Indoor radon/thoron levels and inhalation doses to some populations in Himachal Pradesh, India

Abstract

It is well established that some areas of Himachal Pradesh (H.P.) state of India situated in the environs of the Himalayan mountains are relatively rich in uranium-bearing minerals. Some earlier studies by our group have indicated high levels of radon (>200 Bq m⁻³) in the dwellings. It is in this context that an indoor radon/thoron survey has been carried out in selected villages of four districts in the state of H.P. This survey has been conducted as a part of a national, coordinated project using twin chamber dosemeter cups designed by the Environmental Assessment Division (EAD), Department of Atomic Energy, Govt. of India. The track-etch technique is used for calibration of plastic detector LR-115 type-II which are employed for recording alpha tracks due to radon/thoron and their daughters. Year long radon/thoron data have been collected for seasonal correlations of indoor radon/thoron in the dwellings. The indoor radon levels have been found to vary from a minimum value of 17.4 Bq m⁻³ to a maximum value of 140.3 Bq m⁻³. The indoor thoron levels vary from a minimum value of 5.2 Bq m⁻³ to a maximum value of 131.9 Bq m⁻³. The year average dose rate for the local population varies from 0.03 µSv h⁻¹ to 0.83 µSv h⁻¹. The annual exposure dose to inhabitants in all the dwellings lies below the upper limit of 10 mSv given in ICRP-65.

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Introduction

The measurement of radioactivity in the human environment, in general and in the Himalayan ecosystem, in particular, is of special interest to mankind. It has been established that radon is a causative agent of lung cancer when present in high concentration e.g., in uranium mines where some case studies are reported.^1–3 The health hazard of radon is principally due to its short-lived daughters, especially ²¹⁸Po and ²¹⁴Po. During recent years, several reports have appeared in the literature regarding the ever-increasing interest in monitoring radon in the indoor environment all over the world.^4–10

It is well established that some areas of Himachal Pradesh (H.P.), situated in the environs of Western Himalaya are quite rich in radioactive minerals.^11–12 Some of these sites in H.P. state were exploited by the Atomic Minerals Division of Department of Atomic Energy (DAE) Govt. of India. An earlier survey reported very high values of indoor radon in dwellings around some of these sites.¹³ High values of soil-gas radon varying from 13300 Bq m⁻³ to 75400 Bq m⁻³ were recorded by our group in a previous spot survey.¹⁴ Based on these facts, we decided to monitor radon inside dwellings in some of the villages falling in four districts of H.P., specifically Una, Hamirpur, Kullu and Kangra, as part of a national coordinated radon project sponsored by DAE.

Hamirpur and Una districts lie in the middle and lower Siwalik Himalaya while Kangra valley is enclosed between the middle Siwaliks and the Dhauladhar range of Western Himalaya. Siwalik sediments contain, in general 3–10 ppm of uranium which is much higher than the world average of 2.1 ppm in greywackes and 1.5 ppm in arkoses.¹⁵ Uranium anomalies were also reported in river waters flowing through Western Himalaya.^16,17 Due to this compelling evidence of high radioactivity reported in soil-gas and water channels of H.P., it has become obligatory to undertake this survey for indoor radon monitoring for estimation of radiation inhalation dose delivered to inhabitants due to radon/thoron and their progenies.

Experimental technique

We have used twin chamber dosimeter cups to measure the concentration of radon and thoron separately (Fig. 1). Plastic detector (LR-115 type II) films 2.5 × 2.5 cm² were exposed in these cups, the latter being obtained from Environment Assessment Division, BARC, Trombay. Each cup has two chambers, each with a height of 4.5 cm and diameter 6.2 cm. The detectors were fixed at the bottom of each chamber, the mouth of one chamber being covered with glass fibre filter paper and the other with a semi-permeable membrane. These cups also have provision for exposing the detector in bare mode on the outer side of the cup. The detector that is placed in the chamber covered by a membrane records alpha tracks due to radon (²²²Rn), the membrane only allowing radon to pass through it, suppressing the thoron to less than 1%. The other detector, covered with filter paper, records tracks due to alpha particles from radon and thoron. The bare detector records the tracks due to alpha particles from radon, thoron and their progeny and is used to determine radon and thoron progeny concentration in mWL (milliWorking Level). From this data the inhalation dose due to radon, thoron and their progeny can be calculated.

