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radiocaesium
Editor Neil Crout

 
Entity code RADIO
CAS registry No. 10045-97-3 (Cs-137) and 13967-70-9 (Cs-134)
Family Group 1 Metal
Functions Radionuclide
Chemical name Cs-137 and Cs-134 (EN)
Synonyms
Trade names
Other names
Last updated 23/9/2004

Fate
Fixation in soil

Caesium is generally rather strongly fixed in soil. Downward
migration and availability to plants is thereby reduced. In
mineral soils the movement of 137Cs is appreciably less than that
of 90Sr. Three to four years after deposition on the soil surface,
the median depth to which it has penetrated is usually less than 2
cm [F5]. Its mobility may be somewhat greater in organic soils.
Much smaller amounts of 137Cs than of 90Sr are leached out of the
soil to enter rivers and lakes.

There are exceptional areas, however, where caesium fixation
in soil is much less, allowing enhanced transfer of caesium to
plants. Marei et al. [M2] identified regions in the USSR where the
soil is wet, peaty and podzolic, from which transfer of 137Cs into
the food chain is 10 times higher than for other areas. Other
regions of the world where the soils give rise to high 137Cs
transfer into diet have also been identified, for example, in the
Faroe Islands, New Zealand and Sweden [U8].

It has been reported that in clay minerals the important
factor in fixation of caesium is the ability of certain layered
silicates such as micas, vermiculites and illites to adsorb or fix
trace quantities of caesium [T1]. Caesium ions are trapped in the
interlayer regions of vermiculite or at the frayed edges of illites
and micas. Caesium is thus more strongly retained in soils
containing predominantly micaceous minerals. Soils which do not
contain large quantities of micaceous minerals, such as tropical
soils, peat soils, and podzolic soils, exhibit less retention and
allow greater uptake of caesium by plants.

Fixation of caesium by sediments in aquatic environments
occurs in a similar fashion to fixation in soil. The preferential
adsorption of 137Cs to the micaceous component of sediments has
been demonstrated under environmental conditions.

Transfer to plants

Caesium-137 may be transferred to plants by direct deposition
onto plant surfaces or by root uptake from accumulated deposits in
soil. In general, direct foliar absorption is the predominant mode
of plant contamination when the deposition rate is relatively high.
Root uptake is low except in those cases mentioned above, when soil
conditions allow low fixation of caesium.

Caesium depositing on plant surfaces is retained to the same
extent as other particulate debris. A removal half-time of l4 days
due to weathering is generally assumed. Once absorbed by the
plant, caesium is readily redistributed through the plant.
Relationships between air concentrations and subsequent
concentrations of 137Cs in pasture plants have been discussed by
Hawthorne et al. [H4] and Pelletier and Voilleque [P2].

Caesium may enter plants by plant base absorption before
becoming fixed in soil. Thus, retention of 137Cs in the root mat of
pastures may allow 137Cs to be relatively more available to plants
for a period of a year or more [U3]. A high level of organic
matter in soil can enhance the absorption of caesium by plants
[B1]. Sorption of organic molecules on clay surfaces prevents the
retention of caesium and also of potassium by these minerals.
Thus, for permanent pastures in temperate regions, the frequent
high organic matter content of the upper soil layer allows shallow
rooted grass to absorb 137Cs relatively more freely for a somewhat
more extended period following deposition. Mushrooms have been
shown to concentrate very effectively 137Cs from soil [G2, M1],
which may be associated with the highly organic areas of growing.

Uptake to plants of 137Cs from soil low in available
potassium may be somewhat increased. The addition of potassium may
decrease absorption of 137Cs in this case; however, this has no
effect when the available potassium is high [N5, F4]. Root uptake
of caesium is in general included in the range 0.01 to 1, which is
the ratio of caesium concentrations in the dry plant material to
that in dry soil [M3].

Transfer to milk

The fractional amount of 137Cs transferred into milk is
slightly greater than that of potassium. It has been shown that
some 10% of orally ingested 137Cs is secreted into the milk of
dairy cows, corresponding to 1.3 to 1.5% of the amount ingested per
litre of milk [G1, I1, L2]. The transfer of fallout 137Cs in field
conditions has been found to be somewhat less, ranging from 0.25 to
0.86% of intake per litre of milk [P2, M1, S4].

Transfer to meat

A correlation between 137Cs concentration in beef and in milk
has been noted, the ratio of concentrations in meat (Bq/kg) and in
milk (Bq/1) averaging about 4 for production in the same locality
[L5, E1, J2]. This would correspond to a transfer of about 4% of
the daily intake per kilogram of meat [F5]. Equilibrium conditions
are reached in about 30 days in the cow [F5]. It cannot generally
be expected that there will be a useful relationship between 137Cs
in meat and milk, since animals produced for the two purposes are
frequently provided with different diets and are reared in
different areas.

