Estimation of Thyroid Doses from Inhalation of 131I for Population ofContaminated Regions of Belarus
Vladimir A. KNATKO and Inga N. DOROZHOKThe Institute of Radiobiology, National Academy of Sciences,
Kuprevich Str-2, 220141, Minsk, Republic of Belarus: V_Knatko@radbio.bas-net.by

Abstract
Based on the relationships between 131I and 137Cs content in soil samples and the data on 137Cs
contamination of settlements, the 131I deposition density S n (131 I ) was estimated for the settlements
located in the eastern and the southern areas of Belarus (1,079 and 316 settlements, respectively). The
results show that the 90% interval of the S n (131 I) quantity is about 500 - 2,300 kBq/m2 and 700 3,500 kBq/m2 for the eastern and the southern area, respectively. Using assessments of S n (131 I) ,thyroid doses from inhalation of 131I were evaluated with taking into account the period and characterof radioactive deposition in various parts of the areas. According to the results, the thyroid doses for
adult population of the eastern and the southern areas vary from 3 to 80 mSv and from 40 to 370 mSv,
respectively, while the median doses are about 20 and 130 mSv, for the eastern and the southern area,
respectively. Uncertainties in the calculation of thyroid doses, due to the procedure applied, werediscussed.

Introduction
The data on cancer incidence rates accumulated in Belarus after the Chernobyl accident [1] show astatistically significant increase in the number of thyroid cancer, especially among the children. Accordingto [1], in period 1991-2000 standardized incidence rate for thyroid cancer increased from 3.6 up to 9.2 per
100 000 population (about 2.6 times). The fast increase of thyroid cancer cases evoked interest to the
problem of thyroid dose assessment. The individual thyroid dose estimations in Belarus were derived
primarily from the results of direct 131I thyroid measurements carried out in May and June of 1986 [2].
Note that 131I is the most important radionuclide that caused serious thyroid exposures in the affected areasin the first few weeks following the accident. Thyroid cancers risk assessments obtained in the frameworkof the ICRP recommendations by using the doses derived from thyroid measurements show an excess ofobserved thyroid cancer cases over the expected number [3]. In this connection the assessment of thyroid
doses from inhalation and ingestion of 131I is of interest for a prediction of radiological consequences by
the Chernobyl accident.
The measurements of 131I activity in thyroid of people and determination of 131I concentration in food
stuffs and air were accomplished only in part of the contaminated territory. In order to evaluate the thyroid
doses for the population of those regions where no monitoring was performed, a procedure based onreconstruction of 131I deposition density can be applied. Using the 131I-137Cs relation obtained in [4] and thedata on 137Cs contamination of settlements (see [5]) the 131I deposition density was evaluated in the present
paper for settlements placed in the eastern and the southern areas of Belarus (see Fig.1). Based on theseassessments thyroid doses from inhalation intake of 131I were obtained for adult population.
Reconstruction of 131I Deposition Density
The estimates of 131I deposition density were calculated on the basis of the relationship between 131I
and 137Cs activity concentration in soil samples taken in the contaminated regions under consideration [4].
160M in skM o gilev

BELARUS

Russia50km 100km

C h ern ob yl

50km

100km

Eastern area

G om el0km

0km

UkraineSouthen area

Chernobyl
0km

50km

100km

Fig. 1. Areas under investigation.
The points show settlements where soil samples were taken. Dot line within the southern area shows theboundary of the Polesskyi state radioecological nursery (the zone of evacuation).
Note that the relationship had been deduced in [4] with taking into account the contribution of 137Cs global
fallout that is about 2.2 kBq/m2. The regression function describing the data for the contaminated regions
was written in form:
131

I = A k ⋅[ 137 Cs] Bk(1)where the radionuclides activity in soil is expressed on May 10, 1986, and the parameters Ak and Bk arecharacterized by the following values: A1=33.8 kBq/m2, B1=0.63 and A2=69.4 kBq/m2, B2=0.59 for theeastern (k=1) and the southern (k=2) area, respectively. Starting from the expression (1), the mean densityof 131I deposition S n (131 I) in the territory of the n-th settlement situated in the studied areas was obtainedby using the formula:

S n ( 131 I) = A k ⋅ [ S n ( 137 Cs)] Bk

(2)

137

where S n ( Cs ) is the level of settlement contamination with

137

Cs. Empirical values of S n (137 Cs )expressed on May 10, 1986 were taken from [5].
It should be noted that the settlements situated in the territory of the zone of evacuation (see Fig.1)
were not included in the present calculations of S n (131 I) . Population of the evacuation zone was removed
during the first week of May 1986 when the radioactive release still continued. Dose estimation forremoved inhabitants needs special consideration taking into account a day of evacuation.
The distributions of values S n (131 I) calculated for 1,079 and 316 settlements situated in the easternand the southern areas, respectively, are presented in Fig.2 in comparison with the distributions ofempirical data on S n (137 Cs) . According to the results, the values of S n (137 Cs) and S n (131 I) are describedby a lognormal distribution:

