Abstract

Measurement of indoor radon gas and its progeny was carried out in the rooms and laboratories of Lady Keane College, Shillong, using LR-115 films, Type II, which is a Solid State Nuclear Track Detector (SSNTD). The radon concentration in the different rooms varies from 27.88 Bqm^-3 to 61.01 Bqm^-3, with an average of 42.02 Bqm^-3. The results obtained were well within the prescribed safety limit of 100 Bqm^-3 as laid down by the World Health Organization. The average indoor radon concentration was slightly higher than the world average of 40 Bqm^-3. The Potential Alpha Energy Concentration (PAEC) recorded a maximum of 6.60 mWL and a minimum of 3.01 mWL, with the average being 4.54 mWL. The Annual Effective Dose Equivalent (AEDE) ranges from 0.16 mSv/y to 0.35 mSv/y, which was also well within the prescribed limit of 3mSv/y - 10 mSv/y. The average of the Annual Effective Dose Equivalent was 0.24 mSv/y which was also below the world average of 1.15 mSv/y.

1. Introduction

Radon is a colourless, odourless, tasteless and naturally occurring radioactive gas much denser than air. There are thirty-nine isotopes of radon ¹, and the most stable is Rn-222 which has the longest half-life of 3.82 days. Radon-222 follows the uranium decay series, as shown in Figure 1.

As is evident from the uranium decay series, Radium - 226 emits an alpha particle and decays into Radon-222. Radon - 222 further decays into radioactive products called the progeny of radon which are also radioactive. The progeny of radon is broadly classified into two categories: short-lived decay products such as Polonium-218, Lead – 214, Bismuth – 214, Polonium – 214 and the long-lived products including Lead – 210, Bismuth – 210, Polonium 210. The progeny of radon can easily attach with dust and can be inhaled into the lungs. These, in turn, decay to produce mainly alpha radiations, which irradiate the lungs and over due course of time cause serious health issues. Radon-222 has been classified as a human carcinogen by the International Agency for Research on Cancer. Radon has been placed in the same carcinogenic group as tobacco, asbestos, etc. ^{2, 3}. Indoor radon exposure is one of the significant causes of lung cancer deaths worldwide. The Environmental Protective Agency, USA, estimated that approximately 21,000 deaths attributed to lung cancer are due to indoor radon gas inhalation ^{4, 5}. The risk of having lung cancer from radon arises from the inhalation of radioactive radon and its progeny. No concentration of radon is considered safe, but the guideline provided by the Environmental Protective Agency (EPA), USA, is below 4 picoCuries per litre (pCi/L) ⁴. The World Health Organisation set an upper limit of 100 Bqm^{-3 6}.

Sources of indoor Radon: Most rocks, sand, cement and other building materials have traces of naturally occurring radioactive uranium. As radon is one of the decay products of uranium, radon is always present outdoors and indoors. However, the indoor concentration is higher than the outdoor concentration. The sources of indoor radon generally come from the type of materials that make up the floors, ceiling, walls, soil and groundwater. The indoor radon concentration also depends on several factors such as cracks in the walls, floor and the ventilation in the rooms. Moreover, radon concentration is generally higher in rooms located on the ground floor than rooms above it.

