Introduction

Measurements of thoron (220Rn) have gained momentum in the recent past owing to the development of instrumentation as well as due to their contextual importance. Studies performed in the Indian Himalayan region have shown higher than average concentration of 220Rn and its decay products. These studies covered regions from Uttarakhand1,2,3,4,5,6, Jammu and Kashmir7,8,9 and North-east India10,11. Soil and mud as construction materials have been shown to contribute to the observed higher levels in some of these studies12,13,14. In contrast to the usual seasonal trend, summertime and wintertime levels for some of these regions have shown to be similar. Higher levels observed during the measurements of 220Rn and its decay products in monazite sand areas and occupational environments are not surprising. However, the trends obtained for Uttarakhand state for the same are insightful and require immediate attention15.

Worldwide global averaged radon (222Rn) and 220Rn concentration in ambient atmospheric environments have been reported as 40 and 10 Bq/m3, respectively16. The equilibrium factor relating the gas and decay product’s concentration has been averaged as 0.40 and 0.02 for 222Rn and 220Rn, respectively17. Although no such Indian averaged values are established, a wide variation for concentration and equilibrium factor is reported. Averaged 220Rn concentration came out to be 14.58 Bq/m3, 37.3 Bq/m3, 38.1 Bq/m3, 64.15 Bq/m3, and 105 Bq/m3 for Palakkad (Kerala17), Hyderabad18, Faridabad19, Srinagar8, and HBRA20 (Odisha), respectively. Similarly, a huge variation in equilibrium factor as per 0.001 to 0.07721, 0.01 to 0.2922 and 0.008 to 0.10123 etc. is determining the levels of decay products in the ambient conditions. In terms of radiation dose, approximately 55% of the background radiation dose of 2.4 mSv has been attributed to 222Rn, 220Rn and associated decay products. It is also a fact that most of this attributed dose i.e. 1.26 for 222Rn and 0.07 mSv for 220Rn is for the decay products, not for the gases. Conventionally, the annual average inhaled radiation in the form of 222Rn /220Rn and their decay products is known as the annual inhalation dose (AID). AID has also been shown to be varying significantly in the Indian environment being as low as 0.60, 0.66 mSv24 etc. and as high as 6.3 mSv/y25, 7.4 mSv/y26. As the levels of 220Rn and decay products are higher for some specific regions, the contribution of the same in AID will also be higher.

Exhalation rate, diffusion length and ventilation pattern are major factors determining the levels of the gases and the decay products in an indoor environment. Apart from the source term, soil/building material properties27,28 (such as porosity, water mass fraction etc.) and atmospheric parameters (atmospheric pressure, temperature, humidity, wind pattern etc.) affect these levels in indoor environment29,30,31,32. Recent advancements33,34,35,36 in terms of elaborative measurements, computational fluid dynamic simulations37,38,39 and dosimetric studies18,40 have expanded the database in this direction thus broadening the understanding of the researchers. Studies have also been performed in the direction of interpretation in higher-level regions5,6 (Kumar et al., 2020; Semwal et al., 2019) and towards the mitigation of the activity concentration and/or the attributable dose40. In terms of background gamma radiation, some regions in the world41,42,43 and India25,44,45 have been designated as High Background Radiation Areas (HBRA). It has been shown that a higher gamma dose may not correspond to a higher inhalation dose in HBRAs20,46. Higher concentration levels of 220Rn and Equilibrium Effective Thoron Concentration (EETC) and diverse variation in Equilibrium Factor (EF) in studies conducted in the Uttarakhand region need a collective overview in terms of 220Rn fraction in the dose and the underlying factors. In our earlier field campaigns performed in two nearby districts of Uttarakhand (viz. Almora and Nainital), 220Rn concentration and EETC varied from 5 to 213 Bq/m3 and 0.01–0.68 Bq/m3 for Almora and 6–120 Bq/m3 and 0.03–4.42 Bq/m3 for Nainital, respectively. It is intuitive to corroborate observations of our studies with similar ones conducted in Uttarakhand state and harmonize the findings.

