Current World Environment

Introduction

Since the focus on the health effects of radiation exposure has switched from acute high levels to chronic low levels, both scientists and the general public is becoming more concerned about background radiation¹. Due to higher average concentrations of early radionuclides in soil and the byproducts of their decay, such as ²²²Rn (radon) and ²²⁰Rn (thoron), in the surroundings, many places on Earth have higher natural background radiation levels¹. The background radiation has recently increased due to the technological advancement of naturally existing radioactive material. It's estimated that inhaling ²²²Rn, ²²⁰Rn, and their temporary offspring contributes over 54 percent of the radiation dose from the ambient environment that the general public encounters^{2, 3}. So, the external component also needed to include an inhaling component. No country has an accurate national estimate for this component as of yet³. The ²²⁰Rn issue will also cause issues for businesses who use thorium nitrate³. Thoriated petrol mantle lamps are still used in countries like India for both indoor and outdoor environment, as well as by street vendors in both rural and urban areas³. These businesses handle a lot of thorium nitrate, which means that the amount of ²²⁰Rn gas they emit and the offspring they create may both have a considerable impact on the breathing dose of their staff³. It is untrue that large concentrations of ²²⁰Rn may be present in living and working environments throughout a wide range of countries, and it is increasingly clear that information on ²²⁰Rn in the environment may be required to fully understand inhalation dose⁴ and the new dose conversion coefficients are available in the literature for the half springs⁵. As a result, compared to ²²²Rn⁶, even small quantity of ²²⁰Rn progenies result in a larger radiation dose. As a result, the health sciences community is becoming more interested in ²²⁰Rn exposure. Numerous ²²⁰Rn surveys were recently conducted in various cities and states reported by several researchers^7–11. Additionally, numerous papers¹² have argued for the adoption of trustworthy ²²⁰Rn measurement methodologies. This paper summarizes the existing situation of ²²⁰Rn levels in indoor settings, workplaces, and other substantial amounts of ²²⁰Rn. A description of measurement methods and levels reported in the literature is also provided.

Materials and Methods

According to reports, the inhalation pathway is the main source of the radiation dosage that humans receive from natural sources¹³. Radon and its progeny nuclides are the principal contributors to this route dosage. Thus, measuring the amount of radon in the internal air becomes crucial for assessing the exposure of ambient atmosphere. Because atmospheric radon gases change throughout the day and throughout the seasons, it is crucial to conduct long-term integrated measurements to accurately estimate the gas concentration. For long term measurements cellulose nitrate based dosimeters are used¹⁴, ¹⁵. The detailed explanations are available in the literature ¹⁶, ¹⁷. The activity fractions in relation to the parent gas are recovered through ventilation rate¹⁸. Working level concentrations and equilibrium factor are estimated through the formulas provided by UNSCEAR⁵.The calibration facilities, dosimeter standardization, and dosimetry methodology are all covered in depth elsewhere¹⁹.

Result and Discussion

On the basis of the material we have read and the data we have collected, we project the findings below. We display the ²²⁰Rn readings obtained in a test chamber during a three-year period. All measurements were made on the ground floor. Dosimeters were positioned in lower and upper parabolic arrangements, 60 cm to 450 cm apart from one another, and at equal heights from the floor and the ceiling in the 31 m³ test room. 

Figure 1: Dosimeters spaced equally apart   Click here to view Figure

Figure 2: Dosimeters in a parabolic decline   Click here to view Figure

Figure 3: Dosimeters in parabolic growth       Click here to view Figure

Figures 1, 2, and 3 depict the deployment of dosimeters in an equidistant, lower- and upper-parabolic form, respectively. Figure 4 displays the results of the ²²⁰Rn concentration measurement with equally spaced wall spacing. In the southern part of India, these measurements are the first of their kind.                   

Figure 4: Parallel dispensation of 220Rn   Click here to view Figure

Figure 5: Dispensation of 220Rn from the wall   Click here to view Figure

Figure 6: Distribution of 220Rn and 222Rn   Click here to view Figure

The short half-life of this substance may be the reason for the exponential concentration^{20, 21} drops with distance shown in Figure 4. This suggests that when measuring indoor ²²⁰Rn, it's crucial to maintain a safe distance from the wall of the house. As seen in Fig. 5, the effect of distance on concentrations changes when dosimeter is moved away from one point of the wall, like one point of the north wall, and placed closer towards another point of the wall, like one point of the south wall. In order to monitor the simultaneous dispersion of ²²²Rn and ²²⁰Rn concentrations at various distances from walls, ceilings, and floors, twin cup dosimeter measurements were also made. The distribution of ²²⁰Rn and ²²²Rn concentrations within a home is shown in Figure 6. Locally produced bricks were used to build the walls and floors, and it is assumed that the ²²²Rn and ²²⁰Rn in the bricks came from local soil resources. Due to the small time of ²²⁰Rn, length of moment needed to transit, as seen in Fig. 6, the concentration is waning towards the centre point of the test room. However, due to longer period of time of ²²²Rn, the level is constant throughout the entire house. Figure 7 illustrates that the concentration of ²²⁰Rn is significantly higher than that of ²²²Rn near the walls. More wall distance results in more similar concentrations. The ²²⁰Rn levels fall below the ²²²Rn concentration at a distance greater than 20 cm from the wall, but it stands constant. Our findings concur with other researchers' findings²².

