estimation of annual effective dose from indoor radon ...

Radiation Protection Dosimetry Advance Access published July 30, 2013 Radiation Protection Dosimetry (2013), pp. 1–4

doi:10.1093/rpd/nct191

NOTE

ESTIMATION OF ANNUAL EFFECTIVE DOSE FROM INDOOR RADON/THORON CONCENTRATIONS AND MEASUREMENT OF RADON CONCENTRATIONS IN SOIL Rohit Mehra* and Pankaj Bala Department of Physics, Dr. B .R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India *Corresponding author: [email protected] Received November 15 2012, revised July 10 2013, accepted July 10 2013

INTRODUCTION 222

Radon ( Rn) is a radioactive inert gas produced in the naturally occurring uranium (238U) decay series, following the alpha decay of the radium isotope (226Ra). It directly decays into polonium-218 (218Po) by emitting alpha particles with an energy of 5.5 MeV. The amount of 238U and 226Ra isotopes, presented in the underlying soils and in the building materials, determines the radon release in a house. Thus, the magnitude of 222Rn concentration indoors depends primarily on the construction of the building and on the amount of 222Rn in the underlying soil. The rates of radon release are complex and depend on many factors, such as rock mineralogy and structure, the distribution of parent radionuclides (e.g. 238U, and 226Ra), temperature and moisture content(1 – 4). Radon can migrate from soils and rocks and accumulates in surrounding enclosed areas, such as homes and underground mines. The activity concentrations of radon, thoron and their progenies are largely influenced within the house by factors such as topography, type of houses, building materials, temperature, ventilation, wind speed and even the life style of the people living in the house(5 – 7). The major source of radon in the house is the soil beneath the house but the household water, building materials and cooking gas also influence the indoor radon concentration(8). The assessment of radiological risk related to inhalation of radon and its progeny is based mainly on the integrated measurement of radon(9). Indoor radon and its decay products have been recognised as contributing significantly to an increased risk of lung cancer in the population(10). It is well known that exposure of a population to high concentrations of radon and its daughters for a long period leads to pathological effects like changes in the

respiratory function and the occurrence of lung cancer(11). The most important source of indoor 222Rn is soil gas(12). Measurement of 222Rn concentration in soil gas, in principle, can be used as a method for evaluating the potential of elevated indoor 222Rn concentrations(13). A number of papers have been published dealing with the correlation between the indoor 222Rn and soil 222Rn(14 – 16). The International Commission on Radiological Protection (ICRP), the International Atomic Energy Agency (IAEA) and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) refer to the health hazards due to radon inhalation and define the safe limits for radon concentration(17 – 19). Therefore, the aim of the present work is to study indoor radon/thoron concentrations and measurement of radon concentrations in soil. GEOLOGY OF THE STUDY AREA Una lies between North latitudes 318170 5200 and 318520 000 and between East longitudes 758580 200 and 768280 2500 . The soil found in the districts of Una is generally brown, alluvial and grey brown. As per the investigation of Geological Survey of India, the minerals available in Una include limestone, clays, mica, iron pyrites, salt, gypsum, slate, antimony and lead. Una is in the south-western part of Himachal Pradesh. The Una district has a unique identity by having both plain areas and hilly areas. Alluvial fans, river terraces and gravel beds of recent age and the sandstone, clay stone and conglomerates belonging to Shivalik group are the main formations in this area. Recent deposits constitute gravel beds, alluvial fans and river terraces. Alluvium occupies the vast stretch of the plain. They contain sand, silt and clay in

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Radon short-lived decay products generated from the earth is one of the serious indoor air and soil pollutants. The RAD-7 Electronic Radon Detector with a special accessory is used for the purpose of measurement. The radon and thoron concentrations + 2.3 to 1710+ + 139.36 Bq m23 in the houses of the study area are found to vary from 35+ + 0.5 to 315.2+ + 5.35 Bq m23 and 66.1+ with the average values of 98.65+ + 1.9 and 388.19+ + 11 Bq m23, respectively. From indoor air, the total annual effective dose is calculated and it varies from 0.88 to 7.94 mSv y21. The preliminary investigation shows that the thoron concentration is higher than the radon concentration in the houses of the study area. In general, the values of the indoor air are within the recommended action level of the International Commission on Radiological Protection, 2009.

