-d

United Nations Scientific Committee
on the Effects of Atomic Radiation

1977 report to the General Assembly, with annexes

NOTE
The report of the Committee without its annexes appears as Official Records of the General
Assembly, Thirty-second Session, Supplement No. 40 (A/32/40).
In the text of each annex, Arabic numbers in parentheses are references listed at the end.
The designations employed and the presentation of material in this publication do not imply theexpression of any opinion whatsoever on the part of the Secretariat of the United Nations concerning
the legal status of any country, territory, city or area, or of its authorities, or concerning thedelimitation of its frontiers or boundaries.

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ANNEX A
Concepts and quantities in the assessment of human exposures

CONTENTSParagraphs

Paragraphs

INTRODUCI'JON . . . . • . . . . . . . . . . . . . .

1-2

LBASIC CONCEPTS . • • • . . . . . . . . . . .

3-7

II.

EXPOSURE OF POPULATIONS . . . . . . .

8-30

A.

The collective dose and dose commitment . . • . • • • • . . . . . . . . . . . .
1.
Use of the collective dose andcollective dose commitment in
detriment assessments. . • . . . . .

9-27

17-22

2.

Use of the dose commitment inpredictions relating to continued
practices . • . . . . . . . . . . . . .
Environmental models used in theassessment of dose commitments . . . .

28-30

III. QUANTITIES USED IN THE ASSESSMENT
OF ORGAN DOSES . . . . . . . . . . . . . . .
A. Internal irradiation
B. External irradiation . . . . . . . . . . . .

31-46
32-35
36-46

References . . . . . . . . . . . . . . . . . • . . . . . .

Page
34

B.

23-27

Introduction

I. BASIC CONCEPTS

1.
The purpose of this Annex is to define theconcepts and quantities that are used by the Committee
in collating and reporting data on human radiation
exposures and to qualify their use. These concepts andquantities can be unequivocally defined, but a numberof basic biological assumptions have to be made tojustify their use for particular purposes. While some ofthese assumptions are presented in this Annex, a more
extensive discussion, which a fuller understanding of thequalifications requires, can be found in Annexes G, H, I
and J, which deal with the biological effects of radiation.

3.
The me.an absorbed dose in various human organs
or tissues is the physical quantity usually taken as a basis
for radiation risk estimates, and it is derived by averaging
the absorbed dose over the mass of the organ or tissue
under consideration. The absorbed dose 1 is defined as
the mean energy imparted per unit mass at the point ofinterest (3). In this report, the word "dose", unlessspecifically qualified, refers to the mean absorbed doseover an organ or tissue. ·2.
The concepts and quantities discussed in thisAnnex are designed for the purpose of dealing with low
doses of radiation, as usually encountered in occupational and population exposures.

4.
The use of absorbed dose in tissues as a basis forrisk assessments makes it necessary to present separately
I The SI unit of absorbed dose is the joule per kilogram,the special name for the unit being the gray (1 Gy = 1 J/kg). In
the present document use is made of the previous special unit for
absorbed dose, the rad {I rad= 0.01 Gy).

27

those components of the absorbed dose which are due tohigh-LET 2 radiation, with specification of radiation
type. For low-LET contributions to the absorbed dose,
which comprise the most common types of human
exposures, it is assumed that the influence of radiation
quality and dose rate is negligible.

5.