Aquatic behaviour

Caesium in the aquatic environment is strongly adsorbed by
suspended particulate materials, especially clays. Therefore, the
amount of caesium in the soluble phase decreases with increasing
suspended solid concentrations. Potassium is also sorbed, but to a
much less degree.

Caesium levels in fish are inversely related to the potassium
content of the water [K1]. Because of the high concentration of
potassium in the ocean, the transfer of 137Cs to fish is of primary
concern in the freshwater environment. The activity of fresh water
fish may be 100 times that of ocean fish, given the same caesium
concentrations in water.

The low mineral content of fresh water also enhances the
absorption of 137Cs by aquatic plants. Aquatic plants from fresh
water areas, which are sometimes important in cattle feed, may have
increased levels of 137Cs compared to 137Cs which may have
deposited on nearby pasture ground.

Caesium in aquatic animals is accumulated primarily from the
food chain. Absorption efficiency of potassium and caesium from
food is high. In animals the excretion rate of potassium is about
3 times larger than that of caesium. As a result, the caesium
concentration per unit amount of potassium in tissues increases by
a factor of about 3 with each trophic level [P1].

The food web also accumulates caesium from suspended and
bottom sediments. Filter feeders may accumulate caesium adsorbed
to particulate matter. Benthic invertebrates obtain caesium
absorbed to ingested bottom sediments. Fish ingest those
invertebrates and also some sediment particles along with the prey.
Absorption efficiency in the fish depends on the caesium fixation
ability of the sediment minerals.

The concentration factors (ratio of concentration in organism
to that in the water) for caesium must be related to the potassium
concentration in the water and to the turbidity. From a literature
review of values for fresh water systems [V1], suggested values of
the concentration factors are 1000 for algae and plants, molluscs
and invertebrates in all waters and 5000/Kw and 1500/Kw for non-
piscivorous and piscivorous fish, respectively, in clear waters,
where Kw is the stable potassium concentration of water in µg/g.
The factors for fish are a factor of 5 less in turbid waters (> 50
µg/g suspended solids). The concentration factors for caesium in
the ocean are 10 for algae and molluscs, 30 for fish and 50 for
molluscs [F6].

Toxicology
Transfer to diet

For general contamination situations, as for global fallout
radioactivity, the main contributions to dietary intake of 137Cs
are generally from grain products, meat and milk. Fruit and
vegetables contribute much smaller amounts of 137Cs. This has been
the pattern for western diets, such as Denmark [A1] and the United
States [G4]. In Japan the main contribution has been from cereals
[U1]. For all foods the transfer from a specific deposited amount
seems to be rapid, being essentially completed within the first two
years after deposition.

Marine food chains are of secondary importance in
contributing to dietary intake of fallout 137Cs, even in countries
where fish is widely consumed. In Japan between 1966 and 1971,
only about 8% of the 137Cs in diet came from the consumption of
fish products [U1]. As the contributions from other foods
decrease, however, the relative contribution from fish can
increase. During 1976 in Denmark, 15% of the 137Cs intake was
attributed to fish [A2]. It is possible that individuals consuming
large amounts of freshwater fish may acquire 137Cs burdens several
times greater than individuals eating more diversified diets [G3].

The transfer of 137Cs from deposition to diet has been
studied quantitatively using the following transfer function
between deposition and diet components in regression analyses of
reported data [U8]:



             j         j           j infinite
    Cj(n) = b1 f(n) + b2 f(n-1) + b3 sigma   f(n-m) e-µm

                                     m=1

where Cj(n) is the concentration of 137Cs in the diet component j
during the year (n) in mBq/gK; f(n) is the deposition density of
137Cs in the current year (n) and in previous years (n-1) in Bq/m2;
bjn are the proportionality factors, and µ is the decay constant
accounting for radioactive decay and reduced availability of
deposition in all previous years. The first term of the equations
is the rate term giving the contribution to dietary level from the
current year's deposition amount. The second term is the long term
contribution expressing separately the contribution from the
previous year's deposition including storage of foods by market
practices. The third term gives the contribution from the
cumulative deposit of 137Cs in soil.

The quotient of the time-integrated concentration of 137Cs in
diet and the integrated deposition density defines the transfer
factor P23. The integrals are replaced by summations if, as is
usually the case, the relevant quantities are assessed over
discrete intervals of time, such as annual averages 

              infinite
              sigma   C(n)
              n=1
       P23 =  -----------
              infinite
              sigma   f(n)
              n=1

The transfer factor is usually expressed in mBq a/gK per Bq/m2. It
may be evaluated for total diet or for dietary components.