This work has two objectives namely to perform measurements of 222Rn, 220Rn and decay products in the Bageshwar district of Uttarakhand as a continuation of our field campaigns and to interpret 220Rn dose fractionation for recent studies conducted in the Uttarakhand region. 10 different regions i.e., Almora, Bageshwar, Dehradun, Nainital, Pithoragarh, Rudra Prayag, Tehri, Udham Singh Nagar, Uttarkashi and Yamuna & Tons Valley have been taken as the ‘region of interest’ in this study. The study starts with interpreting the results of the studies conducted in these 10 regions in terms of the fraction of 220Rn and decay products in inhalation dose. The reasons for the observed features have been discussed on the basis of supporting measurements and analysis performed for the current study region (Bageshwar) and study regions chosen in our previous field campaigns (Almora and Nainital).

Materials and methods

Study region

This study compiles results obtained from recent measurement campaigns performed in the Uttarakhand region including the observations made in the measurements performed under this work in the ‘Bageshwar’ region. Bageshwar district (29.40°N to 30.20°N, 79.25°E to 80.10°E) of Uttarakhand state is located in the Himalayan foothill at an elevation of 795 m to 6761 m from the sea level. The average rainfall in Bageshwar district is 1360 mm which mostly (70%) happens from July to September47. This location is close to the two nearby districts viz. Almora5 and Nainital14 were covered in our previous field campaigns. These 3 locations together with other locations of the Uttarakhand region chosen for interpreting the findings have been shown as a map view in Fig. 1. The terrestrial terrain of this region (Almora and Bageshwar) consists of different types of rock units viz. quartzite with associated slate, volcanic, granite gneiss, augen gneiss, mica-talc schist, migmatite, quartzite, limestone, granodiorite, and conglomerate48. The presence of the Main Central Thrust (MCT) in the upper northern part of the Bageshwar and Almora regions is crucial in terms of geological perspective. The rock type for the Nainital district is garnet-ferrous micas chits, which contain layers of quartzites, lenses of graphite schists and a band of gneissic rocks49.

Fig. 1
figure 1

Boundary map of the study region.

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Dwellings of study region

A standard protocol was followed for the selection of dwellings by conducting a primary gamma survey in the study region. The selection criterion was based on the gamma level as well as the construction materials of the dwellings. The entire region has been divided into three zones: zone A where gamma levels were less than 0.10 µSv/h, zone B having gamma levels between 0.10 and 0.20 µSv/h and zone C in which the gamma levels were above 0.20 µSv/h. It was observed that more than 50% local population is still using the local mud, wood and stones to construct dwellings in this remote region. Based on the construction materials, the dwellings have been divided into three categories: Type A is made up of local mud and wood, type B is made of stone and wood and type C is made up of brick, cement and concrete. To cover seasonal variations, dosimeters were deployed inside the dwellings 3 times in the year covering months March-June, July-October and November-February. Figure 1 shows the geological map of the study region. All the maps containing this article are created using QGIS 3.10.10 (A Caruna) LTR open-source GIS application licenced under GNU general public license. The vector shapefiles of administrative boundaries of study areas were ordered from the online digital library of Survey of India (https://onlinemaps.surveyofindia.gov.in) with HQ scale 1:1 M and assigned product code OVSF/1 M/9. The Earth was assumed to be spherical for convenience with a radius of 6371 km. The zero point was set conveniently without falsifying the Northing and Easting. World Geodetic System 1984 (WGS84) is used as a global datum for the projection of positions on Earth’s surface.

