrocuries per  day.  The upper limit of Range II  is  based on an annual RPG (or lower,
in the  case  of  radioactive  strontium) considered  as an acceptable  risk for  a lifetime.
However, it is necessary  to use averages  over periods much shorter than  a lifetime for both
radiation   dose rates  and  rates  of intake for  administrative and  regulatory purposes.   It  is
recommended  that such  periods should be of the order of one year.  It is to  be noted that values in
the remaining  sections are much smaller  than  any single  intake  from which an  individual might
be expected to sustain injury.
   1.23  The  Federal  Radiation Council has developed the guidance presented here to indicate
a general   philosophy  relating the types of actions appropriate  for the different ranges of intake.
It is  the responsibility of the  individual Federal  agency to determine the specific levels  within
this guidance which will actually  be used  for  the various  control efforts.   In some  cases,  action
which have been described in  one range  may  appropriately be taken in another.  The trend of
environmental  levels may  be at least as important as the levels  themselves.  For  example:

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         (1)   Environmental measurements  indicating intake  levels in Range  I but  rising sharply
might suggest actions indicated here for Range II  or Range  III.

         (2)   Measurements  indicating  levels in Range III but known to  be falling and, by action
already taken, expected to be reduced  further in the future  might suggest no actions  beyond
those indicated here for Range  I.

Derivation of Concentration  Values

    1.24   Although concentration values should be  related to appropriate  RPG's, in practice  they
areusually derived from  intake guides.   Thus, the principles which were discussed in connection
with  the  guidance on daily intake are equally applicable  in  the  case  of concentration values.
Specifically,  determination of a concentration value will  be based upon  (1) the choice of a
specific  RPG and range  of intake appropriate  for  the circumstances, and (2) allowance  for the
variability  of intake possible without a resulting exposure exceeding the  specified  RPG.
    1.25   The determination of concentration values involves additional factors,  some of which
are subject to wide variation.   It  is theoretically  possible to  calculate a single  concentration
value for ingestion  to be the average  concentration of  a radioactive material present uniformly
in all sources of ingestion which would meet a given intake  value and  its associated RPG.  Such
a  concentration value however,  would  rarely be applicable  in practice.

    1.26  From the point of view  of the control of general environmental contamination, radioactive
materials  may  enter  the human  body  from  any  one, or  a combination,  of  the  three
environmental media:   air, water, and food.   Before an appropriate concentration value can be
developed  for an environmental medium  in  a specific situation,  the  relative  contribution to
total intake from the  other media must be  determined.   In some situations this determination may
result  in simplification of the problem  of providing  a  concentration value.   For example,  it
might  be  observed that almost all  of the  intake results  from ingestion  of contaminated  water.
In this  case,  the determination of the  concentration value  would depend solely upon factors
associated  with the  determination of water  concentrations which will  deliver a  critical  organ
dose equal to  the RRG.

    1.27   In  many instances, however,  it is found that different environmental media  contribute
to the total intake.  Combinations of intake  from water and food or air  and food may occur, and
intake of the nuclides considered  in this  report may involve such combinations.  Consequently,
concentration values  applying  to the  different sources   of  intake  must  take into  account the
relative contribution of each source to  total intake.   Even in those situations where food is the
only source of intake  of radioactive material, widely varying concentration values  applying to
different items in the  diet may be  appropriate.   For example,  in the case of  intakes in Range
III the necessity may  arise  for  removal  of a  particular  radionuclide  from  certain  major
contributors in the diet, or even elimination of certain dietary items containing high  concentrations
of the nuclide.   The  following  are some  of the considerations  which  may be  involved in the
determination of specific levels at which action such as the condemnation of certain  food supplies
would take place:

         (1)   Relative proportion of the total diet by weight  represented by the item in question.

         (2)   The importance of the particular item in nutrition and the  availability of substitutes
having  the  same  nutritional  properties, or  perhaps  stockpiles  of uncontaminated  food.

         (3)   The  availability of other possible control methods  such as  the  removal  of the
radioactive material  from  the  particular dietary item without affecting its  quality.

         (4)   The half-life of the radioactive material.
         (5)   Other internal or external sources of  radiation exposure  to  the  same  organ.

         (6)   Relative contribution  of other dietary  items to  the total daily intake  of the  nuclide.
         (7)   Physical, chemical, and  other factors affecting  the relationship between intake and
uptake of the nuclide.

         (8)  The time  and  effort required  to  effect   corrective   action.

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In this connection, it is important to emphasize a point made  in paragraph 1.18 in connection
with guidance on intake.   The agencies should bear in mind in  establishing  concentration Values
that  it is possible to have  wide fluctuations  in  daily  intake  which might still result  in  an
annual average dose  within the  RPG. It  can be readily seen that,  since  fluctuations  in
concentration guides  can occur within a given intake value, even wider fluctuations can occur in
concentrations  in  various  foods  and still result in an annual average dose that  does not exceed
the associated  RPG.  In  any specific  instance the greater the variation  in concentrations, the
more difficult  it will be  to  estimate  average intakes.

   1.28  Because  of the  wide difference possible in concentration  values  applying to different
environmental  media  and  depending   on specified  circumstances,  the  Federal Radiation Council
has  not  made  any  specific recommendations  on  such  values.   The responsible  Federal
agencies should develop   specific concentration values  to  apply to appropriate  control  actions
as part of their operating criteria.  The  Federal Radiation  Council will follow  developments in
this  area  and  will promote  the necessary coordination to  achieve  an  effective Federal program.

