Assessment of progeny concentrations of ²²²Rn/²²⁰Rn and their related doses using deposition-based direct progeny sensors

Abstract

Indoor radon and thoron concentrations in the domestic environment result in natural radiation exposure to the public due to the inhalation of their short-lived decay products. Keeping this in view, the annual effective dose and other radiation risks due to radon/thoron progenies have been calculated. In this study, newly developed time deposition-based progeny sensors (DTPS/DRPS) were used for long-term passive determination of progeny concentrations in the environment of Jammu and Kashmir, Himalayas, India. The total equilibrium equivalent radon (EECR_A + U) and thoron (EECT_A + U) concentrations (“A” and “U” referring to attached and unattached fractions) were found to vary from 5 to 38 Bq m⁻³ with an average value of 18 Bq m⁻³ and from 0.48 to 5.49 Bq m⁻³ with an average value of 1.69 Bq m⁻³, respectively. The aerosol concentration, equilibrium factors, and unattached fractions for radon and thoron progeny have been estimated in normal living conditions and their dependence on each others have also been studied. The annual equilibrium factor for radon and thoron progeny has been determined from the calculated data. The estimated annual effective dose due to radon progeny (0.34 to 2.42 mSv y⁻¹) and thoron progeny (0.13 to 1.54 mSv y⁻¹) is found to be below the world’s recommended level. Based on measurements of mean values of the unattached fraction, dose conversion factors (DCFs) in units of mSv per working level month (WLM) has been calculated and the average calculated values of DCFs are 24, 10, and 13 mSv WLM⁻¹. The variability of equilibrium factor and radon/thoron progeny with different seasons, ventilation conditions, and types of houses were also analyzed.

Introduction

Radon (²²²Rn) is naturally existing radioactive gas, with a half-life of 3.8 days, which decays by emission of alpha particles and initiates a series of short-lived progeny: ²¹⁸Po (half-life 3 min), ²¹⁴Pb (27 min), ²¹⁴Bi (20 min), and ²¹⁴Po (164 μs). It is a well-known fact that inhalation doses due to radon and thoron are contributed by their progeny but not by the gas itself. Therefore, these progenies could be inhaled and are the major contributors to the dose from ionizing radiation received by the general population (UNSCEAR 2000). In past studies, thoron (²²⁰Rn) has been neglected due to its short half-life (Steinhausler et al. 1994), but today, it is recognized that it has a special significance in areas which have thorium-rich soil like certain areas in Brazil, China, and India. These are called high background radiation areas (HBRAs) and they are important for epidemiological investigation of the effects of low levels of radiation exposure on human health (Nair et al. 1999).

Inhalation of short-lived decay products of radon (²¹⁴Po) which is responsible for 40% of the total radiation dose received by the population is a major contributor to problems of respiratory system, lung cancer, and damage to sensitive cells in the skin and causes skin cancer but there is no definite identification of the target cells for skin (NRC 1991; Hopke 1992; Charles 2007). The decay products of radon and thoron are positively charged. They combine with negatively charged air ions and transfer their charge to the trace gasses and water vapors in air to form a group or cluster. Decay products having a size or activity median diameter (AMD) of less than 10 nm are referred to as unattached progeny (Reineking et al. 1985; Ramamurthi and Hopke 1989) while those with an AMD in the range 10–1000 nm are called radioactive aerosol and attached progeny (UNSCEAR 1993; Porstendorfer 2000). Unattached fraction, equilibrium factor, and size distribution are the main influent parameters involved in the lung dose calculation (Huet et al. 1999).

In different living/working environments, aerosol particle size and different ventilation rates may bring about a variation in unattached fraction (f_Rn) and since exposure to ²²²Rn progeny contained in attached and unattached fractions is differently diametrically effective, this consequently leads to quite different dose conversion factors (DCFs). The DCF is defined as the dose to the respiratory tract per unit exposure of radon progeny (Guo et al. 2012) and measured in terms of mSv per working level month (WLM). The WLM is the commonly used unit of exposure, even for domestic exposures which corresponds to an exposure of 1 WL during a reference working period of 1 month (2000 working hours per year/12 months = 170 h) (ICRP 1993). The larger the amount of aerosol particles in the air, the larger is the amount of radon progeny which settles on these particles. But these do not settle on furniture, walls, or curtains, in contrast to unattached progeny. Thus, there are more dangerous radon short-lived decay products in a “dusty” room than in a “clean” room with the same concentration of radon in the air (Jasaitis and Girgždys 2011).

The activity concentration of the short-lived radon progeny in air is always less than that of the radon gas. The ratio progeny:gas is called equilibrium factor F, which is a measure of the degree of disequilibrium between the radon gas and its progeny. If the activity concentration of the short-lived radon progeny is equal to the activity concentration of the radon gas (i.e., secular equilibrium has been reached), then F would be 1. But practically, it is always less than 1 (ICRP 1993).

In cold climate countries, the exposure of radon and thoron in indoor environment is considered as an important issue due to bad ventilation rate. Our study region is also a partial cold climate region of India, with towns and villages lying at the altitude of 3000 m above mean sea level. The climate of the study region is of mountainous type.