222Rn and 220Rn activity concentration measurement

Measurement of activity concentration of accumulated 222Rn and 220Rn is determined using distinguished single-entry perforated dosimeters. These dosimeters are devised via. abrogating two chambers built up and down with airtight seals. The air is allowed to be entered through a downward opening of the device only which contains multiple pinholes fleeced by a glass fibre filter (Whatman GF/F 47, pore size: 0.56 μm). Each compartment has a height of 4.1 cm with a radius of 3.1 cm which is enough to exceed the diffusion length of alpha decay from 220Rn, hence, it is preferred to contain both 220Rn and 222Rn within. However, due to the diminutive half-life of 220Rn which is 55.5s, it is likely to disintegrate in the lower compartment completely. This discriminating technique provides the optimum response time for the detection of 222Rn in the upper chamber. The chambers are separated by a metallic coated disk with four pinholes which are efficient to discriminate the 222Rn from the mixture of 222Rn and 220Rn from the exposed area. The chambers are coated with metallic powder to maintain a neutral electric field inside the chamber to minimize the progeny deposition on walls and conserve the homogeneity within chambers. The dosimeter is devised with LR-115 Type II (3 × 3 cm2) as a solid-state nuclear track detector (SSNTD) which is commonly used to detect alpha emitters through the tracks obtained during the impact of the alpha decay. The dosimeters were deployed inside the dwellings for one year and the detectors were retrieved and replaced every four months to obtain the seasonal (Winter, Summer, Rainy) variation of indoor activity concentrations of 222Rn and 220Rn. A detector film was exposed without a dosimeter to quantify the background exposure to retrieved detector films during removal. The properly sealed detectors were then etched in a thermally controlled bath using 2.5 N NaOH solution at 60 °C for 90 min until the residual thickness of the film remains no more than 7 ± 1 μm. The Detector films were then peeled off from their cellulose acetate base and track density was estimated using a spark counter50. The activity concentrations were then determined using Eqs. 1&2.

If CR and CT represent the activity concentrations of 222Rn and 220Rn respectively, then:

$${C}_{R}=\frac{{T}_{1}-B}{\left({d*k}_{R}\right)}$$
(1)
$${C}_{T}=\frac{{T}_{2}-\left(d.{C}_{R}.{k}_{R}^{,}\right)-B}{\left({d*k}_{R}\right)}$$
(2)

where T1 = Track density observed in the 2nd compartment, T2 = Track density observed in 1st compartment. kR = Calibration factor of radon in 2nd compartment (0.017 ± 0.002 tracks.cm[-2 per Bq.m[-3.d), kT = Calibration factor of thoron in 1st compartment (0.010 ± 0.001 tracks.cm[-2 per Bq.m[-3.d). d = Exposure time (in days), k’R = Calibration factor of radon in 1st compartment (0.017 ± 0.002 tracks.cm[-2 per Bq.m[-3.d). B is the background track density in the unexposed LR-115 detector.

Deposition-based progeny sensors (DRPS/DTPS)

The primary contributors to inhalation doses from 222Rn and 220Rn are their decay by-products. Therefore, the cumulative concentrations of these decay products serve as the most accurate indicators of exposure. Traditionally, assessing the dose from these decay products involves measuring gas concentrations and considering equilibrium factors. The Indigenous deposition-based progeny sensors are predominant in estimating time integrating progeny deposition fluxes in the environment. LR-115 type II (3 × 3 cm2) detectors are mounted on the top of the device along with an absorber of appropriate thickness. Deposition-based radon progeny sensor (DRPS) consists of 37 μm of combined aluminized mylar and cellulose nitrate on top of the detector to track the alpha particle of 7.67 MeV emitted from 214Po. The Deposition based thoron progeny sensor (DTPS) component consists of LR-115 type II (3 × 3 cm2) combined with a 50 μm aluminized mylar layer, designed specifically to detect 8.78 MeV alpha particles selectively emitted by 212Po. This sensor offers a unique advantage by eliminating interference from gases, providing a direct estimate of the airborne concentration of alpha-emitter progeny. Airborne concentrations are calculated based on deposited activity using the effective deposition velocity. Calibration of these DTPS and DRPS units has been conducted both in controlled laboratory settings and indoor environments at Bhabha Atomic Research Centre (BARC), India2,50.