Figure 7: Vertical distributions of 220Rn and its progeny      Click here to view Figure

Figure 8: 220Rn level when dosimeter is parabolic decline    Click here to view Figure

Similar findings were also made in a number of homes in the Gansu region, according to Shang et al²³, which indicates that ²²²Rn indoor concentration is generally present. The dramatic increase in concentration close to the wall is shown in Figures 5, 6, and 7. The turbulence flowing from the wall towards the middle of the space diminishes the comparative part of ²²⁰Rn levels towards the wall. Only ventilation rates greater than the exhalation saturation of the entire activity are important for occupant dose assessment²⁴. Interior ²²⁰Rn concentrations increase as a result of ²²⁰Rn-emitting building materials. There is a concentration gradient in the room as a result of the short half-life, with concentrations being greater on the walls, ceiling, and floor.

Figure 9: Waning of 220Rn concentration.   Click here to view Figure

Figure 7 shows the progeny concentration and vertical distributions of ²²⁰Rn. The concentration of ²²⁰Rn decreases rapidly as one gets farther away from the ground. Still, the distance to the wall had little effect on the progeny concentration. Model estimates revealed a potential link between the homogeneity of ²²⁰Rn progeny concentrations throughout a residence and their lengthy half-life (10.64 h)²⁰. The modifications are shown in Figs. 8 and 9. The dosimeters were also arranged in parabolic decline and growth configurations to acquire the measurements. Observing the concentrations in relation to the locations of the dosimeters in reverse order is pretty interesting. This might be as a result of the short half-life of ²²⁰Rn, which causes the concentrations to reverse as moving the detector away from the floor of the room. The dosimeters were also put in 300 residences over 10 distinct Bangalore neighborhoods over a period of more than ten years. With a mean value of 22.8 Bqm^-3 (GM: 18.9) and 0.33 mWL, the observed concentrations of ²²⁰Rn and its progeny ranged from 8.4±0.2 to 69.4±3.5 Bqm^-3, 0.01 to 6.2 mWL. Smaller houses and houses with granite flooring had higher concentrations, which may be connected to the neighborhood's higher ²³²Th activity levels²⁵. The authors²⁵ found that ²³²Th concentrations in 67% of the samples were greater than the 40 Bqkg^-1 global norms. The readings for both India as a whole and the rest of the world were comparable to the average concentration of ²³²Th (53.1 Bqkg^-1). The data also showed a definite seasonal fluctuation with a summer minimum and a winter high. The impact of interior ventilation may have an impact on this trend²⁶. Numerous factors, such as the ²²⁶Ra and ²²⁴Ra present in building materials, the exhalation from walls, and ventilation conditions, have been shown to have an impact on indoor ²²²Rn and ²²⁰Rn concentrations²⁶. The short half life of ²²⁰Rn may make exhalation the most important component. Since dirt walls tend to be more porous than other types of building materials, ²²⁰Rn diffusion is made simpler²⁶. This might be the reason for the elevated ²²⁰Rn content found in brick and earth-walled homes²⁶. For houses based on space, room, season, and wall considerations, the link between ²²⁰Rn and its progeny was found to be 0.84, 0.74, 0.85, and 0.75, respectively. A stronger connection of 0.85 was seen in data that were dependent on the season. This can be due to the variable ventilation conditions. Similar findings were observed by Guo et al²⁶, who also reported a correlation coefficient of 0.79 between the levels of ²²⁰Rn and its offspring. The frequency distribution of the ²²⁰Rn and ²²⁰Rn progeny is shown in Figs. 10 and 11, respectively. 90% of ²²⁰Rn concentrations fell between 10 and 40 Bqm^-3, with 10% of ²²⁰Rn concentrations reaching a maximum of 70 Bqm^-3. Around 66% of ²²⁰Rn progeny concentrations had a minimum of 0.01 mWL, 28% were in the 0.11 to 3 mWL range, and the final 6% were in the 3.1 to 6.2 mWL range.                              