Using the model described above, the evaluation of the
transfer factor reduces to 


                       infinite -µm               e-µ  
    P23 = b1 + b2 + b3 sigma   e   = b1 + b2 + b3 -----
                       m=1                        1-e-µ


The parameters of the transfer function, obtained by regression
fits of the 137Cs/K quotients in milk, dietary components and total
diet from several countries have been reported by UNSCEAR [U8].
Some of these results are given in Table VII.3.

The lowest values for the transfer factor for milk are
obtained for the U.S.A., Denmark and the U.K. Most of the transfer
of 137Cs deposition to milk is from direct deposition. Less than
15% of the transfer is from uptake of 137Cs from soil.
Intermediate values of P23milk are obtained from the USSR and
Argentina. Transfer from direct deposition is increased and uptake
from soil is more significant. The highest values of P23milk are
obtained for New Zealand, Australia, Norway and the Faroe Islands.
A high value has also been reported for Finland, which is 15.8 mBq
a (gK)-1 per Bq m-2 [C2]. These results can be explained by
efficient transfer of direct deposition and soil conditions which
allow only low fixation of 137Cs.

The range of values of transfer factors for milk, 3.4 to 27.5
mBq a (gK)-1 per Bq m-2, indicates that it is not easy to specify a
typical transfer situation. To estimate the 137Cs transfer to milk
in a particular region requires some indication of soil and pasture
conditions.

Using the comprehensive data from Denmark of 137Cs in diet
[A2], values of the parameters of the transfer function of various
components of diet and for the total diet have been determined.
For most foods, the major contribution to the value of the transfer
factor comes from direct deposition. Only a small transfer is
attributed to uptake from soil. The values of the parameters for
the fit to total diet data are: b1 = 1.6, b2 = 2.2, b3 = 0.04 mBq
a(gK)-1 per Bq m-2 µ = 0.11 a-1 and P23 = 4.1 mBq a(gK)-1 per
Bq m-2. Short-term transfer to diet is thus estimated to comprise
93% of the total transfer, (b1 + b2)/P23.

The general pattern of 137Cs transfer to diet can be expected
to be similar to that for Denmark, although increased transfer can
occur in specific areas, depending on soil conditions or particular
consumption habits. UNSCEAR has used a rounded value of P23 of 4
mBq a(gK)-1 per Bq m-2 to assess the dose commitments from 137Cs
for the world population [U8].


Table VII.3 Parameters of the transfer functions between deposition density and 

137Cs/K in milk
-----------------------------------------------------------------------------------------
Parameter  U.S.A.  Denmark  United   USSR  Argentina  New      Australia  Norway  Faroe
a/                          Kingdom                   Zealand                     Islands
           1973    1976     1977     1972  1976       1977     1973       1976    1976
-----------------------------------------------------------------------------------------
b1         2.3     2.2      1.7      4.4   3.2        6.8      7.3        3.5     6.8
b2         1.3     1.2      1.6      0.2   0.0        0.9      4.8        2.4     5.5
b3         0.0     0.01     0.04     0.3   1.7        3.1      0.3        2.5     2.8
µ          0.05    0.03     0.07     0.2   0.3        0.5      0.2        0.2     0.2
P23        3.4     3.7      3.9      6.2   7.5        13.0     13.4       15.6    27.5
-----------------------------------------------------------------------------------------
a/  The unit for parameters b1, b2, b3 is mBq a(gK)-1 per Bq m-2.
    The unit for parameter µ is a-1.
    The unit for the transfer factor P23 is mBq a(gK)-1 per Bq m-2.


The lichen-caribou-man foodchain

A terrestrial situation which allows much greater than usual
transfer of caesium to man is the lichen-caribou-man foodchain.
Caesium depositing on lichens is retained quite effectively. There
is a very slow decrease in activity with time, approximately 5 to
10% of the 137Cs being eliminated annually [U5]. Lichens provide
the food base for grazing caribou and reindeer during the winter.
A proportionality factor of 137Cs in lichen to that in reindeer
meat in northern Sweden during the winter averaged 4.9 ± 0.4 [L4].
In summer also grass and herbaceous plants are consumed by the
animals, and therefore the 137Cs levels in meat show marked
seasonal variation.