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                    SECTION II.- THE THYROID GLAND AND IODINE-131


Introduction

   2.1  This section is concerned with the development of an RPG for the thyroid gland and
guidance in connection with exposure  of the general population to radioactive iodine.   Currently,
the major concern  is environmental  contamination resulting from fallout from  the explosion
of nuclear devices  and  the  release  of  radioiodine  during the use  and processing of  fuel for
reactors.  Fission products  so formed may contain  iodine-131, -132, -133, -134, and -135.  The
dose  rate from  the  shorter-lived  radionuclides  (iodine-132,  -133,  - 134, and  -135 with half-lives
ranging  from  approximately 53  minutes  to  21  hours)  will  decrease  rapidly with  time  in
comparison with iodine-131 (half-life approximately 8 days).   Consequently, guidance on intake of
iodine-131 only is  considered in  this section.  However,  when the  shorter-lived  iodine nuclides
are present and contribute  significantly to the radiation dose received,  they should be  taken
into  account in accordance with the principles for summation  of  dose.

   2.2   Like the naturally occurring stable  isotope of iodine,  iodine-131 when  ingested  or
inhaled concentrates  in the thyroid gland.   Thus the thyroid gland receives  a far  larger radiation
dose  from iodine-131 than any other organ in the  body.  Radiation protection guidance to be
used  in connection  with iodine-131, therefore,  involves the determination  of  RPG's  for  the
thyroid gland.

RPG for the Thyroid Gland

   2.3  Report  No.   1 specifies  a Radiation Protection Guide for the thyroid gland of radiation
workers of 30 rem per year.  It specifies  an annual whole body dose to  individuals in the general
population of 0.5  rem.   The whole  body guide  is  a factor  of  10 below  that  specified for
radiation workers.   If one were  to  assume  that the  thyroid gland of individuals  in the  general
population is no more sensitive when compared with the whole body than is the case in radiation
workers, it might, from the point of view of the risk factor, be reasonable to use a value of 3
rem  per year as an   RPG for the thyroid of individuals in the general population.
   2.4  This, however, may not be the case.   Evidence is summarized below  which has led the
Council to the conclusion that in the development of RPG's  for the thyroid gland it is necessary
to take the  position that a  child's  thyroid  gland, relative  to  other organs  of the  child, is
more sensitive to the carcinogenic effect of radiation  than the  adult thyroid gland compared to
other organs of the adult.   In the development of guidance on intake there is an additional factor
or which must  be considered, i.e., the ratio between size  of thyroid and intake of radioiodine
in children is different from the ratio in adults.

   2.5  In Report No. 1  (paragraph 2.19) it is noted that the child's thyroid is  more sensitive to
the carcinogenic effects of radiation than the adult thyroid.  This  conclusion  is  based upon
several studies  in recent years of the occurrence  of thyroid carcinoma in  children who had
previously received  therapeutic  x-irradiation in the neck  region for enlarged thymus or  for
other benign head and neck conditions.  The incidence of thyroid carcinoma  in these  children
was  significantly higher than in  control  groups who had  not been previously irradiated.

   2.6  In these studies cancer of the thyroid  was observed in children after exposures as low
as approximately  150 rem.   Similar effects have been observed in adults only at much higher
dosages.  Although  these data do not  provide  a  quantitative  relationship, they do  indicate that
the child's thyroid is more sensitive to the carcinogenic effects of radiation  than  is that of the
adult.

   2.7  The  RPG for the thyroid gland  of radiation  workers of 30 rem per  year is twice the
dose  specified for other organs.   This difference is based  on two  factors:  (1) the evidence that
the thyroid gland is  adults is a  particularly radioresistant organ,  and (2) the needs for exposure

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of radiation  workers to radioactive iodine in useful applications of radiation  and atomic  energy.
If it were not for these considerations, no individual treatment would have been given the thyroid
gland  of radiation workers  and  it would have fallen  into the  category of other organs with
ana  RPG of 15 rem per year.

   2.8  From the  point  of view of the biological risk, therefore, the  RPG  for the thyroid  of
individuals in the general population, including children,  should be less than  1/10 of 30 rem
per  year.  On the  other hand,  it is  logical to assume that  the risk associated  with a given
radiation dose to the child's  thyroid gland must be less  than that associated with the same dose
to his whole  body.   Thus the RPG for the thyroid of individuals  in a  population group could  be
higher than the 0.5 rem per year whole  body dose without resulting  in  a  greater biological
risk.

   2.9  The Council has reviewed  data and studied  atomic energy operations involving exposure
of the  thyroid gland of members of the general population.  As noted  in paragraph  2.1,
such operations  usually involve   iodine-131.  It finds  that in general these operations  can be
conducted without, undue difficulty in such a manner that the dose to the thyroid of individuals in
the general population will not exceed  1.5 rem per year.  It  has  been stated  above that,  since
in the situation of environmental  contamination individual  doses  are  not  usually known, this
report will specify both individual doses  and average  doses  to population groups.  Therefore,
the Council  recommends RPG's for the  thyroid gland of 1.5  rem per year for individuals and
0.5 rem per year to be applied to the average  of  suitable  samples of an exposed  group in the
general population as  representing a reasonable balance  between  biological  risk  and  benefit
to be derived from useful applications of radiation and atomic energy.  If specific applications
should be  contemplated which cannot be conducted without exceeding  the dose specified in the
RPG,  an individual assessment of benefit and risk must be made  by the responsible  agency  in
accordance with  the  principles  previously outlined  by the Council.

Guidance on Intake of Iodine-131

   2.10  As  a step in the development of guidance on intake  of iodine-131 it is necessary to
determine  the  average  daily intake which would meet the  RPG for averages in the general
population.  Among the factors to  be considered are:  (1) the weight of the thyroid gland, (2) the
percent of the  iodine  intake  which reaches the  gland, and (3) the  average  retention time.