In the present study, attached and unattached concentrations of radon and thoron were measured separately in 102 dwellings of Udhampur district, Jammu and Kashmir, Himalayas. The estimation of total (attached + unattached) and unattached progeny concentrations was carried out by using LR-115 detectors in DRPS/DTPS bare mode and wire-mesh-capped progeny sensors, respectively. The attached and unattached fractions of radon and thoron progeny were also calculated. The equilibrium factors for radon and its progeny and for thoron and its progeny were studied for each dwelling, and the annual effective doses due to radon and thoron progenies were also estimated. Epidemiological studies are used for the calculations of dose conversion factors and their related doses which depend upon the unattached fraction, aerosol concentration, and tissue weighing factor.

Description of the study area

The state of Jammu and Kashmir has continuous boundaries with Russia, Afghanistan, Pakistan, and China. Due to its geographical situation, the state has become very important geographically, politically, economically, and from the military point of view. The study area is Udhampur district of Jammu and Kashmir state, India, as shown in Fig. 1, which extends from 32°34′ to 33°30′ North latitude and from 74°16′ to 75°38′ East longitude. Udhampur district falls into the submountainous region at the foothill of Himalayas. The study region is a highly hilly and low populated area. Soil of this region is sandy loam to sandy clay loam, mixture of gravels, sand, silt, and clay. The pH of the soil is acidic in nature, which could be due to acidic nature of parent material and leaching of bases due to high rainfall coupled with leaching of Al₂O₃ + Fe₂O₃. Minerals like copper, zinc, lead, and iron are also found in this soil (Gupta and Verma 1975). Most of the houses are made up of mud and bricks and ventilation of these houses is normally good.

Fig. 1

Map showing the 50 sampling sites of Udhampur district, Jammu and Kashmir

Materials and methodology

Radon, thoron, and their progenies (attached + unattached) were evaluated by pinhole single-entry cup dosimeters, direct radon/thoron progeny sensors (DRPS/DTPS), and attached progeny concentrations were measured by wire-mesh-capped DRPS/DTPS sensors. All detectors were calibrated at the Radiological Physics and Advisory Division, Bhabha Atomic Research Centre (BARC), Mumbai, India.

The set of dosimeters (one dosimeter, one bare mode DRPS/DTPS, and one wire-mesh-capped DTPS/DRPS) were suspended in indoor environment, at a distance of 10 to 20 cm from any surface and furthermore from any ventilation for a period of 1 year from November 2015 to November 2016. Detectors were initially installed for a period of 4 months. They were collected and new ones deployed after 4 months to cover three seasons (winter, summer, and rainy) of the year and to assess variability between them. The LR-115 (type II) solid state nuclear track detectors (size 2.5 × 2.5 cm²) were used in DRPS/DTPS. After exposure for 4 months, the detectors were retrieved and etched in 2.5 N NaOH solution for 90 min at 60 °C in a constant temperature water bath unit without stirring. After etching and drying, the detectors were peeled off from their cellulose acetate base. Tracks were counted with a spark counter (Model PSI-SC 1) manufactured by Polltech Instruments Pvt. Ltd., Mumbai (India).

Single entrance pinhole-based ²²²Rn/²²⁰Rn discriminating dosimeter

The radon (²²²Rn) and thoron (²²⁰Rn) gas concentrations were measured by using single-entry pinhole-based dosimeter cups. The details of the dosimeter cup are explained elsewhere (Sahoo et al. 2013). The radon (C_R) and thoron (C_T) gas concentrations were calculated as given by Sahoo et al. (2013)

$$ {\mathrm{C}}_{\mathrm{R}}\ \left(\mathrm{Bq}\ {\mathrm{m}}^{-3}\right)=\frac{{\mathrm{T}}_1-{\mathrm{B}}_1}{\mathrm{d}\times {\mathrm{K}}_{\mathrm{R}}} $$

(1)

$$ {\mathrm{C}}_{\mathrm{T}}\ \left(\mathrm{Bq}\ {\mathrm{m}}^{-3}\right)=\frac{\left({\mathrm{T}}_2-{\mathrm{B}}_2\right)-\left(\mathrm{d}\times {\mathrm{C}}_{\mathrm{R}\kern0.5em }\times {\mathrm{K}}_{\mathrm{R}}^{\prime}\right)}{\left(\ \mathrm{d}\times {\mathrm{K}}_{\mathrm{T}}\right)} $$

(2)

where C_R and C_T are the indoor radon and thoron concentrations (Bq m⁻³). T₁ and T₂ are the track densities recorded by the detectors placed in the radon and radon + thoron chambers. B₁ and B₂ are the back ground track densities for radon and thoron. d is the exposure period. K_R (0.0170 ± 0.002 tracks cm⁻² d⁻¹/Bq m⁻³) is the calibration factors of radon in radon compartment, and K^’_R (0.0172 ± 0.002 tracks cm⁻² d⁻¹/Bq m⁻³) and K_T (0.010 ± 0.001 tracks cm⁻² d⁻¹/Bq m⁻³) are the calibration factors of radon and thoron in the radon + thoron compartment (Sahoo et al. 2013).

Deposition-based progeny sensors

Deposition-based direct progeny sensors DTPS/DRPS were used for the measurement of total (attached + unattached) progeny concentration of ²²²Rn and ²²⁰Rn. The direct progeny sensors are made with detectors (LR-115) mounted with an absorber of appropriate thickness to avoid uncontrolled static charge particles from affecting the deposition rate. For thoron (²²⁰Rn) progeny, the absorber is an aluminized mylar of 50 μm to detect the alpha particles emitted from ²¹²Po having alpha energy 8.78 MeV. The sensors which are used for measurement of thoron progeny are known as direct thoron progeny sensors (DTPS). The absorber used in DTPS is 50 μm thick which does not allow radon progeny to pass through it, and hence, there is no interference of radon progeny with thoron progeny. For radon (²²²Rn) progeny, the effective thickness of 37 μm is used to detect the alpha particles emitted from ²¹⁴Po having alpha energy 7.69 MeV and ²¹²Po having alpha energy 8.78 MeV. The absorber (thickness) is combination of aluminized mylar of 25 μm and cellulose nitrate film of 12 μm (Mishra and Mayya 2008; Ramola et al. 2016). The sensors which are used for the measurement of radon progeny are known as direct radon progeny sensor (DRPS).