The relations between doses in tissues and risks ofdeleterious effects are complex; they are presented indetail and discussed in Annexes G, H, I and J. If theeffects in laboratory experiments under controlled
conditions, as reported in Annexes H, I and J, areplotted against doses, many different types of curves are
obtained, some linear and some curvilinear. A significant
consideration is that in heterogeneous human populations of all ages and states of health and subject to anumber of insults acting with radiation, there may beespecially sensitive individuals, in which case a thresholdmay be less probable. In research it is appropriate to
analyse dose-effect curves and factors relating to them indetail in order to gain an insight into the mechanisms
involved. For purposes of making risk estimates inhumans, however, a different approach has been
employed. As described in Annex G, most of the effectsof radiation in human beings have been observed atdoses (some tens or hundreds of rads) that areconsiderably higher than those near natural backgroundlevels. To estimate risks at low doses the incidence ofeffects at high dose levels is divided by the dose to givean incidence rate per unit dose, w)µch presupposes that
there is proportionality between dose and effect down
to the lowest doses. This assumption will be made forpresent purposes, recognizing that it is also possible that
there could be a much IO\yer effect, and perhaps none. at
low doses. These considerations indicate that the rates ofincidence calculated in these Annexes from observations
at high doses may overestimate the effects of low dosesin the millirad range.
6.
In the present state of knowledge, the possibility
that the dose-effect curve has a zero slope at these low
doses cannot be ruled out. However, at doses aroundnatural background it is assumed that an approximate
proportionality exists between dose increments andcorresponding increments of risk, provided that theincrements are small. The additional assumption is madethat the severity of each type of effect is independent ofdose at low doses.
7.
In the case of inhomogeneous dose distributions
within an organ or tissue, the linear assumption implies
that the inhomogeneity does not affect the relationshipbetween risk and absorbed dose, provided that local
doses are in the range in which proportionality applies
and are small enough not to cause significant cellsterilization. If the latter condition is not met, cellsterilization could reduce the risk per unit dose, andassessments based on the mean absorbed dose over theentire organ or tissue might tend to overestimate therisk.
1

LET (linear energy transfer} is used in the unrestrictedsense as equivalent to the linear collision stopping power (3).
Low-LET radiation, in this report, means radiation for which thelinear collision stopping power in water is Jess than 3.5 keV/µm.

28

II. EXPOSURE OF POPULATIONS
8.
When population groups are exposed to radiationfrom several sources, it is often interesting to compare
the exposures and, under some assumptions. to assess
the relative importance of the potential hazards due toeach source. If the population groups were homogeneously and simultaneously exposed to radiation fromthese sources, the dose due to each source would give
useful information about the relative importance of thesource. In the more general case. however, the size of the
exposed popuJation and the distribution of doses overthis population vary for different sources. Furthermore,
the distribution of the doses in time is an importantfactor to be considered since it may also vary for
different sources.

A. THE COLLECTIVE DOSE AND DOSECOMMITMENT
9.
Practices involving radiation sources can give rise
to a distribution of dose rates in the exposed population.
No single quantity can represent adequately the
distribution in all possible situations. In some cases,
however, a useful quantity for this purpose is thecollective dose rate. The collective dose rate S in a
population from a practice or source is defined by theexpression

S = ["' b N 0 (DJdD.owhere Nb ( /J) is the population spectrum in dose rate,
NiJ (D) d/J being the number of individuals receiving adose rate in a specified organ or tissue, due to a source,in the range /J to /J + d/J. Integrals of this type areusually referred to as "weighted products", thecollective dose rate from a source being therefore theweighted product of dose rate due to the source andnumber of individuals in the exposed population. The
collective dose rate is thus not a dose rate in the sense ofabsorbed dose rate as defined in ICRU report 19 (3).
The actual population exposed should be specified
whenever possible.
10. The collective dose in a population over a specified
period of time is defined as the time integral of thecollective dose rate. The collective dose is an extensive
quantity that can apply to one person, to a populationgroup or to the whole world population. It is expressedin man rad. The period of integration depends upon the
purpose of the assessment. If the period of integration isshort, the population may be assumed to be composedof the same individuals during the whole period.
Conceptually, however, the population for which thecollective dose is assessed could comprise successive
generations.
11. The collective dose in a defined group ofindividuals can also be assessed as the weighted productof individual dose and number of individuals:

S= 1:r;DNv(D)dDJo'

where Nv (D) dD is the number of individuals receiving
a dose, in a specified organ or tissue, in the range D to
D + dD. The defined group may comprise individuals
who live at different times or individuals living in a given
year, depending upon the purpose of the assessment.
12. The average dose to an individual in the defined
exposed group due to a source, called the per caput dose
D from that source. can be calculated as