   2.11  There is wide variation from one individual to another  in the percent of an  ingested
or inhaled quantity of iodine-131 which  appears in the  thyroid gland.   This percentage  uptake
is dependent upon such factors as the amount of stable iodine in  the  diet and the physiological
state  of the  thyroid gland.  In point  of  fact,  certain   pathological conditions in humans  are
manifest by an increase  or decrease in the ability of the  thyroid gland to concentrate iodine. A
review of the data in the United  States  indicates that the  normally functioning thyroid gland
concentrates  at 24 hours on the average  approximately  30% of the initial quantity of iodine-131
taken into the body.   The data also indicate that, while,  as  stated above, there is  wide  variation
from individual  to individual, there  is  no  significant difference  in the  average  between
children and  adults.

   2.12  There is  some  evidence  that suggests that iodine is metabolized  more rapidly in the
child than in  the adult.  This  suggests the  possibility of a somewhat shorter biological half-life.
However,  adequate information concerning  the effective  half-life of iodine- 131 in younger
children is not presently available.   It  is  assumed,  therefore  that  an  effective half-life  of
7.6 days is applicable for all age groups.

   2.13  The average mass  of the thyroid gland in adults is  generally taken  to be 20 grams.
The mass  of the gland  in the child is, of course,  less and depends upon the  specific age.   Since,
as discussed  above under the consideration of the  RPG,  the  child is  taken as the limiting case,
the weight of the child's thyroid  is considered as the  limiting factor in  the  determination of
guidance on  intake.  In calculating the average  daily intake which would  meet the RPG, the
mass  of the  thyroid  gland  is  taken  as  2  grams.  The  resulting  guidance   on  intake should,
theoretically, be applied only to children and  is subject to adjustment upward when applied only
to adults.  In many  practical situations this  adjustment will not  be feasible.   However, when
agencies  develop  appropriate concentration  values  to  refer to  specific modes  of  intake

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(as between inhalation and ingestion) or to different dietary  elements, this consideration should
be kept in mind.
   2.14  Using the known factors and the assumptions enumerated above,  it can be  calculated
that an average  daily intake  of 80 micromicrocuries  of iodine-131 per day would  meet the RPG
for the thyroid for averages  of suitable samples of an exposed population  group of 0.5  rem per
year.   As stated in Section  I, it  is appropriate to specify  three ranges of transient rates  of
daily  intake  in  order to provide guidance  for the  Federal agencies in  the  establishment of
operating criteria.   For this  purpose, the value of 80 micromicrocuries per day has been rounded
off to  100 micromicrocuries  per day as being more  in keeping with  the precision of the data.
Therefore, the following guidance on intake of iodine-131  to be applied to suitable samples of
an exposed population group is recommended:

                   RANGE  I         0 to    10  micromicrocuries per  day
                   RANGE  II        10 to   100  micromicrocuries per day

                   RANGE  III      100 to  1,000  micromicrocuries per  day
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                                 SECTION III.-BONE AND RADIUM-226
Introduction
   3.1  Human  experience with  comparatively large quantities of radium in the skeleton  was
discussed in Report No.  1  (particularly pages  13-15)  and the general practice of establishing
radiation protection  guides for occupational  exposure  to various radionuclides  in  the skeleton
by relating  them to radium-226 was  endorsed.  For this purpose,  0.1  microgram  of radium-226
in the skeleton was adopted as a Radiation Protection Guide for radiation workers.  This value
has  been in  general  use  since  1941. The  discussion in  this  section  is  concerned  with  the
development of an appropriate  Radiation Protection  Guide for bone and of corresponding guidance
on daily intake  for control of exposures of the general  population to radium-226.

   3.2 The  critical organ for radium  in the body is the skeleton.  It is  assumed in this  section
that,  except for radiation  from natural sources other  than radium  and from medical x-rays,  the
total  radiation dose  to  the  skeleton is from  radium-226 and its radioactive  decay  products.   If
other sources of  radiation contribute significantly to the  radiation  dose to the  skeleton,  it is
expected that they will be taken into account.

Considerations in the Development  of RPG's

   3.3  In the consideration of the  risk side  of the risk-benefit balance in the development of
RPG's, Report No.  1 indicated several approaches to aid in the evaluation of the risk.  Comparisons
with  occupational  exposure  guides  and with exposures from natural  background were discussed.
Although  neither  provides  a  quantitative   basis for  the  determination  of population
RPG's, each is  useful.  This is  particularly true in the case of radium-226 because some data
are available on both  occupational  and whole  population environmental  exposure.

   3.4  The  Radiation  Protection Guide  recommended  by the  Council  for the whole  body  of
individuals in the general population is a factor of 10 below the whole body guide for radiation
workers.   There are certain  considerations,  however,  which  indicate that  the  application of the
same factor to the  RPG for occupational  exposure  to  radium-226 to obtain population RPG's
may not  provide the same degree of protection as  in the case of the whole body.  Some of
these considerations are the following:

       (1)  The skeletal content required to  give a particular radiation dose to  the  bone of a
child is less than  for the adult.  Fortunately  (from the point of view of simplicity  of treatment
of the problem), available  data suggest that in an environment in which the average concentration
of radium in the  total diet,  including water,  is  constant,  concentrations  of radium-226  in
the skeletons  of humans  who have lived their entire lives  in the environment are found to be
relatively independent of age.