EETC and EERC were estimated by using Eqs. 3 and 4 respectively.

$$EETC\left(Bq/{m}^{3}\right)=\frac{{T}_{T}-B}{{S}_{T}*d}$$
(3)
$$EERC\left(Bq/{m}^{3}\right)=\frac{{T}_{R}-B-\left({S}_{T{\prime}}\times EETC\left(Bq.{m}^{-3}\right)\right)}{{S}_{R}*d}$$
(4)

where ST and ST’ are the sensitivity factors for thoron progeny {0.09 Tracks cm− 2 d− 1 (Bqm− 3)−1} in DRPS and {0.94 Tracks cm− 2 d− 1 (Bqm− 3)−1} in DTPS. SR is the sensitivity factor for radon progeny {0.94 Tracks cm− 2 d− 1 (Bqm− 3)−1}.

Estimation of dose values

The annual effective doses due to the exposure of indoor 222Rn & their decay products (AEDR) and 220Rn & their decay products (AEDT) have been estimated using Eqs. 5 and 6, respectively16.

$$AEDR\left({mSv.y}^{-1}\right)=EERC\left(Bq{m}^{-3}\right)\times8760h{y}^{-1}\times0.8\times9nSv{(Bq.h.{m}^{-3})}^{-1}\times{10}^{-6}$$
(5)
$$AEDT\left({mSv.y}^{-1}\right)=EETC\left(Bq{m}^{-3}\right)\times8760h{y}^{-1}\times0.8\times40nSv{\left(Bq.h.{m}^{-3}\right)}^{-1}\times{10}^{-6}$$
(6)

In the above equations, dose conversion factors for 222Rn and its decay product concentrations are 0.17 and 9, respectively50,51,52. These values are 0.11 and 40 for 220Rn and its decay products concentrations. An occupancy factor of 0.8 has been taken in the above estimations50,53.

Estimation of radionuclide concentration

The samples were taken from undisturbed land by removing the upper 10 cm surface. The collected samples were dried in an electric oven at 110 °C and packed in airtight polythene bags for carrying them to the laboratory. The samples were prepared as per IAEA protocols for radionuclide analysis and kept of the establishment of secular equilibrium of radium, thorium and decays products13. The analysis of primordial radionuclides in soil samples was carried out using gamma-ray spectroscopy.

The employed spectrometer system manufactured by ATOMTEX (Belarus) makes use of a NaI (Tl) scintillation detector of crystal size 63 mm × 63 mm for identifying and quantifying the desired radionuclide in the soil sample. Data acquisition and analysis of soil samples were performed in its software AT1315. The energy and efficiency calibration of the detector system was done prior to radionuclide analysis. The detection limits for different radionuclides are as follows:

  1. a.

    40K: 20 − 2 × 104 Bq/kg.

  2. b.

    232Th: 3–104 Bq/kg.

  3. c.

    226Ra: 3–104 Bq/kg.

Results and discussions

As stated previously, this work dwells on the interpretations of the results of measurements in the Bageshwar, Almora and Nainital districts (current work) and other locations of Uttarakhand state. These locations include Almora and Nainital districts wherein measurements were carried out by our research group in past as well as other locations from the open literature.

Fractionation of AEDR and AEDT in Uttarakhand region

Averaged worldwide fraction of AEDR and AEDT in total inhalation dose has been reported as 94% and 6%, respectively53. Figure 2(a) represents the fraction of AEDR and AEDT obtained in 10 different studies conducted in Uttarakhand region.

Fig. 2
figure 2

(a) Percentage contribution of AEDR and AEDT for all locations, (b) AEDR and AEDT values for all locations: Map view.

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In the above figure, study locations are arranged alphabetically moving up on the y-axis. AEDT can be observed to be varying from 21% (Nainital14 and Dehradun3) to 48% (Tehri54) in the above plot. This is significantly higher than the 6% value for the worldwide average. The above figure also includes the result obtained for the location (Bageshwar) of the current work (29% AEDT). The absolute average values of AEDR and AEDT for all the districts of Uttarakhand are shown in Fig. 2(a) and represented in a map view drawn for the Uttarakhand region in Fig. 2(b). These values and the associated range are also compiled as Table 1 which also adds a few more locations from India and the world.