Fig. 1 Rn–Tn discriminating dosemeter.

Theoretical formulation

The methodology proposed by Mayya et al.¹⁸ for mixed field situations has been used to calculate the dose rate as well as equilibrium factors. Here we let T₁, T₂, T₃ be the track densities for the membrane chamber film, filter paper chamber film and bare film, respectively. S₁ and S₁′ are the calibration factors for the radon in the membrane and filter compartments, respectively, S₂ is the calibration factor for thoron in the filter compartment, and d is the exposure period in days. Thus the radon concentration

C_R = T₁/(dS₁), (1)

and the thoron concentration

C_T = (T₂ − dC_RS₁′)/(dS₂) (2)

The bare track density T₃ is related to the concentration of both the gases and their daughters through the equation:

T₃ = S₃d[{C_R + C_R-A + C_R-C} + {2C_T + C_T-C}] (3)

where S₃ is the calibration factor for the bare film, C_R-A and C_R-C are the concentrations of the radon daughters ²¹⁸Po and ²¹⁴Po, respectively, and C_T-C is the concentration of the thoron daughter ²¹²Po.

The activity fractions of the progeny are controlled by their wall loss rates for the fine fractions (λ^F_W), coarse fractions (λ^C_W) and ventilation rate (λ_V), through use of the following formulae:

For radon progeny:

F_R-A = λ_R-A/[λ_R-A + f_Aλ^F_W + (1 − f_A)λ^C_W + λ_V] (4)

F_R-B = F_R-Bλ_R-B/[λ_R-B + f_Bλ^F_W + (1 − f_B)λ^C_W + λ_V] (5)

F_R-C = F_R-Bλ_R-C/[λ_R-C + f_Cλ^F_W + (1 − f_C)λ^C_W + λ_V] (6)

where f_A, f_B, f_C are the unattached fractions for the respective species and λ_R-A, λ_R-B, λ_R-C are the decay constants for Ra-A (²¹⁸Po), Ra-B (²¹⁴Pb) and Ra-C (²¹⁴Po) respectively.

For thoron progeny:

F_T-B = λ_T-B/(λ_T-B + λ^C_W + λ_V) (7)

F_T-C = F_T-Bλ_T-B/(λ_T-C + λ^C_W + λ_V) (8)

assuming that thoron progeny unattached fractions are negligible.

In eqns. (7)–(8), λ_T-B, λ_T-C are the decay constants for Th-B (²¹²Pb) and Th-C respectively.

With the use of these, eqn. (3) can be written as

T₃ = S₃d[C_R{1 + F_R-A + F_R-C} + {2C_T′F_T-C}] (9)

In the above equation, C_T′ is the room average concentration of thoron. Since thoron has a short half life of only 55 s, its concentration cannot be uniform in the room. It will have a concentration profile while its relatively longer-lived daughters will mix more or less uniformly in the room. So it is assumed that they will be fractions of average thoron conentration in the room, C_T′ rather than C_T.

The spatial distribution of thoron near any emitting surface can be approximated by a one dimensional profile given by eqn. (10).¹⁹

C(x) = C₀exp(−x/ξ) (10)

where C₀ is the concentration at the surface, x is the normal distance and ξ is the characteristic length of the profile. It is assumed that all the surfaces emit thoron at same rate. The characteristic length is given by ξ = (K/λ_T)^1/2 where K is the eddy diffusivity in the room and λ_T is decay constant of thoron. Doi et al.¹⁹ reported the value of K to be 5.4 cm² s⁻¹ for Japanese houses. To include the ventilation effects on the profile, it is assumed that ventilation would increase ξ. If λ_V is the air change rate for the room and L is the average wall to wall extension of the room, the average convective flow velocity will be v = Lλ_V. Then solving the convection diffusion equation i.e.,

KC″(x) − vC′(x) − λ_TC(x) = 0. (11)

The ventilation dependent characteristic length is calculated to be

ξ(λ_V) = (L/2λ_T)[λ_V + {λ_V² + 4Kλ_T/L²}^1/2] (12)

where λ_V is air exchange rate and L is average wall to wall extension of the room.