D= .CDN 0 (D)dDI.CN (D)dD
0

and since rt' ND dD = N, the total number of individuals
in the defined group, it follows that S = DN.

from a given event, decision or fmite practice. Thisquantity is the dose commitment Dt defined as theinfmite time-integral of the per caput dose rate due toevent, decision or finite practice k:D{ = ["'bk(t) dt.owhere the per caput dose rate Dk(t) is the quotient of
the collective dose rate and the population size at time t,i.e., Dk(t) = Sk(t)/N(t). If the population size remains
unchanged over the time period contributing to theintegral, we have~ = !JfcN.

I. Use of the collective dose and collective dose
13. The collective dose and the collective dose rate areparticularly useful quantities in source-related assessments. The collective dose rate from a source k, Sb isobtained by including in the population underconsideration all individuals simultaneously being
exposed by the source k. As the value of Sk remains
unchanged if the population is made arbitrarily larger
than the actual exposed group by adding unexposed
individuals, it is convenient for the purpose of assessing
the collective dose rate from a source to define thepopulation as the world population. This specification is
not necessary when the exposed group is small and welldefmed in a way that every exposed person can be
accounted for.
14. In some cases, the world population may be
conveniently subdivided into several groups labelled by
j = 1, 2, 3, .... the collective dose rate then being
calculated as$ =~ lbN1.vfD)dD =~sj

where N;b (b) is the population spectrum in dose rateof group j, and S; is the contribution of group j to thecollective dose rate. The collective dose rate from source
k can be assessed assk = I l>k. 1 N1jwhere Dk; is the per caput dose rate contributed bysource k in group j, and Ni is the number of individuals
in group j. All these quantities are functions of time.
15. In order to have a measure of the total exposure of
the population caused by a source, the collective dose
commitment is used. The collective dose commitment
S~ due to a given event, decision or finite practice k isdefined as the infmite time-integral of the collective doserate, Sk(t), caused by that event, decision or finite
practice:

s~ = ["' StftJ dt.o

As discussed above. the collective dose rate Sk(t) at agiven time can be assessed by integration or summation
over the world population.
16. As in previous reports. the Committee has also
found it convenient to define a per caput quantity forthe purpose of assessing the total exposure resulting

commitment in detriment assessments
17. Under the assumptions given in paragraphs 5 and6, the collective dose can be used in assessments of therelative detriments from several radiation sources. ICRP
has introduced the concept of "detriment" as aquantitative measure of the expected harm in a group ofpeople as a result of a given radiation exposure (4). The
detriment is defmed as the mathematical expectation ofharm, taking into account the probability of occurrenceof each type of deleterious effect and the severity of theeffect. If the group is composed of N persons and if Pi is
the probability of incurring the effect i, the severity ofwhich is expressed by a factor g;, the detriment G is

G=NLpigi
I

18. Considering only individual exposures to the same
type of radiation, and making the assumption that theprobability of each effect i attributable to the source
under consideration is directly proportional to the dosefrom that source (p; = riD, where r; is the risk factor) andthe further assumption that the severity of the effects is
independent of their frequency in the dose range ofinterest, the health detriment in a homogeneously
exposed group becomes G = ND"'i,;riKi· In the generalcase where there is a distribution of doses over the world
population, the detriment to health due to source k isgiven by