       (2)  The distribution of radium-226  in  the skeleton of an individual  who  has lived his
entire life in an environment constant with respect  to  small quantities of radium in his diet will
be much more uniform than  that of radium deposited in the skeleton as the result of occupational
exposure.   How  the   degree  of  hazard  from radium  in the  skeleton  might   depend  upon
non-uniformity ofdistributionisnotknown.

       (3)  The radiation  dose to the bone from radium deposited in the  skeleton under  constant
environmental  conditions  is relatively constant throughout life.   On the other hand, the dose
resulting from deposition under controlled occupational exposure increases with length of exposure.
Constant  environmental  exposure,   therefore,  results  in   a larger  lifetime  dose  per unit
quantity  of radium-226  in the skeleton than occupational exposure in which the specified quantity
is assumed  to be  reached only near  the end  of  life.  Furthermore because of the  long latent
periods  characteristic  of  carcinogenesis  at   low  dose  levels,  it  appears   reasonable  to

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assume that  the  earlier  in  life the radiation dose  from  radium is received the more likely the
individual will  live until any carcinogenic effect can become manifest.

   3.5  Turning  to the second approach,  that of comparing the radiation doses  to the skeleton
from  radium-226 with radiation doses  normally received from all  natural sources of  radiation,
it  is  immediately apparent  that bases for  comparisons are,  at best, uncertain.   In  physical
units  of radiation dose (e.g.,  rads)  the  dose to the skeleton  from all  natural sources of radiation
averages between 0.1 and 0.15 rads per  year.  The  quantities of radium-226  in the adult skeleton
which,  with its  radioactive   decay products,  are  required  to give  corresponding  physical
doses range  from about 0.003 to 0.005 micrograms.  There  is insufficient  information  on the
relative biological effectiveness of the  radiation from  radium to attempt a  realistic conversion
of this dose in  rads  to the  skeleton  from radium and  its decay  products into rems.

   3.6  Because  of the uncertainties involved in comparing radiation from radium with total radiation
to the  skeleton  from  natural  sources,  it  is  useful  to  consider  the natural  occurrence  of
radium in the  skeleton.  In  most  areas of the  United  States, the  radium content  of the adult
human skeleton is found to range from  about 0.0001 microgram of radium-226 to some two or three
times this amount.  In such  areas, the  radium content of drinking water is generally so low that
the skeletal content is believed to be almost entirely due to the occurrence  of sufficient radium-226
in food to  result in a  daily intake  of from  1  to 2 micrograms.   In  some  areas,  however,
concentrations  of radium-226 in drinking water are sufficiently high to  result in  much larger daily
intakes and  correspondingly higher amounts in the skeleton.  There are communities in  which
unusually high radium concentrations in  supplies  of drinking water result  in adult skeletal  levels
which range upward to amounts  of the order of  0.001  microgram.   A  program is  underway to
determine whether any biological effects  of such amounts of radium can be detected by epidemiological
studies  with methods currently  available.   However,   it  is  expected  that a  number  of
years will be required to  reach any useful conclusions.

   3.7  These approaches give two reference  points for use  in  comparison of biological  risk with
reasons for acceptance of risk.  In the case of  radium,  reasons for acceptance  of  risk involve
consideration of the  difficulty  of  meeting possible RPG's  and the  impact of this  difficulty  on
industry and  the  community.   Before this comparison  can be  made it is necessary to consider the
relationship between  environmental levels  and body content of radium since this relationship
vitally affects the difficulty of meeting  any  RPG.

   3.8   The data which  are most relevant  to  the  determination  of the relationship between
environmental levels and body content are the observations of the relationships between concentrations
of radium-226  in community water  supplies  and corresponding  quantities  in the  skeleton
of persons using  the  water.   Estimates of average concentrations  in normal  United States  diets
and   corresponding  average  skeletal  contents,  while  less  firmly  supported,  are  reasonably
consistent with  these observations.   These data  indicate that on the average  the concentration of
radium-226 in  the skeleton of individuals of any age  does not exceed  a value corresponding to
a total quantity in the adult skeleton of about fifty times the  daily intake.

   3.9   The Council has  considered  operations  involving  the  release  of  radium-226  to  the
environment.  These can be conducted, in the opinion of the Council, without undue difficulty in such
a manner that  average daily intake of radium-226  in an exposed population group will not exceed
20 micromicrograms.  The  Council has  also reviewed  available data on radium-226 concentrations
in public water  supplies  in the  United States.   The  overwhelming  majority  of  the  population
consumes water  from supplies  corresponding to  daily intakes of radium-226  well below  this
level.  In those  situations where  this may  not be  the case,  the extremely  small  risk associated
with  intakes  above this  level  should be considered by the appropriate authorities in light of
difficulties which may be  associated with any  modifications in the water  supply.

   3.10  In view of the above  considerations, the Council recommends as  an alternate RPG for
bone   for  individuals  in  the  general  population  a skeletal concentration of radium-226
corresponding to 0.003 microgram in the  adult skeleton.  The RPG to be applied to  the average of
suitable  samples  of an exposed  population  group  is  a  skeletal  concentration  of  radium-226
corresponding to 0.001 microgram  in the adult skelton.  These values are considered  by the
Council to represent an appropriate balance between  biological risk and reasons for acceptance
of risk.