Table 1 Summary parameters for AEDR and AEDT for different locations.
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Figure 2(b) represents the values of AEDR and AEDT as blue and red colour bars for different locations. The maximum values of AEDR (2.2 mSv/y) and AEDT (1.1 mSv/y) have been obtained for the Dehradun and Tehri locations, respectively. As such, no geographic trend was observed in terms of higher AEDR or AEDT values. AEDT values were also observed to be on the higher side (> 0.5 mSv/y) for the locations where AEDR values were higher (> 1 mSv/y) barring the Nainital location. The highest AEDR obtained for Dehradun corresponds to the main boundary thrust and tectonic activities might have contributed towards this feature3. Relatively lower values have been measured for Rudraprayag (AEDR: 0.5, AEDT: 0.3) but still have the AEDT fraction as 29%. For Tehri, values for AEDR and AEDT are observed to be quite close to each other (1.1 and 1.1 mSv/y, respectively). The ranges correspond to the averaged values shown in Fig. 2(b). We have added a few more locations from different studies conducted for India and the world to obtain additional insights on the observed trends. These locations have been picked from recently conducted studies following the trend of higher than worldwide AEDT fraction in total inhalation dose.

The fractions of AEDT in total inhalation dose for the above added locations are 47%, 38%, 25%, 31%, 26%, 25% and 39% for Delhi, Aligarh, Jammu, Udhampur, Srinagar, Shivalik hills and Kabini river basin of Karnataka region, respectively. These values are similar to the observed range of 17–33% for the locations of Uttarakhand region and indicate the possibility of pan-India signature of higher thoron contribution to inhalation dose. The values for Delhi and Karnataka region at 47% and 39% are above the highest fraction obtained for the Uttarakhand region. In fact, this fraction crossed 50% for Kerala69 (65%) and Odisha61 (72%) which was expected as these regions are marked as thoron rich locations where thoron and decay products are shown to be a major contributor to total inhalation dose. Although the worldwide AEDT fraction has been averaged at 6%, several locations of the world have shown significantly higher fractions as well. These values are 24%, 18%, 30%, 42%, 64%, 79% and 95% for Fukushima (Japan), Hengyang (China), Hungary, Yangdong (China), Northeast Iran and Kenya, respectively. The last 3 locations here represent the high background radiation areas from different parts of the world.

The above interpretation based on the presented data indicates that the Uttarakhand region shows distinct features characterized in terms of significant thoron contribution to total inhalation dose. In order to probe into this, we first present the summary results obtained for the study region chosen for the present work.

Combined inferences from 3 nearby locations

Subsequently, we combine the results obtained based on measurements made in the Bageshwar location with those gathered from our previously conducted field campaigns in Almora and Nainital. The values of AEDR have been found to range from 0.6 to 2.7 mSv/y, 0.4 to 3.5 mSv/y and 0.6 to 2.8 mSv/y with an average of 1.4, 1.7 and 1.5 mSv/y in Nainital, Almora and Bageshwar district respectively. The values of AEDT have been found to range from BDL to 1.1 mSv/y with an average of 0.3 mSv/y, BDL to 1.1 mSv/y with an average of 0.5 mSv/y and 0.2 to 1.6 mSv/y with an average of 0.6 mSv/y in Nainital, Almora and Bageshwar district respectively. The average ratio of AEDT and AEDR was found maximum (0.5) in Bageshwar and a minimum in Nainital (0.2). The values of radon, thoron, EERC, EETC, AEDR and AEDT are summarised in Table 2.

Table 2 Statistical values of radon, thoron EERC, EETC, AEDR and AEDT in Nainital, Almora and Bageshwar regions.
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Reasons for a higher fraction of AEDT to the inhalation dose?

To clear the complete picture, there is a need to analyse the different perspectives. The first factor is the role of the gamma level. As previously mentioned, the entire study region was divided into three different zones based on the gamma level in that region. The data of radon, thoron, EERC, EETC, AEDR and AEDT in different zones is summarised in Table 3. The average values of all the measured factors were comparatively high in Zone C. The relative difference between the thoron and the relevant dose was higher than the radon and its relevant dose.