161

Relative
Frequencies

A: Eastern

0.20

0.18
Model lognormal
a=0.78(0.04)
b=5.49(0.04)----------------E0 =307.960
M0 =256.632
N=10790.16

Model lognormal
a=0.48(0.03)
b=6.99(0.04)------------E0 =1175.30
M0 =1113.74
N=1079

0.15

0.12

0.12

137

Cs

0.08

131

I

0.09

0.06

0.04
0.03

0.00

0.00
0

400

800

1200

1600
137

2000

0

500 1000 1500 2000 2500 3000 3500 4000 4500

2131

Sn( Cs), kBq/m
Relative
Frequencies

2

Sn( I), kBq/m

B: Southern

0.25

0.20
Model lognormal
a=0.72(0.04)
b=5.26(0.05)-------------------E0 =296.29
M0 =192.03
N=316

0.20

Model lognormal
a=0.46(0.03)
b=7.33(0.04)--------------E0 =1849.19
M0 =1543.86
N=316

0.15

0.15

137

Cs

0.10

131

I

0.10

0.05
0.05

0.00

0.00
0

300

600

900

1200
137

1500

0

1000

2000

3000

4000
131

2

5000
2

Sn( I), kBq/m

Sn( Cs), kBq/m137

Fig. 2. Distributions of Cs contamination level and theestimated 131I deposition density for settlements situated in
the eastern (A) and the southern (B) areas of Belarus.

f(x) =
The

exp[ −(ln x − b) 2 / 2a 2 ].
2πa xparameter(3)

a in (3) characterizing skewness of the distribution is defined asa = [2(ln E − ln M )] , where E is the average, and M = exp(b) is the median of the distribution. The
1/ 2

analysis of the distribution parameters shows that the median value of S n (137 Cs ) obtained for thesettlements situated in the eastern area is about 35% larger than in case of the southern area. At the same2

time the variation V = σ/E = ( e a − 1) 1 / 2 of values S n (137 Cs) takes the close values for the studied areas.
Note that in accordance with [6] the quantity of variation of radionuclide deposition density reflects theheterogeneity of Chernobyl fallout.
According to the distribution parameters derived (see Fig.2), the median value of S n (131 I) obtained
for the settlements situated in the southern area is about one third larger than in case of the eastern area.
The 90% interval of the S n (131 I) quantity is about 500 - 2,300 kBq/m2 and 700 - 3,500 kBq/m2 for theeastern and the southern area, respectively. The comparison of the distribution characteristics given in
Fig.2 shows that the ratio of median values Rmed = med[ S n (131 I) ]/med[ S n (137 Cs) ] for the eastern area isabout 1.8 times smaller than for the southern area. It should be stressed that the present values of S n (131 I )162

Relative
Frequencies
0.25

A: Eastern

0.20
Model lognormal
a=0.27(0.01)
b=1.49(0.02)-------------E 0 =4.65
M 0 =4.34
N=1079

0.15

0.10

0.05

0.00
0

2

4

6

8

10

12

14

16

18

20

R

0.15

B: Southern
Model lognormal
a=0.37(0.03)
b=2.07(0.04)---------------E 0 =8.07
M 0 =8.04
N=316

0.12

0.09

0.06

0.03

0.00
0

2

4

6

8

10

12

14

16

18

20

22

R

Fig. 3. Values of the ratio R=Sn(131I)/Sn(137Cs) obtained for settlements situated in
the eastern (A) and the southern (B) areas.
were obtained by using the radionuclides activities expressed on May 10, 1986.
The distributions of the isotope ratio R = S n (131 I ) / S n (137 Cs) are given in Fig.3. According to the results,
about 90% of ratio values obtained for the eastern area range between 3 and 7. In case of the southern area
the quantity R takes a value mainly in the interval 4 < R < 13. The estimations of R agree with the isolinesof the isotope ratio 131I/137Cs in the Chernobyl fallout at Belarus territory (see [7]). The observed
difference between isotope ratio 131I/137Cs in deposition of Chernobyl radionuclides can be caused byseveral factors, in particular, various gaseous and aerosol iodine fractions in radioactive discharges of theChernobyl NPP as well as the deference of wash-out rate between aerosol and gaseous iodine (see [8]).
Evaluation of Thyroid Dose from Inhalation of 131I
Thyroid doses from inhalation intake of 131 I were estimated in a framework of the approachdescribed in [9]. The formula used for calculation of the inhalation dose D was written as(4)
D = d ⋅ A = d ⋅ f ⋅C ⋅ r ⋅ t ,where d is committed effective or organ dose equivalent per unit intake via inhalation, Α is activity
intake over the time period t , C is the mean activity concentration of radionuclide in air, r is inhalation
rate, and f is the shielding factor of inhabitants. The values of quantities d and r for different age
groups of population are presented in [10] and [11], respectively. Following to [9] it was conservatively
assumed for the purposes of dose assessment that f = 1.
163

