Quantification of an alpha flux based radiological dose from seasonal exposure to 222Rn, 220Rn and their different EEC species

This study summarizes the seasonal experimental data on the activity concentrations of indoor 222Rn (Radon), 220Rn (Thoron) and their progeny in Mansa and Muktsar districts of Punjab (India) using LR-115 solid state nuclear track detector based time integrated pin-hole cup dosimeters and deposition based progeny sensors for the assessment of radiological dose. The indoor 222Rn concentration was observed higher in the rainy and winter seasons while 220Rn concentration was observed higher in the winter season. However, Equilibrium Equivalent Concentrations (EECs) of 222Rn and 220Rn exhibited distinct seasonal behaviour unlike their parent nuclides. The average equilibrium factors for 222Rn (FRn) and 220Rn (FTn) were found 0.47 ± 0.1 and 0.05 ± 0.01, respectively. The annual arithmetic means of unattached fractions of 222Rn (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${f}_{p}^{Rn}$$\end{document}fpRn) and 220Rn (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${f}_{p}^{Tn}$$\end{document}fpTn) were found to be 0.09 ± 0.02 and 0.10 ± 0.02, respectively. The attachment rate (XRn) and attachment rate coefficients (β) of 222Rn progeny were also calculated to understand the proper behaviour of progeny species in the region. A new alpha flux based technique has been proposed and used for the assessment of absorbed dose rate and annual effective dose rate for radiation protection purpose.


Geology, Climate and Seasons of Studied Area
Punjab is in north western India and has an area of 50, 362 square kilometers. Geologically, Punjab is formed by alluvial deposits of various rivers flowing in the region. The rocks of Aravalli -Delhi subgroup and the Malani igneous suite comprising greywacke, Ortho -quartzite carbonate sediments, calcareous shales and slates, high heat producing granites and felsites form the basement in the region [22][23][24][25] . The scattered outcrops of the Aravali-Delhi Subgroup occur at Tosham (Haryana) just south of the study area i.e. Mansa and Muktsar Districts of Punjab (India) as shown in Fig. 1. The other geological parameters, area and population are given in Table 1 26,27 .
The climate of Punjab is determined by the extreme hot and cold conditions. The Himalayas in North, Deserts of Rajasthan in south, three rivers and famous Indian monsoon influencing the climate and environment of Punjab. The seasons of Punjab can be categorized into summer, rainy, winter, pre -summer and post monsoon seasons. In Punjab, the summer season commences at the start of March. Punjab's rainy season begins in last week of June monsoon. Three quarters of the total rainfall is concentrated during the three months of southwest monsoon winds and the rest comes during the winter months. There is a wide difference in the amount of rainfall experienced in east and west Punjab. The monthly average rainfall in the studied area has been shown in Table 1 and it has been seen that gap between October and November is a transitional period between monsoon and winter 26,27 . The minimum temperature in studied area has been recorded in months of January -February and maximum in the month of June. The maximum temperature usually occurs in the months of May -June and during this period temperature remains greater than the 40-45 °C in studied area.

Materials and Methodology
The seasonal behaviour of 222 Rn, 220 Rn, unattached fraction and equilibrium factors of 222 Rn and 220 Rn have been studied in 42 villages of Mansa and Muktsar districts of Punjab (India). The acquired data points were processed on computer using IDW (Inverse Distance Weighted) algorithm on Arc map GIS (Geographical Information System) 10.3 software 28 . The months of March to mid-June as summer, Second half June to mid-October as rainy and Second half October to February as winter were taken for the seasonal estimation of 222 Rn, 220 Rn and their daughter products 27 . The 222 Rn/ 220 Rn based dosimeters and DRPS/DTPS were suspended in the way to 20 cm away from the adjacent walls. 222 Rn and 220 Rn. The 222 Rn and 220 Rn concentrations in the air were estimated by using a pin hole-cup dosimeter (Fig. 2). The gas enters through the bottom of the dosimeters in the lower chamber ( 222 Rn + 220 Rn chamber) and diffuses to upper chamber ( 222 Rn chamber) through four pin holes (2 mm length and 1 mm diameter). The glass fiber filter (0.60 µm) paper has been used to stop the entry of progeny nuclides into the chamber as shown in Fig. 2. LR-115 detector films (3 × 3 cm 2 ) have been installed in both chambers of the dosimeter. The chambers are of cylindrical shape having a length of 4.1 cm and radius 3.1 cm. The dosimeters were deployed in the indoor environment for different seasons of a year. After stipulated time of exposure, LR-115 films were retrieved, chemically etched (2.5 N NaOH solution at 60 °C for 90 minutes) and track densities on LR-115 detectors were counted using the spark counter. The concentrations of 222  www.nature.com/scientificreports www.nature.com/scientificreports/ Figure 1. Map of the investigated area of Punjab (India).