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Guidance on Intake

   3.11 The relationship between environmental levels and body content referred to in paragraph
3,8 indicates  that an  average daily  intake  of 20 micromicrograms of  radium-226 corresponds
to the  RPG  for  suitable  samples  of exposed  population  groups.   Therefore,  the  Council
recommends the  following  guidance on  transient rates  of  daily intake  of radium-226 to be
applied to the  average of suitable  samples  of an exposed population group:

                    RANGE  I         0 to    2 micromicrograms per  day

                    RANGE  II        2 to   20 micromicrograms per  day
                    RANGE III       20 to   200 micromicrograms per  day

   It is important to emphasize that the risk associated with  this intake guidance is, in the opinion
of the Council, much lower than has generally been considered.  The skeletal content associated
with  a daily  intake  of 20  micromicrograms  is  about  an  order  of magnitude  lower  than
that which would  be implied by extrapolation from  current occupational  standards for radium.
The  Council  considers, however, that the data from the environmental studies,  though limited,
represent a more  valid basis  for derivation of the relationship between continuous exposure
and body content.
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       SECTION IV.-BONE MARROW, BONE AND RADIOISOTOPES OF STRONTIUM


Introduction

   4.1  In this section, RPG's for bone marrow  and bone  and guidance for the protection of
individuals in the general population against  excessive exposure to radioisotopes  of strontium are
developed.  The  chemical and  physical characteristics are such that, for this purpose,  our  principal
interest is  in the irradiation of bone and bone  marrow  as  the result  of deposition of strontium-90
and   strontium-89  in  the  skeleton.   Because  such  deposition results   from  the  occurrence
of the radioisotopes in ingested food and water and in inhaled air, protection is achieved by limiting
average concentrations  in  food,  water,  and air used  by humans.   Thus,  while the  hazard  to
the individual  results from radiation emitted over  long periods of time by  material  actually in
his skeleton, for purposes of control it is  necessary  to specify  guidance on intake of the isotopes
which will not result in excessive irradiation of body tissues.  In applying such  guidance to
actual environmental situations,  it is necessary to convert intake values to concentration  values
applicable to specific items in the total diet  (both  food and water)  and in inhaled air according
to the general principles in Section I.

Derivation of RPG's for Bone Marrow and Bone

   4.2  Report No.  1  recommended an RPG  for the  whole body of individuals in the general
population of 0.5 rem per year as representing  an appropriate balance between the requirements
of health  protection  and of the  beneficial  uses of  radiation and atomic energy.   Basic to the
considerations involved in a guide for whole body dose were the factors  associated with exposure
of bone marrow.   Thus RPG's for the  bone marrow of 0.5 rem per year for individuals  in the
general population and  0.17 rem  per year as an average to be applied to suitable samples of an
exposed population  group  are  considered by the  Council  to  represent  a similarly appropriate
balance of benefit and  risk.

   4.3  Experience  indicates that bone is relatively insensitive to X  and gamma radiation when
compared with bone marrow.   Groups  exposed  to  X  and gamma radiation in which a higher than
normal incidence of leukemia  has been observed have  not shown corresponding increases  in
bone  tumors.   Although these  data do not provide a quantitative  basis  for relating the  sensitivity
of bone and bone marrow they do indicate  that  from the point of view of the risk it is  reasonable
to permit  a larger dose to bone than to bone marrow.

   4.4  In the  case  of  strontium-90, the  dose rate  to bone from  a given skeletal content is three
times  the average dose rate to bone marrow.   Other beta emitters  of similar distribution  in
bone  and  comparable energy  would  yield  similar  factors.   The Council considers that  Radiation
Protection Guides for the bone of 1.5  rem per  year for individuals in the general population and
0.5 rem per year as an average to be  applied to suitable samples  of an exposed population
group  represent an  appropriate balance between the  requirements  of health protection  and of
the beneficial  uses  of radiation and  atomic energy.

The Development of Guidance  on Intake  of Strontium-89 and Strontium-90

   4.5  The considerations  involved  in the development of guidance  on  intake  of strontium-89
and  strontium-90 are summarized  in the following paragraphs.   The guidance  is  applicable only
under the  conditions specified  in their derivation, i.e., continous exposure to  radioactive  strontium
in food,  water, and  air throughout the lifetimes of  the individuals  involved and under  normal
peacetime  operations.   The guidance  is  based on  the  assumption   that the  exposure  source
it covers is the only source of exposure  of the skeleton to radiation other than natural background
and  medical  and   dental  exposures.   Where  actual  exposure   involves  both  strontium-89
and  strontium-90, or where the  skeleton is also exposed to significant amounts  of  radiation

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from  other  sources, such  as barium-140 or abnormal  quantities  of radium-226, it is  expected
that these will be taken into account.  Likewise, where there is significant intake  through  both
ingestion  and  inhalation,  it  is expected  that  the  total deposition  in the skeleton will be
considered.

Biological  Effects

   4.6  No  effects  in  humans attributable to the ingestion  or inhalation of radioactive strontium
have  been  observed  from  the  levels  of  radioactive   strontium which  have occurred  in the
environment nor does  it appear from our present knowledge that it would be possible to observe
any.  Consequently, evaluation of the hazard to humans  is primarily dependent upon extrapolation
and  dose  interpolation  from  the  effects  on  experimental  animals  exposed  to   far greater
quantities of  radioactive strontium, or from the effects of other sources of radiation on humans.

   4.7  Experimental  animals given large doses of radioactive  strontium  have developed
osteogenic sarcomas, and  it might be expected that this would occur in a human  group under
similar    circumstances

   4.8  Some small laboratory animals have developed leukemia following large  injected  doses
of radioactive strontium, presumably from irradiation of the  bone marrow, ,although the  causative
relationship  is  not clear.   Extrapolating  animal experience  to  humans is  very uncertain.
Data  obtained as a result of exposure of humans to  external radiation  indicate that at levels of
exposure  much higher than those  under consideration  here,  the bone marrow is  significantly
more  radiosensitive than the bone.

