Seasonal variation of indoor radon/thoron and their progeny levels in lesser-Himalayas of Jammu & Kashmir, India

Abstract

In this investigation, the passive estimation of radon (Rn²²²), thoron (Rn²²⁰) and their progenies have been measured in the dwellings of Reasi district of Jammu & Kashmir for a period of 1 year. These estimations have been done with the help of latest developed single entry Pin-hole based dosimeters and progeny sensors based on deposition. The annual Equilibrium factors for ²²²Rn, ²²⁰Rn, and their progenies have been calculated separately for each dwellings The average annual effective dose was found to be 0.9 ± 0.2 mSv/y for ²²²Rn, which is less than prescribed limit of ICRP. The results obtained indicate no vital health hazards because of exposure of Rn²²², Rn²²⁰ and their progenies.

Introduction

Due to natural radiations, inhalation of ²²²Rn, ²²⁰Rn, and their decay-products contribute about 50% of world-wide effective dose to the general population [1]. Various case–control investigations of residential exposure to ²²²Rn have been completed in different parts of the globe to enhance our understanding of the health risks of ionizing radiations. These controlled instigations provide the knowledge of an enchanting the risk of lung malignant with the expansion in exposure of ²²²Rn [2]. ²²⁰Rn, then again, has not been concentrated in detail because of reference to lung cancer risk. Recently, ²²⁰Rn contribution is only recognized in the radiation dose [1, 3, 4].

The Inhalation dose due to ²²²Rn and its short-lived progeny are the primary source and about 40% of the total radiation dose taken by the overall populace is the significant supporter to the issue in the respiratory tract, lung malady and sensitive tissue of the skin and cause skin disease [5, 6]. In Past decades, an equilibrium factor (fixed value = 0.4) (ratio of Equilibrium Equivalent Concentration of the short-lived to the Concentration of Radionuclide) can be utilized to measure the decay products of the radionuclide’s [7], but in now a days, direct ²²²Rn/²²⁰Rn progeny sensors (DTPS/DRPS) have been utilized in this work for the progeny estimation. Unattached part, size distribution, and equilibrium factor are also the essential influent parameters related to the lung dose computation [8].

Radon (²²²Rn) and thoron (²²⁰Rn) decay into various short-lived radio-isotopes. After the decay of ²²²Rn, the recently framed radio-active nuclides react with environmental gases and vapors and form a cluster of particles of size around 1 nm, which are Un-attached particles. These unattached radio-active nuclides may likewise combine with existed aerosols presented in the atmosphere within a time period of 1–100 s, framing the attached particles [9]. The buildup of activity of ²²²Rn gas and its short-lived alpha emitters inside enclosed spaces may increase the radiation risk to the public. This applies especially to work environments like, underground mines, visitor surrenders, and water supply offices which deal with high radon ground water sources. By and large, health risk by radon (²²²Rn) is considerably more far-reaching than by thoron (²²⁰Rn). Since thoron (²²⁰Rn) has a short span of life, it is less capable than the ²²²Rn to move from the point where it is shaped [10]. As an outcome, materials used for building purposes are the most regular source of ²²⁰Rn exposure. Conversely, radon (²²²Rn), have the half-life of 3.8 days can diffuse in the soil in excess of meter from where it’s shaped. As the result, the under-ground structures are normally the essential wellspring of indoor ²²²Rn [11].

The northern part of India is well studied both in terms of radioactive nuclides and ²²⁰Rn/²²⁰Rn concentrations [1, 3, 4, 12,13,14,15]. However, such studies in Jammu and Kashmir region are scarce. No such type of study of natural radio-activity or indoor ²²²Rn, ²²⁰Rn and progeny concentration has been so far reported in Reasi district of Jammu & Kashmir. The most important geographical features of the study area are Reasi inlier and various fault lines. The fault is generally accompanied with specific changes in geographical qualities, such as an extensive increment in the porosity and porousness of deformed shakes along this zone. Hence this part of Himalayas is very significant for the study of ionizing radiations. Several studies have been conducted that elevated concentration of radon gas in soil and groundwater could be signs of an imminent earthquake. It is believed that the radon is released from cavity and cracks as the Earth crust is strained prior to the sudden slip of an earthquake [16].The Present investigation includes the estimations of ²²²Rn, ²²⁰Rn, and their progeny concentration by utilizing deposition based time integrating passive technique. We aimed to see the effect of various factors like types of dwellings, the seasonal effect on observed values.

