Introduction

Because of the carcinogenic effects of radon, thoron and their daughters on humans, it is important to determine their levels [1, 2]. Human always been exposed to the radiation which mainly comes from natural and human sources. Natural radiation significant as it constitute major source of radiation doses to the people. Natural background radiation is generated by cosmic radiation, internal radiation (radionuclide’s presents in body), inhalation and external terrestrial radiation [3, 4]. The natural background radiation predominantly depends upon the geological condition, altitude and house type. Radon and its offspring account for more than half of all radiation [5,6,7,8,9]. The alpha decay of 226Ra in uranium decay chain produces radon (222Rn). As radium emits alpha particles, it transforms into radon, a gaseous substance. This gas can break away from the soil grain and move through pore spaces to the surface, where it can be released into the living atmosphere. Thoron (220Rn), another radon radioisotope, is found in the thorium series and contributes significantly to radiation doses. Previously, researchers had completely ignored thoron, and studies had only concentrated on 222Rn and its effects. However, in recent decades, research has focused on the 220Rn as well [10,11,12,13]. Due to gaseous nature of radon it can accumulated in the dwellings. The indoor level of radon depends upon the several meteorological parameters like ventilation rate, construction materials, humidity, wind rate, life style of inhibitors etc. [14, 15]. It is important to estimate the 220Rn level as it is found everywhere along with 222Rn and sometimes the quantity is much greater than that of 222Rn, which contributes a significant amount to the total radiation dose [16, 17].

This study was performed in the district Udham Singh Nagar (USN) of Uttarakhand. The USN is third most populated district of Uttarakhand. The location of district Udham Singh Nagar is shown in Fig. 1. The latitude and longitude of Udham Singh Nagar district are 28° N to 58° N, and 78° E to 81° E, respectively. No previous studies for radon & thoron were reported from this district so far. The aim of this research is to determine the levels of 222Rn, 220Rn, and progenies, as well as their seasonal variations, which will be useful in calculating radiation doses and their effects on human health. This data will add to the national database of radon, which can will be useful for the purpose of radiation protection.

Fig. 1
figure 1

Map and location of study area

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Experimental techniques

Exposure of detectors

The passive techniques had been used to evaluate the 222Rn, 220Rn, EERC and EETC values. A total number of 102 houses were covered for this study. For the 222Rn and 220Rn concentration, cylindrical pin-hole-twin-cup dosimeters were used and for progeny concentration direct progeny sensor (DTPS/DRPS) was used. Sahoo et al. (2013) and Mishra et al. (2008) explore the details of these instruments [18,19,20].

In both pin hole dosimeter (Fig. 2) and DTPS/DRPS (Fig. 3) techniques cellulose nitrate detectors (LR-115) were utilized. These detectors were installed in houses following standard protocol and exposed for the period of 90 days. Dosimeters were retrieved and replaced with new ones after the exposure cycle was completed. The recovered dosimeters were then analysed in lab, where the registered tracks on LR-115 film were etched in a constant temperature bath with sodium hydroxide (2.5 N) to achieve good visibility of the tracks.

Fig. 2
figure 2

Pin-hole-twin cup dosimeter

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Fig. 3
figure 3

Direct progeny sensor (DTPS/DRPS)

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Estimation of 222Rn and 220Rn concentrations

After chemical etching of LR-115 films from pinhole dosimeters were counted with a spark counter [21], and the counted tracks were transformed into concentrations using the Eqs. (1) and (2) [18]:

$$C_{{{\text{Rad}}}} \left( {{\text{Bq}}\,{\text{m}}^{{ - {3}}} } \right) = T_{{r{1}}} /\left( {D_{{{\text{exp}}}} *k_{{{\text{rad}}}} } \right)$$
(1)
$$C_{{{\text{Tho}}}} \left( {{\text{Bq}}\,{\text{m}}^{{ - {3}}} } \right) = \left( {T_{{r{2}}} - T_{{r{1}}} } \right)/\left( {D_{{{\text{exp}}}} *k_{{{\text{tho}}}} } \right)$$
(2)

where CRad, and CTho are the 222Rn and 220Rn concentration, Tr1 and Tr2 are track density in Radon-chamber and Radon + Thoron-chamber, and Dexp is the number of days of exposure. Krad and Ktho are the calibration factors for 222Rn (0.172 tr cm−2 day−1/(Bq m−3)), and 220Rn (0.010 tr cm−2 day−1/(Bq m−3)) respectively.