The radionuclide concentration in air C was estimated on the basis of the data on deposition density
S, deposition velocity v , and the time period of deposition T

C=

S
.
v ⋅T

(5)

According to the data presented in [12], in general the contamination of air in the southern area continued
for nine days. About 90% of 131I activity fell out during the period 8:00 April 26 - 8:00 April 29, 1986.
The territory of the eastern area was contaminated by radioactive fallout mainly in two days, namely 8:00
April 27 - 8:00 April 29. The fraction of 131I activity which fell out during the pointed period on the
territories of different administrative districts situated in the eastern area range from 75% to 95%. The
contamination of the eastern area persisted until May 10, 1986.
Data from [12] show different characteristics of radioactive deposition among the areas underconsideration. The territory of the southern area was contaminated by dry deposition, whereas wet
deposition prevailed in the eastern area. The contribution of wet deposition to the overall contamination of
territory of several districts in the eastern area was about 75%. The velocity values of dry (vd = 0.4 cm/s)
and wet (vw = 3.0 cm/s) deposition of 131I used in the present work were taken from [13].
The expression for evaluation of settlement contamination with 131I is written as

S n ( 131 I) = ∑ S n (t m ) ⋅ k (t m ) ,

(6)

tm

where S n (131 I) is the density of 131I deposition at the territory of the n-th settlement on May 10, 1986,
k (t m ) = exp(−∆t m / τ) , ∆t m = t f − t m is the period from the final day (tf) of deposition (i.e. May 10, 1986)

and the m-th day of deposition, and τ is half-life time of 131I. In order to use the data on daily depositionof 131I the quantity S n (t m ) was expressed

S n ( t m ) = a (t m ) ⋅ ∑ S n (t m ) ,

(7)

tm

where a(t m ) is daily fraction of summarized deposition activity of 131I. The values of a(t m ) characterizes
daily variation of 131I deposition in the period April 26 - May 10, 1986 (see [14]). Taking into accountexpressions (6) and (7) the relationship between S n (131 I) and ∑ S n ( t m ) is written astm∑ S n (t m ) = S n (131 I)/[∑ a(t m ) ⋅ k (t m )] .

(8)

tm

On the basis of Equations (4) - (8), the expression for estimation of thyroid dose from daily (t = T = 1day) inhalation of 131I was defined as

Dn (t m ) = d ⋅ f ⋅ r ⋅ h ⋅ S n ( 131 I)/v (t m ) ,(9)where h = a(t m ) /[∑ a(t m ) ⋅ k (t m )] . Distributions of S n (131 I) values used for dose calculations are given in
Fig.2. Starting from equation (9), the doses from inhalation during the main period of radioactive
deposition ∆t = April 26 - May 10, 1986 were estimated as a sum of doses from daily inhalation intake of131I:

Dn ( ∆ t ) = ∑ Dn ( t m ) .

(10)

tm

Results and Discussion
The calculation results show (see Fig.4) that thyroid dose equivalent for adult population of the areas
under investigation varies from a few to a few hundreds of mSv. Dose values obtained for 95% of thesettlements situated in the eastern and southern the areas are smaller than 45 and 300 mSv, respectively.
The maximal dose is about 80 mSv in the eastern area and about 370 mSv in the southern area. The
164

median value characterizing the dose distribution obtained for the southern area is about 6 times larger
than in case of the eastern area. The computations of thyroid dose for children show that the dose
assessments obtained for children of 10 years old are 1.8 times larger than the results for adult population.
It should be emphasized that present dose assessments correspond to conservative (largest) value of thefactor f. Note that thyroid doses from inhalation intake of 131I are smaller then doses from 131I intake withfoodstuff. According to [12], 131I intake with cow milk evaluated for adult residents of settlements locatedin Gomel district (southern area in our case) are higher than 500 mSv.
It should be emphasized that the present dose estimation based on the relationship between 131I and137
Cs activities in soil does not takes into account local features of 131I deposition and inhalation. As aresult the spatial distributions of the reconstructed 131I deposition densities S n (131 I) and the thyroid doses
Dn are similar to the distribution of the137

