Radiological characterization of building materials used in Malaysia and assessment of external and internal doses

Radiological characterization of building materials used in Malaysia and assessment of external and internal doses 1 3 first-author 1 2 corresp Corresponding author: aznan@ukm.edu.my Corresponding author: aznan@ukm.edu.my aznan@ukm.edu.my 2 Nuclear Science Program, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Nuclear Science Program, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Centre for Frontier Science, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Centre for Frontier Science, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Department of Physics, Faculty of Science, Gombe State University, P.M.B. 127 Gombe, Nigeria Department of Physics, Faculty of Science, Gombe State University, P.M.B. 127 Gombe, Nigeria In this study, the activity concentrations of 226 Ra, 232 Th, 222 Rn, and 40 K, emanation fractions ( P ), equilibrium equivalent concentration ( EEC ), and mass exhalation rates ( E m ) of radon released from building materials used in Malaysia were studied using gamma-ray spectrometer with HPGe detector. Radiological parameters (activity concentration index ( ACI ), indoor air absorbed dose rate ( D in ), annual effective dose ( AED in ) from external and internal ( E Rn ), soft tissues ( H ST ) and lung ( H L ) and effective dose equivalent ( H eff )) were estimated to evaluate radiological hazards due to use of these building materials: sand, cement, gravel, bricks, tiles, fly ash, white cement, and ceramic raw materials. The measured P , EEC , and E m vary from 10 to 30 %, 0.9 to 22 Bq m -3 , and 33 to 674 mBq h -1 kg -1 , respectively. While the calculated ACI and AED in vary from 0.1 ± 0.01 to 2.1 ± 0.1 and 0.1 ± 0.01 to 2.4 ± 0.6 mSv y -1 , respectively. On the other hand, the internal annual effective dose ranges from 0.1 to 1.4 mSv y -1 . The estimated radiological risk parameters were below the recommended maximum values and radiological hazards associated with building materials under investigation are therefore negligible. Indoor radon Emanation fraction Equilibrium equivalent concentration Mass exhalation rate Annual effective dose effective dose equivalent 1 Introduction 1 All materials present in the Earth’s crust contain some amount of natural radioactive elements. However, the most common Naturally Occurring Radioactive Materials (NORM) found in the environment are Uranium ( 238 U), Thorium ( 232 Th), and their progeny and isotope of potassium ( 40 K) [ 1 1 - 5 5 1-5 ] . Some materials such as industrial wastes and/or industrial by-products may contain high radioactive elements called Technologically Enhanced Naturally Occurring Radioactive Materials (TENORM). Radium ( 226 Ra) is mostly used as a reference radionuclide in the measurement of 238 U [ 5 5 - 7 7 5-7 ] . Most of the radiation exposure received by the general public from natural sources comes from the decay of 226 Ra which produces radon ( 222 Rn) gas and its decay products [ 8 8 ] . Radon is an immediate decay product of 226 Ra and is naturally occurring radioactive noble gas with important features: it is odorless, it is colorless, it has about 3.8-day half-life, it is present in the air and all building materials, and it has four important immediate decay products including 218 Po, 214 Pb, 214 Bi, and 214 Po [ 3 3 ] . 222 Rn gas is the most important source of an indoor radiation exposure and account for about half of the worldwide background radiation dose received by human population [ 9 9 , 10 10 ] . It emanates from rocks, soils, and building materials and is accumulated in enclosed places such as underground mines and houses [ 9 9 - 11 11 9-11 ] . Various international organizations such as the World Health Organization (WHO), International Commission on Radiological Protection (ICRP), International Atomic Energy Agency (IAEA), International Agency for Research on Cancer (IARC), and US National Toxicology Program (US NTP) have recognized 222 Rn as human carcinogen based on sufficient evidence [ 4 4 , 11 11 - 14 14 11-14 ] . Long term exposure to indoor 222 Rn concentrations could cause pathological effects and changes in the respiratory function and, thus, could increase the possibilities of developing lung cancer [ 15 15 ] . Some parts of 222 Rn decay products will also infiltrate into the blood from the lungs, thereby irradiating the whole human body [ 9 9 ] . The first cause of cancer mortality in the world is lung cancer. And smoking is the main cause of lung cancer, followed by indoor radon exposure. Furthermore, a statistics study showed that, worldwide, about 10 to 25 % of lung cancer cases occur among people with no history of smoking [ 16 16 , 17 17 ] . Moreover, from a radiation viewpoint, lung cancer risks from the exposure of 222 Rn and its progeny is not the only concern: other forms of cancer are also associated with radiation exposures such as X-radiation and gamma (γ) radiation. The exposure to the aforementioned radiations is strongly associated with