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ORIGINAL ARTICLE
Year : 2021  |  Volume : 44  |  Issue : 3  |  Page : 161-166  

Assessment of radon exhalation rates in mineral rocks used in building decoration in Nigeria


1 Department of Science Laboratory Technology, Ladoke Akintola University of Technology, Ogbomoso, Nigeria
2 Pure and Applied Physics, Ladoke Akintola University of Technology, Ogbomoso, Nigeria
3 Department of Physics, Federal University Oye-Ekiti, Ekiti, Nigeria

Date of Submission18-Oct-2021
Date of Decision26-Nov-2021
Date of Acceptance27-Nov-2021
Date of Web Publication04-Jan-2022

Correspondence Address:
Paul Sola Ayanlola
Department of Pure and Applied Physics, Ladoke Akintola University of Technology, Ogbomoso
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.rpe_39_21

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  Abstract 


The recent interest in the applications of mineral rocks for interior building decoration has been a major concern from the radiological perspective because the breaking down of rocks into various shapes and sizes releases radioactive gaseous element that is harmful to human health. Hence, this study assessed the radon exhalation rates of different rock types in Nigeria and their implications on human health. A total of 45 samples comprising five samples per rock type were prepared, packed, and sealed inside a modified closed can and thereafter subjected to radon concentration measurement using a RAD7 detector. The results of the study showed that granite and limestone rock types had high radon concentrations and exhalation rates as compared to other rock samples assessed. The radon concentration for granite and limestone rockfalls within the action level limit. Thus, effort should be made to ensure adequate ventilation of any building that uses these rock types in building decoration. The results obtained can be used as baseline data for future investigation of the rock types in any locality.

Keywords: Building decoration, mineral rocks, Nigeria, radon exhalation


How to cite this article:
Lawal MK, Ayanlola PS, Oladapo OO, Oni MO, Aremu AA. Assessment of radon exhalation rates in mineral rocks used in building decoration in Nigeria. Radiat Prot Environ 2021;44:161-6

How to cite this URL:
Lawal MK, Ayanlola PS, Oladapo OO, Oni MO, Aremu AA. Assessment of radon exhalation rates in mineral rocks used in building decoration in Nigeria. Radiat Prot Environ [serial online] 2021 [cited 2022 Jun 28];44:161-6. Available from: https://www.rpe.org.in/text.asp?2021/44/3/161/334782




  Introduction Top


Humans are continuously exposed to radiation from natural and artificial sources spatially distributed in the ecosystem. Materials originating from the rock and soil contain natural radionuclides of the decay series headed by uranium-238, thorium-232, and the singly occurring radionuclide of potassium-40.[1] Rocks consisting of the aggregate of minerals, including granite, marble, pegmatite, lamprophyres, calcite, kaolin, shale, limestone, gypsum are being employed extensively in diverse domestic and industrial building constructions and other applications.[2],[3],[4],[5] The advent of modern design and beautification of buildings has resulted in the recent usage of these mineral-induced rocks. These minerals either during breaking down into fragments or cutting to specific shapes and sizes exhale radioactive elements most especially radon-222 (Rn-222) which is harmful to human health.

Due to the differences in geology and geochemical composition of the Earth, the level of the radioactive element varies from location to location and from material to material.[6] Building materials to which rocks and soil constitute a larger share have been identified to be a potential source of either internal or external radiation exposure of humans. Internal exposure occurs through the inhalation of radon gas and their short-lived decay products while external exposure occurs due to the emission of penetrating gamma rays. Radon and its progeny are constituent of the atmosphere, and their concentrations tend to be higher in confined buildings than in the open air. The air within a building contains radon which enters from outside, together with radon from the ground beneath and from different building materials that make up the structure.[7]

Radon is an inert and naturally occurring radioactive gaseous element that can traverse freely through porous media. Radon exists in three different isotopes, namely radon-222 (222Rn), radon-220 (220Rn), and radon-219 (219Rn) of the uranium, thorium, and actinium series, respectively. Of radiological concern in terms of public exposure is 222Rn, which has been identified and classified to be a class I carcinogen by different international bodies including International Agency for Research on Cancer, World Health Organization, Environmental Protection Agency. 222Rn is the decay product of 226Ra, which is the part of the long decay chain of 238U. Since 238U is found everywhere in the Earth's crust, 226Ra and 222Rn are present in almost all rocks, soil, and water. 222Rn decay to short-lived radioactive elements 218Po, 214Pb, 214Bi, and 214Po called radon progenies. These progenies get attached to the existing aerosols, suspended particulate matter in the atmosphere, therefore, the inhalations of 222Rn and its progeny are the most common source of irradiation of the human respiratory. Studies reported that 222Rn is the single largest contributor to an individual's radiation dose.[1]