Metabolic   Factors

   4.9  Ingested strontium  is concentrated  in the  mineral  skeleton, as is calcium and  several
other  alkaline  earth elements.  Under equilibrium conditions,  essentially  all   strontium in the
body  is  in  the  skeleton.  The mineral skeleton appears during intra-uterine  life,   and  increase
in mass  until about age twenty years.   Another process of bone metabolism is the continuous
replacement of the  mineral  portion at  a  low  rate on a  microscopic  scale throughout life.   Thus,
there  is a  continuous  exchange of mineral elements between  the environment  and  the blood,  and
a continuous  exchange between the blood and the skeleton.

   4.10   Strontium is similar but  not identical biochemically to  calcium.   Therefore, although
some ingested  strontium  is deposited in  bone  in  a  manner  similar  to calcium, there are
metabolic mechanisms which perform some  discrimination between the two elements,  so that their
relative concentration  when deposited  in bone  is different from  their  relative concentration  in
the diet.  The similarities in metabolic pathways of strontium  and calcium make  it meaningful
and convenient to  use ratios of the two  elements in  evaluating the deposition of radioactive
strontium.

   4.11  Newly formed bone has about the same  strontium to calcium ratio as is  in the blood
circulating  at the time  of formation.  There is  some  discrimination against  strontium  between
ingested material  and blood,  which  results primarily  from  preferential  renal excretion of
strontium,  but which  may also be  influenced  by  preferential absorption of calcium through the
gut.

   4.12  Data on humans  and laboratory animals indicate rather well that there  is a discrimination
factor against strontium  of about four  in the  strontium  to  calcium  ratio  between  diet  and
bone.  Although some  experimental evidence  suggests that there may be periods  during infancy
and  adolescence in which the discrimination factor is  less than  four, observations of the  ratio
of natural  strontium to calcium in the human  skeleton as a function of age indicate no practical
difference,  The strontium to  calcium  ratio  of the  embryo and  fetus  is affected not only by  the
maternal discrimination factor of four between diet and blood,  but by a placental discrimination
factor of  about two.  The  resultant  discrimination  between  maternal  diet and  fetal   bone
would therefore be about eight under conditions of equilibrium.  Presently, the  observed occurrence
of strontium-90 in fetal bone  is  somewhat less  than predicated  for conditions of  equilibrium,
probably  because  of  a calcium   contribution from  the maternal  skeleton,  which  is  not now
in equilibrium with the strontium-90 in  the diet.

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   4.13  Under  constant intake  conditions throughout life, and with the  exception of the infant,
whose skeletal level  of strontium would be in transition from the prenatal to the postnatal
equilibrium values, evidence  indicates that  the distribution  of strontium in  bone mineral  would
be reasonably uniform both throughout the bone and throughout life.   For  example, measurements
of the  ratio of natural  strontium  to calcium  in  over  200  skeletons   of persons  ranging  in
age  from stillbirths to  eighty years,  reported by  the  Medical Research  Council  of the  United
Kingdom,  November 14, 1960, indicate  that the  mean  ratio of strontium to  calcium in humans
does  not increase more than  about  25 percent after  the age of two years.

Radiation  Dose  Factors

   4.14  Strontium-90  in the  skeleton exists in secular  equilibrium with its daughter,  yttrium-90.
These nuclides emit beta radiation  with a  maximum  range  of about  six millimeters in bone
and  one centimeter in  soft tissue.   For a non-uniform  distribution  of the nuclides in bone, they
would deliver a substantially more  uniform radiation  dose  than a similarly distributed alpha
emitting material.  When the macroscopic  distribution of  strontium-90 in bone  is  reasonably
even, the  radiation dose can be considered as essentially uniform.

   4.15  Because  of  the greater range of beta  radiation, bone marrow would receive  a greater
portion of the  radiation dose from  strontium-90 than from  an alpha-emitting material  in  bone.
The  dose to a  small bit of bone marrow  surrounded  by a large  mass of  dense bone would
approach the dose to  the bone.  However, the average bone  marrow dose from strontium-90 would
be substantially less  than the bone  dose.   Similar  considerations  apply to  strontium-89.

Application of RPG's to Strontium -90

   4.16  The Council has considered the basis  for  evaluation of the  biological  risk associated
with  exposure  of population groups  to  strontium-90  under  the conditions stated in paragraph 4.5.
This  consideration included comparison  with the RPG  for  bone  marrow and bone recommended
in paragraphs 4.2 and 4.4 and comparison  with  the  alternate guide for bone in Section III.

   4.17  For those radionuclides  for which the skeleton is  considered  to be the critical organ,
occupational standards  commonly have  been derived by estimating body burdens considered to
be no more hazardous  than  0.1 microgram of radium.  Two of the  reasons  for adopting  this
approach were:   (1)  experience with radiation injury to the  human skeleton is largely limited to
cases in which  relatively large quantities of radium  have been introduced  into  the skeletons of
adults,  whether as a  result  of occupational  exposure  or  for medical  reasons;  and  (2)  it is
considered that,  in general, the distribution of radionuclides deposited   in the  skeleton under
occupational conditions of exposure  may be  of such  a nature as to  make direct comparison  with
X and gamma radiation uncertain.

   4.18  In  addition  to the considerations which normally arise  in making comparisons between
exposures of population groups and  exposures  for occupational reasons, the  manner  in which
occupational standards for strontium-90 have been derived appears to make them  less appropriate
as a basis  for  comparison than the  RPG's  for bone marrow and bone  given  in  paragraphs
4.2  and 4.4. Basically,  derivation  of occupational  standards for strontium-90 has  involved
experimental determination of  relative   quantities of  strontium-89 and radium-226  in  small
laboratory animals required to produce biological  damage considered to be comparable.  It  was
then assumed (for lack of more certain  information) that,  except  for an adjustment to allow for
the higher  retention of  radon  in the  human skeleton,  the same ratio would hold  for man.   The
corresponding ratio for strontium-90 and radium-226 was  estimated to  be twice  as  large as
that  for  strontium-89 and radium-226 because  the  combined energy  emitted  by strontium-90
and  yttrium-90 per  disintegration  of  strontium-90  is approximately  twice that  emitted   per
disintegration by strontium-89.