Gk = [xiI(DkrigJN0,,(DJdD 1 =SkLrigi
- 0

i

i

since the sum 'i.;TiK; is independent of the dose
distribution under the assumptions made.
19. The validity of this expression depends therefore
on the assumed linearity, with no threshold, between
dose and risk, as has been discussed in paragraph 5. The
Committee has stressed that this is a cautious
assumption, .the validity of which has not yet been
established. The risk factors ri may vary both with thepreviously accumulated dose to which the dose D fromthe source under consideration is added and also withthe dose rate and radiation quality. It cannot be assumed
that the values of r; which can be derived fromobservations at high doses and dose rates also apply tosmall dose increments at low dose rates.
20. However. for practices resulting in small dose
increments to doses of similar order of magnitude, e.g.,29small dose increments above the natural background, as
well as for the background doses as such, and provided
there is no large dose-rate dependence, it may be
assumed that, in the first approximation. one and thesame value of "i:,;r;K; would apply. In these conditions it
would be possible to assess the relative detriment fromtwo practices as the ratio of their collective doses:
Gi/G2 ""'Si/S2.
21. The use of the collective dose for relative
detriment assessments would also be.valid in cases where
the practices under comparison generate similar dose
distributions over the population. Furthermore, if thereal dose-risk relationships have an approximate
proportional region, the comparison of collective doses
would be valid in cases in which all the individual doses
fall in the region of proportionality.
22. The evaluation of the relative health detrimentresulting from practices which deliver the doses overquite different times can be done, provided that theassumptions of paragraphs 20 or 21 apply, by comparing
the collective dose commitments from these practices.
2. Use of the dose commitment in predictions
relating to continued practices
23. Both the dose commitment and the collective dose
commitment are usually proportional to the size of theoriginating event. For example, if the event underconsideration is the release of a quantity of aradionuclide to the environment, the dose commitment
and the collective dose commitment are proportional tothe activity released, provided all other influencing
factors remain constant.
24. In these conditions, the values of the per caput
dose rate at given times after the originating event arealso proportional to the size of that event. It is therefore
29ssible to define for a type of practice a function
Di(t), giving the per caput dose rate per unit oforiginating event (e.g., per unit of activity released) as afunction of time elapsed since the event. It isconsequently possible to define the dose commitment
per unit originating event D~, and the collective dose
commitment per unit originating event~, asDf =["'b (t) dt

.o

1

and

Sf= ["'bi(t)N(t)dt.owhere N(t) is the population size. If the population size
remains constant over the period contributing to theintegral, we have Sf = ND f. In the more general case, thecalculation of collective dose commitments often
requires the use of a population growth model. For shortprojections into the future an approximate exponential
growth has been assumed in this report. On the other
hand, for exposures delivered over very long periods oftime, the assessments are based on an assumed upper
limit for the world population of 10 1 0 individuals.
25. A continued practice causing radiation exposures
can be considered as a sequence of originating events

30

discussed in the previous paragraph. The dose commitment per unit practice can be used for predictions of the
per caput dose rate in the future for the case of arepeated practice at a known and constant rate R. The
per caput dose rate will increase and for giort-lived
radionuclides eventually reach a steady value b .. , which
can be calculated by integrating the dose rate
contributions at an arbitrary time T, when a steady state
has been reached:

b"'=[

00

Ri\(T-t)dt=R[ Di(t)dt=RD1.-~.o

In the simplified case of a cons.!_ant population size, wehave ~ = NDL and therefore b ... = (R/N)~. In manycases it is possible to m_ake rough projections of thepractice rate per caput R/N. It is then possible to predictthe maximum per caput dose rate that will be
experienced in the future due to the expanded andcontinuing practice.
26. For doses delivered over a very long time after asingle originating event, it would not be realistic topostulate a continued practice for the long times
required for the per caput dose rate to approach thesteady state value. Again, it is in~resting to assess the
maximum per caput dose rate, Dmax., caused in the
future by the practice. If the practice continues to atime T at rate R and then is discontinued, the per caput.
dose rate at any time T> T is given by

D(T) = [' Rl\ (T-t)dt = R [., O

Di(c)dt.. T-t

It can be shown that if the function bi(t} has a single
maximum at time 8 and then declines monotonically,
the maximum per caput dose rate occurs at a time Tmsuch that T <Tm < T + 8. Since in most practical cases
T >> 8 (and therefore Tm==< T), themaximumpercaput
dose rate is approximately given by

Dmax~R ['Di(t)dt.o

27. The integral Jo Ddt)dt = Df does not relate to thetotal detriment per unit practice. but it is useful inassessing the maximum per caput dose rate that will
result from a continuing but finite practice. If the
population size remains constant during the time
contributing to the integration, it is convenient toexpress the maximum per capui dose rate as a functionof the practice rate per caput R/N as/jmax = ( R/N} S',.

sr

where
= NDI will be referred to in this report as theincomplete collective dose commitment per unit
practice.