Recently, there has been considerable public concern about radon exhalation from building materials and its contribution to indoor radon level. Radon exhalation designates the rate at which radon escapes from any environmental matrices such as rock or soil into the surrounding air, most especially indoor radon. The dominant contributor to indoor radon level has been identified to emanate from soil and fractured bedrock close to the surface.[8] As individuals spend more than 80% of their time indoors, the internal and external radiation exposure from building materials creates prolonged exposure situations.[9] Based on epidemiological studies, it has been established that the enhanced levels of indoor radon in buildings can cause health hazards and lead to serious diseases such as lung cancer in humans.[10] Several studies have been conducted on radon exhalation rate in building materials most especially on soil, both locally and international,[11],[12],[13],[14] yet database on radon exhalation rate resulting from the recent usage of mineral-induced rock samples as building materials is very sparse to the best of our knowledge. Hence, this study assesses the radon exhalation rate in mineral-induced rocks used in building construction and decoration in our locality. The present study also assesses the possible radiological hazards to human health and develops a baseline yardstick for the use and management of these materials.


  Materials and Methods Top


Sample collection and processing

Nigeria is endowed with lots of natural minerals that can boost the income generation of the country. Nine types of mineral-induced rocks, namely granite, limestone, kaolin, gypsum, marble, calcite, shales, pegmatite, and lamprophyre were selected for investigation. From each type, five samples were collected and further processed. Most of the mineral rocks considered for the study can be found in a large deposit in the different parts of Nigeria, most especially the South-western and North central parts of the country [Figure 1]. The geology of these areas consists of Precambrian rocks that are typical of the basement complex terrain of Nigeria.
Figure 1: Map of Nigeria showing the locations of mineral rock deposits

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Experimental setup

Among the different techniques available for radon measurement, the use of closed-can technique, usually made of polyvinyl chloride container of different geometry and dimensions with Solid State Nuclear Tracks Detectors (SSNTDs), has been the most widely employed method, and the most commonly used SSNTDs are CR-39. Nuclear track detectors are plastic detector that is used to register alpha particles in form of tracks, which will only become visible under the optical microscope upon suitable chemical etching of the SSNTDs. Can technique has the advantage of being simple, efficient, and relatively inexpensive.[12] This technique is also referred to as diffusion chamber, accumulation chamber, radon exposure chamber, and time-integrated passive radon dosimeter and emanation container.[15]

However, this study employed a modified can technique, made from a cylindrical plastic container of 18 cm high and 10 cm in diameter. Each container consists of a one-way inlet and outlet probe connected to the top and base of the container, respectively [Figure 2]. The containers were decontaminated by washing them with dilute nitric acid and thorough rinsing with distilled water.[14] The background radon concentration of each container was measured before packing the sample into each container. Each sample was broken down into fragments and 600 g of each sample was packed separately inside five different containers and then sealed for 30 days to prevent the escape of radon gas and ensure that radon and its progenies are confined within the sample and that the radon activity reaches an equilibrium state. This equilibrium state is very important for the determination of the radon concentration in the can technique. In all, a total of 45 containers comprising of five samples per rock type was prepared for the study.
Figure 2: Experimental setup

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Measurement of radon concentration

The prepared samples were analyzed using a well-calibrated active electronic solid-state detector RAD7 (DURRIDGE Company Inc., USA). The detector is a highly versatile instrument that can form the basis of a comprehensive radon measuring system and may be used in many different modes for different purposes. The detector uses the alpha spectrometry technique to continuously sniff and grab out radon gas, the result of which are displayed in real-time monitoring on the detector liquid crystal display screen, printed out after the completion of the cycle or stored in the RAD7 memory for later referencing (an advantage over the SSNTDs technique). The detector was powered by an inverter to provide a continuous flow of electricity throughout the measurement.

The detector was firstly purged with the outdoor air to provide clean, desiccated, radon-free air to the inlet of the RAD7 detector as discussed by the manufacturer. This is to ensure that any radon gas that was previously sampled was pushed out. After the successfully purging of the detector, the probe of each sealed sample container was connected to the detector through a desiccant tube containing calcium sulfate and an inert filter having a pore size of 1 μm for sucking radon gas from the sample. The radon gas was then sucked through the tube pipe into the detector for 2 h pumping phase and counted for 12 rounds which gives a total of 24 h count cycle to obtain results. The procedure was repeated for all five samples of each rock type, and the average radon concentration was determined.

Estimation of radon exhalation rates

The study of radon exhalation is important in understanding the relative contribution of the building decoration materials to the total radon concentration found in the dwellings. Conventionally, the surface and mass radon exhalation rate has been estimated using Equations 1 and 2, respectively, by different studies.[14],[16],[17],[18],[19]





However, it was observed that the units on both sides of the equations are not the same. Hence, to overcome this difficulty, this study adopts Equations 3 and 4 for the estimation of the radon surface exhalation rate, ES and mass exhalation rate, EM respectively.[15]





where: CRn (Bqm−3) is the average radon concentration (Bqm−3) of each rock type measured with the RAD7 detector, V (m3) is the effective volume of the sample container, λ (h−1) is 222Rn decay constant (7.55 × 10−3 h−1), Teff (h) is the effective exposure time, τ (h) is the mean lifetime of 222Rn (5.5 days = 132 h), Ms (kg) is the mass of the sample, T (h) is the period of exposure and As (m2) is the surface area of the sample. The value of Teff, V, and As were determined using Equations (5), (6), and (7) respectively:







Where r and ht is the radius and total height of the sample container.