The average thoron concentration C_T′ in the room is calculated using

C_T′ = (1/L)∫^L₀C_T(x) dx = C₀(ξ(λ_V)/L)[1 − exp( − L/ξ(λ_V))] (13)

Mayya et al.¹⁸ approximated the measured concentration by that in the central region in the room and using this approximation calculated the room average thoron concentration. Since in actual conditions, the dosemeter is never placed at the centre of the room, so we worked out a correction factor by altering one parameter i.e. the distance of the dosemeter from the nearest surface. Here we have used the actual distance of dosemeter from the nearest thoron emitting surface i.e. roof or wall. So, if Y is the distance of the dosemeter from the nearest surface and L taken as the perpendicular distance between this surface and the opposite surface,

C_T = C_T(Y) = C(L/z) = C₀ exp(−L/[zξ(λ_V)]) (14)

using (13) and (14)

C_T′ = [C_Tξ(λ_V)/L] [exp(L/[zξ(λ_V)]) − exp(L(1 − z)/[zξ(λ_V)])] (15)

where z is a constant factor such that Y = L/z

Using the value of C_T′ from eqn. (15) in eqn. (9) and using the subsequent equation and eqn. (12), the value of λ_V can be found out.

Using the value of λ_V in eqns. (4)–(8), F_R-A, F_R-Betc. can be calculated which lead to equilibrium factors for radon and thoron.

The progeny working levels are determined using equations:

WL_R = C_RF_R/3700 = C_R(0.104F_R-A + 0.518F_R-B + 0.37F_R-C)/3700 (16)

WL_T = C_T′F_T/275 = C_T′(0.908F_T-B + 0.092F_T-C)/275, (17)

where C_T′ is the room averaged thoron concentration, calculated taking into account the spatial profile of thoron.

The dose rate is calculated by use of a formula given in UNSCEAR(1993):²⁰

D/μSv h⁻¹ = 10⁻³[(0.17 + 9F_R)C_R + (0.11 + 32F_T)C_T′] (18)

A computer program Track1 has been developed at BARC to carry out these calculations and we have used the following typical values for the various parameters: f_A = 0.2, f_B = 0.025, f_C = 0.001, λ^F_W = 20 h⁻¹and λ^C_W = 0.2 h⁻¹, S₁ and S₁′: 0.019 tracks cm⁻² Bq⁻¹ d⁻¹ m³, S₂: 0.025 tracks cm⁻² Bq⁻¹ d⁻¹ m³, S₃: 0.021 tracks cm⁻² Bq⁻¹ d⁻¹ m³.