B. ENVIRONMENT AL MODELS USED IN THE
ASSESSMENT OF DOSE COMMITMENTS
28. The chain of events leading from the primary
release of radioactive substances to the irradiation ofhuman tissues can be schematically represented bycompartment models, in which the rates of transfer ofradioactivity between compartments are specified byconstants or by time functions. The use of compartment
models, even when very complex, implies considerable
simplification of the real transfer processes.
29. Since the dose commitment from a given source is
the integral over infinite time of the per caput dose rate
result~g from the input, steps in the sequence from
input to the final dose commitment can be conveniently
described in terms of the quotient of the infinite
time-integral of the appropriate quantity in step j of thesequence to the infinite time-integral of the appropriatequantity in the preceding step i. These quotients define
the transfer factors Pij in the pathway from input ofradionuclides into the environment to the subsequent
radiation dose in man:
P;i =

1:

Mlt)dt/1:

M;(t)dt

where P,j is the factor relating compartment j and thepreceding compartment i, and Mtft} and M/t) are theappropriate quantities (e.g. activity concentrations) in
the respective compartments at time t. The factors P1i
must be expressed in terms of the dimensions of the twoquantities they link.
30. The network of pathways linking the release ofradioactive material (input) to the dose commitment
consists of pathways in series and in parallel. The totaltransfer factor of a branch (pathway in series) is the
product of the transfer factors involved; the totaltransfer factor of several branches in parallel is the sumof the transfer factors of the branches. The dose
commitment, therefore, can be assessed from the input
Y by the relation

A. INTERNAL IRRADIATION

32.
In the case of internal irradiation, the doses in
the organs of interest of a given individual may be
estimated from one or more of the following basic
determinations:(a)Measurements of activity concentrations in
the environment and in diet components, leading toestimates of the intake and, by the use of appropriate
metabolic models, to estimates of the residence of theactivity in the tissues of interest;(b) Assessments of the activity in the body or in
the relevant tissues, made on the basis of measurementsof radiation emitted from the body or of measurementsof activity in tissue samples;(c)Measurements of the activity of excreta or
exhaled air leading, by the use of appropriate metabolicmodels, to estimates of the activity in the body.

33. The use of these three types of information for the
purpose of dose assessments requires the postulation ofmodels to describe the body and tissue radioactive
contents as a function of time. For example, forcontamination through the diet, the activity in the bodyor a tissue, as a function of time, is given byQ(t) =
where Q(t)

I
C(T)
A

D'=Y1: parallel IT.
P
scnes

r(t)

Transfer factors, their estimation for a number of
nuclides, and the uncertainties involved in thisestimation because of the variability of the transfer
processes were discussed extensively in the 1969 and1972 reports of the Committee (10, 11). The essential
information from those reports is summarized in therelevant annexes of this report.

ill. QUANTITIES USED IN THE
ASSESSMENT OF ORGAN DOSES
31. In assembling information on radiation levels, theCommittee, as in its previous reports, has selectedprimary data which can be used for the derivation ofabsorbed doses in human tissues, collective absorbed
doses and absorbed dose commitments. Data which
describe conditions of exposure to radiation in a usefulway for the Committee's purposes are released
activities 3 and activity concentrations, from whichinternal or external irradiation may be assessed, andquantities specifying ambient radiation levels, fromwhich external irradiation may be assessed.
3
The SI unit of activity is the second to the power minus
one, the special name of the unit being the becquerel (lBq =1
l s· ). In the present document use is made of the previous special
unit for activity, the curie (l Ci = ~.7 10' 0 Bq).

f

JC(r)Ar(t-r)dris the activity in the body or tissue ofinterest at time t,
is the diet consumption per unit time,is the activity concentration in the diet attime T.
is the fractional absorption into blood of
the ingested activity, andis the fraction of the activity absorbed
into blood that is retained in the body or
the tissue under consideration at a time t
after a single uptake.