R. MEHRA AND P. BALA

varying proportions. Almost all the dwellings are old and the construction of most of the houses is identical, using local rock and mud for flooring and wood for roof. Only a few houses have cemented flooring and walls. The ventilation level in houses is comparatively low compared with residential accommodation in plain areas. METHOD OF MEASUREMENT

RESULTS AND DISCUSSION The results of the measurement of indoor air radon/ thoron concentration levels using RAD-7 of the study area are shown in Table 1. The measured radon and thoron concentrations in the houses of the study area found to vary from 35+0.5 to 315.2+5.35 Bq m23 and 66.1+2.3 to 1710+139.36 Bq m23 with the

Hair ¼ Rnair F O ðDCF) where Hair (mSv y21) is the effective dose, Rnair the indoor radon concentration (Bq m23), F the equilibrium factor between radon and its decay products (0.4), O the average indoor occupancy time per person (7000 h y21) and DCF is the dose conversion factor for radon exposure of 9 nSv h21 (Bq m23)21. The calculated average annual effective dose from the measured radon concentration in indoor air varies from 0.88 to 7.94 mSv y21 with a mean value of 2.49 mSv y21. The calculated values of the annual average indoor radon concentration in the study area are higher than those reported for indoor radon in dwellings of the Malwa region of Punjab(20) and also for some areas of Sirsa district of Haryana(21). The measured indoor radon concentration values are higher in comparison with the values for radon in dwellings reported for the houses of the Chhatrapur area of southeastern coast of Orissa, India(22). The measured indoor radon concentration values are higher in comparison with the values for radon in dwellings reported for the houses of the Malwa region of Punjab(23). The measured indoor radon concentration values are lower in comparison with the values for radon in dwellings reported for the houses around the Tusham Ring Complex, Bhiwani, Haryana and Amritsar District of Punjab(24). The relatively high concentrations of radon and thoron in some houses are because of poor ventilation. It is observed that the soil of the study area have a significant contribution to the indoor radon concentration in the area. The ICRP has recommended that remedial action against radon is always justified above a continued effective dose of 10 mSv y21, while an action level within the range of 3–10 mSv y21 has been proposed(17). The houses in the village of Jowar gave anomalously high values of indoor radon and thoron concentrations, which may be due to the high radium and thorium contents in the soil or due to its geological location(25). The results of the measurement of soil gas radon concentration levels using RAD-7 of the study area are shown in Table 2. The variation of the radon concentration in soil varies from 35+0.5 to 473+16.3 Bq m23 with the average values of 117.42+2.8 Bq m23. The soil gas radon concentration of the Una district is found to be lower than that reported by Choubey et al.(26) in the Doon Valley, India, which varies from 1.00 to 19.68 kBq m – 3. The measured value of soil gas radon concentration is lower in comparison with that in Ljubljna, Slovenia, which varies from 2 to 14 kBq m – 3(27). The measured value of soil gas radon concentration is also lower in comparison

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The RAD-7 Electronic Radon Detector (Durridge Co.) is a solid-state detector. The solid-state detector is a semiconductor material (usually Silicon) that converts alpha radiation directly into an electrical signal. The internal sample cell of RAD-7 is a 0.7-dm3 (0.7 l) hemisphere, coated on the inside with an electrical conductor. A solid-state, ion-implanted, planar silicon alpha detector is at the centre of the hemisphere. The high-voltage power circuit charges the inside conductor to a potential of 2000–2500 V relative to the detector, creating an electric field throughout the volume of the cell. The electric field pushes the positive charges onto the detector. The indoor air samples are taken in different houses with a special accessory for the purpose of measurement by the RAD-7 detector. The radon content in air is measured by the RAD-7 detector, a high-performance continuous radon measuring instrument. Air is drawn in by the built-in pump, which is passed through the drierite to the solid-state detector for the measurement of the radon concentration. The detector distinguishes alpha particles from 218Po and 214Po with the energies of 6.0 and 7.9 MeV, respectively, in the respective windows. The soil gas samples at each site is collected with the help of a probe, immersed in the soil to a depth of 1 m, which is then connected to the RAD-7 detector with a special accessory for the purpose. The probe is penetrated into the soil with a rotating handle or immersed with gentle strokes of a hammer. The water lock and measuring instrument are then attached to the probe for drawing in the soil gas from the deep soil. The soil gas is drawn in through the tube pipe into the measuring instrument and then the data along with the respective bar charts and cumulative spectra of each sample are printed out on the printer attached with the instrument. The sniff protocol and Grab mode are used for the soil gas samplings on the RAD-7 detector at each site.