The total equilibrium equivalent concentration of radon (EECR_A + U) and thoron (EECR_A + U) progeny was estimated by the alpha tracks deposited on LR-115 detectors using the sensitivity factors given by Mishra and Mayya (2008).

$$ {\mathrm{EECR}}_{\mathrm{A}+\mathrm{U}}=\frac{{\mathrm{T}}_{\mathrm{R}}-\mathrm{B}}{\mathrm{t}\times \mathrm{S}}-{\mathrm{EECT}}_{\mathrm{A}+\mathrm{U}} $$

(3)

$$ {\mathrm{EECT}}_{\mathrm{A}+\mathrm{U}}=\frac{{\mathrm{T}}_{\mathrm{T}}-\mathrm{B}}{\mathrm{t}\times \mathrm{S}} $$

(4)

where EECR_A + U and EECT_A + U are the equilibrium equivalents of ²²²Rn and ²²⁰Rn progeny concentration (Bq m⁻³); T_R and T_T are the track densities (tracks cm⁻²) observed in DRPS and DTPS; and B, t, and S are the background counts, exposure time, and sensitivity factors, respectively. The sensitivity factor (S) is 0.94 tracks cm⁻² d⁻¹/EECT_A + U (Bq m⁻³) and 0.09 tracks cm⁻² d⁻¹/EECR_A + U (Bq m⁻³) for ²²⁰Rn and ²²²Rn progeny, respectively.

Wire-mesh capped DRPS/DTPS

Attached and unattached progeny concentrations were measured using wire-mesh-capped DTPS/DRPS with a 200 mesh type wire (79 mesh cm⁻¹, wire diameter 0.005 cm) screens to detect the fine fractions. The unattached fraction of progeny atoms was trapped on the wire mesh due to Brownian diffusion and only the attached fraction of progeny atoms passes through the mesh and get deposited on the absorber of DTPS/DRPS (Mayya et al. 2010). The measurement technique is explained elsewhere in detail (Ramola et al. 2016; Mayya et al. 2010).

The attached equilibrium equivalent concentration of radon (EECR_A) and thoron (EECT_A) progeny has been calculated as:

$$ {\mathrm{EECR}}_{\mathrm{A}}=\frac{{\mathrm{T}}_{\mathrm{R}}-\mathrm{B}}{{\mathrm{S}}_{\mathrm{R}}\times \mathrm{D}}-{\mathrm{EECT}}_{\mathrm{A}} $$

(5)

$$ {\mathrm{EECT}}_{\mathrm{A}}=\frac{{\mathrm{T}}_{\mathrm{T}}-\mathrm{B}}{{\mathrm{S}}_{\mathrm{T}}\times \mathrm{D}} $$

(6)

where EECR_A and EECT_A are attached equilibrium equivalent concentrations of radon and thoron (Bq m⁻³). T_R and T_T are track densities (tracks cm⁻²) of attached radon and thoron progeny in wire mesh DRPS/DTPS. The background tracks in the detector during the preparation as well as during handling should be subtracted from the observed track density. S_R (0.04 (tracks cm⁻² d⁻¹)/(Bq m⁻³)) and S_T (0.33 (tracks cm⁻² d⁻¹)/(Bq m⁻³)) are the sensitivity factors for attached fraction of EECT_A and EECR_A (Mayya et al. 2010).

Some alpha particles from thoron progeny with energy 8.78 MeV have impact on radon sensor with absorber energy 7.69 MeV. So, to calculate accurate radon progeny alpha particles from DRPS, tracks of thoron progeny have to be subtracted from it using the following equation (Mishra et al. 2009)

$$ \mathrm{Tracks}\ \frac{\mathrm{only}\ {\mathrm{R}}_{\mathrm{n}}\mathrm{P}}{\mathrm{DRPS}}=\mathrm{Tracks}\ \frac{\mathrm{Total}}{\mathrm{DRPS}}-\frac{\upeta_{\mathrm{R}\mathrm{T}}}{\upeta_{\mathrm{TT}}}\ \mathrm{T}\mathrm{racks}\frac{\mathrm{Total}}{\mathrm{DTPS}} $$

where η_RT and η_TT are the track registration efficiencies for thoron progeny in DRPS (0.01 ± 0.0004 per alpha particle emitted from thoron progeny) and that in DTPS (0.083 ± 0.004 per alpha particle emitted from thoron progeny) (Mishra et al. 2009), respectively. Therefore, the DRPS should be always deployed along with DTPS to achieve this radon progeny and thoron progeny discrimination.

Calibration and sensitivity factors for dosimeters, bare, and wire mesh DTPS/DRPS will rely on the experimental conditions such as operational characteristic of the spark counter or etching conditions beside the design of the dosimeter and DTPS/DRPS. The calibration factors exhibited here are pertinent to single entrance type pinhole-based ²²²Rn and ²²⁰Rn discriminating dosimeter cups, bare mode, and wire-mesh-capped DTPS/DRPS. The mentioned calibration factors depend upon etching procedure (without stirring) and spark counter technique (pre spark voltage = 900 V and operating voltage 500 V) (Sahoo and Sapra 2015). It is suitable to set up from the earlier relative variables for changed measurement conditions through inter-comparison exercises before using these calibration factors.