The basic data for this equation are the dietconcentrations as a function of time. Similar formulations can be derived to make use of basic information oftypes (b) and (c) referred to in paragraph 32.
34. For the purpose of assessing dose commitments.
the population weighted average of Q(t) is needed. The
parameters of the expression given in paragraph 33, as
well as the function r(t), are usually functions of age,and this must be taken into account in the derivation of
the per caput value Q(t).
35.

The dose commitment is calculated as
D'=K[""Q(t)dt.owhere K is a conversion factor appropriate to the
population group. The conversion factors involved in thecalculation of absorbed dose from
Q(t)dt do notnecessarily relate to the same tissue or organ for which
Q(t) is given. For example, the dose in the red bonemarrow and the endosteal cells may be caused byirradiation from radionuclides in bone. Also, differentJo31

parts of one and the same organ or tissue may receive
different doses, but as already mentioned, it is the meandose in the specified organs and tissues which is ofinterest to the Committee.

B. EXTERNAL IRRADIATION
36. The assessment of organ doses from external
irradiation requires, in principle, the knowledge of theenergy and angular differential distribution of thefluence rate of each component of the radiation field.
These differential distributions can be obtained byspectrometric measurements, but in general only
calculated values, for cases where the source distributions are known, are available.
37. Most reported field measurements provide valuesof integral quantities, usually describing the field in theabsence of an exposed person-in "receptor-free"conditions. These quantities, if specified unambiguously,
can be used for the assessment of organ doses of aperson introduced in the radiation field, provided some
information is available on the characteristics of the
field. The present report uses two quantities to describe
receptor-free irradiation situations: the absorbed dose
rate in air D8 and the absorbed dose index rate D1.
38. The absorbed dose rate in air Da is used in thisreport to describe environmental exposure situations
resulting from gamma-emitting nuclides, and it isunambiguously specified when it may be assumed thatfull secondary electron equilibrium exists in air. In thiscondition, the absorbed dose rate in air can be derived
from the exposure rate X as Da = aX, where the value ofthe conversion factor a is 0.869 rad/R. The absorbed
dose rate in air will usually be expressed in microrad perhour.
39. For describing receptor-free irradiation situations
due to cosmic radiation it is convenient to select aquantity which, for the purpose of comparison, could
equally be used at any altitude and in space. The
absorbed dose rate can be used for this purpose but its
magnitude depends upon the geometry of the material inwhich it is determined. Selection of a specified geometry
for such a determination provides an unambiguous value
for the absorbed dose rate. Such a specified geometry is
contained in the definition of the absorbed dose index
rate D1 which is used in this report to describe
receptor-free environmental radiation levels due tocosmic rays. The absorbed dose index rate is defined as
the maximum absorbed dose rate that would occur in a
30-cm diameter tissue-equivalent sphere if the sphere
were located with its centre at the point of interest (3).
The D1 also will usually be expressed in microrad perhour.
40. From the environmental quantities, the absorbed
doses in a number of human organs of interest will beassessed, usually as annual doses or as dose commitments
per unit practice. These assessments necessitate anumber of assumptions, both about factors which
influence the depth dose calculations and on the periodsof time during which persons are exposed to the various
radiation fields.
32