Results and discussion

For Hamirpur and Una districts, the total exposure period of one year, from August 1997 to September 1998, was divided into two three-month surveys and one six-month survey. The exposure period of one year from 18 November 1998 to 20 November 1999, for dwellings of Kangra and Kullu districts was surveyed in three cycles of four months each. Utilizing the software supplied by DAE and subsequently modified by us, radon and thoron gas concentrations were calculated. Results from 22 dwellings in the Hamirpur and Una districts are reported in Table 1. The values in the tables are the yearly averages for individual quantities. The radon concentration has been found to vary from a minimum value of 36.2 Bq m⁻³ to a maximum value of 140.3 Bq m⁻³. The thoron concentration varies from 16.5 Bq m⁻³ to a maximum value of 92.4 Bq m⁻³. It may be mentioned here that the thoron values inside the dwellings strongly depend upon the distance at which the dosimeter is placed, from any thoron-emitting surface, i.e. roof, walls or floor, because of its very short half-life compared with radon. Hence the room averaged thoron concentrations are considered as true representative values of indoor thoron. The room averaged thoron concentration is found to vary from a minimum value of 3.2 Bq m⁻³ to a maximum value of 54.9 Bq m⁻³. From Table 1, it is evident that radon values are below 200 Bq m⁻³ in all the dwellings of two districts, the lower limit for action level proposed by ICRP-65. The equilibrium factor for radon in dwellings is found to vary from a minimum value of 0.08 to a maximum value of 0.53 while the equilibrium factor for thoron is found to vary from a minimum value of 0.01 to a maximum value of 0.24. Radon and thoron progeny concentrations are also given in Table 1. Thoron progeny contributes predominantly to exposure dose rates in dwellings varying form 0.5 mWL to a maximum value of 32.2 mWL. The total dose due to radon and thoron progenies is estimated for all 22 dwellings considering an annual occupancy of 80% i.e., 7000 h. The exposure dose rates for inhabitants are found to vary from a minimum value of 0.06 µSv h⁻¹ to a maximum value of 0.59 µSv h⁻¹ that correspond to an annual dose rate of 0.42 mSv a⁻¹ and 4.13 mSv a⁻¹ respectively. The total annual exposure dose delivered to inhabitants lies below the upper limit of 10 mSv, prescribed by ICRP recommendations.²¹

Table 1 Yearly average radon/thoron concentration levels, equilibrium factors, progeny levels and dose rates in some dwellings of Hamirpur and Una districts, Himachal Pradesh

Sample no.	C R/Bq m^−3^a	C T/Bq m^−3^b	C T′/Bq m^−3^c	F R ^d	F T ^e	mWLR/mWL^f	mWLT/mWL^g	D/μSv h^−1^h
a Radon concentration. b Thoron concentration. c Room averaged thoron concentration. d Equilibrium factor for radon. e Equilibrium factor for thoron. f Radon progeny concentration. g Thoron progeny concentration. h Inhalation dose rate.
1	50.9 ± 4.6	17.6 ± 1.9	3.2	0.53	0.24	7.2	2.8	0.27
2	137.7 ± 9.4	73.7 ± 11.6	26.9	0.48	0.16	18.0	17.1	0.77
3	47.1 ± 3.2	24.6 ± 1.9	4.8	0.53	0.24	6.7	4.2	0.27
4	140.3 ± 13.6	65.3 ± 7.2	20.3	0.32	0.13	11.6	6.8	0.47
5	89.5 ± 7.6	72.9 ± 4.9	49.9	0.08	0.01	2.5	0.5	0.11
6	128.1 ± 16.1	53.9 ± 8.1	19.4	0.17	0.06	6.6	8.8	0.33
7	113.2 ± 10.0	20.7 ± 2.4	9.0	0.24	0.09	10.5	7.0	0.42
8	61.2 ± 2.4	36.8 ± 2.7	6.0	0.43	0.13	7.3	2.4	0.27
9	68.3 ± 4.7	48.7 ± 6.1	54.9	0.44	0.15	8.3	32.2	0.59
10	62.8 ± 6.2	92.4 ± 10.0	20.3	0.44	0.12	7.6	8.3	0.34
11	85.5 ± 4.4	61.1 ± 6.7	33.6	0.31	0.13	5.4	3.0	0.22
12	41.3 ± 2.4	54.1 ± 5.0	22.2	0.14	0.01	1.3	0.3	0.06
13	37.2 ± 2.4	27.4 ± 2.7	5.4	0.42	0.18	4.2	2.1	0.17
14	45.4 ± 1.8	26.9 ± 2.6	9.7	0.53	0.24	6.4	8.5	0.30
15	56.1 ± 5.7	66.9 ± 5.2	11.6	0.32	0.05	4.7	1.6	0.18
16	36.2 ± 1.2	27.0 ± 3.1	5.3	0.49	0.18	4.8	3.7	0.20
17	36.3 ± 2.0	16.5 ± 1.2	6.5	0.53	0.24	5.2	5.7	0.23
18	37.5 ± 1.5	29.9 ± 1.6	11.2	0.51	0.21	5.2	7.8	0.25
19	39.0 ± 2.5	20.3 ± 3.2	7.6	0.53	0.24	5.6	6.7	0.25
20	60.7 ± 1.5	49.7 ± 4.6	18.0	0.45	0.17	7.2	9.5	0.33
21	47.0 ± 2.6	40.2 ± 3.1	6.6	0.20	0.02	2.7	0.5	0.10
22	49.3 ± 2.5	17.1 ± 2.1	3.6	0.49	0.18	6.6	1.8	0.25
Mean	66.8 ± 4.8	42.9 ± 4.3	16.2	0.39	0.14	6.6	6.4	0.29
s	33.9 ± 4.0	22.2 ± 2.8	14.4	0.14	0.08	3.5	7.0	0.16