High concentrations of 137Cs arise in Lapp and Eskimo
populations who eat the caribou and reindeer meat. Levels of 740-
1300 Bq per kg of body weight were observed in individuals during
1964 from fallout [H1, M4], about a factor of 100 greater than
burdens of individuals from temperate northern hemisphere regions.
Other fallout radionuclides, such as 54Mn and 55Fe, and the natural
isotopes 210Pb-210Po are also concentrated along the lichen-
caribou-man foodchain.

Absorption and distribution in man

As a general rule, caesium compounds are soluble in body
fluids. Intestinal absorption is complete (100%) under
experimental conditions [R2, S3], but from normal diets is probably
less efficient, ranging from 50 to 80% [F5]. In man caesium is
secreted into the gastrointestinal tract, between the stomach and
small intestine, and is readily reabsorbed [I2]. One basis for
therapeutic treatment in internal contamination cases is to
administer solutions of Prussian blue, which binds with caesium in
the gastrointestinal tract preventing reabsorption [I2, D1].

Caesium migrates rapidly into cells of the body following
intake and becomes relatively uniformly distributed in soft tissues
[R2, L3, R4]. The metabolism in mothers and infants has also been
studied. There seems to be no placental discrimination, as the
newborn has 137Cs concentrations about equal to that of the mother
[B3].

Concentrations of caesium and potassium are low in fat
tissues. Therefore, for equal 137Cs concentrations in intake, the
concentrations of 137Cs (Bq per kg body weight) in males are higher
than in females, due to the higher average proportion of fat tissue
in the female body. However, a difference is also expected due to
longer retention time of caesium in males. Expressed in Bq 137Cs
per gK, the difference between males and females is somewhat
reduced.

It has been inconclusive for some time whether 137Cs
concentrates in bone. One study reported that the concentration of

137Cs in rib bones, which were free of muscle but not of marrow,
was comparable to the concentration in soft tissues [Y2]. The
results were variable, and subsequent studies both did and did not
confirm these results [A3, H3, N4]. From the results of a recent
study it appears that caesium associated with bone is present in
the marrow portion with only slight uptake by the hard tissue [H2].

A slower turnover of caesium in bone could allow
concentrations in bone to lag behind those in tissue, causing
higher relative concentrations in bone during periods of decreasing
intake. A longer retention half-time would eventually be noted in
whole-body measurements. However, such a component has not been
identified in over 1000 days of measurement following an acute
intake case [R4].

Retention half-time

A great many investigations of the biological half-time in
man have been conducted. Many of the references are collected in
the discussion by Lloyd [L6]. The half-time in man varies
considerably, depending on age and other factors. The half-time is
less in women than in men, and the half-time in children and
infants is less than in adults. Pregnant women have shorter
caesium half-times than in their non-pregnant conditions. Table
VII.4 shows the summary of 137Cs retention half-time reported
recently by the NCRP [N1], using the data of Lloyd et al. [L7] and
Zundel et al. [Zl]. 

Table VII.4  Retention half-time of  137Cs in the 
human body [N1]
--------------------------------------------------
Subjects         Number   Age        Half-time (d)
--------------------------------------------------
Men              26       23-55 a    105 ± 25
Women            15       20-51 a    84 ± 20
Pregnant women   24       16-39 a    49 ± 16
Children         7        5-17  a    57 ± 20
Infants          5        17-143 d   19 ± 8
--------------------------------------------------

The biological half-time for caesium in man can be considered
a function of age for juveniles and of sex for adults, but it is
not determined by body mass [L6]. Half-time and body mass may,
however, be dependent on some other common factors. The rate of

caesium turnover may be under hormonal influence or control or may
reflect the general metabolic rate [L6].

Shorter half-time components of 137Cs in man have been
reported, including one of only 2 to 3 hours [N3]. In general, two
components of the 137Cs half-time in man have been established: a
small fraction (10 to 15%) excreted with a short half-time (1 to
1.5 days) and the remainder excreted more slowly (50 to 150 days)
[R1, R3]. The ICRP suggests representative values of retention of
10% with a half-time of 2 days and 90% with a half-time of 110
days. Integral retention is 143 Bq d per Bq intake, contributed
almost entirely by the long-term component.

Transfer factor

The value of the transfer factor P34 relating concentrations
of 137Cs in diet and man can be derived from the integral retention
by dividing by the potassium content of the body (140 g) and
multiplying by the daily potassium intake (3.3 g d-1) [I5]. The
result is 3.4 Bq a (gK)-1 in man per Bq a (gK)-1 in diet.

The relatively short biological half-time of caesium in the
body makes it possible to assess the transfer factor P34 from the
measured 137Cs/K quotients in diet and man integrated over a few
years. Using this procedure, an average value of 3 Bq a (gK)-1 per
Bq a (gK)-1 diet is derived [U6, U7, U8].

