However, there is a small probability of radon decaying before it is exhaled. Therefore the radioactive progeny, rather than radon itself, presents health hazard.
Indi an Journal of Pure & Appli ed Physics Vol. 39, November 200 I, pp . 738-745
Studies on indoor radon/thoron and their progeny levels at My sore city, Karnataka state LA Sathi sh, J Sannappa, L Paramesh, M S C handras hekara & P Venkataramaiah Department of Studies in Physics. University of Mysore, Manasagangotri. Mysore 570 006 Received 17 April 200 I ; rev ised 2 July 200 I; accepted 20 September 200 I Radon. thoron and their progeny concentrations have been measured in different types of buildings at different locations in Mysore city over a period of 2 years using solid state nucl ear track det ector (SSNTD) method. Th e dose to the selected popul ation of Mysore city has bee n estimated. The arithmetic mean (AM ) values of concentrations of rado n and thoron in indoor atmosphere ranged from 9.20 to 58.02 Bq m·3 with a median of 34 Bq m·3 and 7.2 1 to 59.27 Bq m·3 with a median of 33 Bq m·', respectively. The arithmeti c mean of progeny concentration s varies from 0.033 to 2.52 mWL with a median of 1.29 and 0.074 to 47.04 mWL with a median of 20.7, respectively. Th e dose to the selected popul ation of Mysore city has been found to be 1.58 mSv /.The concentrati ons of radon/thoron and th eir progeny vary with the type of floori ng and venti lation conditions. Diurnal and seasonal variations have also been observed . Hi gher radon concentrati on has been found durin g ni ghts and early morning hours.
1 Introduction
mu
and 232 Th are naturally occurring radioisotopes of uranium and thorium present in soi l and rocks at varying concentrations . They generate radon (222 Rn) and thoron (Z 20 Rn ), through radioactive decay, which are noble radioactive gases. These gases diffuse continuously from the soil to the atmosphere. Both radon ( 222 Rn ) and thoron (Z 20 Rn) have relatively short radioactive half-lives and decay to isotopes of so lid daughter products known as radon or thoron progeny. When the atoms of progeny are first produced they will be positive ions and they are very reactive and will mostly attac h themselves to water molecules and aerosols in air. In the atmosphere we have radon progeny in excess and are present in attached and un attached form s. Whe n inhal ed both attached and unattached radon progeny will be deposited in respiratory tract. In the decay of the progeny, alpha particles are e mitted , whic h irradiate the cell s of the lung ti ss ues. If radon gas present in the atmosphere is inh aled it is largely breathed out. However, there is a small probability of radon decaying before it is exhaled. Therefore the radioactive progeny, rather than radon itself, presents health hazard . To obtain an accurate estimate of radon re lated lung cancer risk and to plan proper control measures the population dose must be determined. The soil in
the Mysore city is red sandy loam . The soi ls being porous permit free internal and downward movement of water and also diffusion of radon into the atmosphere depending on water content.
2 Experimental Methods 2.1 Radon concentration (LLRDS) Estimation of radon concentration in ind oor atmosphere was made using the Low Level Radon Detection System (LLRDS). The system used in the experiment was fabricated by Srivastava 2• Th e procedure consists of co llectin g the air sample in an evacuated chamber and exposing a circular metallic (aluminium or stainless stee l) disk of 50mm diameter to the air containing radon inside the co llection chamber. The di sc is mai nta in ed at a negative potential of -800V with respect to the bod y of the chamber, which is grounded. Radon decay products, as they are produced, are known to be positively charged. The decay products form ed inside the chamber get attracted to the disc which is negativel y ch arged. The decay products are collected for 75 min. The di sc is then ta ken out and counted for a lph a act ivity typica lly for about 5000 s. Radon concentration in the c hamber is given by 2 :
Rn (Bq. m) = I OOOC E.F.V.Z
... ( I )
SATISH et al: RADON, THORON & THEIR PROGENY
L=Samp ling rate in litre per minute (LPM) , and
where C=The net total number of counts observed during the counting period ,
F (T,t)= Workin g leve l factor correspondin g to a sampling timeT min and counting de lay oft min .
£=The alpha counting efficiency (26%),
2.3 Measurement of radium and thorium in soils
F=The efficiency of collection of RaA-atom s on the disc and is empirically related to humidity by F=0.9 { 1-exp (0.039H-4. 11 8) }, where H is re lative humidity (%),
The gamma ray spectrometry was employed to estimate the acti vity of 226R a and m Th in soil s6 .