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The radon/thoron concentrations, equilibrium factors, progeny concentrations and dose rates for 25 dwellings in Kullu and Kangra districts are summarized in Table 2. Radon concentration varies from 17.4 Bq m⁻³ to 75.1 Bq m⁻³ while the thoron concentration varies from 5.2 Bq m⁻³ to 57.5 Bq m⁻³. The equilibrium factors for radon and thoron vary from 0.12 to 0.53 and from 0.01 to 0.24 respectively. Radon/thoron progeny concentrations vary from 0.5 mWL to 8.3 mWL and from 0.4 mWL to 60.0 mWL, respectively. Due to large variations in equilibrium factors and ventilation rates of dwellings, the indoor thoron progeny levels show wider fluctuations compared with radon. The inhalation dose rates vary from 0.03 µSv h⁻¹ to a maximum value of 0.83 µSv h⁻¹ which corresponds to an annual dose rate of 0.21 mSv a⁻¹ and 5.81 mSv a⁻¹, respectively. Hence, for all the dwellings surveyed in Kullu and Kangra districts, the total annual dose delivered to inhabitants lies within safe limits (3–10 mSv) as recommended by ICRP-65.

Table 2 Year average radon/thoron concentration levels, equilibrium factors, progeny levels and dose rates in some dwellings of Kangra and Kullu districts, Himachal Pradesh

Sample no.	C R/Bq m^−3	C T/Bq m^−3	C T′/Bq m^−3	F R	F T	mWLR/mWL	mWLT/mWL	D/μSv h^−1
1	29.4 ± 2.5	11.8 ± 1.8	56.8	0.28	0.09	2.2	45.6	0.49
2	31.7 ± 2.9	13.1 ± 2.0	9.3	0.39	0.09	3.6	3.9	0.16
3	17.4 ± 2.2	6.5 ± 6.3	28.9	0.27	0.12	1.2	8.5	0.12
4	54.8 ± 3.3	57.5 ± 6.3	83.2	0.18	0.02	2.9	7.5	0.18
5	28.9 ± 2.6	26.8 ± 3.8	31.6	0.19	0.02	1.5	4.0	0.09
6	38.0 ± 3.3	7.4 ± 1.0	3.1	0.27	0.09	2.9	0.8	0.11
7	41.7 ± 2.3	10.2 ± 1.0	7.9	0.53	0.24	5.9	6.9	0.27
8	63.3 ± 3.6	35.3 ± 4.2	25.5	0.23	0.03	3.6	2.3	0.15
9	18.7 ± 1.0	22.2 ± 1.5	14.6	0.12	0.01	0.7	0.4	0.03
10	75.1 ± 2.5	19.7 ± 1.5	92.2	0.41	0.17	8.3	60.0	0.83
11	60.2 ± 4.2	13.2 ± 1.7	6.0	0.30	0.06	4.0	1.2	0.15
12	43.2 ± 3.6	12.3 ± 1.4	86.7	0.42	0.12	4.9	49.3	0.61
13	26.3 ± 2.2	7.4 ± 1.2	2.0	0.36	0.10	2.8	0.8	0.10
14	32.2 ± 3.1	33.2 ± 3.9	55.5	0.28	0.07	2.6	15.5	0.24
15	50.6 ± 3.0	14.3 ± 1.7	9.2	0.21	0.03	2.7	0.9	0.11
16	43.7 ± 2.2	36.7 ± 2.9	24.4	0.20	0.03	2.3	1.3	0.10
17	23.2 ± 1.8	5.2 ± 0.8	3.8	0.47	0.15	2.9	1.9	0.12
18	27.1 ± 1.3	20.4 ± 2.0	14.7	0.19	0.08	0.5	3.0	0.05
19	28.8 ± 3.0	25.8 ± 4.2	13.0	0.37	0.12	3.1	5.7	0.16
20	23.9 ± 1.0	17.3 ± 1.3	44.7	0.53	0.24	3.4	39.1	0.47
21	27.7 ± 1.0	34.1 ± 3.2	36.2	0.43	0.17	3.4	23.5	0.33
22	31.9 ± 1.6	24.5 ± 2.8	6.8	0.53	0.24	4.5	6.0	0.21
23	53.7 ± 3.3	33.4 ± 3.7	9.2	0.40	0.11	5.7	5.0	0.24
24	33.1 ± 1.6	17.8 ± 2.0	10.2	0.29	0.12	1.9	2.1	0.09
25	51.7 ± 2.0	11.2 ± 0.8	5.4	0.07	0.01	1.0	3.1	0.04
Mean	38.3 ± 2.4	20.7 ± 2.5	27.2	0.32	0.10	3.1	11.9	0.22
s	15.0 ± 0.9	12.5 ± 1.6	27.6	0.13	0.08	1.8	17.3	0.19