Geography of study area

Reasi district lies between 33°05″ North latitude and 74°50″ East longitudes. The district imparts its limits to Udhampur district in the South, Ramban in the east, Shopian in the north and Rajouri in the west. The study zone is watershed of the waterway Chenab and its tributaries (Ans, Rudd, Plassu, Banganga, Pai, and Anji). It falls in the area which can be termed as Outer Hill Region, comprising the slopes and hills of Siwalik, Lesser Himalaya. The areas within the jurisdiction of the present study are hilly, comprising several off-shoots of great mountains inter-woven closely. The hills are of moderate heights and are surmountable. Though the areas in the north are very high, rising to the heights of above 4256 m but present study areas have a normal height ranging between 400 and 900 m. This region is on the southern side of the Pir Panjal. Out of four rock zones defined in the district according to Census report of 2011, the present study falls in two rock zones namely

• 1. The Reasi Limestone Inlier and

• 2. The Siwalik belt

Another important geographic feature of the study area is Reasi inliers which are about 80 km long and 8–20 km wide. In the Reasi fault, the Sirban arrangement is compared against the Tertiary sedimentary progressions of the Subathu—and the Murree formations in the northern part and against the Siwalik formation in the south [17]. Main boundary thrust and Medicott–Wadia thrust passes through this region, raising the interest of geologist across the world to study this area in detail especially after the devastating earth quake of 2005. Map of the study area is shown in Fig. 1.

Fig. 1

Map showing the total area covered for the present study

Materials and methods

Selection of locations

The study area is a cross segment of Medlicott–Wadia Thrust (MWT) near its interaction with Chenab river. MWT is the real dynamic (active) out-of-succession thrusts in Himalaya. In this region, MWT is generally referred to as Reasi Thrust. The determination of houses in the examination zone was taken to considering that cover the significant part of the study locale as logically feasible. The efforts were taken to select the dwellings with all impacting factors, such as building materials (cement, mud, marble, etc.) and ventilation conditions. The measurements were made in 87 houses of 28 villages for investigation of seasonal varieties in radon/thoron and their alpha emitter concentrations. Whole year was partitioned into three periods of 4-month time frame as set 1 (November–March); set 2 (March–July); and set 3 (July–November), respectively. The primary points are to cover diverse kind’s types of houses, with the goal that the reported results can be utilized as representative estimations of ²²²Rn and ²²⁰Rn in the locale considered for the study.

Measurement of ²²²Rn/²²⁰Rn concentrations

Measurements of ²²²Rn and ²²⁰Rn have been done by pin-hole based ²²²Rn/²²⁰Rn discriminating cup by utilizing LR-115 Type II detector. The discriminating cup has the one passageway through which the gas goes to the primary chamber to be specific as “radon + thoron” chamber through a filter paper of 0.56 µm and then diffuses to 2nd part called “radon” chamber. LR–115 of size 3 × 3 cm² is lodged in both the chambers, such that LR-115 in primary chamber detects the tracks because of both ²²²Rn and ²²⁰Rn, while that in the 2nd part detects the tracks only because of ²²²Rn gas. The reason is that only the radon gas enters the second chamber through 4 pin-holes of 2 mm in length and 1 mm in diameter made on a round circular disc owing to the very short half-life of ²²⁰Rn (55 s). The alpha emanations from ²²²Rn and ²²⁰Rn creates the tracks on LR-115 detector lodged at the end of the primary chamber while tracks are enlisted on LR-115 which is at the top of the 2nd chamber due to the alpha’s of ²²²Rn only. The schematic graph of the pin-hole based ²²²Rn/²²⁰Rn discriminating cup is shown in Fig. 2.