Several works have been carried out to determine the indoor radon concentration in dwellings across the world. However, enough studies have not been conducted in schools and colleges, especially in Shillong, Meghalaya. Studies conducted in schools in the northeast of Portugal ⁷ using CR-39 detector reported that 92% of the schools have radon concentration far exceeding the WHO recommended limits. A similar study was conducted in schools in Korea situated in radon prone areas using a CR-39 detector enclosed in a diffusion chamber. It was found that 16% of the schools have radon concentration above 170 Bqm^{-3 8}. A nationwide survey of radon in Iceland was conducted using a CR-39 detector to obtain average radon concentrations and Autoradon, a liquid scintillation counter for continuous radon measurements ⁹. In this study, all kindergartens were reported to have very low radon concentrations with a mean of 11 Bqm^-3. Recent studies were also conducted in Finland's daycare centres and schools located in radon prone areas ¹⁰. It was found that 8% of daycare centres and 14% of schools reported radon levels greater than 300 Bqm^-3. Several studies were also conducted in individual colleges and universities worldwide, which used either short term measurements or long term measurements or a combination of both ^{11, 12, 13, 14}. Some countries like Switzerland, Norway and Finland make radon testing in schools mandatory ¹⁵. In the United States of America, only nine states make radon testing mandatory in schools ¹⁶. However, in India, radon testing and monitoring in schools and dwellings are not mandated by law. In Meghalaya, radon measurements were conducted in nine schools in Jowai located about 66 km from Shillong using LR-115 detectors ¹⁷. All schools under the study in this region have radon levels within the recommended limits. In normal circumstances, the average hours spent by a pupil in a school or college in Meghalaya in a day is 6 – 8 hours, accounting for 25% to 33.33% of the time. As the pupils' exposure to radon and its progeny could prove detrimental to their health, so this is one of the compelling reasons for undertaking this study. The main objective of the current work is to determine the concentration of indoor radon expressed in Bqm^-3 in the rooms of Lady Keane College, Shillong. The exposure to the harmful progeny of radon can be quantified by a quantity known as the Potential Alpha Energy Concentration (PAEC) expressed in Working Level (WL). Historically, the Working level is generally used as a unit to describe the decay products of radon in a uranium mining site. One Working Level is defined as the concentration of the potential alpha energy corresponding to the progeny of radon that is in equilibrium with 3700 Bql^-1. This concentration is equivalent to 1.3×10⁵ MeVl^-1 Nowadays, Working Level is defined as the concentration that is equivalent to 1.3×10⁸ MeVm^-3. The conversion factor from Working Level to S.I. unit is 1 WL = 3.54 mJhm^-3; where 1 month of exposure is taken to be equal to 170 hours ¹⁸. The Annual Effective Dose Equivalent (AEDE) as a result of radon exposure and its progeny was also determined and expressed in mSv/y.

2. Materials and Methods

Lady Keane College is located in Shillong, Meghalaya, at 25.57°N latitude and 91.88°E longitudes. The study was conducted on three main academic blocks of the institution: Academic block I, Academic Block II and Science Block. Three rooms were selected from each block located on the ground, first and second floors. The details of the rooms selected are given in Table 1. In each room, the type of materials that make up the floors, walls and ceiling were identified. The rooms have a natural ventilation system where outdoor and indoor air exchange relies mainly on purpose-built openings such as doors and windows. The ventilation system in different rooms is classified as poor, satisfactory and good based on the ratio of the total areas of the doors and windows to the total area of the floor in each room.

The detection of indoor radon is achieved using various techniques. The easiest yet most reliable method is the Solid State Nuclear Track Detector (SSNTD) LR-115 films that have a high Detector Time Efficiency (DTE), ease of usage and do not require calibration ¹⁹. In the present study LR-115 films Type II, non-strippable (Kodak Pathe Co, F 93270 Severan, France) were used. These films consist of a 100 μm thick polyester base coated with a 12 μm thin film of red coloured cellulose nitrate. The red coloured cellulose nitrate is responsible for radon detection. Heavy energetic particles such as alpha particles leave damaged tracks when passed through these films. These tracks can be rendered visible under an optical microscope of appropriate magnifying powers by chemical etching.

The films of dimensions 2cm x 2cm, fixed on cardboard, were exposed in the bare mode for 55 days in the rooms mentioned above of Lady Keane College, Shillong. The range of alpha particles in the air is less than 10cm, so the films were kept at relatively large distances from the walls, floor and ceiling to prevent these films from detecting alpha particles emitted by other radioactive sources such as uranium apart from radon. Films were placed in the centre of the room and at the height of 1.5 m above the ground. After exposure, the films were collected and brought to the Department of Physics, Lady Keane College, for analysis. The films were detached from the cardboard and then etched with a chemical reagent. The etching process is done with 2.5N NaOH solution at 60±1°C for 90 minutes in a constant temperature water bath ²⁰. The films were then washed with distilled water and left to dry in a desiccator. After the films were dried, they were then viewed under a binocular optical microscope with magnifying power 400X. The tracks were visible and counted for further analysis.

The radon concentration in Bqm^-3 was calculated by the formula given in equation (1) ²¹

(1)

where k is the Calibration factor whose value is 0.02 Tracks cm^-2 d^-1(Bqm^-3)^{-1 22}, ρ is the number of tracks per square cm, and t is the time of exposure of the films (= 55 days)

The Potential Alpha Energy Concentration (PAEC) to measure the radon progeny can be calculated in terms of Working Level units using equation (2) ²¹

(2)

where F is the Equilibrium factor between radon and its progeny and is taken to be equal to 0.4 ²¹

The Annual Effective Dose Equivalent (AEDE) in terms of (mSv/y) units was obtained using relation (3) ²¹

(3)

Where H is the Occupancy factor taken to be 0.8 ²¹, T is the time in hours spent in the college in a year, (T=2000 h/y), D is the Dose Conversion Factor [D = 9×10^-6 (m Sv) / (Bqm^-3h)] ²¹

Readings were presented for all rooms selected and the mean was calculated. A one way Analysis of Variance (ANOVA) was used and a probability P<0.05 was considered significant.