District Area Population
Average Rainfall (mm)

Estimation of Equilibrium Equivalent Concentrations of 222 Rn and 220 Rn (EERC/EETC). The total
unattached and attached EECs of 222 Rn and 220 Rn have been measured by DRPS and DTPS progeny sensors as shown in Fig. 3. Both DRPS and DTPS element was made up of LR-115 (2.5 × 2.5 cm 2 ) mounted with absorbers of appropriate thickness as shown in Fig. 3. The EEC has been estimated from the tracks registered on LR-115 films using sensitivity factors used in Eqs (3) and (4) [30][31][32] . The coarse (attached) fraction of progeny concentration was measured by the wire-mesh capped DTPS/DRPS (mounted with 200 mesh type wire screens) as shown in Fig. 4.

Theoretical Formalism
Attachment Rate and attachment coefficient. The activity size distribution f a (d) of the radionuclides and the number size distribution Z(d) strongly depend on attachment process and a function of particle size. The following expression can be used to evaluate f a (d) 34 .
The function β(d) is attachment coefficient given by where, D 0 = 6.8 × 10 −2 cm 2 s −1 is diffusion coefficient, v 0 = 1.72 × 10 4 cm s −1 is mean thermal velocity and δ = d/2 + l 0 with l 0 = 4.9 × 10 −6 cm is mean free path of the unattached decay product cluster 34 . Function Z(d) was normalized to unit aerosol concentration (N 0 ) as: p p Here, β(d) is the attachment coefficient of attached 222 Rn progeny to aerosol and Z(d) represents aerosol concentration (cm −3 ). In the present investigation, the attachment rate of 222 Rn (X Rn ) has been calculated from the measurement of unattached and attached activity concentrations of 214 Po as follows:  www.nature.com/scientificreports www.nature.com/scientificreports/ The average attachment coefficient is defined as to be ratio of the attachment rate and aerosol number concentration given as Equilibrium Factor (F Rn , F Tn ). Equilibrium factor for both 222 Rn and 220 Rn has been calculated using following equations:

Results and Discussion
Seasonal variation of 222 Rn and 220 Rn. Prior to statistical analysis, the reliability of the data has been tested. The observed value of Kronbach alpha (=0.816 > 0.7) revealed that the data was statistically significant. Few outliers (cases with standardized residual greater than ±3 standard deviations) are found ( Table 2) and further data have been tested for normality (Kolmogorov-Smirnov). A robust statistics has been used to study www.nature.com/scientificreports www.nature.com/scientificreports/ the descriptive analysis of the different radiological parameters and recorded the lower quartile (25%), median quartile (50%) and upper quartile (75%) using tukey's Hinges and visualized by box -whisker plots as depicted in Figs 5 and 6. The mean, trimmed mean (5%), Inter-quartile range (IQR) corresponding to the dispersion, standard deviation (S.D.) and variance are also reported in Table 2. The asymmetry and tailness of the distribution is indicated by the skewness (S k ) and kurtosis (K) of the data. Relative variability has been analyzed by the absolute median deviation (AMD). All the above tests have been performed at 95% confidence interval and no missing values have been observed.
The descriptive statistics for seasonal variation of 222 Rn concentration (C Rn ) and 220 Rn concentration (C Tn ) in Mansa and Muktsar districts are given in Table 2. It is vivid that the overall mean C Rn in three seasons was 45 Bq m −3 with high 222 Rn concentration during rainy season (54 Bq m −3 ) and the lowest concentration in summer season (32 Bq m −3 ). The average C Rn in rainy season is 1.1 fold higher than the winter season. The ratio of rainy to summer 222 Rn level is 1.7 due to the variation of the soil moisture in different seasons. The soil moisture content is higher in the rainy season as compared to summer or winter season. The rainfall sealing the outer soil surface and elevate the negative pressure field in the room. The negative pressure field generated by the house is responsible for the transportation of soil gas from large distances. The overall mean of C Tn was found to be 44 Bq m −3 across all seasons. The C Tn was highest during winter season with mean value of 54 Bq m −3 and lowest concentration in summers season (34 Bq m −3 ). The seasonal variation of C Rn is different as compared to C Tn because of shorter diffusion length and half -life of 220 Rn. As C Tn is not affected by ventilation rate 35 , even then C Tn is higher in the winter season as compared to rainy and summer seasons.