Fig. 2

Schematic diagram of pin-hole based dosimeter

The ²²²Rn and ²²⁰Rn concentrations in an indoor environment were figured as given by Sahoo [18]

$$^{222} {\text{Rn }}\left( {{\text{Bq/m}}^{3} } \right) \, = \frac{{T_{1} - B_{1} }}{{d - K_{\text{R}} }}$$

(1)

$$^{220} {\text{Rn}}\,\left( {{\text{Bq/m}}^{3} } \right) = \frac{{\left( {T_{2} - B_{2} } \right) - \left( {d \times 222_{\text{Rn}} \times K_{\text{R}}^{'} } \right)}}{{\left( {d \times K_{\text{T}} } \right)}}$$

(2)

where T₁ and T₂ = tracks seen in ²²²Rn and ²²²Rn + ²²⁰Rn chambers. B₁ and B₂ = back-ground track obtained for ²²²Rn and ²²⁰Rn. d = presentation time frame.

Measurement of attached/un-attached ²²²Rn/²²⁰Rn progeny

For the estimation of ²²²Rn/²²⁰Rn alpha emitters (progeny), LR-115 (12 μm cellulose nitrate covered on a 100 μm thick poly-ester base) SSNDTs based direct ²²²Rn/²²⁰Rn progeny sensor techniques were utilized for present examination. Samplers are made up of latent atomic track identifier (SSNDTs-LR) mounted with an absorber of suitable thickness for ²²⁰Rn progeny, an absorber which is aluminium coated sheet of 50 μm thickness. It specifically recognizes just 8.78 MeV α-particles radiated from ²¹²Po. For radon descendants, absorber comprises with the suitable match of an aluminized coated sheet of 25 μm and cellulose nitrate of 12 μm of effective thickness 37 μm, which mostly recognizes α’s produced from ²¹⁴Po (α energy = 7.69 MeV). For estimation of the only Attached ²²²Rn and ²²⁰Rn progeny to wire-mesh sensor were utilized as a part of the detached mode. Wire-mesh sensors comprise of direct sensors with a two hundred mesh wire-screen. The Un-attached part of the progenies is trapped on these wires and just attached part of the descendants get deposit on these sensors. The emitted alphas are deposited on wise caped progeny sensors is an estimation of the attached part of progeny concentration. Progeny concentration is ascertained by suspending DRPS/DTPS in the indoor environment far from entryway and windows [19].

Deployment details and analysis

The dosimeters both pinhole, as well as DTPS/DRPS along with WM-DTPS/WM-DRPS, were deployed in indoor environments of 28 villages in Reasi district, such that they were at least 1.5 m above the from any level and no less than 10 cm far from any of the surfaces for the time of 4 months. After the environmental exposure, the exposed detectors were retrieved and etched in 2.5 N NaOH solutions at 60 °C for 90 min without stirring [1, 4, 20]. The indicators were then cleaned in running water, dried, peeled and the track tallying was done using a spark counter with a voltage of 500 V. The tracks obtained from exposed films are converted into activity concentration using appropriate calibration factors.

Radon/thoron progeny

The total (attached + un-attached) and attached equilibrium equivalent concentration EEC_(A+U) were computed using the relations given below [18, 21]:

$${\text{EERC}}(i,j) ( {\text{Bqm}}^-3) = \frac{{T_{{{\text{R}}i,j}} - T_{\text{B}} }}{{D \times S_{{{\text{R}}i,j}} }}$$

(3)

$${\text{EETC}}(i,j) ({\text{Bqm}}^- 3) = \frac{{T_{{{\text{T}}i,j}} - T_{\text{B}} }}{{D \times S_{{{\text{T}}i,j}} }}$$

(4)

where EERC(i, j) and EETC(i, j) = total equilibrium equivalent concentration of ²²²Rn and ²²⁰Rn progeny, i.e., i, and j stands for attached part. T_Ri,j and T_Ti,j = tracks (Tr cm⁻²) produced on total and attached ²²²Rn and ²²⁰Rn progeny received in bare and wire-mesh sensors. T_B = back-ground received because of sensor timeframe of realistic usability and amid travel, D = the introduction of detector in given time frame (120 days). S_Ri,j and S_Ti,j are sensitivity coefficients for ²²⁰Rn and ²²⁰Rn progeny [22].