2.4 Solid state nuclear track detector (SSNTD)
Z =The theoretical correcti on factor for build -up of radon daughter atoms on the di sc and decay during exposure and counting period , and
The concentrations of radon, thoron and their progeny are measured in some of the dwellin gs of the Mysore city using solid state nucl ear track detectors (SSNTD), which are thin sheets of di e lectric materi als such as cellul ose nitrate (CN) and polycarbon ates. They are sensitive to alpha but not to beta and gamma radiations. They are unaffected by moderate humidity, heat and li g ht. For indoor measu re ments normall y LR-115 TYPE II (Kodak Pathe, France) plastic track detector (C N film) is preferred.
V =The volume of LLRDS chamber (litres).
2.2 Radon daughter products (Kusnetz's method) Diurnal variations of concentrations of radon daughters in indoor and outdoor air were measured usi ng Ku snetz's method. Air was drawn through a glass fiber filter paper by means of a suction pump at a known flow rate. The decay products of radon in air get deposited on the filter paper. The filt er paper was then alpha counted after a spec ific time delay. Rad on daughter concentration (in working leve l unit) was calcul ated usin g the Ku snetz's equation modified by Raghavayya4 . The express ion used to calculate the radon daughte r concentration is:
c
The doubl e c hamber dosimeter c up used for monitorin g radon , thoron and their progeny is shown in Fig. I . Each chamber has a le ngth of 4 .5 e m and a radius of 3. 1 em. Th e SSNTDs used are 12 J..Lm thi c k. The SSNTD I placed in compartment I measures onl y radon which diffuses into it from the ambie nt air through a semi-pe rmeable membrane (e.g. latex , cellulose nitrate, etc.) . These membranes have permeability constants in the range of IO-x- 1o-7 cm.s- 1 and a llow more than 95 % of the radon gas to diffu se and suppress th oron gas to less th an I %. On the other hand , the glass fibre filter paper in
... (2)
Ru(WL) = - - -E.L.F(T,t)
739
where C=The count rate, £=Efficiency of a lpha countin g system (26 %),
~~~ '/"'"/.
~//~,;P;
~
~
CD
©
f~
(%)
@
'
~
Fig. I -
v
--
~
Exp loded vi ew of th e double chamber dosimeter cup :
A-Membrane; ( I) CN film in Rn chamber. B-Filtcr paper; (2) CN film in Rn + Tn chamber, C-Perforated covers: (3) CN film , bare detector
©
INDIAN J PURE & APPL PHYS, VOL 39, NOVEMBER 2001
740
compartment 2 allows both radon and thoron gas to diffuse in and hence the tracks on SSNTD2 are related to the concentration of both gases. The SSNTD3 exposed in the bare mode (placed on the outer surface of the dosimeter) registers alpha tracks attributable to the airborne concentrations of both the gases and their progenl. These dosimeters are suspended from the mid-point of the house at a height of 2 metres from ground level. At the end of the stipulated period of exposure, usually about I 00 days, the dosimeters are retrieved and 3 SSNTDs are etched with I 0 % of NaOH solutions for I hr at a bath temperature of about 60 oc. The track density of alphas in the film was determined using a spark counter. This exposure cycle has been extended in a time integrated four quarterly cycles to cover all the four seasons of a calendar year to evaluate the annual indoor radon I thoron and their progeny levels. The radon/thoron levels and their progeny working level concentrations are calculated by the following relations~:
T
CR(Bq.m·')=-m-
... (3)
d.Sm CR(Bq.m·')= Tr -d.CRSrr
... (4)
d.S,r
d=Period of exposure( days), Sm=Sensitivity factor of membrane compartment, T,=Track density of the film in filter compartment, S,,=Sensitivity of radon in filter compartment, and CR=Radon concentration CT=Thoron concentration
c F. Rn (mWL)=~
... (5)
3.7 CF R ( rn WL) = ____I____I_ T 0.275
... (6)
where Rn=Radon progeny concentration, RT=Thoron progeny concentration, FR=O.I 04 !RA + 0 .518 !RB + 0 .37JR(",
FT=0.9l !TB + 0 .09 fm where !Rc etc are activity fractions with respect to parent gas, FR and FT are the equilibrium factors for radon and thoron progeny respectively, corresponding to the extracted ventilation rate. From their working leve l equilibrium fac tors are concentrations the calculated. Finally an estimation of the inhalation dose in mSv.y-' may be provided using the UNSCEAR formula 7
D = JO·'{ (0.17 + 9FR)CR +(0. 11 + 32FT)CT}
where Tm=Track density compartment,
of
the
film
m
membrane
3 Results and Discussions Fig. 2 and Table
I show the average (6
Radon concent:ra.tion 1.2
.., .... / /
1.0
-- -.- -- - -
E
Radon progeny
0 .8
/
...,. -·
/
-
...