The mean and median values of 222 Rn and 220 Rn are indicated that the distribution is normal and can be justified by skewness (S K ), kurtosis values (range between −2 and +2) as given in Table 2. The Q -Q plots (probability plots) have been used to confirm the statistical distribution of data around their mean. Figure 7 demonstrates normal Q-Q plots of C Rn and C Tn during rainy, winter and summer seasons. During rainy season, 222 Rn has normal distribution (S K = −0.21) while 220 Rn is rightly skewed (S K = 0.49). In winter season, both 222 Rn (S K = 0.42) and 220 Rn (S K = 0.61) appears to be rightly skewed, with more variation seen in 220 Rn distribution. In summer season,  www.nature.com/scientificreports www.nature.com/scientificreports/ 222 Rn has normal spread with an outlier (S K = −0.01), however, 220 Rn is rightly skewed (S K = 0.21). The results from these Q-Q plots are corroborating with box-plots. The Q-Q plots of C Rn and C Tn lie on a straight diagonal line with minimal deviations indicating normality. Tests of normality (Kolmogorov-Smirnov test) showed that C Rn and C Tn in three seasons follow normal distributions and have a further possibility to proceed with parametric statistics. Figures 5 and 6 revealed the box-plots of C Rn and C Tn in three seasons (range of data, R) and it has been found that variation in C Rn was less as compared to C Tn as indicated by size of the boxes. 222 Rn has less spread of data than 220 Rn, with smallest box being that of summer season (IQR = 10, R = 38) and 220 Rn has widest distribution during winter season (IQR = 49, R = 108). The only one outlier in the overall data has been found in summer season for 222 Rn. The median values of C Rn in three seasons are nearly in the middle of their boxes. For C Rn the median values are shifted towards 3 rd and 1 st quartiles in summer and winter seasons, respectively. However, the amount of data on both sides of the boxes is largely unequal for C Tn than C Rn .
Seasonal Variation for EEC of 222 Rn and 220 Rn progeny. Unlike their parent nuclides, the seasonal demeanor of progenies for 222 Rn and 220 Rn has shown distinct consequences in the studied region as given in Table 2. During the rainy season, the average EERC A+U and EETC A+U were 22 Bq m −3 and 1.7 Bq m −3 . In winter and summer seasons, the average EERC A+U and EETC A+U were 24 Bq m −3 , 1. Statistically, the 5% trimmed mean value and the mean value of EERC in rainy, winter and summer season indicated no outliers (<Q1-1.5 * IQR; >Q3 + 1.5 * IQR) in the data. The EETC also has a normal distribution in different seasons as explained by descriptive statistics (S k , K and IQR) in Table 2. The box-whisker plots revealed more data spread in EERC as compared to EETC (Figs 8 and 9). In general, 95% attached EEC present in the indoor atmosphere and the seasonal behaviour of total EEC (EERC A+U and EETC A+U ) relies on attached EEC. The EERC A was greater in winter season as compared to rainy or summer season due to the poor ventilation in the winter season and faster formation process of attached progeny aerosols as shown in Figs 10 and 11. The aerosol concentration increases with the decrease of temperature in the winter season (India) 36 . The higher aerosol concentration in winter season tends to increase the 222 Rn and 220 Rn attached progeny concentration in the winter season. However, a seasonal pattern for EETC A+U and EETC U has been not observed in the studied region.