Since α energy of ²¹²Po (²²⁰Rn progeny) is greater than the ²¹⁴Po (²²²Rn progeny), the α’s radiates from both ²²²Rn and from ²²⁰Rn progeny go through the absorber (37 μm) used in the DRPS. To compute exact ²²²Rn progeny α’s from DRPS, tracks of ²²⁰Rn progeny must be subtracted using the following figures:

$${\text{Tracks}}\,\frac{{{\text{only\;Rn}}\;P}}{{\text{DRPS}}} = {\text{Tracks}}\,\frac{{\text{Total}}}{{\text{DRPS}}} - \frac{{\eta_{\text{RT}} }}{{\eta_{\text{TT}} }}\,{\text{Tracks}}\,\frac{{\text{Total}}}{{\text{DTPS}}}$$

(5)

where η_RT and η_TT = track enrolment efficiencies for ²²⁰Rn progeny in DRPS (0.01 ± 0.0004 for each α radiates from ²²⁰Rn progeny) and that in DTPS (0.083 ± 0.0004 per each α’s radiates from ²²⁰Rn progeny [23], respectively

Calibration factors

The calibration factors used for the pinholes dosimeters are:

• For ²²²Rn + ²²⁰Rn section (K_T) (0.010 Tr cm⁻² d⁻¹/Bq m⁻³) and that for only ²²²Rn section (K_R) was 0.017 Tr cm⁻² d⁻¹/Bq m⁻³ [18].

• For direct DTPS (direct thoron progeny sensors) and DRPS (direct radon progeny sensors), (0.94 Tr cm⁻² d⁻¹/Bq m⁻³ and 0.09 Tr cm⁻² d⁻¹/Bq m⁻³) respectively [19].

• For wire–mesh capped DTPS and DRPS, the calibration factors were 0.33 Tr cm⁻² d⁻¹/Bq m⁻³ and 0.04 Tr cm⁻² d⁻¹/Bq m⁻³ respectively [20].

Un-attached equilibrium equivalent concentration and its un-attached part

The un-attached ²²²Rn and ²²⁰Rn progeny concentrations EC_(U) have been figured out by just subtracting the attached EEC_(A) from the total (attached + un-attached) concentration EEC_(A+U) utilizing an expressions 6 and 7:

$${\text{EERC}}_{{({\text{U}})}} = {\text{EERC}}_{{({\text{A}} + {\text{U}})}} {-}{\text{EERC}}_{{({\text{A}})}}$$

(6)

$${\text{EETC}}_{{({\text{U}})}} = {\text{EETC}}_{{({\text{A}} + {\text{U}})}} {-}{\text{EETC}}_{{({\text{A}})}}$$

(7)

The unattached portion of potential alpha energy concentration (PAEC) of ²²²Rn and ²²⁰Rn progeny has been estimated utilizing the Eqs. (8) and (9) [1, 24]:

$$f_{\text{p}}^{\text{Rn}} = \frac{{{\text{EERC}}_{({\text{U}})}}}{{{\text{EERC}}_{\left( {{\text{A}} + {\text{U}}} \right)}}}$$

(8)

$$f_{\text{p}}^{\text{Tn}} = \frac{{{\text{EETC}}_{({\text{U}})}}}{{{\text{EETC}}_{({\text{A}} + {\text{U}})}}}$$

(9)

where $f_{\text{p}}^{\text{Rn}}$ and $f_{\text{p}}^{\text{Tn}}$ = unattached portions of ²²²Rn and ²²⁰Rn progeny, respectively

Equilibrium factor (EF)

The activity concentration of the short-lived radon progeny in air is always less than that of the radon gas. Equilibrium factor is the ratio of radon and its short lived radioactive decay products, which is a measure of the degree of disequilibrium between the radon gas and its progeny [11]. 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 [12]. The variation of EF mainly depends on environmental conditions like humidity, sort of houses, and ventilation rate, etc. [25]. Therefore, EF for ²²²Rn and ²²⁰Rn have calculated as:

$${\text{EF}}_{\text{Rn}} = \frac{{{\text{EERC}}_{{{\text{A}} + {\text{U}}}} }}{{222_{\text{Rn}} }}$$

$${\text{EF}}_{\text{Tn}} = \frac{{{\text{EETC}}_{{{\text{A}} + {\text{U}}}} }}{{220_{\text{Rn}} }}$$