According to Mishra and Mayya (2008), the sensitivity analysis for progeny sensors will greatly rely upon ventilation rate and aerosol concentration. Assuming that the if total particle concentrations are typically ~ 20,000–30,000 cm⁻³ (10–200 nm range), one can expect an uncertainty of about 40% in sensitivity factor for DTPS and DRPS due to the possibility of a range of values for ventilation rates in uncharacterized indoor environments. Therefore, if the ventilation rate is in the range of 0.5–1.5 h⁻¹ and the particle concentration is around 20,000 to 30,000 particles/cm³, therefore, sensitivity factor can be assumed to be constant as mentioned above.

Theoretical formalism

Estimation of equilibrium factor

The equilibrium factor depends upon environmental conditions such modes of ventilation, humidity, type of houses, place, etc. (Prasad et al. 2016). The equilibrium factors for radon and its progeny and for thoron and its progeny were calculated for individual dwelling by using the following expressions:

$$ \mathrm{Equilibrium}\ \mathrm{Factor}\ \mathrm{for}\ \mathrm{Radon}\ \left({\mathrm{F}}_{\mathrm{Rn}}\right)=\frac{EECR}{C_R} $$

(7)

$$ \mathrm{Equilibrium}\ \mathrm{Factor}\ \mathrm{for}\ \mathrm{Thoron}\ \left({\mathrm{F}}_{\mathrm{Tn}}\right)=\frac{EECT}{C_T} $$

(8)

where the quantities radon concentration, thoron concentrations, EECR, and EECT represent the average values for the measurement period (1 year).

Attached and unattached fraction

The time-integrated fine fraction or unattached fraction for radon (f_Rn) and thoron (f_Tn) was calculated as (Knutson 1988)

$$ \mathrm{Unattached}\ \mathrm{fraction}\ \mathrm{of}\ \mathrm{radon}\ \left({\mathrm{f}}_{\mathrm{Rn}}\right)=\frac{{\mathrm{EECR}}_{\mathrm{U}}}{\ {\mathrm{EECR}}_{\mathrm{A}+\mathrm{U}}} $$

(9)

$$ \mathrm{Unattached}\ \mathrm{fraction}\ \mathrm{of}\ \mathrm{thoron}\ \left({\mathrm{f}}_{\mathrm{Tn}}\right)=\frac{{\mathrm{EECT}}_{\mathrm{U}}}{{\mathrm{EECT}}_{\mathrm{A}+\mathrm{U}}} $$

(10)

$$ \mathrm{Attached}\ \mathrm{fraction}\ \mathrm{of}\ \mathrm{radon}\ \left(\mathrm{f}{\ast}_{\mathrm{Rn}}\right)=\frac{{\mathrm{EECR}}_{\mathrm{A}}}{{\mathrm{EECR}}_{\mathrm{A}+\mathrm{U}}} $$

(11)

$$ \mathrm{Attached}\ \mathrm{fraction}\ \mathrm{of}\ \mathrm{thoron}\ \left(\mathrm{f}{\ast}_{\mathrm{Tn}}\right)=\frac{{\mathrm{EECT}}_{\mathrm{A}}}{{\mathrm{EECR}}_{\mathrm{A}+\mathrm{U}}} $$

(12)

where EECR_U and EECR_A are the equilibrium equivalent concentrations of unattached and attached ²²²Rn progeny, and EECT_U and EECT_A are the equilibrium equivalent concentrations of unattached and attached ²²⁰Rn progeny. EECR_A + U and EECT_A + U are the total (attached + unattached) EEC of ²²²Rn and ²²⁰Rn progeny, respectively.

Estimation of annual effective dose

The annual effective dose due to exposure of radon progeny was calculated by the relation given by UNSCEAR (2000):

$$ \mathrm{AEDR}\ \left(\mathrm{mSv}\ {\mathrm{y}}^{-1}\right)={\mathrm{EECR}}_{\mathrm{A}+\mathrm{U}}\left(\mathrm{Bq}\ {\mathrm{m}}^{-3}\right)\times 8760{\mathrm{hy}}^{-1}\times 0.8\times 9\mathrm{nSv}\ {\left(\mathrm{Bq}.\mathrm{h}/{\mathrm{m}}^3\right)}^{-1}\times {10}^{\hbox{-} 6} $$

(13)

Similarly, the effective dose due to exposure of thoron progeny was calculated by the relation:

$$ \mathrm{AEDT}\ \left(\mathrm{mSv}\ {\mathrm{y}}^{-1}\right)={\mathrm{EECT}}_{\mathrm{A}+\mathrm{U}}\left(\mathrm{Bq}\ {\mathrm{m}}^{-3}\right)\times 8760{\mathrm{hy}}^{-1}\times 0.8\times 40\mathrm{nSv}{\left(\mathrm{Bq}.\mathrm{h}/{\mathrm{m}}^3\right)}^{-1}\times {10}^{-\mathbf{6}} $$

(14)

where EERC_A + U and EETC_A + U are the equilibrium equivalent concentrations of radon and thoron, respectively. The numerical quantity 0.8 is the annual indoor occupancy factor, and 9 and 40 are the dose conversion factors for radon and thoron progeny in nSv units, respectively (Mayya et al. 1998). The number of hours per year is 8760. The multiplication factor 10⁻⁶ is used to convert the nSv units into mSv unit.