Pearson correlation analysis has been performed to determine the interrelation between the 222 Rn, 220 Rn and their attached/unattached EEC in different seasons. A significant and positive correlation has been observed between 222 Rn, 220 Rn and different seasons respectively (Table 3). This suggests that 222 Rn and 220 Rn separately varied during three seasons with no impact on each other and have an influence on their respective concentrations. 222 Rn concentrations during the three seasons was significantly associated with attached and unattached EEC of 222 Rn (r = 0.37-0.72, p < 0.05). However, there was no relationship between 220 Rn and EEC of 220 Rn. www.nature.com/scientificreports www.nature.com/scientificreports/ Seasonal variation of Equilibrium factor for 222 Rn and 220 Rn. From the last few decades, UNSCEAR specified the value of 0.4 for 222 Rn and 0.02 for 220 Rn 37,38 . For an accurate dose assessment, it is necessary to estimate the equilibrium factor of each dwelling due to different ventilation conditions and building material. The   www.nature.com/scientificreports www.nature.com/scientificreports/ equilibrium factor for 222 Rn (F Rn ) and 220 Rn (F Tn ) has been calculated season-wise. The F Rn during rainy, winter and summer seasons was 0.42 ± 0.15, 0.51 ± 0.14 and 0.49 ± 0.15 respectively. It has been observed that F Rn was lower in the rainy season due to higher C Rn and lower EERC A+U in the same season. The overall arithmetic means of F Rn and F Tn was 0.47 ± 0.04 and 0.05 ± 0.01, respectively. The F Tn during rainy, winter and summer seasons was 0.04 ± 0.03, 0.05 ± 0.03 and 0.05 ± 0.04, respectively. Due to the much smaller value of F Tn , a proper relation has not been observed seasonally. However, F Rn varies in different indoor environments due to variation in environmental parameters (ventilation rate, temperature etc). Further, it is worth highlighting that F Tn varies in the same indoor environment due to the fact of 220 Rn gas is not uniformly distributed in a room but 220 Rn progeny having relatively high half-life is uniformly distributed 39 . Hence, it is problematic to define season wise F Tn due to the difference in physical properties of 220 Rn and its progeny.

Seasonal variation of Unattached Fraction for 222 Rn ( f p Rn ), 220 Rn ( f p Tn ).
Most of the unattached 222 Rn and 220 Rn progeny is deposited in the respiratory tract during breathing whereas 80% of the attached progenies get exhaled without deposition 40 . So, the estimation of unattached fraction is necessary for accurate dose assessment. The unattached fractions for 222 Rn ( f p Rn ) and 220 Rn ( f P Th ) have been calculated (using Eq. 12, 12a) season wise.
It has been found that unattached fraction is slightly higher in summer season as compared to winter and rainy seasons. However total EERC A+U and EETC A+U were higher in the winter season, but f p Rn and f p Tn are lower in the winter season. In the winter season, the aerosol concentration is higher which is attributed to high values of attached progeny in the dwellings. The aerosol concentration decreases during rainfall in India 36 . The EECs for 222 Rn and 220 Rn are lower in summer season due to good ventilation and higher exchange rate between indoor and outdoor environments. The overall arithmetic means (rainy, winter and summer seasons) of f p Rn and f p Tn were 0.09 ± 0.02 and 0.10 ± 0.03, respectively. A relation = f Z 400/ p Rn has been used to calculate the aerosol concentration (Z) from the unattached fraction of 222 Rn for the estimation of attachment coefficient 21,41 . In the past studies, the aerosol concentration for a residential environment was considered to be 10,000 cm −3 with = . f 0 07 p Rn for an indoor environment 42 . UNSCEAR suggested a central value of 0.05 (in homes) for f p and it can vary by a factor of 2 depending on air filtration and local source 15 . The observation for f p Rn in the present manuscript is in agreement with literature (Table 4).  www.nature.com/scientificreports www.nature.com/scientificreports/ Variation of attachment rate (X Rn ) and attachment coefficient (β) of 222 Rn. While studying the seasonal behaviour of attached and unattached progeny, it is necessary to discuss the attachment rate and attachment coefficient. The X Rn and β have been calculated using Eqs (5)(6)(7)(8)(9)(10). The X Rn in the studied area is 64 h −1 in winter, 38 h −1 in summer and 47 h −1 in rainy season. Attachment coefficient is defined as the attachment rate per unit aerosol concentration. In past studies, the ratio X Z / Rn was denoted as the average attachment coefficient (β) 34 . The attachment rate coefficient increases with AMAD (Activity median aerodynamic diameter) and larger AMAD describes aerosols with a larger diameter which induce increasing attachment rate 42 . The value of β is slightly greater in winter (0.010 cm 3 h −1 ) than summer (0.009 cm 3 h −1 ). The average calculated attachment rate coefficient is greater than 0.005 cm 3 h −1 as reported by Porstendorfer 43 and almost equal to 1.45 × 10 −9 m 3 h −1 given by Stevanovic et al. 44 . The average estimated F Rn , X Rn and β are concerned as per the values given by other investigators in Table 4. A positive correlation (R 2 = 0.4) has been observed between F Rn and X Rn (h −1 ) and the best fit is shown in Fig. 12. A correlation between f p Rn and F Rn has been already studied in the past studies 45,46 .  15 . In the present manuscript, direct energy deposition rate (alpha fluence), D ab (nGyh −1 ) and effective dose have been estimated from total EEC. The energy deposition rate in bronchial epithelium has been calculated using Eq. (13). Where Φ (αcm −2 s −1 ) is an alpha flux, R t = Range of α-Particle in tissue, ρ t = Density of tissue. A 50% of this flux has been taken due geometry consideration of basal and secretory cells. For the indoor environment, an occupancy factor of 0.8 has been used to calculate the total dose 16 . The absorbed dose rate for lungs (D lung (nGy h −1 )), for trachea-bronchial region (D TB (nGy h −1 )) and for the pulmonary region (D P (nGy h −1 )) have been estimated for comparison using UNSCEAR report using Eqs (14-16) 47 .