Annual effective dose

²²²Rn and ²²⁰Rn doses depend basically on ²²²Rn progeny and the duration of exposure, the breathing rate and airborne molecules including the activity size dissemination of ²²²Rn descendant’s aerosol and the un-attached part [26]. The dose transformation factor given by UNSCEAR [7] has been utilized to evaluate the Annual Inhalation dosage. The annual effective dosage (Sv y⁻¹) for ²²²Rn (AE_Rn) and that of ²²⁰Rn (AE_Tn) were assessed utilizing the equations [7]:

$$\begin{aligned} & {\text{AE}}_{\text{Rn}} = {\text{EERC}}_{{({\text{A}} + {\text{U}})}} \times {\text{DCF}}_{\text{Rn}} \times {\text{OF}} \times T_{{{\text{Rn}}(\exp )}} \\ & {\text{AE}}_{\text{Tn}} = {\text{EETC}}_{{({\text{A}} + {\text{U}})}} \times {\text{DCF}}_{\text{Tn}} \times {\text{OF}} \times T_{{{\text{Tn}}(\exp )}} \\ \end{aligned}$$

where DCF_Rn (9 nSv y⁻¹ Bq⁻¹ m⁻³) and DCF_Th (40 nSvh⁻¹ Bq⁻¹ m⁻³) = radon and thoron dose conversion factor, OF = indoor occupancy factor, and T_Rn(exp) and T_Rn(exp) = exposure span per year (7000 h y⁻¹) respectively

Annual inhalation dosage

The total Annual Inhalation dosage because of introduction of indoor ²²²Rn, ²²⁰Rn and their progenies has been determined by utilizing the following expression [7]

$$D\left( {\text{mSv/y}} \right) = \left\{ {\left( {0.17 + 9 \times {\text{EF}}_{\text{Rn}} } \right) \times C_{\text{Rn}} + \left( {0.11 + 40 \times {\text{EF}}_{\text{Tn}} } \right) \times C_{\text{Tn}} } \right\} \times 8760 \times 0. \, 8 \times 10^{ - 6}$$

where EF_Rn = equilibrium factor for ²²²Rn and EF_Tn = equilibrium factor for ²²⁰Rn. C_Rn = radon concentration and C_Tn is ²²⁰Rn concentration. 0.17 and 0.11 (nSv/Bq/m³/h) = the dose transformation co-efficient for ²²²Rn and ²²⁰Rn, 9 and 40 (nSv/Bq/m³/h) = the dose transformation factors for ²²²Rn and ²²⁰Rn progenies, 8760 h/y = indoor inhabitancy time, 0.8 = the Indoor occupancy factor.

Results and discussion

²²²Rn/²²⁰Rn in an indoor environment

The overall results of ²²²Rn and ²²⁰Rn in indoor environment of 28 villages (approximate two to three dwellings in each village) of Reasi district (lesser Himalayan region) of Jammu & Kashmir, India are presented in Table 1. The range of indoor radon concentration in studied dwellings have been from 18 ± 3 to 59 ± 13 Bq m⁻³ with arithmetic mean (AM) of 29 ± 9, and geometric mean (GM) of 28 Bq m⁻³, respectively. The obtained results of indoor ²²²Rn are much less than the suggested reference range (100–300 Bq m⁻³) prescribed by ICRP [27]. In three villages, the indoor ²²²Rn concentration is greater than the world average value given by UNSCEAR, but the average value of indoor ²²²Rn in studied area is less than the global average [28].

Table 1 ²²²Rn, ²²⁰Rn, EEC, and annual effective dose in 28 villages of Reasi District, Jammu & Kashmir

The calculated results of indoor ²²²Rn are compared with neighboring states of the studied region. The obtained results of indoor radon in the study region (18–59 Bq m⁻³) are comparable to that of Jammu district (16–50 Bq m⁻³) [29] and Udhampur district (11–58 Bq m⁻³) [4] of Jammu & Kashmir and Jalandhar (6–47 Bq m⁻³) [3] and Kapurthala district (19–63 Bq m⁻³) [30] of Punjab. But observed concentration is less than that of Tosham region (37–80 Bq m⁻³) [31] of Haryana, Hamirpur region (25–208 Bq m⁻³) of Himachal Pradesh [32] and Garhwal Himalayas (13–291 Bq m⁻³) [1]. A high value of ²²²Rn concentration in Tosham Haryana is due to the presence of granite rocks while in Garhwal Himalayas, it is due to active boundary thrust.