Estimation of other radiological indices

The effective radium content, CRa was found using the following relation:[11],[15],[20]



Where hds is the distance between the detector and the top of the sample.

An important problem in epidemiological radon studies is to determine the radon levels that existed indoor; hence, the annual effective dose equivalent, AEDE which gives the annual indoor exposure to potential alpha energy was estimated by Equation (9):



Where ϵ is the equilibrium factor (0.4) of indoor exposure, fRn is the conversion factor (9 nSv/Bqhm−3), T is the number of hours spent indoor per year (8760 h), Of is the indoor occupancy factor (0.8), all of which were recommended[1] and CRn is the average radon concentration (Bqm−3).


  Results and Discussion Top


The results obtained for the analyzed mineral-induced rocks are presented in [Table 1] which shows the average radon concentrations of each rock type, their corresponding average exhalation rates, and other radiological indices. For the average radon concentration, the value obtained ranges from 30.21 to 585.16 Bqm−3, with granite having the highest radon concentration while pegmatite has the least. This variation in the radon concentration can be attributed to the origin of the rock type, geological and geochemical characteristics of the location from which the rocks were obtained. Comparing the range of radon concentrations obtained in this study with the international level, it was observed that the radon concentrations of the samples fall action level within the limit of 200–600 Bqm−3 recommended for dwelling.[21] These values represent the action level which requires initiating intervention to remedial action in existing dwellings. However, except for granite (585.16 Bqm−3) and limestone (237.50 Bqm−3), the radon levels recorded for other rock types are <100 Bqm−3 reference level[22] [Figure 3]. Hence, the result shows that kaolin, gypsum, marble, calcite, shales, pegmatite, and lamprophyre are safe as far as the health hazards due to 222Rn are concerned.
Table 1: Results of average radon concentration, exhalation rates and other radiological indices

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Figure 3: Comparison of radon concentration of the rock samples with the reference level

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The surface exhalation rate in the rock samples varies from 5.73 to 111.00 mBqm−2 h−1. The mass exhalation rate ranges from 0.69 to 13.40 mBqkg1 h−1. Both the surface and mass exhalation rates also revealed that granite has the highest exhalation rate while pegmatite has the least. Granite is a coarse intrusive igneous rock that is rich in quartz and feldspar; it is the most common plutonic rock of the Earth's crust, forming by the cooling of magma at a particular depth. The high radon concentration recorded in the granite rock can be attributed to the relatively high uranium content in its natural formation,[1] which may be low in other samples. The estimated radium content ranges from 23.40 to 453.00 Bqkg−1, with only pegmatite having less value than the global value of 30 Bqkg−1 for building materials. The results obtained for the annual effective dose equivalent vary from 0.76 to 14.80 mSvy−1, with only pegmatite (0.76 mSvy−1), shale (1.11 mSvy−1), and calcite (1.10 mSvy−1) having less value than the average worldwide exposure (1.15 mSvy−1) to 222Rn.[1] The average AEDE values obtained for the remaining rock are above action level, hence necessary safety protocol such as adequate cross ventilation is required to mitigate exposure to radon gas. The uranium content of other rock types is reflected by the high radium content and radon concentration, both of which play an important role in radon exhalation and the equivalent exposure.

The results showed that the radon concentration is directly proportional with the radon exposure, when the equilibrium factor and exposure time are constant. This suggests that any individual who decided to use only one of all these rock types in the interior decoration of a building will only be exposed based on the radon concentration of the selected rock type. This relationship between the indoor radon exposure and the concentrations emanating from the rock samples may not be totally correlated, but it does describe an existing pattern for the mineral-induced rocks. However, it will be advisable that any individual who wishes to beautify and decorate the interior of its building should go for the rock type with the least radon concentration to avoid and mitigate exposure to 222Rn.





The close can technique (modified) connected to the RAD7 detector have been employed to assess radon exhalation rate in different rock samples to assess the contribution of mineral induced rock type to indoor radon exposure and its implication on human health. The results obtained from the study showed that the radon exhalation rates from granite and limestone have relatively high values when compared to other rock samples and fall within the action level limit. Thus, it can be concluded that granite and limestone samples are not safe for use as building materials; however, adequate cross ventilation must be ensured to diffuse the radon gas appropriately if other rock types are to be used for building decoration. The procedure employed in this study can be used as reference information in assessing the radon exhalation rate. The results obtained can be used as baseline data for future investigation of radon exhalation rates in rock types in any locality.

Acknowledgments

The authors acknowledge the Department of Earth Science, Ladoke Akintola University of Technology, Ogbomoso, Nigeria in making the rock types available for research purposes.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1]



 

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