41. When the receptor is located in the region ofinterest, the procedure of assessing organ dose rates fromabsorbed dose rates in air involves the following
considerations: Absorbed dose rates in air are multipliedby the s-factor4 to transform them to tissue dose rate.
The change from receptor-free conditions to thesituation where a person is present in the radiation field
will influence the field at the surface of the body.
Back-scattered radiation may increase the dose rate at
the surface, but the body will also act as a shield andreduce the dose rate. The influence of body shielding
will depend upon the location of the organ of interest,
and the energy and angular distribution of the radiation.
42. Except for the rare case where the radiation field
is monodirectional and the irradiated person is notmoving, the "depth" of an organ would be the weighted
average of the distance in the body that would have tobe penetrated by rays entering from different directions.
The difference in absorbed dose rate at the surface ofthe body when the radiation is monodirectional andexposing the surface under consideration, and when thefield is isotropic, may be substantial. The ratio of the"monodirectional dose rate" to the "isotropic dose rate"
at any depth, when the fields produce one and the sameabsorbed dose rate in air under receptor-free conditions,
may be referred to as the isotropy factor k at that depth.
The isotropy factor cannot exceed 2 for points near thesurface of the body, and it approaches 1 near the centreof the body and at all locatio!}S if the absorbed dose rateis constant throughout the body.

43. The assessment of organ doses requires therefore
that the absorbed dose in air (after having beentransformed to tissue dose by multiplication by thes-factor) be then multiplied by the depth transmission
factor d, the backscatter factor b and a shielding factor,
which would be the inverse of the isotropy factor 1/k.
The absorbed dose rate in an organ would be

Dori= sbdk- 1 i>0 = sgi>awhere the product of the factors b, d and k- 1 could becalled the geometry factor g.
44. In the 1972 report, the Committee referred togonad dose calculations by Bennett (2), indicating thatsg = 0.82 for external outdoor exposure {21r geometry),and to gonad dose measurements by Spiers and Overton(9), indicating that g = 0.63 (sg = 0.69) for exposure
indoors. By transport calculations, O'Brien and Sanna(6) have assessed absorbed doses in several organs (in the
MIRD phantom (8)) per unit exposure, as a function of
the energy of gamma rays. Combining these calculations
with the environmental gamma-ray exposure spectrum inenergy (]), they have calculated organ absorbed doses
per unit exposure for environmental conditions. Fromthese calculations sg for gonads can be estimated to beabout 0.8 in males and about 0.55 in females. As themathematical phantom, however, uses the Reference
Man mass (5) in all calculations, the factor for female
gonads is somewhat underestimated. O'Brien (7) has• The s-factor is the ratio of the mass energy absorption
coefficients for tissue and air. A value of s = 1.10 is correct
within :t I per cent for photon energies between 0.1 and 4 MeV
for soft tissues.

recently calculated the organ doses for a female
phantom. From these calculations the value of sg forfemale gonads is estimated to be about 0.6.
45. If the occupancy factor (i.e., the fraction of theyear during which a person is actually exposed to a given
source) is q, the annual gonad dose, assuming constantenvironmental irradiation levels, can be derived from theabsorbed dose rate in air as

D=c1 qDa
where c,. = 7.2 mrad µrad- 1 h for outdoor exposuresand 6.0 mrad µrad- 1 h for indoor exposure, using thevalues of sg given in the 1972 report. The calculations ofO'Brien and Sanna (6) give a value of sg equal to about
0. 7 for the ratio between the average absorbed dose in
the body and the absorbed dose in air, and show that thedoses in other organs of interest to the Committee arevery nearly equal to the gonad dose. It will be assumed
that the equation given above applies also to organs suchas the breast, the lung and the red bone marrow.
46. In the case of exposures to cosmic radiation, it will
be assumed that the absorbed dose index rate represents
sufficiently well the relevant organ dose rates in an
individual introduced at the location of consideration. If
the occupancy factor for that location is q, the annualorgan dose can be derived from the absorbed dose index
rate, assuming constant environmental levels, as
D = c0 qD 1
where D is the annual organ dose, and Cc is 8.76 mradµrad- 1 h.

33

REFERENCES

1.

Beck, H. L. The physics of environmental gamma
radiation fields. p. 101-134 in Proceedings of theSecond International Symposium on the Natural
Radiation Environment , Houston , Texas. UnitedStates Energy Research and Development Administration report CONF-720805 (1972).

2.

Bennett, B. G. Estimation of gonadal absorbed
dose due to environmental gamma radiation.
Health Physics 19: 757-767 (1970).

3.

International Commission on Radiation Units andMeasurements. Radiation Quantities and Units.
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