Similarly, indoor thoron concentrations in same dwellings have been found to vary from 30 ± 1 to 204 ± 19 Bq m⁻³ with an AM of 85 ± 42 Bq m⁻³ and GM of 76 Bq m⁻³. The obtained results of ²²⁰Rn concentration are much greater than the global value given by UNSCEAR [33]. The ²²⁰Rn concentration is comparatively much greater than the ²²²Rn concentration in each dwelling due to diffusive transportation of radon, suggesting high thoron emanation rate in soil and material used for building purposes in the examined region. Therefore, high thoron content might be due to thorium-rich soil [12]. Figure 3 represents the variation of indoor ²²²Rn and ²²⁰Rn concentration in studied locations of Reasi district, Jammu & Kashmir.

Fig. 3

Variation of indoor ²²²Rn/²²⁰Rn concentration in studied locations

The radon level in study area can be reduced by taking some short term steps. These general steps are to seal cracks and holes found in walls, floors, drains and pipes. Renovate existing basement floors, particularly earth floors. Increase ventilation in the subfloors beneath the basement. Install a device that sucks the radon from the lowest space in the basement (radon sump system). Avoid using exhaust fans for a continuous amount of time. When you are not using the fireplace, shut the chimney damper. By using these steps, we can help to minimize radon in our daily life.

²²²Rn/²²⁰Rn progeny concentration

Table 1 gives the knowledge about the calculated results of progeny concentrations of ²²²Rn and ²²⁰Rn in examined dwellings of the studied region. The total equilibrium equivalent (EERC_A+U) of ²²²Rn and (EETC_A+U) ²²⁰Rn concentration were found to vary from 10 to 30 Bq m⁻³with a mean of 18 ± 4 Bq m⁻³ and from 0.9 to 3.3 Bq m⁻³ with a mean of 1.8 ± 0.6 Bq m⁻³, respectively. Majority of EERC_A+U lies in the range of 14–25 Bq m⁻³ and about 4% of total dwellings have radon progeny concentration above the 25 Bq m⁻³, respectively. But the overall results of radon progeny concentration in buildings lie within the range (2–50 Bq m⁻³) prescribed by ICRP [11]. Similarly, most of the thoron progeny concentration lies in the range of 1.1–2.1 Bq m⁻³ and about 7% dwellings have higher thoron progeny level than 2.1 Bq m⁻³. However, 28% of the locations have higher thoron progeny than the range suggested by ICRP [11]. The variation seen in progeny of radon and thoron is might be the presence of radium content in bedrocks, different material used for construction as well as decorative purposes, ventilation rate, and type of houses. Figure 4 represents the variation of ²²²Rn and ²²⁰²Rn progeny in studied houses. Longer the whisker plot in a positive direction and mean is larger than median shows that ²²²Rn and ²²⁰Rn progeny was well positively skewed.

Fig. 4

a Box Whisker plot for radon progeny. b Box plot for thoron progeny

The attached progeny concentration (EERC_A) of ²²²Rn and (EETC_A) of ²²⁰Rn in examined locations has been varying from 7 to 25 Bq m⁻³ with an AM of 14 ± 4 Bq m⁻³ and from 0.6 to 3.1 Bq m⁻³ with an AM of 1.4 ± 0.3 Bq m⁻³, respectively. Similarly, EERC_U and EETC_U in the dwellings ranged from 1 to 6 Bq m⁻³ with an AM of 4 ± 2 Bq m⁻³ and from 0.1 to 1.0 Bq m⁻³, respectively.