Table 3 Distribution of radon, thoron, EERC and EETC in different zones of Nainital, Almora and Bageshwar.
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The spatial distribution of the chosen dose parameters for the present study locations i.e., Nainital, Almora and Bageshwar are shown in Fig. 3. The maps were generated by interpolating the data points over the spatial boundaries of Bageshwar, Almora, and Nainital using Inverse Distance Weightage (IDW). This plot potentially visualizes any geographical variation of parameters within the administrative boundaries and gives a wholesome idea. At some highlighted locations (Kanda, Baijnath, Sirkot and Kausani in Bageshwar; Someshwar, Basoli and Vishwanath in Almora; Kakrighat, Khairna, Ramgarh, Jeolikote, Pratapgarh and Naya Gaon in Nainital) thoron and its relevant dose were found higher than the other places.

Fig. 3
figure 3

Spatial distribution of AEDT and AEDR in Bageshwar, Almora and Nainital districts.

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Although no clear geographical bias could be attributed to the variations in dose parameters, few insights can be obtained. Relatively lower values of AEDT were found towards the western direction. Lesser values for AEDR and AEDT were obtained for the western region and higher values for the same were scattered in the western and central regions. It is to be noted that the colour indexing chosen for the dots in this figure is towards the higher side.

In the next step, the effect of construction materials was analysed. For this, AEDR’s and AEDT’s % contribution distribution of all dwellings segregated based on their construction material was plotted. Figure 4 shows this plot for the Bageshwar, Almora and Nainital regions where the x-axis shows the % levels and the y-axis represents the dwellings.

Fig. 4
figure 4

(a-c): Variation of % levels segregated concerning the type of dwellings for (a) Bageshwar, (b) Almora and (c) Nainital.

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The curve showing the variation in the above plot has been drawn by arranging the dwellings in the order of maximum to minimum AEDR (or minimum to maximum AEDT) in the positive x-direction. Hence, the topmost plot drawn for mud dwellings in Fig. 4 (a) has a variation of AEDR from ≈ 82% to ≈ 43% while moving up on the x-axis covering all mud dwellings (n = 31) for this region. This corresponds to the variation of 18 to 57% for AEDT in the plot. It is illustrated that AEDT varied from 18 to 57, 19 to 41 and 6 to 37% for mud dwellings (Type A), stone and wood dwellings (Type B, n = 30) and brick and cement dwellings (Type C, n = 31), respectively for Bageshwar region (Fig. 4 (a)). For the Almora region, it varied from 5 to 58, 5 to 38 and 3 to 19% for type A, type B and type C dwellings, respectively (Fig. 4 (b)). This variation was observed from 15 to 60, 13 to 33 and 2 to 40% for type A, type B and type C dwellings, respectively for the Nainital region (Fig. 4 (c). It can be seen that the variation of AEDT for mud dwellings was nearly similar for all 3 locations. However, this fraction (maximum: 19%) was distinctly lesser for cement-based dwellings in the Nainital region. This was consistent with the lesser values of AEDR and AEDT for the Nainital region shown in Fig. 2.

It is now clear that average dose estimates for all indices were higher in mud houses. This was expected as several other studies (Kandari et al., 2016; Prasad et al., 2010; Ramola et al., 2016) have shown that dwellings made up of mud as construction material shows higher 222Rn and 220Rn concentration values due to higher source term (Kandari et al., 2018; Kumar et al., 2021; Semwal et al., 2018). However, the variation of AEDR (24.8% higher relative value in mud houses compared to cement houses) comes out to be much less than the observed variation of AEDT(107.7% higher relative value in mud houses compared to cement houses). This together with the higher AEDT fractions observed in Fig. 4 (b) and 4 (c) for the Almora and Nainital regions indicate that dwellings made up of mud could be responsible for higher AEDT fraction in the Uttarakhand region.