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Radon/thoron concentrations will depend upon the type of dwelling, particularly the building materials used for construction. Most of the dwellings are constructed using local sandstone and mud-mortar as the main building materials. Some kutcha houses have mud flooring while others have wooden or cemented flooring. Another factor that influences the radon/thoron concentration inside the dwellings is the nature of the soil at the plinth level and the nature of the crustal rocks underneath. High radon/thoron levels are recorded in the dwellings constructed near the vicinity of uranium/thorium-bearing soils in H.P. state.

From the point of view of construction, we have divided the dwellings into two categories, those with mud floor as type I and others as type II. The concentration of both radon and thoron in both types of the dwellings is given in Table 3. The average concentration of radon is higher in type I dwellings as compared with type II dwellings, indicating that subsurface soil may be the predominant source of indoor radon. However in case of room averaged thoron, the values are very nearly equal for the two types of dwelling. Since the dosemeter was generally placed at a height of about 2.5 m from the floor, any effect of subsurface soil on indoor thoron concentration could not be detected. The main sources of indoor thoron at this level are the walls of the room.

Table 3 Distribution of radon/thoron in different types of houses

Type of house	No. of houses	Average concentration/Bq m^−3
		Radon	s	Thoron	s
I (mud floor)	21	57.7	28.4	23.0	25.5
II (concrete or other floor)	26	47.0	19.5	21.3	20.9

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Conclusions

1. The source of high natural radioactivity in H.P. state of Western Himalaya is the presence of U/Th bearing minerals in the form of sandstones and sedimentary rocks.

2. Thoron concentrations are significant and a predominant source of inhalation dose inside dwellings.

3. The indoor radon levels have been found to vary from 17.4 Bq m⁻³ to 140.3 Bq m⁻³ and thoron levels from 5.2 Bq m⁻³ to 92.4 Bq m⁻³ inside dwellings.

4. The integrated annual exposure dose rate varies from 0.03 µSv h⁻¹ to 0.83 µSv h⁻¹ which corresponds to an annual dose rate of 0.21 mSv a⁻¹ and 5.81 mSv a⁻¹, respectively.

5. This is the first systematic survey in dwellings located in Western Himalaya where radon and thoron contributions are taken into consideration separately. Evidence for thorium bearing rocks is established.

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