Cs contamination levels of the settlements S n ( 137 Cs) in the areaof investigation. In particular, two maxima revealed in the statistical distribution of dose Dn calculated for
population of the eastern area may be caused by the peculiarities of location of settlements in this area (seeFig.2 and Fig.4).
Based on the expressions (9) the behavior of the normalized thyroid dose D n0 = D n / S n ( 137 Cs) can bemodeled by the power function, which is simply deduced using the isotope ratio R = Sn (131I) / S n (137 Cs ) :Dn0 ∝ [ S n (137 Cs)]Ck(11)

Relative
Frequencies
0.15

A: Eastern

0.12

Model lognormal
a=0.48(0.03)
b=3.03(0.03)---------------E 0 =22.91
M 0 =21.40
N=1079

0.09

0.06

0.03

0.00
0

20

40

60

80

100

D

0.18

n

,mSv

B: Southern

0.15

Model lognormal
a=0.45(0.03)
b=4.85(0.04)---------------E 0 =155.29
M 0 =129.65
N=316

0.12

0.09

0.06

0.03

0.00
0

100

200

300

400

500

D

n

,mSv

Fig. 4. Distributions of thyroid dose Dn from inhalation of 131I calculated for adult
population of the settlements located in the eastern (A) and the southern (B)areas.
165

where Ck = Bk − 1 < 0 (see (1)). According to equation (11), the normalized dose Dn0 decrease withincreasing of 137Cs contamination density. Equation (11) can partly explains the decreasing trend in thenormalized mean values of thyroid dose measured for population of 70 settlements with the increase of the137
Cs contamination level [2].
The present estimats of thyroid dose from inhalation of 131I are associated with uncertainties arisingfrom different sources. One of them is the 131I deposition velocity v depending on type of deposition (wetor dry). No data about the quantity v in the areas of investigation are available. For this reason the velocity
values vw = 3.0 cm/s and vd = 0.4 cm/s derived in [13] for radioactive deposition were used for the present
calculation. The value 3.0 cm/s agrees with estimation 2.6 cm/s (the difference is about 15%) obtained forvelocity of 131I wet deposition in the town Obninsk situated on the territory of Kaluga region (Russia)
bordering upon the eastern area under investigation (see [15]). The velocity value for dry deposition (0.4cm/s) is two times larger than estimation 0.2 cm/s derived in the measurements carried out in the ResearchCenter of Seibersdorf (Eastern Austria) in April-July 1986 [8]. Considering that the radionuclide
concentration in air is directly related with deposition velocity, the value of deposition velocity would be
the important source of uncertainty in dose estimation, in particular for the southern area where dry
deposition was dominant.
The next source of uncertainties is dose dependence on the period of 131I deposition. In thisconnection it should be noted that the previous thyroid dose estimation for population of the eastern area(see [16]) was carried out in proposal of constant daily wet deposition of 131I during April 28 - May 2,
1986. The comparison of the dose distribution given in Fig. 4 with data from [16] shows that dose values
obtained in [16] in average are about 20% smaller than the present estimations. The similar uncertainties
in dose estimation can be caused by errors in the parameters Ak and Bk characterizing the relationshipbetween 131I and 137Cs deposition densities, or errors in the level of settlements contamination with 137Cs(see [16]). Note that despite the uncertainties mentioned above, the assessment of 131I deposition density
and thyroid exposure based on the procedure described seems reasonable taking into consideration thelack of empirical data on 131I content in air and soil.
Conclusion
Using the relationships between 131I and 137Cs activity concentration in soil samples and the data on
the level of settlements contamination with 137Cs, the 131I deposition density S n (131 I) was evaluated for thesettlements situated in the territory of the eastern and the southern areas of Belarus. The results show thatthe median values of Sn (131 I) obtained for settlements located in the eastern and the southern area areabout 1.1 and 1.5 MBq/m2, respectively.
Based on the assessments of S n (131 I ) thyroid doses from inhalation of 131I were calculated for adult
population of the studied areas. The results show that thyroid doses Dn for population of the eastern areachange in region 3 < D n< 80 mSv. In case of the southern areas the dose values vary from 40 to 370 mSv.
The median doses are about 20 and 130 mSv, for population of the eastern and the southern area,
respectively. The observed difference between doses is caused mainly by different character of 131I
deposition in the studied areas.
The values of thyroid dose normalized to the level of settlement contamination with 137Cs tend to
decrease with increasing 137Cs deposition density. This tendency in behavior of the normalized dosereflects the non-linear relationship derived between 131I and 137Cs concentration in soil.
The results of the present estimation of 131I deposition density and thyroid doses from inhalation of131
I will be included in wide-scale computations of thyroid dose for population of the contaminatedregions of Belarus.

166

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