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The
disaster at the Chernobyl Nuclear Power Plant in 1986 led to a release
of at least 180 million Curies of radioactive substances (letting alone
the radioactive fallout of several tons of nuclear fuel around the station
V.B. Nesterenko, 1992). Among all the radloisotopes released into the atmosphere
the major contributors into the dose rate are Iodine- 133, Cesium 137 and
134, Strontium-90.
Iodine-131
has the half-life 8.05 days, Cesium-134 - 2.06 years, Cesium-137
-30 years, Strontium-90 - 29.12 years, meanwhile Plutonium-239 - 24,390
years V.B. Nesterenko, 1992).
While
the short-living isotopes (iodine-131, strontium-90), inert gases (krypton,
xenon, etc.) were the main contributors into the dose rate, Cesium 137
and 134 became the governing contributors in the second period. Also the
effect of strontium-90 and transuranium radioisotopes (Plutonium) included
into the composition of "hot" particles should be taken into consideration.
These radioactive elements are incorporated by the organism with food,
water and air, in addition to creating the external gamma-background.
Iodine-131
has the half-life 8.05 days and the main paths of its incorporation
are gastric, inhalation (absorption through outer skin makes up 1-204)
V.A. Bazhenov et al., 1990).
Significant
sources of penetration of radioactive iodine into the human organism are
foodstuffs of plant and animal origin, specifically milk, fresh milk products,
leaf vegetables. Iodine-131 concentration in the organisms of goats and
sheep exceeds that in cows several times. Meat basically does not contain
this radioisotope, yet its significant quantities are contained in bird
eggs.
Iodine-131
is rapidly absorbed by blood and lymph. The quantity and rate of absorption,
accumulation of this radioisotope in organs and tissues, the rate of its
excretion from the organism depend upon age, sex, concentration of stable
iodine in food (V.B. Bazhenov et al., 1990). The thyroid gland manifests
its maximum concentration. Already two hours after introduction of the
isotope its concentration in the thyroid amounts to 5-1004, and
after 24 hours 25-3~4 of the total amount V.A. Bazhenov et al.).
Other
organs (kidneys, liver, muscle and bone tissues) accumulate this radioactive
isotope in much smaller quantities.
It
has been revealed that iodine-131 crosses the placental barrier and penetrates
from the mother's organism into the fetus predominantly accumulating in
the thyroid (V.A. Bazhenov et al, 1990).
Primarily
kidneys excrete the radioactive iodine from the organism. Iodine-131 effects
upon the human organism during the first days after the disaster are injuries
of the thyroid. The dose burden upon this organ due to the incorporation
of radioactive iodine by humans is produced within a relatively short period
of time 2.5-3 months after the disaster because of the short half-life
of this element.
It
has been revealed that the thyroids of children (aged 0-17 years) living
in the Khojniki district received the highest doses of radiation (242-527
cGr). The thyroid doses in Gomel amount to 15-60 cGr, in Minsk - from 2.4
to 9.2 cGr.
Cesium
has 23 known radioactive isotopes, yet at present the current situation
is primarily determined by 137 and 137 isotopes.
7
mushrooms 9,787 Bq/kg (V.B. Nesterenko, 1986). Small quantities are incorporated through respiratory tracts (0.5%). After peroral incorporation and absorption by blood significant quantities of Cesium-137 are secreted into the intestinal lumen and reabsorbed by the colon. Such natural incorporation of the radioisotope together with food (grain crops grown in the areas contaminated with radloisotopes) leads to its different accumulation in tissues and organs.
Feeding
female and male common breed albino rats and Vistar line rats with oats
containing about 400 Bq/kg of 137Cs would result in its
significant accumulation in the organism after several weeks.
The
maximum 137Cs concentration has been registered in the tissue
of the myocardium (Fig.1), while its concentration
in bone and muscle tissues is much less (Yu.I. Bandazhevsky, G.S. Bandazhevskaya,
1995). Radiometric measurements of the autopsied material have revealed
the maximum quantities of this radioisotope in the thyroid gland per unit
of the organ weight, somewhat less in the heart, liver, skeleton muscle
(Yu.I. Bandazhevsky, A.M. Perepletchikov, 1996). These results are confirmed
by the studies of E.F. Lushnikov et al. (1996) who have demonstrated that
the maximum quantities of 137Cs are registered in the internal
organs among the residents of the Bryansk and Kaluga Region.
Pronounced
differences in the accumulation of 137Cs by males and females
should be emphasized. When the radioisotope is incorporated enterally male
organisms accumulate it more intensively than females. It is confirmed
by the results of numerous experimental studies of laboratory animals (Yu.I.
Bandazhevsky, N.E. Fomchenko et al., 1995) and by radiometric measurements
among the residents of the Gomel Region (Yu.I. Bandazhevsky et al., 1996).