Equilibrium factors

The annual estimation of equilibrium factor (EF) for ²²²Rn and its progeny and ²²⁰Rn and its progeny have been ranged from 0.2 to 0.9 with a mean of 0.6 ± 0.2 and from 0.01 to 0.04 with an average of 0.02 ± 0.01 respectively as given in Table 1. The seasonal variations of EF for ²²²Rn and ²²⁰Rn are graphically presented in Fig. 5 and tabulated in Table 1. The annual average of equilibrium for ²²²Rn and its progeny has been observed to be little higher than its global value (0.4) as detailed by UNSCEAR [34]. These values are calculated for the first time for this part of Himalayas. The annual average of EF for ²²⁰Rn and its progeny have been observed to be lower than the all-around accepted value (0.02) as detailed in UNSCEAR [7]. Radon progeny and the EF depend to a great extent on the environmental conditions, which may result in the variation in dosage calculations. Due to the comparatively short half-life of ²²⁰Rn as compared to its decedents results in the non-uniformity of ²²⁰Rn EF even in the natural environment. The large variation in estimated results of EF suggests that while calculating the radiation dosage because of the exposure of ²²²Rn, ²²⁰Rn and their decedents, the EF ought to be determined separately for an individual houses.

Fig. 5

Variation of equilibrium factor in different ventilation conditions, seasons, and type of houses

Estimation of an unattached fraction

Unattached fractions have also been calculated using progeny concentrations of ²²²Rn and ²²⁰Rn. In normal conditions, the unattached fractions of Rn²²² ($f_{\text{p}}^{\text{Rn}}$) and Rn²²⁰ ($f_{\text{p}}^{\text{Tn}}$) have been found varying from 0.1 to 0.5 with an AM of 0.2 and from 0.05 to 0.5 with an AM of 0.3, respectively. These obtained values are close to ²²²Rn and ²²⁰Rn progeny concentrations. The obtained average result of radon unattached fraction is comparable to the prescribed value (0.15) reported in the literature [35,36,37]

A weak positive correlation has been seen between $f_{\text{p}}^{\text{Rn}}$ and EF_Rn with Pearson’s coefficient of 0.22 as shown in Fig. 6. This weak relationship is due to low particle concentration inside the homes. The reason for this low attachment rate is might be due to high particle concentration in selected dwellings. Among these lines, ²²²Rn progeny are for the most part free and in this way plate out on surfaces prompting an imperative dis-equilibrium amongst ²²²Rn and its progeny [38].

Fig. 6

Correlation between EF_Rn and $f_{\text{p}}^{\text{Rn}}$

Seasonal variation

The temperature of the studied area generally varies from minimum of 6 °C in winter nights to maximum of 39 °C during peak summers. The annual average concentrations of ²²²Rn during winter, summer and rainy seasons have been found to be 38 ± 21, 26 ± 7 and 23 ± 7 Bq m⁻³, whereas for ²²⁰Rn they were found as 127 ± 73, 68 ± 38, and 81 ± 34 Bq m⁻³ as shown in Table 2. Graphical representation of a variation of indoor ²²²Rn and ²²⁰Rn and progeny concentration is given in Fig. 7. Results of seasonal variations of ²²²Rn reveal maximum concentrations during the winter season. This might be due to temperature inversion which is generally expected in winter. Also, houses are kept closed during this season for most of the time which leads to poor ventilation [1]. The concentration gradually decreases during summers and rainy seasons, lowest being in the rainy season. ²²²Rn concentration is found a bit more in summers than in rainy season in contrast to the usual trend of winter maximum and summer minimum as observed elsewhere. This might be due to the fact that temperature variation is almost the same during these two seasons in the studied area so ventilation rate is almost the same. But during rainy season soil becomes saturated with water and hence less concentration is exhaled.

Table 2 Seasonal, ventilation and dwelling type variation of radon, thoron and its progeny (attached, unattached and total) concentration along with equilibrium factors

Fig. 7

Seasonal variation of indoor radon/thoron and its progeny

Radon progeny concentration is also observed to be maximum in winters and minimum in rainy season. A similar trend is observed for ²²⁰Rn and its progeny concentrations. As decay constant of ²²⁰Rn is significantly larger than the ventilation rate, the difference in ventilation conditions does not influence ²²⁰Rn concentrations. It is seen that the thoron concentration in all seasons is higher than the corresponding indoor radon concentration. This might be due to thorium-rich construction material used to build houses or thorium-rich soil in the region.