   4.19  This estimate of the relative quantities of  strontium-90 and radium-226 required to
produce  radiation hazards  or  effects considered to be  equivalent  for purposes  of  radiation
protection to those of radium was found to depend upon the conditions of the experiment, particularly
dose  rate,  and  upon   the  effect  chosen  as  a measure  of  injury.   The  ratios  chosen  as
representing  the  relative  hazards  of strontium with respect to radium  were those corresponding
to massive  acute doses. The experimental  observations  indicated  that  for  chronic  exposure  at

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lower  dose rates  the  relative  hazards of radiostrontium  are  smaller by  factors which  range
downward to  less than one-tenth and perhaps  to  one-hundreth of those observed for acute doses.

   4.20  Studies  of individual and  relative  radiotoxicities of radium-226 and  strontium-90 using
large  laboratory  animals  are  nowprogress.  It  is   expected  that  such  studies   will no  only
provide better  comparisons  of the  relative hazards  of strontium  and  radium  to experimental
animals under conditions more nearly approaching  those  of interest,  but will provide better
independent data on the nature and degree of hazard  from radioactive  strontium.   In  addition,
the use   of  larger  animals and  several species  of animals  is  expected  to reduce  the
uncertainties inherent in extrapolation to man.  However, the nature of such investigations is such
that periods of time comparable to the normal lifetimes  of the animals are  required to obtain
a sufficient amount of useful information on  which  to base sound  conclusions.
   4.21   It appears that comparisons  with the  bone marrow and bone RPG's  given inparagraphs
4.2 and 4.4 can  be made  with less uncertainty and are more meaningful than comparisons
with occupational standards for strontium-90 which have been, in turn, based upon comparisons
with radium-226.   It is  assumed  that the total intake of  strontium-90 by individuals is such  that
the average ratio of strontium-90 to calcium  in the blood is  constant  throughout life.   This is
considered to be approximately true if the ratio of strontium-90 to calcium in the  total diet
(that is, in the total amount of food and water ingested by the individual)  remains constant.  In
line with  the principles in  Report  No.l of  control of exposure  of members  of the public to
radiation,  ratios may be  averaged  over periods  of time of the  order of one year.

   4.22   Under the conditions assumed,  experience with  stable strontium in the normal diet as
well as  such  data  on  the uptake of  radioactive  strontium  from  the diet  indicate that the
distribution of strontium-90  in the skeleton will be reasonably uniform.   The ranges of the beta rays
from  strontium-90  and  its   radioactive decay product, yttrium-90,  are sufficiently  large  that
the microscopic distribution of radiation  dose to the  bone (except for losses  of radiation  near
the surface) will be even  more uniform.  Under these conditions, the  RBE  (relative biological
effectiveness)  of the beta radiation does not differ  markedly from that of X and gamma radiation
of quantum energy  in the  range between two hundred  and  several  hundred  Kev.

  4.23  It has been estimated that  the average dose to bone marrow from strontium-90 and
yttrium-90 in  a skeleton of average  density  is about one-third of the dose to bone.  Data on
experimental  animals indicate that the protection of a small portion of bone marrow  from  a high
dose of radiation may markedly lower the incidence of leukemia.  This suggests that in the
case of non-uniformity of radiation  dose to  the bone  marrow, the average dose is  a more
meaningful index of hazard than the  maximum local dose  and  that,  for a  given average, a
non-uniform distribution of dose may be less hazardous than a  uniform distribution.  Thus, the
RPG's  for bone marrow and bone recommended in paragraphs 4.2 and  4.4 appear  appropriate
as a  basis for the evaluation of  the  risk  associated  with  exposure  of population  groups to
strontium-90.

   4.24   The  Council  has  emphasized, however, that in the application of general RPG's, both
the risk and  the  reasons for accepting the  exposure should be  considered.  The Council  has,
therefore,   reviewed  past and  current activities  resulting  in release  of strontium-90 to  the
environment, and given some consideration to future developments.   This review  indicates that
in general these  activities  can be conducted  without undue difficulty at exposures  lower than
those  corresponding  to the  RPG's.   Therefore,  in the  development of the guidance  on intake,
doses corresponding to one-third the RPG's  for bone marrow and bone to be applied to the
average of suitable samples of an exposed population group have been used.

Guidance  on Intake  of Strontium-90

   4.25 As a  step in the  development of guidance  on intake of strontium-90,  it is  necessary to
determine  the  average daily  intake  of strontium-90 which  would  correspond to doses  of
one-third the RPG's to be applied to  suitable samples of an exposed population group.  The nature
of the  relationship between the  ratio of strontium  and  calcium in  the  human diet and  in the
human skeleton has been discussed  in paragraphs 4.9 - 4.13.   The data referred to  in paragraph
4.13 not only indicate that  the ratio of natural strontium  to calcium in the skeleton does not
increase  significantly with  age but  they  show  that  within a general geographical  area  natural


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differences  in  dietary habits do  not result  in  a large  spread  in the values  observed in the
skeletons of individuals of all ages.
   4.26  The average  ratio  of strontium to calcium in the human  skeleton is  estimated to be
about one-fourth of the  ratio in the diet.  On this basis, a continuous dietary  ratio  of 200
micromicrocuries of  strontium-90 per  gram of calcium is estimated to result in a  skeletal
concentration  of 50 micromicrocuries per  gram  of  calcium  and to produce   radiation  doses,
averaged over any age group  of a uniformly exposed population group,  corresponding to approimately
one-third  of the  appropriate  RPG's.  This  level  in  the  maternal diet   would  give  about
one-sixth the RPG to the prenatal individual.