Dose conversion factors

ICRP has recommended dose conversion factor (4 mSv WLM⁻¹) for indoor environment (ICRP 1993). The following models are used for calculating dose conversion factors for mouth and nasal breathing for the estimation of effective dose.

$$ {\mathrm{DCF}}_{\mathrm{M}}=101\times {\mathrm{f}}_{\mathrm{Rn}}+6.7\times \left(1\hbox{--} {\mathrm{f}}_{\mathrm{Rn}}\right) $$

(Porstendorfer 1996)

$$ {\mathrm{DCF}}_{\mathrm{N}}=23\times {\mathrm{f}}_{\mathrm{Rn}}+6.2\times \left(1\hbox{--} {\mathrm{f}}_{\mathrm{Rn}}\right) $$

(Porstendorfer 1996)

$$ {\mathrm{DCF}}_{\mathrm{b}}=11.35+0.45\times \mathrm{P} $$

(Marsh and Birchall 2000)

where P is the percentage of total potential alpha energy concentration and calculated according to Mehra et al. (2016) and DCF_b = is the dose conversion factor using Marsh and Birchall model.

Combined DCF is calculated by taking 60% mouth and 40% nasal contributions to breathing (Bennett et al. 2003)

$$ {\mathrm{DCF}}_{\mathrm{C}}=0.6\times {\mathrm{DCF}}_{\mathrm{M}}+0.4\times {\mathrm{DCF}}_{\mathrm{N}} $$

where DCF_C is the dose conversion factor for combined breathing (mSv WLM⁻¹).

Inhalation dose

The inhalation dose due to mouth and nasal breathing is estimated by the lung based on human respiratory tract model (HRTM) of ICRP (ICRP 1993):

$$ \mathrm{ID}={\mathrm{C}}_{\mathrm{R}}\times \frac{{\mathrm{F}}_{\mathrm{R}\mathrm{n}}}{3700}\times \frac{7000\ {\mathrm{ha}}^{-1}}{170}\times {\mathrm{DCF}}_{\left(\mathrm{M},\mathrm{N},\mathrm{b}\right)} $$

(15)

where

DCF_M:

Dose conversion factor for mouth breathing

DCF_N:

Dose conversion factor for nasal breathing

DCF_b:

Dose conversion factor using Marsh and Birchall model

The unattached fraction (f_Rn) is defined as the fraction of the potential alpha energy concentration (PAEC) of short-lived progeny that is not attached to the ambient aerosol. The f_Rn depends on the concentration of particles of ambient aerosol (Z) and can be estimated by Porstendorfer (2001).

$$ \mathrm{Z}\ \left({\mathrm{cm}}^{-3}\right)=\frac{414}{{\mathrm{f}}_{\mathrm{Rn}}} $$

(16)

The measurement of Z is necessary to evaluate the impact of aerosol concentration on variation of radon short-lived decay products.

Results and discussion

Indoor ²²²Rn and ²²⁰Rn concentrations

Basic statistics of the results obtained for indoor radon, thoron, and their progeny in 102 houses of Udhampur district, Jammu and Kashmir, Himalaya, is shown in Table 1. The variability as measured by SD/AM or GSD reflects true geographical variability and statistical measurements uncertainties. The annual average indoor radon concentration in studied dwellings varied from 9 to 58 Bq m⁻³ with an average value of 28 ± 9 Bq m⁻³, which is less than the reference level (100 Bq m⁻³) suggested by ICRP (ICRP 2010) and WHO (WHO 2009). Out of 102 dwellings, only 10.7% of the values are higher than that of the global average of 40 Bq m⁻³ (UNSCEAR 2009; Ramola et al. 2016). The annual average indoor thoron concentration is found to be in the range of 25 to 185 Bq m⁻³ with an average value of 83 ± 32 Bq m⁻³. Due to diffusive transportation of thoron in indoor environment, the average thoron level comparatively double the radon level. This may be due to thorium-rich soil used in building construction in the study region resulting in more thoron emanation. Overall thoron concentration is higher than the global average of 10 Bq m⁻³ (UNSCEAR 2006). The most of the higher high values of the ²²²Rn and ²²⁰Rn concentration was found in mud type of dwellings, which is the main source of uranium and radium content.

Table 1 Statistical parameters of annual average of equilibrium equivalent concentrations for radon (EERC), thoron (EETC), and their equilibrium factors in 102 dwellings of Udhampur district, Jammu and Kashmir, Himalayas

The indoor ²²²Rn and ²²⁰Rn concentrations of the study region were also compared with the data reported from the adjoining Jammu district (Kaur et al. 2017a), Garhwal Himalayas (Ramola et al. 2016), Tosham region of Haryana (Singh et al. 2015), and Hamirpur district of Himachal Pradesh (Singh et al. 2016). The average indoor ²²²Rn and ²²⁰Rn levels of study region are comparatively lower than those of the Garhwal Himalayas, Hamirpur, and Tosham ring. The high levels of indoor ²²²Rn and ²²⁰Rn in Garhwal Himlayas are due to its geology and soil substructure (Ramola et al. 2016), in Hamirpur due to thorium-rich soil (Singh et al. 2015) and in Tosham due to the presence of granitic and felsite rocks.