Variation according to types of houses

Observed values of ²²²Rn, ²²⁰Rn and progeny concentration levels in dwellings for different construction materials used covering all the three seasons are tabulated in Table 2. Graphical representation of observations is shown in Fig. 8. It is observed that the average ²²²Rn and ²²⁰Rn concentrations are greater in the Mud type (Mud floor + Mud wall) dwellings. Likewise, the average values of EERC and EETC are also found maximum in the Mud type abodes. This is in accordance with the trend reported elsewhere and this might be explained on the criteria of high porosity and permeability in these types of houses as radon easily enters these houses from the ground below due to high porosity [39]. On the other hand, modern housing techniques using types of cements blocks or marbles have shown low values of ²²²Rn and ²²⁰Rn concentration. This may be due to low porosity and low diffusion rate in cemented houses.

Fig. 8

Variation of radon/thoron and its progeny in different type of dwellings

Houses in which floors are constructed using local construction material are found to have high thoron concentration than those constructed using marbles, which are mainly brought from outside. Due to its short half-life (55.6 s), ²²⁰Rn in soil gas underneath a building, in most cases can’t survive sufficiently enough to enter the building and contribute to the indoor ²²⁰Rn level. In this way, indoor ²²⁰Rn is ordinarily due to the exhalation from thorium, which might be available in materials utilized on the inside surfaces of the building. Also cemented floors are more porous than marble floors. Seeing it in combination with the fact that high thoron concentration is observed in comparison to radon concentration for all types of dwelling during all seasons, we might conclude that high indoor thoron concentration is due to thorium-rich bricks and local construction material like sand used in the construction of walls.

Dependent of radon, thoron, and its progeny on ventilation condition of houses

In mud houses, inhabitants used to keep a low number of doors and windows, which give poor ventilation conditions to a house. An attempt has been made to study variation in ²²²Rn, ²²⁰Rn, and their progeny concentration according to ventilation conditions. It is observed that values are high in the poorly ventilated room in comparison to average and good ventilated rooms. Average ²²²Rn and ²²⁰Rn concentration is found lowest in well ventilated rooms in accordance with the trends observed in other studies. Figure 9 shows variation in radon, thoron and its progeny concentration according to ventilation conditions of the rooms selected. Figure 9 shows the graphical representation of data observed.

Fig. 9

Variation of gas and its progeny concentration in different type of dwellings

Estimation of radiation doses

The annual effective dose due to indoor ²²²Rn and its progenyare found to vary from 0.5 to 1.5 mSv y⁻¹ with an arithmetic mean of 0.9 ± 0.2 mSv y⁻¹ while the annual effective dose due to thoron and progeny are found to vary from 0.2 to 0.8 mSv y⁻¹ with an arithmetic mean of 0.4 ± 0.2 mSv y⁻¹. The total annual inhalation dose (D) due to exposure of indoor ²²²Rn, ²²⁰Rn, and their progenies is found to vary from 1.07 to 2.68 mSv y⁻¹ with an average value of 1.72 mSv y⁻¹. These values are less than the safe limit (3–10 mSv y⁻¹) [40] and pose no health risk to the population of the examined area. The contribution of indoor ²²⁰Rn and its progeny to total dose is about 1/4th. Thus thoron can’t be ignored while evaluating radiation measurements.

Conclusions

The annual average values of indoor radon concentration are found lower than the world-wide average of 40 Bq m⁻³, whereas thoron concentration is found to be higher than the world-wide average of 10 Bq m⁻³ as well as the national average of 12.2 Bq/m³. The ²²²Rn, ²²⁰Rn, and progeny concentrations have been found to be relying on type of season, ventilation conditions and type of building materials used to construct houses. Houses in which floors are constructed using local construction (mud and rocks) material are found to have high thoron concentration than those constructed using marbles, which are mainly brought from outside. Also, overall very high thoron concentration levels point to the fact that soil of the region is thorium-rich. The annual equilibrium factor for radon and its progeny and thoron and its progeny have been within the globally expected value prescribed by UNSCEAR. The large variation in the thoron equilibrium factor even for the similar natural conditions is due to its short life.

Moreover, radiation dose is within safe limit and poses no health risk to the population of the study area. The contribution of indoor thoron and its progeny to total inhalation dose is about 1/4th. Thus thoron cannot be neglected while assessing radiation doses.