0 .6
-
0
0
6
8
10
12
14
16
"18
e
0..
c
""'... CG 0
0.4 0.2
4
>. c u
co
~
. - - - -s-
2
~
20
22
Time of the day in hours
Fig. 2 - Diurnal variation of indoor radon concentration and indoor radon progeny
24
SATISH et al: RADON, THORON & THEIR PROGENY
74 1
Table 1 _Diurnal variation of rad on and its progeny concentration in the indoor and outdoor atmosphere Time of the day (Hr)
Indoor
Rn Cone. (Bq m' 3 ) 10.50 12.5 1 12.00 12.98 I 1.80 I 1.00 10.85 8.98 7.60 7. 10 6.58 9.58
2 4 6 8 10 12 14 16 18 20 22 24
Outdoor
Rn Progeny (mWL) 0.65 0.98 0.87 0.97 0.87 0.72 0.61 0.57 0.45 0.44 0.52 0.68
Rn Cone. (Bq .m-3 ) 5. 10 8.60 9.5 8 3.30 2.80 3.40 2.60 2.80 4.80 5.35 4.88 8.80
Rn progeny (mWL) 0.45 0.86 0.88 0.78 0.75 0.55 0.42 0.30 0.27 0.22 0.40 0.48
Pressure (mm)
Wind velocity ( m.s- 1)
Temperature Humidit y ("C) (%)
701.4 702.8 702 .8 703.0 703.0 703.0 703.5 701.2 700.9 700.5 700.0 701.0
0.8 0.5 calm 1.5 1.8 2.1 2.6 2.3 calm 1.5 0.3 calm
27 26 25 .5 28 30 32 33 35 34 31 30 29
65 70 70 50 26 22 20 18 24 54 57 60 8
Rodon c:on.c:c::ntration I
E ci~
'5
6
~ E
= .2
~ g
tO
,- -- -- -·
8
-§ """
'
'
/
S'" ~
4
, g
,
~
Radon progeny p-roc:luct.s
5
2.
0
w
s
"""
0 R
Sc:.a...sona.J -...,a.ria.tion. o£ radon c o n e . a.n.d it.s
A.
progeny
Fig. 3(a)- Seasonal variation of indoor radon concentrati on and its progeny
measurements in the month of August and September) diurnal variation s of radon and its progeny. During night time, concentrations of radon and its daughter products are usually higher than those in day time. In outdoor, the maxima occur in the early hours of the day (0400 to 0600 hr). In the indoor (all the windows are closed) the maximum concentration occurs during the early morning hours up to mid-day (0400 to 1200 hr) . The existence of higher concentrations in the outdoor atmosphere during early morn ing hours may be due to inversion conditions in the _lower atmosphere and also low wind speed . But m the indoor atmosphere higher concentrations occur during early hours up to mid-d ay becau se, after sunrise, convective current develops s lowly.
In the outdoor during invers ion, upward move ment of atmospheric air is low. As a result , the concentration at the ground level is higher. After sunri se the convective current of the atmosphere transports the radon and its progeny to the hi gher levels decreasing the concentrations at lower leve ls. After sun set, when the atmospheric temperature begin s to decrease the inversion condition s lowl y set in. As a result the concentration of radon and its progeny starts increasing. Thi s process gets repeated. The seasonal variation of concentration of radon and thoron progeny in ind oor atmosphere in different types of buildings is shown in Figs 3 (a) and (b) . The concentration is m ax imum durin g the
742
INDIAN J PURE & APPL PHYS , VOL 39, NOVEMBER 200 1
-
16
8
12
6
·('f1
I
E
g ,..
"' "' "'
C"
co .._
·c
c
·-.0.. ...,..
>.
c
~
8
~
0 () ,..