Indoor ²²²Rn and ²²⁰Rn level of Jammu district was comparatively less than the Udhampur district (study region) which may be due to subsoil structure, construction materials, and geology of the area. The study region is exposed with slates, phyllite lithology (Kumar et al. 2017), and this kind of lithology contains uranium in trace amounts.

Radon and thoron progeny concentrations

The total (attached + unattached), attached, and unattached equilibrium equivalent radon and thoron concentrations are given in Tables 1 and 2. The total (attached + unattached) equilibrium equivalent ²²²Rn (EECR_A + U) and total (attached + unattached) equilibrium equivalent ²²⁰Rn concentration (EECT_A + U) were found to be varying from 5 to 38 Bq m⁻³ with an average concentration of 18 ± 6 and 0.48 to 5.49 Bq m⁻³ with a mean value of 1.69 ± 0.75 Bq m⁻³, respectively. The attached equilibrium equivalent ²²²Rn (EECR_A) and ²²⁰Rn (EECT_A) concentration varied from 3 to 25 Bq m⁻³ with an average of 13 ± 4 and 0.19 to 3.68 Bq m⁻³ with an average of 1.3 ± 0.59 Bq m⁻³. The unattached EECR_U and EECT_U in studied dwellings ranged from 0.04 to 25.93 Bq m⁻³ with an average of 4.66 ± 4.96 and 0.01 to 2.1 Bq m⁻³ with an average value of 0.40 ± 0.44 Bq m⁻³, respectively. The estimated range of ²²²Rn and ²²⁰Rn progeny concentrations (EECR_A + U & EECT_A + U) in buildings lies between 2 to 50 Bq m⁻³ and 0.04 to 2 Bq m⁻³ (ICRP 2014). The results obtained for EECR_A + U are within the ICRP range while for EECT_A + U, 26 dwellings have higher values than ICRP range. But overall, the average value of EECT_A + U lies within the suggested range (ICRP 2014). The average of total thoron/radon progeny ratio is 0.10, within the worldwide range of 0.01–0.5 according to data given by United Nation Scientific Committee on the Effect of Atomic Radiations (UNSCEAR 2000) (Table 1). The statistical variations of radon and thoron progeny concentrations are given in box-whisker plots shown in Figs. 2 and 3. Longer whisker in positive direction and larger mean than median indicate that radon and thoron progeny concentration was positively skewed.

Table 2 Calculated values of different indoor parameters of the studied dwellings

Fig. 2

Box-whisker plot for thoron progeny

Fig. 3

Box-whisker plot for radon progeny

The frequency distributions of total (attached + unattached) equilibrium equivalent of radon (EECR_A + U) and thoron concentration (EECT_A + U) in 102 dwellings are given in Fig. 4. It is clear from Fig. 4a that about 70% of EECR_A + U are found to vary from 10 to 20 Bq m⁻³. The frequency distribution is highest at 15 to 20 Bq m⁻³ and minimum at more than 35 Bq m⁻³. All dwellings have concentrations within the recommended range. From Fig. 4b, about 74% of EECT_A + U are found to vary from 1 to 3 Bq m⁻³. The frequency distribution of thoron progeny is highest at 1 to 2 Bq m⁻³ and minimum at more than 4 Bq m⁻³.

Fig. 4

a Frequency distribution of EECR_A + U in 102 dwellings of Udhampur district. b Frequency distribution of EECT_A + U in dwellings of Udhampur district

Concentrations of total (attached + unattached) equilibrium equivalent of radon and thoron concentration and attached equilibrium equivalent of radon and thoron concentration are positively correlated. From Fig. 5a, it can be seen that the positive correlation has been observed between EECT_A + U and EECT_A with Pearson’s coefficient 0.80. Similarly, Fig. 5b also gives the positive correlation between EECR_A + U and EECR_A with Pearson’s coefficient 0.54.

Fig. 5

a Correlations between EECT_A + U and EECT_A. b Correlations between EECR_A + U and EECR_A

Unattached fraction, aerosol concentration, and equilibrium factor

The results of unattached fraction, attached fraction, and equilibrium factor for radon, thoron, and its progeny measurements are summarized in Tables 1 and 2. Under ambient conditions, the unattached fraction for radon (f_Rn) and thoron (f_Tn) ranged within 0.002 and 0.87 with a mean value of 0.24, relative GSD of 3.05 and from 0.001 to 0.91, with a mean value of 0.26, with GSD of 3.37, respectively (Table 2). The values are very similar for Rn and Tn. According to literature, the unattached fraction in indoor air is usually below 0.15 (Kojima and Abe 1988; Reineking and Porstendorfer 1990; Hopke et al. 1995). But the mean value of f_Rn determined in the present study is higher than the data reported in the literature. Attached fractions for radon (f^*_Rn) and thoron (f^*_Tn) vary from 0.13 to 0.99, with an average value of 0.76 and from 0.08 to 0.98, with an average values of 0.75, respectively.

Aerosol particle concentration in the ambient environment has also been determined. The higher the aerosol particle concentration, the higher is the radioactive equilibrium factor and the process of attachment becomes faster than the deposition of unattached particles on the surface (Jasaitis and Girgždys 2011). Under natural conditions, the aerosol concentration altered from 0.4 × 10³ to 39 × 10³ cm⁻³ with an average value of 4.9 × 10³ cm⁻³, with relative GSD of 2.82 (Table 2). The calculated data for aerosol concentration agrees fairly with the range 2000 < Z < 7 × 10⁵ cm⁻³ (Porstendorfer 2001).