Estimation of 222Rn/220Rn progeny concentration

LR-115 from DTPS/ DRPS were etched and counted using spark counting technique [21]. For measuring progeny concentration, Eqs. (3) and (4) has been used to convert tracks into concentration [19, 20]:

$${\text{EETC }}\,\left( {{\text{Bq}}\,{\text{m}}^{{ - {3}}} } \right) = T_{{{\text{tho}}}} /\left( {d_{{{\text{exp}}}} * \, S_{{{\text{tho}}}} } \right)$$
(3)
$${\text{EERC}}\,\left( {{\text{Bq}}\,{\text{m}}^{{ - {3}}} } \right) = T_{{{\text{rad}}}} /\left( {d_{{{\text{exp}}}} * \, S_{{{\text{rad}}}} } \right) - {\text{ EETC}}$$
(4)

where Ttho and Trad are tracks in DTPS and DRPS, Stho and Srad are the sensitivity factor for 220Rn (0.94 ± 0.027 Tr cm−2 day−1/EEC (Bq m−3) and 222Rn (0.09 ± 0.0036 Tr cm−2 day−1/EEC (Bq m−3) respectively.

Equilibrium factor for 222Rn and 220Rn

Estimation of equilibrium factor between 222Rn, 220Rn and their progenies is always important for assessment of inhalation dose. The Equilibrium factor (‘F’) is the ratio of total PAEC (potential alpha energy concentration) for the actual progenies to the total PAEC of the progenies which would be equilibrium with the 222Rn and 220Rn [22]. An equilibrium factor 1 defines the equal amount of 222Rn and its progeny.

In past years, equilibrium factor F for indoor radon (0.4) was globally recommended by ICRP and UNSCEAR. With the time, new DTPS/DRPS passive detection techniques were discovered for measuring the short lived daughters of 222Rn and 220Rn [23]. The equilibrium factor for 222Rn (F-Radon) and 220Rn (F-Thoron) was estimated by Eqs. (5) and (6) respectively [24]:

$${\text{F}}_{{\text{-}}_{{\text{Radon}}}} = {\text{EETC}}\,\left( {{\text{Bq}}\,{\text{m}}^{{ - {3}}} } \right)/C_{{{\text{Rad}}}} \left( {{\text{Bq}}\,{\text{m}}^{{ - {3}}} } \right)$$
(5)
$${\text{F}}_{{\text{-}}_{{\text{Thoron}}}} = {\text{EERC}}\,\left( {{\text{Bq}}\,{\text{m}}^{{ - {3}}} } \right)/C_{{{\text{Tho}}}} \,\left( {{\text{Bq}}\,{\text{m}}^{{ - {3}}} } \right)$$
(6)

Estimation of annual effective dose (AED) due to inhalation

AED due to inhalation of indoor 222Rn, 220Rn, and their progeny has been calculated using the Eqs. (7) and (8) [25].

$${\text{AED}}_{{{\text{radon}}}} \,\left( {{\text{mSv}}\,{\text{y}}^{{ - {1}}} } \right) = \, \left[ {\left( {C_{{{\text{rad}}}} \times 0.{17}} \right) + \left( {{\text{EERC}} \times {9}} \right)} \right] \, \times { 7}000 \times { 1}0^{{ - {6}}}$$
(7)
$${\text{AED}}_{{{\text{thoron}}}} \,\left( {{\text{mSv}}\,{\text{y}}^{{ - {1}}} } \right) = \left[ {\left( {C_{{{\text{tho}}}} \times 0.{11}} \right) + \left( {{\text{EETC}} \times {4}0} \right)} \right] \times {7}000 \times { 1}0^{{ - {6}}}$$
(8)

Results and discussion

222Rn and 220Rn concentrations, EERC and EETC were measured in the total numbers of 102 houses in Udham Singh Nagar district. This study was carried out in winter, summer and rainy season. Table 1 illustrates the results of these three seasons.

Table 1 Seasonal variation in 222Rn, 220Rn and progeny concentration
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222Rn in winter season found to be in the range 15.3–84.1 with a mean of 46.3 ± 15.7. In summer, it is found to be in the range 7.3–84.1 with a mean of 24.9 ± 11.0, and the value found to be in the range 11.6–81.0 with a mean of 38.4 ± 12.6 in rainy season. The 220Rn value found to be in the range 11.7–149.6 with a mean of 61.6 ± 32.2, 7.2–151.5 with a mean of 43.8 ± 26.7, and 11.4–82.3 with a mean of 41.3 ± 16.6 within winter, summer and rainy seasons, respectively.

The value of EERC found to be 9.0–41.3 and a mean of 20.8 ± 6.7 in winter, 2.4–37.5 with a mean of 13.9 ± 5.7 in summer, and 4.2–56.3 with a mean of 17.1 ± 8.2 in rainy season. The EETC value in winter found to be 0.8–4.4 through a mean of 2.0 ± 0.7, in summer it is found to be 0.4–3.3 through a mean of 1.5 ± 0.7, and in rainy season it is found to be 0.5–4.8 through a mean of 1.7 ± 0.7.