Figure 10: Frequency distribution 220Rn   Click here to view Figure

Figure 11: Frequency distribution 220Rn progeny   Click here to view Figure

The average concentrations of ²²⁰Rn and its progeny at the observed sites were found to be 22.8 Bqm^-3 and 0.33 mWL, respectively. Zhuo et al²⁷ measured the concentrations of ²²⁰Rn and its offspring in both indoor and outdoor contexts using the grab sampling approach. 48.1± 20.0 and 22.1±10.7 Bqm^-3 were the results, respectively. The average equilibrium-equivalent concentrations for the same environment were 1.13±0.80 and 0.50±0.29 Bqm^-3, respectively. For all types of wall construction, homes with soil/mud walls had the greatest ²²⁰Rn concentration; the reported average value for this kind of home was 104 Bqm^-3. The ratio of exposure to ²²²Rn, ²²⁰Rn, and their offspring to that from ²²⁰Rn and its offspring was reported²⁵ to be 30.5% in Guangdong province. Martinez et al²⁸ conducted extensive research on the levels of indoor ²²⁰Rn in residences in Mexico City, and the findings revealed a broad log-normal distribution of integrated concentration with annual geometric and arithmetic averages of 82 and 55 Bqm^-3, respectively. It deviates by 8 to 234 Bqm^-3 from the global average of 3 Bqm^-3. The observations made in Bangalore's surroundings have been recorded, and they align with those made elsewhere^27–29.

Conclusion

²²⁰Rn levels were highest on the walls, ceiling, and floors of the test room and rapidly fell away from them. The concentration of ²²⁰Rn offspring indoors is unaffected by the separation between walls. It is essential to analyze public exposure to natural radiation more carefully across the nation, with a focus on ²²⁰Rn and its offspring.

Acknowledgments

The cooperation extended by all the Principals and the heads of the departments is acknowledged for allowing us to use the instruments to take the data in the test room.

Conflict of Interest

Author(s) do not have any conflict of interest.

Funding Sources

The author is thankful to the Research Funding Council, University Grants Commission, New Delhi, India for providing the funds to carry out research work.