The calculated equilibrium factor for both radon and its progeny (F_Rn) varied from 0.28 to 0.99, with a mean value of 0.6, with relative GSD of 1.56 and for thoron and its progeny (F_Tn) ranged from 0.01 to 0.09, with an average value of 0.02, with GSD of 1.37, respectively (Table 1). The equilibrium factor for radon and its progeny was found to be ˂ 0.30 in 4 houses and ≥ 0.6 in 42 houses. However, in remaining 56 houses, this factor lies in the range of 0.30–0.60. The calculated annual average value of the equilibrium factor for radon and its progeny is 0.61, which is greater than globally assumed value of equilibrium factor 0.4 (UNSCEAR 2000). The annual average value of equilibrium factor for thoron and its progeny is 0.02 which is comparable with world’s average value (UNSCEAR 2008).

A low positive correlation with Pearson’s coefficient 0.28 was observed between unattached fraction (f_Rn) and equilibrium factor of radon (F_Rn). This low value can be due to the low particle concentration. Therefore, the reason for the low attachment process is the concentration flame of particles. So, radon daughters are mainly free and thus plate out on surfaces leading to an important disequilibrium between radon and its daughters (Huet et al. 1999).

Annual effective dose

The values of annual effective dose obtained from attached and unattached progeny of radon (AEDR) and that from thoron (AEDT) shown in Table 2 were found to be in the range 0.34 to 2.42 y⁻¹, with geometric mean of 1.05 ± 0.36 mSv y⁻¹ and from 0.13 to 1.54 mSv y⁻¹, with geometric mean of 0.45 ± 0.22 mSv y⁻¹, respectively. The calculated values for the annual effective doses are below the reference value of 3 to 10 mSv y⁻¹ (ICRP 2010), and WHO recommended reference level of 10 mSv y⁻¹ (WHO 2009). The frequency distributions of annual effective doses due to radon (AEDR) and thoron (AEDT) are given in Fig. 6. About 87% of AEDR are found to vary from 0.5 to 1.5 mSv y⁻¹ as given in Fig. 6a. From Fig. 6b, about 78.43% of AEDT is found to vary from 0.3 to 0.9 mSv y⁻¹. Although, radon progeny is considered as the major source for the total dose, about 31% of the total dose is received from thoron progeny by residents of this region, which we do not consider negligible. The estimated values of annual effective doses have shown no significant health risk due to exposure of radon and thoron progeny in the study area.

Fig. 6

a Frequency distribution of annual effective dose due to radon progeny. b Frequency distribution of annual effective dose due to thoron progeny

Dose conversion factor and effective lung dose

The dose conversion factors (DCFs) have been calculated by two different models (Porstendorfer 1996; Marsh and Birchall 2000). The dose conversion factor for mouth (DCF_M) and nasal (DCF_N) breathing varies from 6.90 to 45.10 mSv WLM⁻¹ with an arithmetic mean of 23.44 ± 12.10 mSv WLM⁻¹ and from 6.23 to 20.79 mSv WLM⁻¹ with an average of 10.22 ± 3.21 mSv WLM⁻¹. The DCF_b (Marsh and Birchall 2000) has been found to vary from 11.98 to 15.81 mSv WLM⁻¹. The inhalation dose for mouth (ID_M) and nasal (ID_N) breathing has been calculated by Singh et al. (2016) and the average value of inhalation dose for mouth breathing (ID_M) is approximately twice as large as that of nasal breathing (ID_N), and our results are in good agreement with the ones reported by other investigators (Singh et al. 2016; Bangotra et al. 2015). The inhalation dose (ID_b) due to Marsh and Birchall (2000) is also calculated and has the similar variance as that of Porstendorfer (1996) (Table 2). The variation of ID_M, ID_N, and ID_b is given in box-whisker plot (Fig. 7) and is positively skewed. The observed average values of DCFs using different models are 24, 10, and 13 mSv WLM⁻¹. Our values also lie in the range reported by Marsh et al. (2010).

Fig. 7

Box-whisker plot of inhalation doses with different models

The effective lung dose has been calculated by using the Eq. 15 given by ICRP (ICRP 1993). The effective lung dose values due to radon (ELD_Rn) and thoron (ELD_Tn) (calculated including the ICRP tissue weighting factor of 0.12 for the lung) by using DCF of 5.7 mSv WLM⁻¹ for indoor radon and 25.4 mSv WLM⁻¹ for indoor thoron (UNSCEAR 2008; ICRP 2007) have been found to vary from 0.35 to 2.11 mSv y⁻¹ with an average value of 1.07 ± 0.31 mSv y⁻¹ and from 0.13 to 1.55 mSv y⁻¹ with an average of 0.5 ± 0.22 mSv y⁻¹, respectively. The effective lung doses for radon and thoron are similar with annual effective dose. The difference in them is just because of different dose conversion factors. The variations of ELD_Rn and ELD_Tn between dwellings are given in Fig. 8. It is important to calculate the lung doses; during inhalation, lungs receive the maximum dose from short-lived progenies which are 100 times greater than that of radon gas (ICRP 2007). The obtained results of ELD are within the reference level given by ICRP and WHO.