The F-Radon was found to be varying from 0.17 to 1.20 with a mean of 0.51 ± 0.23 in winter, 0.10–1.48 with a mean of 0.60 ± 0.28 in summer and 0.07–1.45 with a mean of 0.48 ± 0.26 in rainy season. The F-Thoron was found to be varying from 0.01 to 0.18 with a mean of 0.045 ± 0.03 in winter, 0.01–0.19 with a mean of 0.049 ± 0.04 in summer and 0.01 to 0.18 with a mean 0.049 ± 0.03 in rainy season.

Figure 4 represents seasonal variation in 222Rn, 220Rn and their progeny concentration. The higher concentration is found in winter season because of the cold climate, people trying to close doors and windows which leads to the poor ventilation condition and more radionuclide’s accumulated insight the houses. Due to good ventilation rate, lower value was found in summer season (except 220Rn) which allow the dilution of indoor air by outdoor air.

Fig. 4
figure 4

Seasonal variations in concentration of 222Rn, 220Rn, EERC, and EETC

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The annual average concentration for 222Rn, 220Rn, EERC and EETC is found to be 36.7 ± 9.7, 48.8 ± 21.2, 17.1 ± 4.5 and 1.7 ± 0.5 respectively. The annual average 222Rn value (36.7 ± 9.7 Bq m−3) found less than the worldwide standard value (40 Bq m−3) as well as the national standard value (42 Bq m−3). The 220Rn level (48.8 ± 21.2 Bq m−3) found higher when compare with worlds standard value (10 Bq m−3) as well as national average value (12 Bq m−3).

Figure 5a–d represents the frequency distribution of 222Rn, 220Rn, EERC and EETC. Frequency distribution of 222Rn shows that 42 houses fall between 31–40 Bq m−3. And only 9 houses has the value more than 50, but no house has the value equal to or more than 100 Bq m−3. For 220Rn level most of the houses fall between the range 21–40 and 41–60 Bq m−3. Most of the 222Rn progeny and 220Rn progeny fall in range 16–20 and 1.6–2.0 Bq m−3 respectively.

Fig. 5
figure 5

a Frequency distribution of 222Rn. b Frequency distribution of 220Rn. c Frequency distribution of EERC. d Frequency distribution of EETC

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Variation with type of houses

Table 2 shows the 222Rn, 220Rn, EERC and EETC in different house type. Of the study area divided into three categories; Pakka House, Kachha house and grass house. The pakka houses were made of cement, bricks etc., and the kachha houses were made of mud, stones etc., and the grass houses were made from grass and bamboos. The results for these three kinds of houses are shown in Table 2. As can be seen, high concentration of 222Rn, 220Rn has been estimated in Kachha houses and the lower value estimated in Grass houses. The Kachha house has mud floor while Pakka houses has cemented/tile/marvel floor, which seal the entry of 222Rn/220Rn gas from underneath soil. The Grass houses generally have lower value as compared to both Pakka as well as Kachha house may be due to the wall and also roof made from bamboos and grasses, which are not a source of radon.

Table 2 222Rn, 220Rn, and their progeny level (in Bq m−3) in different house type
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Estimation of radiation doses

The annual radiation doses were calculated to the individuals of the study area by using yearly average concentration of 222Rn, 220Rn and their progenies. Figure 6 represents frequency distribution of dose in studied house. The AED due to inhalation of 222Rn and its progeny was found to be 0.50–1.95 with a mean value of 1.13 ± 0.29 mSv year−1. The AED due to 220Rn and its progeny was found to be 0.29–0.93 with an average of 0.53 ± 0.14 mSv year−1. It can be seen from the frequency distribution diagram (Fig. 6) that large number of houses found between the ranges 1.51–2.0 mSv year−1.

Fig. 6
figure 6

Frequency distribution of AED due to inhalation

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Conclusions

The findings of this study reveals that the annual average 222Rn and 220Rn values did not exceed the WHO recommended safe limits of 100 Bq m−3 [26]. The design of house also has an effect on the level of 222Rn, 220Rn and progenies. The concentration of these radio-elements also shows the variation with the season. The F-Radon and F-Thoron values were 0.53 (greater than the worlds average, 0.4) and 0.048 (lower than the worlds average, 0.10), respectively. The dose (AED) obtained by local residents from indoor 222Rn, 220Rn, and their progenies were found to be less than the UNSCEAR acceptable limits and well below the action limit of ICRP (3–10 mSv year−1) [25, 27]. 220Rn and its progeny cannot be disregarded when evaluate the doses, as they contribute significantly (about 1/3rd) to the total radiation doses received by individuals. Although from the data presented in this study may assure that the level of 222Rn and 220Rn is not so high so peoples are safe from the harmful effect of radiation.