Detection
Radiocaesium (both Cs-137 and Cs-134) are readily detected by gamma analysis. This can be accomplished using a variety of detection systems (e.g. NaI or Hyperpure Ge detectors) and is reasonably straightforward at relatively high levels (e.g. in radio-tracer experimental work). Typically (and fortunately) most environmental levels of these nuclides are low and this increases the technical difficulty in their measurement.
Key considerations in analysis of environmental samples include detector efficiency, background radiation, sample preparation and geometry. A thorough description of the issues involved are given by Sutton (1993). In the case of very low level samples the efficiency of gamma detection may not be high enough and the use of beta analysis may be appropriate (see for example Carpenter et al 1995).
In the specific context of assessing the transfer of radionuclides from soils to plants the concept of a transfer factor‚ is often used. This is simply the ratio of plant concentration (Bq/kg) to soil concentration (Bq/kg). However care must be taken to define to depth of soil sampled as this simple operational factor can often have an effect on the resultant value. A good review of the use and determination of soil-plant transfer values can be found in Frissel et al (2002).

Sampling

Methods
Detailed description of considerations and methods for gamma analysis of environmental materials is published by the Centre for Environment Fisheries and Aquaculture Science. An automated procedure is also described.

MRLs

Uses
Caesium (Latin caesius meaning "sky blue") was spectroscopically discovered by in 1860 in mineral water. Its identification was based upon the bright blue lines in its spectrum and it was the first element discovered by spectrum analysis. Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical applications. Caesium is one of the only three metals liquid a room temperature (melting point 28.5 C).
Caesium has 32 known isotopes which is more than any other element. The atomic masses of these isotopes range from 114 to 145. Nevertheless it only has one naturally occurring stable isotope, Cs-133. The radiogenic isotope Cs-137 has been used in hydrologic studies, analogous to the use of H-3. Cs-137 (and to a lesser extent Cs-134) is produced from detonation of nuclear weapons and emissions from nuclear power plants, most notably the 1986 Chernobyl explosion. Throughout the era of atmospheric testing of nuclear weapons radiocaesium was released into the atmosphere where it can be deposited onto surfaces.

Notes
Dosimetry

Dose per unit intake

The dose from 137Cs in tissue is due to the beta particles
from 137Cs decay and to the photon, x-rays, and conversion and
Auger electrons from decay of the daughter, 137mBa. A portion of
the photon energy will escape from the body, depending on the body
size. Calculations have been performed for uniform distributions
of 137Cs in the body for a range of proportions and masses
corresponding to infants, children and adults [N1, F1].

The average dose rate within the body for a uniform 137Cs
concentration of one Bq per kg body weight is about 3.5 nGy/d from
beta particles in both adults and infants plus 3.2 nGy/d from
photons in the adult and about 1.6 nGy/d in infants. The totals
are 6.7 nGy/d per Bq/kg in the adult and 5.1 nGy/d per Bq/kg in the
infant.

For 140 gK in the 70 kg adult body, the dose rate
corresponding to 1 Bq 137Cs per gK would be 


            µGy d-1   140 gK  365 d       µGy/a
    0.0067  --------  ------  ----- = 4.9 -------
            Bq(kg)-1  70 kg   a           Bq/gK

Spiers [S2] also obtains this result and estimates the
corresponding dose rate in a child weighing 8 kg of 

                µGy/a
          4.1  -------
                Bq/gK

UNSCEAR [U6, U7, U8] assessed the transfer factor between
tissue and dose, P45, as 4.9 µGy per Bq a (gK)-1. The value is
nearly independent of age, being only slightly less for the child.
For a single uptake of 137Cs, the integral retention per unit
intake is 143 Bq d per Bq intake (paragraph 317). The average
absorbed dose in the body per unit intake is thus 

          µGy/d     Bq d  1
   0.0067 ----- 143 ----- ----- = 1.4 10-8 Gy per Bq intake.
          Bq/kg     Bq    70 kg

This result applies to intake by ingestion. For inhalation
the value is less by a factor of 0.63 (for particles of 1 µm size)
due to fractional deposition in the lungs of inhaled amounts.
Following inhalation there is only a short retention in the lung
(half-time = 0.5 d) for the soluble caesium compounds and a small
dose primarily from the beta particles before the 137Cs becomes
distributed throughout the body.

Bibliography

Mode of action

Ecotoxicology

Plant metabolism & residues

Physicochemical properties
Property Value

Notes

Toxicological end points
Property Value

Notes

Environmental fate values
Property Value

Notes

Ecotoxicological end points
Property Value

Notes