0
4
Thoron concentration
co 0
~
0. ·
·
0 () c o·
-. l= ~
c 4
'' '
c
·0~ 0
''
2
'
' , Tho ron pr9geny product~ 1
~
I , J
0
0 W
S
R
A
Seasonal variation of thoron cone. and its progeny products Fig. 3(b)- Seasonal variation of ind oor th oron concentration and its progeny: W-winter, S-summer, R- rain y season . A-autumn
winter periods of November-February as observed elsewhere 1• It is essentially influenced by the temperature inversion and also because a lmost all windows are closed during winter season. But in summer low concentrations of radon , thoron and their progeny were observed because of the vertical mixing and di spersion. Further during summer fans are used and all windows are kept open. During the rainy season and autumn the radon and thoron daughter concentrations do not show much variations. Table 2 gives the results of measurements of annua l average concentration s of radon/thoron and their progeny in different types of bu ildings at different locations of the Mysore city. Table 3 gives
the di stribution of 226 Ra and m Th in different parts of the Mysore city. The data show slightly hi gher concentration of radon/thoron and their progeny in K G Koppal , Vivekanandanagar and J C Nagara (southern part). In K G Koppal the hou ses are very old, and they have mud walls, loose cement flooring and very poor ventilation . The gap between the one house to the other is very narrow. In Yivekanandanagar and J C Nagar the houses are relatively new and partially ventilated . In thi s region 226 Ra and m Th concentrations are slightly high er compared to other parts of Mysore ci ty. In northern part we observe low concentration of radon/thoron and their progeny. This may be due to the low concentration of 226Ra and m Th presen t in the soil.
SATISH et al. : RADON , THORON & THEIR PROGENY
743
Table 2 - Annual average radon/thoron and their progeny concentrations and the dose rates
Location
Type of houses (flooring and roof)
No. of hou ses
Concentration (Bq m·J) Rn
.
14 4 2 23 12 10 8 7 5 6 5 3 4
Mosaic' Lashkar Mohalla
Cement Cement
..
...
Cement' Cement'' Siddiquenagar
Mosaic '
Naidunagar
Mosaic ·
Mandimohalla
Cement
Tilaknagar Badamakkan
Cement' Cementttt
N R Mohalla
Cement
Rn
Progeny concentrations (mWL)
Dose (mSv.Y-1)
Tn
Rn
Tn
0.133 0.109 0.003 0.278 0. 102 0.265 0.391 0.041 0.131 0.094 0.001 0.391 0.196
1.819 1.088 0.189 2.124 0.840 1.846 2.520 0.694 1.156 1.363 0.033 1.756 1.218
7.435 3.105 0.079 19.0 15 5.793 16.472 20.716 1.319 13.480 7.096 0.077 47.038 13.447
0.926 0.654 0.702 0.970 0.876 0.922 0.858 0.593 1.396 1.0 13 0.920 1.264 0.961
(A) Year 1998-99 (Northern region)
. Cement Stone
Tn
Eq. Factor
..
...
14.65 13.04 14.97 12.89 12.79 12.98 13.2 1 10. 12 17.39 12.62 9.25 9.20 12.52
15.29 7.78 7.2 1 18.79 15.58 17.11 14.57 8.96 28.30 20.69 21.15 33.08 18.87
0.460 0.309 0.046 0.61 0.243 0.526 0.706 0.254 0.246 0.400 0.013 0.706 0.36
(B) 1999-2000 (Central. South-western and Southern region) Central Agrahara
Mosaic Red oxide'
.
South-western Saraswathipuram
Mosaic
KG Koppal
Red oxide Marble' Red oxide'
Jayanaga r Kuvempunagar Sriramapura R K Nagar
.
.
Granite Red oxide'
.
Mosaic Red oxide' Mosaic Mosaic' Red oxide'
Vivek ananda nagar Red oxide' Mosaic'
.
Southern J C Nagara * Concrete roof, t Wooden roof,
Cement Mosaic'
**B it tiled roof, tiled roof,
tt Small
8 10
38. 1 40.68
20.21 0.078 45.46 0.013
0.00625 0.801 0.001 0.143
0.459 0.165
1.844 2.661
6 10 2 12 2 7 6 5 7 14 10 5 6
36.80 44.4 35.27 54.51 50.35 30. 18 26 .65 38.76 29.98 34.04 30.08 52.89 37.04
15.66 38.56 59.27 42.09 43.21 20.30 22.05 40.58 19.24 19.27 19.24 40.48 27.63
0.003 0.001 0.0618 0.001 0.001 0.001 0.0045 0.0045 0.001 0.0015 0.001 0.00125 0.001
0.502 0. 166 1.670 0.192 0.177 0. 137 0.448 0.608 0.116 0.237 0. 106 0.289 0.160
0. 171 0. 140 13.309 0.153 0. 157 0.074 0.361 0.664 0.070 0. 105 0.070 0.184 0. 100
1.669 2.591 2.882 3.018 2.917 1.583 1.5 18 2.457 1.548 1.682 1.553 2.919 2.026
7 5
58.02 41.76
40.20 0.025 27.28 0.017
0.0015 0.001
0.392 0. 192
0.219 0.099
3.081 2. 170
*** Asbestors roof, "' Mangalore tiled roof
0.051 0.014 0. 175 0.013 0.013 0.017 0.062 0.058 0.014 0.026 0.013 0.020 0.016
INDIAN J PURE & APPL PHYS, VOL 39, NOVEMBER 2001
744
In the south-western part (Ramakrishnanagara, Sriramapura, Saraswathipuram and Kuvempunagara) houses are almost of new type and partially ventilated. In these regions concentration of 226 Ra and 232 Th do not vary significantly. The concentrations of radon/thoron and their progeny also do not vary significantly.