References

1. Burchfield L.A., Akridge J.D., Kuroda P.K. Temporal  Distributions of  Radio Strontium  Isotopes and  Radon  Daughters in Rain  Water  during a  Thunderstorm. J Geophy Res.1983; 88: 8579 – 8584.
   CrossRef
2. United Nations Scientific Committee on the Effects of Atomic Radiation, (2019) sources, Effects and Risk of Ionization, Report to the General Assembly, Scientific Annexes B: Lung Cancer from exposure to Radon, 195.
3. Ramola R.C., Mukesh Prasad, Tushar Kandari, Preeti Pant, Peter Bossew. Rosaline Mishra, Tokonami S. Dose estimation derived from the exposure to radon, thoron and their progeny in the indoor environment, Scientific Reports. 2016; 6: 31061.
   CrossRef
4. Ramachandran T.V., and Sathish L.A. ²²⁰Rn in indoor Environment of India: A Review. Uni J Env ResTech. 2011; 1: 7-21.
5. International Commission on Radiological Protection . Annals of the ICRP 46 (3/4) ICRP Publication 137; London, UK: 2017. Occupational intakes of radionuclides: Part 3. [Google Scholar]
   CrossRef
6. Tokonami S. Characteristic of Thoron (²²⁰Rn) and its progeny in the indoor environment. Int. J. Environ. Res. Public Health. 2020;17:8769. doi: 10.3390/ijerph17238769. [PMC free article] [PubMed] [CrossRef] [Google Scholar]
   CrossRef
7. Bineng G.S., Saïdou S.T., Hosoda M., Siaka Y.F., Issa H., Suzuki T., Kudo H., Bouba O. The importance of direct progeny measurements for correct estimation of effective dose due to radon and thoron. Front. Public Health. 2020;8:17. doi: 10.3389/fpubh.2020.00017. [PMC free article] [PubMed] [CrossRef] [Google Scholar]
   CrossRef
8. Omori Y., Tokonami S., Sahoo S.K., Ishikawa T., Sorimachi A., Hosoda M., Kudo H., Pornnumpa C., Nair R.R.K., Jayalekshmi P.A., et al. Radiation dose due to radon and thoron progeny inhalation in high-level natural radiation areas of Kerala, India. J. Radiol. Prot. 2017;37:111–126. doi: 10.1088/1361-6498/37/1/111. [PubMed] [CrossRef] [Google Scholar]
   CrossRef
9. Nyambura C., Tokonami S., Hashim N.O., Chege M.W., Suzuki T., Hosoda M. Annual effective dose assessment due to radon and thoron progenies in dwellings of Kilimanbogo, Kenya. Radiat. Prot. Dosim. 2019;184:430–434. doi: 10.1093/rpd/ncz090. [PubMed] [CrossRef] [Google Scholar]
   CrossRef
10. Smetsers R.C.G.M., Blaauboer R.O., Dekkers F., Slaper H. Radon and thoron progeny in Dutch dwellings. Radiat. Prot. Dosim. 2018;181:11–14. doi: 10.1093/rpd/ncy093. [PubMed] [CrossRef] [Google Scholar]
    CrossRef
11. Saputra M.A., Nugraha E.D., Purwanti T., Arifianto R., Laksmana R.I., Hutabarat R.P., Hosoda M., Tokonami S. Exposures from radon, thoron, and thoron progeny in high background radiation area in Takandeang, Mamuju, Indonesia. Nukleonika. 2020;65:89–94. doi: 10.2478/nuka-2020-0013. [CrossRef] [Google Scholar]
    CrossRef
12. Hosoda M., Kudo H., Iwaoka K., Yamada R., Suzuki T., Tamakuma Y., Tokonami S. Characteristic of thoron (²²⁰Rn) in environment. Appl. Radiat. Isot. 2017;120:7–10. doi: 10.1016/j.apradiso.2016.11.014. [PubMed] [CrossRef] [Google Scholar]
    CrossRef
13. UNSCEAR (2000) United Nations Scientific Committee on the Effects of Atomic Radiation, Sources, Effects and Risks of Ionizing Radiation, Report to the General Assembly, (United Nations,New York).
14. Fleischer, R.L., (1988) Radon in the environment-Opportunities and hazard, Nucl Trac Rad Meas. 14: 421-435.
    CrossRef
15. Ilic R., and Sutej T.,(1977) Radon Monitoring devices based on Etched Track Detectors, In: ²²²Rn Measurements by Etched Track Detectors, Applications in Radiation Protection, EarthSciences and the Environment, (Eds., S.A. Durrani.and R. Ilic), World Scientific, (Singapore), 103-128.
    CrossRef
16. Ramachandan T.V., and Sathish L.A., (2011a) 220-Rn in the indoor environment and work places: a review: Proceedings of the 2011 AARST International Symposium, Vol 1, 9-10. October, 2011
17. Miles J.C.H., Calibration and standardization of etched track detectors in Radon measurements by etched track detectors, applications in radiation protection, Earth Sciences and the Environment (Eds, Durrani, S.A., and Ilic, R) World Scientific, (Singapore), 1997; 43-176.
    CrossRef
18. Mayya Y.S., Eappen K.P., Nambi K.S.V., Methodology for mixed field inhalation dosimetry in monazite areas using a twin cup dosimeter with three track detectors. Rad Prot Dosim. 1998; 77: 177-184.
    CrossRef
19. Ramachandan T.V., and Sathish L.A., Nationwide indoor ²²²Rn and ²²⁰Rn map for India: a review. J of Environ Radio act. 2011; 102(11): 975-986. Doi:https://doi.org/10.1016/j.jenvrad.2011.06.009 
    CrossRef
20. Tso M.W., Li C. Indoor and Outdoor ²²²Rn and ²²⁰Rn Daughters in Hong Kong. Healt Phys. 1987; 53: 175–180.
    CrossRef
21. Yamasaki T., Guo Q., Iida T. Distributions of thoron progeny concentrations in Dwellings. Rad Prot Dosi. 1995; 59(2): 135–140. 
    CrossRef
22. Tetsuya S., Measurement of Indoor Thoron Gas Concentrations Using a Radon-Thoron Discriminative Passive Type Monitor: Nationwide Survey in Japan. Int J Environ Res Pub Healt. 2021; 18(3): 1299.
    CrossRef
23. Shang B., Chen B., Gao Y., Wang Y., Cui H.,  Zhou Li. Thoron levels in traditional Chinese residential dwellings. Rad Environ Biophy. 2005; 44: 193–199.
    CrossRef
24. Tschiersch J., Li W.B., Meisenberg O. Increased ²²⁰Rn indoor concentrations and implication to inhalation Dosimetry. Rad Prot Dosi. 2007; 127: 73-78. 
    CrossRef
25. Shiva Prasad N.G., Nagaiah N., Ashok G.V., Karunakara N. Concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in the soil of Bangalore region, India. Healt Phys. 2008; 94: 264- 271.   
    CrossRef
26. Guo Q., Iida T., Okamoto K. Measurements of Thoron Concentration by Passive Cup Method and Its Application to Dose Assessment.  J Nucl Sci Tech. 1995; 32: 794-803.
    CrossRef
27. Zhuo W., Iida T., Yang X. Environmental Radon and Thoron Progeny Concentrations in Fujan Province of China. Rad Prot Dosi. 2000; 87: 137–140.
    CrossRef
28. Martinez T., Navarrete M., Gonzalez P., Ramirez A. Variation in indoor thoron levels in Mexico City Dwellings. Rad Prot Dosi. 2004; 111: 111–113.
    CrossRef
29. Rishi Pal C., Amit K., Neetika C., Distribution of indoor thoron in dwellings under normal and turbulent flow conditions using CFD simulation technique. Nucl Tech & Rad Prot. 2017; 32(2):180-184.
    CrossRef