3. Results

The indoor radon concentration, the Potential Alpha Energy Concentration (PAEC) and the Annual Effective Dose Equivalent (AEDE) in different rooms are presented in Table 1. The radon concentration has a maximum value of 61.01 Bqm^-3 and a minimum value of 27.88 Bqm^-3 with an average of 42.02 Bqm^-3. The radon progeny concentration expressed as Potential Alpha Energy Concentration (PAEC) is maximum in room 11 with a value of 6.60 mWL and room 46 recorded a minimum of 3.01 mWL average being 4.54 mWL. The Annual Effective Dose Equivalent (AEDE) received by the staff and students of the college ranges from 0.16 mSv/y to 0.35 mSv/y with an average of 0.24 mSv/y.

4. Discussion

The average radon concentration in the ground, first and second floor was 52.39 Bqm^-3, 36.90 Bqm^-3 and 36.77 Bqm^-3 respectively. No significant difference was noted for radon concentration between the first floor and second floor (P>0.05).

The concentrations of radon in Academic Block I and the Science Block follows the expected pattern, with rooms on the ground floors recorded higher concentrations and decreases as we move towards the second floor. In Academic Block II, the concentration of radon in room 28, located on the first floor, is 42.02 Bqm^-3. In contrast, in room 37, located on the second floor, the concentration is 46.46 Bqm^-3, deviating from the expected result. One of the reasons may be because room 37 is directly on top of room 28, and since the floor of room 37 is made up of wood, there were many gaps between the woods. The architectural design of the rooms causes the radon gas from room 28 to seep through the gaps on the floor of room 37, resulting in a build-up of radon in room 37. Another reason is the role played by the ventilation system in the two rooms. From Table 1, it was observed that the ventilation in room 28 was satisfactory compared to room 37. So whatever radon was generated in room 28 could not be adequately expelled, resulting in radon gas escaping through the ceiling into room 37.

The average values of PAEC in the ground, first, and second floors are 5.67 mWL, 3.99 mWL and 3.97 mWL, respectively. However, there was no significant difference in the PAEC values between the first floor and second floor (P>0.05).

The average dose received by the pupils and staff of Lady Keane College due to the exposure to radon and its progeny is maximum on the ground floor with an average dose of 0.3 mSv/y. No significant difference in exposure was recorded between the first and second floors as pupils and staff alike in both floors received an average dose of 0.21 mSv/y (P>0.05).

The study has its limitations that need to be addressed in future investigations. The scope of future studies might include the seasonal variation of radon concentration, contributions of indoor radon from water sources such as tap water, role of environmental factors, influence of ventilation rate and estimation of the lifetime fatality risk posed by radon and its progeny. Therefore, a future study of this kind could better understand the college's indoor radon map.

5. Conclusion

The radon concentration in the rooms, as mentioned above, was below the prescribed safety limits of 100 Bqm^-3 as laid down by the World Health Organization ⁶. The average radon concentration was 42.02 Bqm^-3 which was higher than the world average of 40 Bqm^{-3 21}. A typical indoor PAEC value on the ground floor is 8 mWL and on the first floor is 4 mWL. It was observed that the ground floor and the first floor have PAEC values within the recommended range. The Annual Effective Dose Equivalent (AEDE) ranges from 0.16 mSv/y to 0.35 mSv/y, which was also well within the prescribed limit of 3 mSv/y - 10 mSv/y ¹⁸. The Annual Effective Dose Equivalent (AEDE) average of 0.24 mSv/y was also well below the world average of 1.15 mSv/y ¹⁸.

Acknowledgements

The authors are grateful to the Principal and Governing Body, Lady Keane College, Shillong, for the support received for the project. The authors are also indebted to Prof. A Saxena, Department of Physics, North-Eastern Hill University, Shillong, Dr Y. Sharma, and Dr D. Maibam, Department of Physics, Don Bosco College Tura, for their support during the work.

References