Fig. 8

Variation of effective lung dose (ELD) for radon and thoron progeny

Ventilation conditions

The results of measurements of annual indoor radon and thoron progeny concentrations under different ventilation conditions of the various dwellings are presented in Table 3. The selected dwellings were divided into three types of categories on the basis of ventilation like good ventilation (two or three window and one door), average ventilation (one window and one door), and bad ventilation (one door and no window), respectively. The storage rooms are mostly closed due to low usage in daily routine and ventilation rate is poor in these rooms. Thus, these are considered as bad ventilators. On the other hand, the bedroom, living room, and hall are expected to have high ventilation and on the basis of room appearance, they are categorized in good and average ventilation. As could be expected, the EECT_A + U and EECR_A + U were found to be higher in bad ventilation rooms than average and good ventilation rooms due to difference in their ventilation rates. The annual variation of EECT_A + U and EECR_A + U in different ventilation conditions is shown in Fig. 9.

Table 3 Annual average concentration of equilibrium factor for radon and thoron progeny concentrations in different ventilation conditions

Fig. 9

Variation of EECR_A + U and EECT_A + U with ventilation conditions

Household statistics

The results of the measurement of indoor radon and thoron progeny concentrations corresponding to different type of dwellings are listed in Table 4. The total dwellings are divided into four categories according to their construction materials such as cemented + concrete, marble + concrete, mud + mud, and tiles + concrete. The indoor radon and thoron progeny concentrations were found to be maximum in mud + concrete type of houses. The concentration of EECT_A + U and EECR_A + U in cemented and marble type of houses was in between the mud and tiles type of houses as shown in Fig. 10. The mud of which the houses have been constructed makes up mud bricks and soil floor. Higher Rn and Tn concentrations in these houses may be due to radium and thorium content in soil (Kumar and Chauhan 2014) and having larger exhalation rate than other type of houses. On the other hand, cemented and marble types of houses also exhale radon and thoron but less than soil.

Table 4 Annual average concentration of equilibrium factor and radon and thoron progeny concentrations in different types of dwellings

Fig. 10

Variation of EECR_A + U and EECT_A + U with construction materials

Seasonal variation

The range of radon and thoron progeny concentration not only depends upon ventilation effects and types of houses but also on different seasons (winter, summer, and rainy). The variation of EECR_A + U and EECT_A + U is shown in Fig. 11a. It is evident from Table 5 that seasonal levels of radon progeny show maximum values in winter season. This is because the doors and windows of the dwellings are mostly closed in winter season as compared with summer, and hence, the ventilation is poor in winter season. The seasonal variation of thoron progeny shows higher values in winter season and low value in rainy season. This random behavior of thoron progeny is due to short diffusion length (4 to 5 cm) and short half-life of thoron. Our results of seasonal variations of radon and thoron progeny agree with the results of Ramola et al. (2016) and Kaur et al. (2017b) for the dwellings of Garhwal Himalayas and Jammu district of Jammu and Kashmir state. The seasonal variations in equilibrium factor of radon and its progeny and thoron and its progeny in the houses of Udhampur district are also presented in Table 5. The graph between equilibrium factor of radon and thoron for different seasons is presented in Fig. 11b. The number of houses with higher equilibrium factors was greater in the rainy than in the summer or winter season. This variation in equilibrium factor with seasons is mainly due to the ventilation of the houses during the different seasons. However, the equilibrium factor for thoron and progeny has been found to be nearly same for rainy, winter, and summer seasons. The annual average equilibrium factor for radon and its progeny of the study region is comparatively higher than the regions (Tosham, Hamirpur, and Garhwal Himalayas) having almost same geographical characteristics but the annual equilibrium factor for thoron and its progeny of the study region is comparatively lower than the abovementioned regions.

Fig. 11

a Seasonal variation of EECR_A + U and EECT_A + U. b Seasonal dependence of equilibrium factor for radon (F_Rn) and thoron (F_Tn)

Table 5 Seasonal variations of radon and thoron progeny concentrations, unattached fractions, and equilibrium factors in 99 houses of Udhampur district, Jammu and Kashmir, Himalaya

Conclusions

The measurement of indoor radon and thoron progeny concentration in different types of dwellings for different seasons and for different household conditions was carried out. The following conclusions are drawn from the study:

• 1) ²²⁰Rn gas concentration was higher as compared to ²²²Rn gas in most of the houses.

• 2) The arithmetic mean value of the total (attached + unattached), attached, and unattached of radon progeny concentration was within the estimated range of ICRP (ICRP 1993), and for thoron progeny, 25% of the dwellings exceeded from the reference range of ICRP. The concentrations of total and attached progenies are positively correlated.

• 3) The annual effective dose rate (AEDR and AEDT) in the study area was less than the recommended reference level of ICRP and WHO and does not imply significant health risk due to exposure to radon and thoron progeny.

• 4) The calculated DCFs from different models are in satisfactory agreement previously reported data (Marsh et al. 2010) and their related doses are below the reference level.

• 5) The average equilibrium factor for radon and thoron was 0.61 and 0.02, very close to the globally reported values of UNSCEAR.

• 6) The values of radon and thoron progeny concentration were found to be higher in mud type, than in other dwellings. The high levels can attribute to the direct emission of radon to the environment from soil and building material.

• 7) It has been found that the seasonal variation of equilibrium factor for radon and its progeny depends strongly on environmental conditions but in case of thoron and its progeny, the values are same in winter and rainy seasons but slightly differ in summer season.

Results of the present study will be helpful for mapping of radiological assessment in indoor environment of the mountainous region of India. Therefore, the present calculations are representative not only for the national but also for international audience for comparison as well as for the estimation of radiation hazard in ambient environment. Hence, in the light of above results, the indoor environment of the study region is not much affected by the radioactive radon gas and its decay products. Therefore, the air of the study region is safe for breathing purpose as far as the radioactivity is concerned.