In UNSCEAR 1988 report 47 , the D lung (nGyh −1 ) was estimated from 222 Rn, D TB (nGyh −1 ) and D P (nGyh −1 ) was estimated from EEC as given in Eqs (14)(15)(16) respectively. These results showed a huge gap between these two approaches and in the same report, it was further assumed that bronchial dose is better related to 222 Rn gas concentration directly instead of from total EEC. However, in the later reports of UNSCEAR, a previous DCF of 5.7 mSv WLM −1 (based on EEC) has been considered for the estimation of radiological dose. In the present manuscript, D ab (nGyh −1 ) is floating in between both approaches. A comparison between these doses has beengiven in Table 5. The calculated D ab (nGyh −1 ) shows similar variance as reported by UNSCEAR (2000) report for indoor gamma exposure rate 6 . The effective dose in the studied area varied from 1.9 mSv a −1 to 7 mSv a −1 with an average of 3.4 mSv a −1 respectively. The calculated effective dose is in consent with past studies and these different doses have been calculated using Eq. (17) from DCFs discussed in different model asreported in  [22][23][24][25] . The observed pattern of seasonal variation in different parameters is not in agreement with the past studies (due to imperfect selection of seasons and directly used bare mode LR-115 as detector) 49 . In general, indoor 222 Rn and 220 Rn concentration in the studied area are lower than the recommended action level of 200-300 Bq m −3 7 . The attached EEC of 222 Rn (EERC A ) is high in winter season as compared to other seasons due to faster formation process of attached progeny aerosols and poor ventilation in winter.
The calculated values of F Rn (0.47 ± 0.04) and F Tn (0.05 ± 0.01) in the studied area are slightly greater than those given by UNSCEAR and ICRP 21,37,38 . The calculated value of F Rn is slightly higher in summer season as compared to rainy and winter seasons. Due to the very smaller value of F Tn , a proper seasonal relation has not been observed for F Tn . The overall arithmetic means (rainy, winter and summer seasons) of unattached fraction for 222 Rn   is an important parameter that have correlation with an unattached fraction ( f p Rn ), attachment rate (X Rn ) and aerosol concentration (Z).
The average D ab (nGyh −1 ) (absorbed dose) in the studied area is 60 nGyh −1 that minimizes the gap to estimate the absorbed dose rate from 222 Rn and EEC approach, as given in UNSCEAR 15 . The annual effective dose rate in the studied region is 3.4 mSv a −1 . Figure 13 shows that there is significant contribution of 220 Rn to total inhalation dose which suggests that simultaneous measurements of 222 Rn and 220 Rn are important for an accurate dose assessment. The calculated value of effective dose in the present investigation is greater than that reported by UNSCEAR 37 and Marsh et al. 45 approach. However, it is almost equal to dose given by Nikezic et al. 48 . The estimated dose is greater than average worldwide dose of 1.26 mSv a −1 for 222 Rn 46,50 . The present approach followed dosimetric approach rather than epidemiological approach. The dose estimated in the present study is 3.8 fold higher than epidemiological approach. This discrepancy may be due to the use of revised alpha radiation weighting factor in the previous report of ICRP and mostly dosimetric models except ICRP 24 were developed after 1980 13,14 . The other aspect of this discrepancy may be unregistered cancer cases under epidemiological approach. This discrepancy can be solved to deploy long-term measurements of both approaches in active mines rather to assumed previous epidemiological based unregistered data in official reports.