average values of 98.65+1.9 and 388.19+11 Bq m23, respectively. The annual average effective dose for indoor radon is calculated using parameters introduced in the report by UNSCEAR(19).

ANNUAL EFFECTIVE DOSE FROM INDOOR RADON/THORON CONCENTRATIONS Table 1. Radon/thoron concentrations in indoor air and estimation of annual effective dose of the studied area. S. No

Location

Radon concentration in air (Rnair) (Bq m23)

Thoron concentration in air (Thair) (Bq m23)

Annual mean effective dose (Hair) (mSv y21)

Temperature (8C)

Relative humidity (%)

1 2 3 4 5 6 7 8 9 10

Jowar Lohara Daulatpur Gagret Oela Panjawar Saloh Haroli Kuriala Kotlakala

315.2+5.35 70.4+1.35 70.4+1.35 70+1.33 35+0.59 35.5+0.59 70+1.35 70+1.35 144+3.37 106+2.24

1710+139.36 723+42.44 131+4.67 327+14.74 130+3.40 131+4.67 262+11 65.8+2 336+17.3 66.1+2.3

7.94 1.77 1.77 1.76 0.88 0.89 1.76 1.76 3.63 2.67

18.5 24 28 20.4 19.7 22.5 18.2 14.3 20 19.4

12 8 9 9 9 15 12 21 8 10

S. No

Location

Radon concentration in soil (Rnsoil ) (Bq m23)

Temperature (8C)

Relative humidity (%)

1 2 3 4 5 6 7 8 9 10

Jowar Lohara Daulatpur Gagret Oela Panjawar Saloh Haroli Kuriala Kotlakala

316+6.6 35+0.5 70+0.9 35+0.5 35+0.5 34.8+0.5 70+0.9 35+0.5 473+16.3 70.4+1.3

18.2 23 28 22.8 21 23.1 19.4 15.5 18.8 19.7

35 25 26 22 17 22 20 24 25 18

with the radon in the soil gas in coastal regions in Mexico city in the range of 0.1–12.0 kBq m – 3(28). So one can say that the soil gas radon concentration of the Una district is found to be lower than that in some parts of the world. Other similar measurements performed by various researchers showed that the soil gas radon concentration may vary over a wide range depending on weather conditions, climatic factors and soil type(29, 30). Since soil has been found to be a most important source of indoor radon(29, 31) a large number of papers deal with the relationship between the geological features of the areas under study and indoor radon levels(30 – 33). In some locations where the soil gas radon is high, the indoor radon is also found to be relatively high, indicating that soil is also the source of the increase of radon in the dwellings at these locations. CONCLUSION The average values of radon and thoron concentrations in the studied area are 98.65+1.9 and 388.19+11 Bq m23, respectively. The measured values of the effective dose is well within the action level

(3–10 mSv y21)(17), but the measured indoor radon concentration values for the studied area are on the higher side than the world average value of 40 Bq m23. The relatively high concentrations of radon and thoron in some houses are because of poor ventilation. The soil gas radon concentration of the Una district is found to be lower than that in some parts of the world. The present systematic investigations on radon/thoron activity concentrations in indoor air and radon activity concentration in soil gas clearly demonstrate that there is no radon risk in the high-population-density area of the Una district. ACKNOWLEDGEMENTS The authors thank the residents of the Una district in the selected houses who gladly helped during the field work. They also thank the laboratory staff of NIT, Jalandhar, Punjab, India. REFERENCES 1. Barretto, P. M. C. Radon-222 emanation characteristics of rocks and minerals. In: Proceedings of the Panel on Radon

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Table 2. Radon concentration in soil gas of the studied area.

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