Another way to interpret the observations could be the segregation of dose values concerning the altitude of the dwellings. The variation of AEDR and AEDT % contribution as a function of altitude (height from sea level) has been shown in Fig. 5 (I-III) for Bageshwar, Almora and Nainital districts. This plot is similar to Fig. 4 replacing the dwelling label with the altitude of the dwelling as a y-axis date-point/ordinate. No definite trend was established for the altitude-based trend for the Nainital district (Fig. 5 (III)). For the Almora district (Fig. 5 (II)), a slight increase in AEDT fraction for higher altitude dwellings could be noted. This trend became more prominent in Bageshwar district where AEDT fraction increased as a function of altitude. These observations are again consistent with the interpretations made based on Figs. 2 and 4. Being located in more remote conditions, dwellings located at higher altitudes in the Bageshwar district seem to be utilizing local mud as the most contributing construction material. This then results in higher dose estimates in particular higher AEDT as well when viewed cohesively. This reasoning can be confirmed by looking at the radioactivity content of samples taken from all 3 districts. Figure 6 depicts the concentration (in Bq/kg) of 226Ra, 232Th and40K in samples taken from sources nearby dwellings for Bageshwar (n = 75), Almora (n = 24) and Nainital (n = 26), respectively.

Fig. 5
figure 5

(I-III): Variation of % levels for mud dwellings with concerning altitude for (I) Bageshwar, (II) Almora and (III) Nainital.

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Average 226Ra concentration was measured as 33, 42 and 49 Bq/kg for Bageshwar, Almora and Nainital districts, respectively. The averaged values for 232Th and40K came out to be (55,43,40 Bq/kg) and (1567, 2004,1983 Bq/kg) for these districts, respectively. Higher values of 232Th for the Bageshwar region follow the trend of higher AEDT observed for this region. The peak of the distribution for 232Th concentration can also be seen to be towards the higher concentration region in Fig. 6 for the Bageshwar region.

Fig. 6
figure 6

Distribution of radionuclides in Almora, Nainital and Bageshwar districts.

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To identify the natural punctuation of radiological parameters attributable to the radon and thoron with corresponding radionuclides, correlational studies have been performed. Figure 7 depicts a diagram of the correlation between radon, thoron, radium, thorium, and potassium40K). The Pearson method of linear fitting endeavoured to obtain a correlation coefficient (r2) which is shown numerically in the upper right triangle of the diagram. A strong correlation (r2) has been found between radon and radium; thoron and thorium viz., 0.74 and 0.59 respectively. Meanwhile, a weak but positive correlation is found between radon and thoron i.e., 0.15. Radon isotopes are weak-negatively correlated with potassium40K) concentrations.

Fig. 7
figure 7

A diagram (Correlation Matrix) showing correlations between radon, thoron, radium, thorium, and potassium40K) along with corresponding histograms and geometric jitter of sample points for the collective dataset of all three districts (Almora, Nainital, and Bageshwar).

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The inferences obtained based on the analysis performed above suggest that higher radioactivity content in the building construction material could be the major contributor to the observed higher thoron fraction in inhalation dose. The remoteness of the location affecting the unavailability of the resources for modern housing material could be one reason for the higher dose estimates.

Conclusions

This study compiled the results of the measurements of 222Rn, 220Rn and their decay products conducted in Bageshwar, Almora and Nainital districts of Uttarakhand state. The main focus of the compilation is to interpret the variation of AEDR and AEDT obtained for these regions and 7 other regions of this state. To check the significant role of thoron and its decay products in natural background radiation dose, four key factors (geography, construction material, altitude and radionuclide concentration) were analysed by the measured data. This study confirms that the value of AEDT in the entire Uttarakhand Himalayan region is multiple times of the world average value. The geographical study pointed out some highlighted regions in Bageshwar, Almora and Nainital for the higher thoron values. This study concludes that in traditional mud houses contribution of thoron is enhanced. However, in cemented and stone houses its contribution reduces due to the short diffusion length of the thoron. As the pressure and temperature changes with the altitude. There is no exact pattern was found but in the Bageshwar it can be seen that thoron’s contribution increases with altitude. The study of radionuclides in the respective regions confirms that the source is the key factor for this scenario. From a future perspective, thoron should be analysed as a significant contributor. To conform to the role of altitude of the atmospheric parameters on the journey of thoron a deep and concentrated study is required in the Indian Himalayas.