Table 3 - Distribution of radium-226 and thorium-232 Concentration (Bq. Kg. 1)
Location
A Northern region Lashkar mohalla Siddique nagar
15 .5 1 9.85
26.59 20.50
Naidu nagar
9.65
Tilak nagar Bada makkan
7.85
18.68 15.80
10.12
19.20
8 Central region 16.98 Agrahara C South western region 13.85 Saraswathipurm 17.55 KG Koppal 12.80 Jayanagar 13.1 0 Kuvempunagar 15.12 Srirampura 14. 15 R K Nagara D Southern region. 24.00 1 C Nagara 13.85 AM 4.10 SD
22.00 25.00 29.00 13.25 15.50 17.55 15.20 55.90 22.66 10.00
Table 2 also shows that in the houses with stone and marble flooring concentration of radon/thoron and their progeny is less but in the houses with granite flooring it is slightly higher because granite contains high concentrations of 226 Ra and 232Th . The data show that red oxide flooring houses have slightly higher concentration compared to mosaic flooring houses. This is due to low diffusion coefficient and less porosity resulting in less radon exhalation in mosaic-flooring houses. The long-term measurements during a period of two years in about 200 dwellings can be expected to be representative data for Mysore city. From this data, the population dose may be estimated. Following this, a median value for radon progeny (1.29 mWL) and that for thoron progeny (20.7 mWL) is used to calculate the dose rates. The inhalation dose to the selected population of Mysore
city due to radon/thoron progeny activity is found to be 1.58 mSv.y- 1•
4 Conclusion Radon/thoron and their progeny concentrations in Mysore city vary with ventilation conditions, types of the flooring , and types of the materials used for construction of houses. High concentration is observed in very poorly ventilated and loosecemented flooring houses. The average annual dose to the population of Mysore city due to radon and thoron daughter activity works out to be 1.58 mSv.y·1·7·x . All India mean effective dose equivalent is 2.49 mSv.y- 1 and the global effective dose equivalent is 2.4 mSv.y- 1• Seasonal variations in the concentration of indoor radon/thoron and their progeny concentrations are observed. Higher values were found in winter and lower in summer. Also maximum concentrations have been observed in rainy and autumn seasons in some dwellings. Indoor radon/thoron concentrations at Mysore are comparable with the global average value; the global average values of indoor radon/thoron concentrations being 40 Bq.m·' and I 0 Bq.m·l, respectively 7 •
Acknowledgements The authors express their profound gratitude to Dr M C Subba Ramu, Retired Scientific Officer, BARC, Mumbai , presently at Mysore and M Raghavayya, Scientist, Rare Earth Materi al Projects, BARC, Mysore, for useful discussion s and constant encouragement. Thi s work was carried out under a Coordinated Radon Project (CRP) sponsored by the Board of Research in Nuclear Science, Department of Atomic Energy, Government of Indi a.
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Srivastava G K, A study of the potential international radiation hazards and their control in Uranium mines and milling, PhD thesis, University of Bombay, Part II, Chapter 6-12, 1994.
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Kusnetz H L, Indian Hyg As soc Quarterly, 17 ( 1956) 85.
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Ragavayya M. Bull Radial Prot Environ, 27 ( 1998) 3.
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IAEAIRCA , Regional work on environmental sampling and measurement of radioactivity for monitoring purposes.
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Report to th e UN General Assembly with Sciemi(ic Annexes, 1993.
Health Physics Division, BARC, Kalpakkam, ( 1989) pp. 85-95. 6
Mayya Y S, Eappan K P & Nambi K S V, Radial Protect Dosimetry, 77 ( 1998) 177.
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UNSCEAR, Sources and effects of ionising radiation,
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Narayanan K K, Krishnan D K & Subba Ramu M C, Indian Society for Radiation Physics (ISRP CK BR-3). 1991 , p. 6.