   4.27  The  similarity  between the chemical properties  of  strontium and those  of calcium
makes the average ratio of strontium-90 to calcium in the diet a useful device in the  development
of guidance  on intake.   In  some  situations,  it may be desirable to  consider concentrations
of strontium-90 and calcium  in individual items of diet.  However, in  general it is useful to
use intake  values based  on average calcium content of the diet.

   4.28  Appropriate intake values will depend upon the composition of the diet and  the average
consumption.   The minimum calcium requirement in  the  American diet is considered to be of
the order of one gram per  day.  The average  intake may be considerably  in excess of this
amount, although in some  areas it is found to be somewhat less.   For the derivation of intake
guidance, the  Council adopts the  figure  of  one gram of calcium per day.   On this  basis,  a
continuous  dietary intake of 200 micromicrocuries per  day would  generally  correspond  to  the
radiation doses discussed above.

   4.29  It  is therefore  recommended that the  following guidance  on  transient rates of daily
intake of strontium-90 to be applied to the average of suitable samples of an exposed population
group  be   adopted   for  normal  peacetime  operations:

                  RANGE  I         0  to     20 micromicrocuries per day
                  RANGE  II       20  to    200 micromicrocuries per day
                  RANGE  III      200  to  2,000 micromicrocuries per day

Strontium-8 9

   4.30  Occupational standards have related  body burdens  of strontium-89  and strontium-90
in such a manner as to  permit the  same total absorption  of  energy by the skeleton from one as
from  the other. This  results  in a body burden for strontium-89 two times that for strontium-90.
Because  of the  shorter half-life  of strontium-89,  52  days  as compared to  27 years, the
corresponding  ratio  of permissible concentrations has  been estimated  to  be about  100.

   4.31  Because of  the manner in which the Council  has derived guides  for exposures  of
population  groups to  strontium-90,  it is  not possible  to  relate the two on the  basis  of energy
comparison alone with as high a degree of confidence as is  involved  in the development of the
guide for strontium-90.  The guides for strontium-90 depend  upon the validity  of the assumption
of reasonable  uniformity of concentration  in  the  skeleton.   Because of the  relatively  short
half-life of  strontium-89, and  hence the  relatively  short  time  in  which strontium-89 atoms
exist  in the body, the distribution  of dose is necessarily  much less uniform than that from
strontium-90.   It is,  however, possible to derive, by  comparison  with strontium-90, guides
which represent no greater hazards than  those  for strontium-90 and which are  not  excessively
restrictive

   4.32  For this  purpose,  we take advantage  of the  current practice of  permitting  population
exposures to be averaged  over periods of up to one year.   The maximum  dose  rate  will be
experienced in  areas in which new bone is being formed.  Our  objective is  to limit the dose in
any one year to the  value which would have been permitted if the  radioactivity were  strontium-90.
For  simplicity, consider a  section of  "bone"  of reasonable  size and  suppose that  it has
been  "formed" of calcium, strontium-89, and other  appropriate  elements by normal process
of metabolism  in  a  period  of time  short in comparison with the  half-life of strontium-89.  It
may be shown that  the  decay rate of strontium-89  is  such that the average dose rate to the
bone  over a period  of one  year after formation  will he only one-fifth of the  initial dose rate.

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Because  the  average  energy absorbed per  disintegration of strontium-89 is  only half that per
disintegration  of strontium-90 and its yttrium daughter, in this  hypothetical case ten times
as much  strontium-89, measured  in  terms  of activity,  could  be permitted as of strontium-90
without increasing the  average  dose  in  one year. In subsequent years,  of course  the dose to
this section of the bone would  be essentially zero.

   4.33   It is apparent that if such  a section of bone were to be  built up  slowly  instead of
instantaneously,  the average dose  to this section  of the bone during  the ensuring year would be
somewhat less.   This may be demonstrated in the following manner.   If the section of bone
added  is  reduced in thickness, a larger fraction of the total radiation emitted by the strontium-89
in this section  escapes to  adjacent  material.   While  this escape  may  be compensated for in
part by absorption  of radiation  from adjacent material,  if such  adjacent material  is  older  than
the section  under consideration, it must have a  lower concentration of strontium-89  and,  hence,
the compensation cannot be complete.

   4.34  On  the  basis of the above argument, since strontium-89 follows  the  same metabolic
pattern  as strontium-90,  guidance on  intake  of ten  times that used   for  strontium-90 will
result  in  dose rates to bone marrow and  bone  which,  in any  area  of the  skeleton,  will  not
exceed  in any  one  year those  permitted from  strontium-90.   While  these  dose  rates
represent  hazards which, over a  period  of years,  are  certainly  much less  than those from
continuous exposure to strontium-90 at one-third the  RPG,  the reasons for accepting comparable
risks   from   strontium-89   are  generally  less.

   4.35   Therefore the following guidance on transient rates of  daily intake  of strontium-89 to
be applied to the average of suitable samples  of an  exposed population  group is recommended
for normal  peacetime  operations:

                 RANGE I            0  to     200 micromicrocuries per day

                 RANGE II        200  to  2,000 micromicrocuries per day
                 RANGE  III       2,000 to  20,000 micromicrocuries per day
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