A
comparative analysis of accumulation of 137Cs by children has
manifested that its concentration increases as a function of age (Fig.
2). In particular, children born in 1978-1981 had the 137Cs
concentration about 120 Bq/kg, meanwhile the children born in 1989-1996
had it equal to 60 Bq/kg. Laboratory
newly-born animals manifest the least 137Cs s concentration
in experiments (Fig. 3) (Yu.I. Bandazhevsky, T.S. Ugolnik, 1995). It is
confirmed by the results of radiometric studies of the autopsied material
of the children of the first year of life (Yu.I. Bandazhevsky9 A.M.
Perepletchikoy, 19%; E.F. Lushnikoy et al, 19%). Meanwhile, the concentration
of gamma-sources (137Cs and 134Cs) increases
strongly in the mother's organism during pregnancy (Yu.I. Bandazhevsky,
T.S. Ugolnik, 1995) exceeding the accumulation by non-pregnants (Fig. 3). Analysis
of accumulation of radioisotopes in the organisms of the offspring manifests
its rise in the areas with heavier 137Cs contamination (Yu.I.
Bandazhevsky et al., 1996). In
particular, the average 137Cs concentration in the organisms
of Gomel children per unit of body weight amounts to 30.0 Bq/kg (at a contamination
level with this element 1-5 Ci/km2), meanwhile its accumulation
is much larger in the areas with a much higher contamination level Fig.
4). A
high probability of incorporation of radioisotopes with mushrooms and berries
in the radiation contaminated areas should be taken into consideration. Mainly
kidneys are responsible for the excretion of 137Cs from the
organism when up to 8~/o of
137Cs introduced within a single time is excreted within a month.
The period of half-excretion of 137Cs from human organism is
70 days, from mice 3 days, from rats 18 days, from guinea pigs 19-25
days, from rabbits 19 days (N.V. Zhuravlev,1990). There
is a number of reagents influencing the process of incorporation of radioactive
cesium by human and animals organisms, enterosorbents, in the first place,
which combine radioactive elements, microelements, bacterial preparations,
chemical compounds in the lumen of the gastrointestinal tract which excrete
them. A
variety of different groups of such compounds have been proposed. Yet,
not all of them satisfy the requirements: (1) to reduce the accumulation
of radioisotopes in the organism; and (2) to restore the processes of metabolism. A
number of enterosorbents have been tested at the Gomel Medical Institute
in experiments with laboratory animals to assess their effectiveness in
respect to 137Cs. The sorbent containing 60% of modified clay
and 40% dextrin has been rated as the most promising enterosorbent which
never aggravates the effect of radioisotopes upon the liver and kidney
tissues, unlike other sorbents, such as organic silica or charcoal (N.E.
Fomchenko, 1997). Experiments
with sorbents containing pectins, have manifested their high effectiveness
in excreting radloisotopes from the organism. Pectopal belongs to this
group of effective compounds (Yu.I. Bandazhevsky, I.K. Lyakhova, 1997). Strontium
(90Sr) is a radiation source having half-life 28.5 years (V.F.
Zhuravlev, 1982). Strontium is a stable microelement actively involved
in the metabolism of plants, it is constantly present in the tissues and
organs of man and animals. Being analogous to calcium, upon incorporation
it gets involved in the mineral metabolism. 'It
penetrates into the organism through the intestinal gastric tract, lungs
and skin (V.5. Kalistratova, 1990). The greatest risk is produced when
irradiation is due to the penetration of 90Sr
through the nutrition tract (V.I. Ternov, 1988) when the levels of absorption
of strontium vary from 5 to 100% (V.S. Kalistratova, 1990). This
process is influenced by a number of physiological factors (age, pregnancy,
lactation, condition of vital systems of the organism). Irrespective of
the way and rate of incorporation, soluble 90Sr compounds are
accumulated by the bone tissue. Concentration
of this isotope in the bones of rat is 40-40 times higher already after
24 hours than in kidneys, spleen and muscles (V.S. Kalistratova, 1990)
with skeletons of male rats accumulating more than the skeleton of female
rats. Introduction of 90Sr
into the organism of pregnant animals leads to its accumulation in the
bone tissue of the offspring (V.S. Kalistratova, 1990). The
rate of strontium metabolism in the bone tissue of man is directly proportional
to age: it is 100% among
infants (up to one year of age), 40% among children and adolescents, 20% among
adults (V.I. Ternov, 1988). Thus,
continuous incorporation of 90Sr
leads to its maximum accumulation in childhood. Paroxysmal incorporation
of this radioisotope leads to its greater concentrations among senior age
adults.
In
particular, the average 137Cs concentration per body weight
in the organism of Gomel children in 1994 amounted to 30.0 Bq/kg (with
the soil contamination 1-5 Ci/km2), meanwhile in the areas with
a higher level of contamination, its accumulation is significantly higher
(Fig. 5).