leukemia and cancer of the thyroid, breast, and lung. The evidence was discovered at absorbed doses of less than 0.2 Gy. The tendency to develop these cancers depend on age. Other cancers such as the salivary glands, urinary bladder, stomach, ovary, colon, skin, and central nervous system are associated with high absorbed doses up to 1 Gy [ 12 12 ] . In Malaysia, cancer is one of the leading causes of death [ 18 18 ] . According to a Malaysian National Cancer Registry (MNCR) report on cancer incidences from 2007 to 2011, a total number of 103,507 was recorded as new diagnosed cases of cancer and 45.2 % of these cases were male while 54.8 % cases were females. And, whereas colorectal, lung, nasopharynx, lymphoma, and prostate are common cancers among males. Breast, colorectal, cervix uteri, ovary, and lung are common cancers among females [ 18 18 ] . The report also showed that the number of deaths caused by cancer within this period is 64,275. And this cause-caused death trend increased from 2007 to 2011. In Malaysia, lung cancer was ranked second and fifth most common cancers among males and females, respectively [ 18 18 ] . Therefore, studying possible substances associated with cancer risks in Malaysia such as radiological health risks attributed to building materials and fractional contribution of each of the building materials to the exposure caused by inhalation of indoor radon and its progeny can be beneficial to local authorities and general public. The aim of this study is to evaluate the level of natural radioactivity in Malaysian building materials and their associated radiological hazards to the human health by assessing external and internal doses resulted from the use of these materials under investigation. 2 Materials and Method 1 2.1 Sample Preparation 2 The building materials under study are the common construction materials used by the Malaysian people. One hundred and two (102) samples were source from various locations within Malaysia such as Selangor, Kuala Lumpur, Johor, Pahang, Terengganu, Kelantan, and Putrajaya. The selected building materials with sample identity (ID) in parentheses are tiles (1-40), red brick (41-45), cement brick (46-50), sand (51-60), cement (61-70), gravel (71-80), white cement (81-85), fly ash (86-90), feldspar (91-93), limestone (94-96), kaolin (97), pottery stone (98), clay (99-100), glaze (101), and talc (102). The samples were oven-dried at 105  o C for 48 hours to ensure significant moisture content was removed, grinded, and then sieved using 500 µm-sized mesh. The grain size was carefully chosen to optimize the filling of the marinelli beakers. They were then packed in a marinelli beaker in three replicates, air-tight sealed, labelled, weighed, and kept in a laboratory for 30 days to achieve radioactive secular equilibrium. The present study followed the standard sample preparation procedure as mentioned in the IAEA technical report series no. 295 [ 19 19 ] . 2.2 Instrumentation 2 The gamma-ray spectroscopy consisted of a closed-end coaxial high purity germanium (HPGe) detector, lead shielding, and electronics to process the signals produced by the detector (pulse sorter and related amplifiers). Additionally, it also consisted of read out devices such as a computer analyzer using Genie 2000 software to generate, display, and store the final gamma-ray spectrum. The 10 cm thick cylindrical lead shield is coated with 1 mm tin and enclosed in a 1.6 mm copper HPGe detector capsule. The HPGe detector is mounted in a vacuum chamber inserted into a liquid nitrogen Dewar at a temperature of 77 K as a cooling medium for the system [ 20 20 ] . Other components include high voltage supply, depletion voltage, +2500 V, recommended bias voltage +3000 V and pulse shaping time of 4 µs. The relative efficiency and full width at half maximum (FWHM) energy resolution at 1.33 MeV are 30 % and 1.8 keV, respectively. The spectra data were analyzed using Genie 2000 software made by Canberra. 2.3 Detector Calibration and Measurement of Natural Radioactivity 2 The building materials are measured using gamma-ray spectroscopy employing a HPGe detector located within the Science Nuclear building at the Universiti Kebangsaan, Malaysia. The absolute efficiency of the counting system was evaluated using a mixed standard source containing 22 Na, 57 Co, 60 Co, and 137 Cs. Also, the activities of the standard source at the time of study are 9.86 kBq, 0.03 kBq, 8.36 kBq, and 1.18 kBq for 22 Na, 57 Co, 60 Co, and 137 Cs, respectively. Routine background measurements were carried out before commencing the counting of the samples. Reference materials (International Atomic Energy Agency: IAEA-375 Soil) of known activities of the desired natural radionuclides. The IAEA-375 soil were prepared in three replicates and counted several times prior to counting of the samples with a view to compare the measured activity concentrations with the known activity concentrations of the IAEA-375 soil. The activity concentrations of 226 Ra, 232 Th, and 40 K of the IAEA-375 soil are 20, 20.5, and 424 Bq kg -1 , respectively. Each sample was prepared in three replicates and counted for 43200 seconds. The acquisition of data and identification of radionuclides based on their gamma-ray energy peaks were carried out using Canberra Genie 2000 software. An empty Marinelli beaker was counted for 43200 seconds to obtain the background spectrum. The analyzed results of the HPGe detector were printed and the identification of radionuclides of each sample was performed by selecting the net peak area of specific known gamma-ray energy peaks. The gamma-ray energy peaks used for identifying 226 Ra and 232 Th are 1764.5 keV ( 214 Bi) and 2614.5 keV ( 208 Tl), respectively, while singly occurring 40 K was identified directly from its gamma-ray energy peak of 1461 keV. The activity concentration of individual radionuclides was calculated using Eq. (5) [ 5 5 , 21 21 ] . The minimum detectable activity (MDA) was calculated based on Curie’s derivation [ 22 22 ] . The average MDA for the counting time of 43200 seconds was 6.2 ± 0.1, 5.3 ± 0.1, and 22.1 ± 0.2 Bq kg -1 for 1764.5, 2614.5, and 1461 keV, respectively. 3 Results and Discussion 1 3.1 Determination of Radon Emanation Fraction (P) 2 The measurement of radon emanation fraction was carried out using gamma-ray spectroscopy coupled with a high-purity germanium (HPGe) detector. The samples were sealed in a counting bottle and emitted gamma-rays from 214 Pb and/or 214 Bi were counted in an interval of two days before and after achieving radioactive equilibrium between the 226 Ra and its progeny. Therefore, the samples were counted 25 times in 50 days; the goal for extending the days up to 50 is to ensure the secular equilibrium between the parent nuclide and their decay progenies are clearly observed as shown in Fig. 1 Fig. 1 . When the 222 Rn and its progeny are in secular equilibrium, then the activities of 222 Rn and its progeny are equal [ 23 23 ] . Moreover, the activity concentration of 226 Ra was also investigated from various building materials. The choice of this method is born out of two major advantages: non-destructive and indirect measurement through the decay products of the 226 Ra nuclide. Since only relative counts are needed, no calibration is required for this method [ 24 24 ] . The expression in Eq. (1) shows the change in the total radon concentration C Rn (Bq m -3 ) with respect to time t (s) in solid and gas phases enclosed in a container [ 25 25 ] : <math display="block" id="M1"><mrow><mfrac><mrow><mtext>d</mtext><msub><mi>C</mi><mrow><mtext>Rn</mtext></mrow></msub></mrow><mrow><mtext>d</mtext><mi>t</mi></mrow></mfrac><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mi>E</mi><mi>V</mi></mfrac><mtext> </mtext><mo>−</mo><mtext> λ</mtext><msub><mi>C</mi><mrow><mtext>Rn</mtext></mrow></msub><mo>.</mo></mrow></math> It follows that, <math display="block" id="M2"><mrow><msub><mi>C</mi><mrow><mtext>Rn</mtext></mrow></msub><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mi>E</mi><mrow><mi>λ</mi><mi>V</mi></mrow></mfrac><mo> </mo><mrow><mo>(</mo><mrow><mn>1</mn><mtext> </mtext><mo>−</mo><mtext> </mtext><msup><mi>e</mi><mrow><mo>−</mo><mi>λ</mi><mi>t</mi></mrow></msup></mrow><mo>)</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><msub><mi>C</mi><mrow><mtext>Rn</mtext></mrow></msub><mrow><mo>(</mo><mn>0</mn><mo>)</mo></mrow><mtext> </mtext><msup><mi>e</mi><mrow><mo>−</mo><mi>λ</mi><mi>t</mi></mrow></msup><mo>.</mo></mrow></math> Eq. (4) was further deduced as: <math display="block" id="M3"><mrow><mi>N</mi><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow><mo> </mo><mo>=</mo><mtext> </mtext><msub><mi>N</mi><mrow><mtext>eq</mtext></mrow></msub><mo> </mo><mrow><mo>{</mo><mrow><mn>1</mn><mtext> </mtext><mo>−</mo><mtext> </mtext><msup><mi>e</mi><mrow><mo>−</mo><mi>λ</mi><mi>t</mi></mrow></msup></mrow><mo>}</mo></mrow><mtext> </mtext><mo>+</mo><mtext> </mtext><msub><mi>N</mi><mn>0</mn></msub><mo> </mo><msup><mi>e</mi><mrow><mo>−</mo><mi>λ</mi><mi>t</mi></mrow></msup><mo>,</mo></mrow></math> where E (Bq s -1 ) is the radon exhalation rate from the sample, V is the volume of the sample container or counting bottle, and λ (s -1 ) is the radon decay constant. Integrating Eq. (1), the radon growth is presented in Eq. (2), where C Rn (0) is the radon concentration at t = 0. At this point, all radon atoms exist in the solid phase. In other words, all radon atoms are present in the mineral grains. Whereas N (t) is the net count rate of γ-rays, N eq and N 0 are the net count rates at t > 30 days and t = 0, respectively. The net count rate N was obtained by fitting a series of N eq and N 0 to Eq. (3). Eq. (3) was used to determine the grow of 222 Rn decay daughters in a closed counting container. An example of measured data and fitting curves are shown in Fig. (1) Fig. (1) . It could be observed that the fitting was successful. Hence, the radon emanation fraction P (%) was calculated using Eq. (4) [ 24 24 , 25 25 ] : <math display="block" id="M4"><mrow><mi>P</mi><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mrow><msub><mi>N</mi><mrow><mtext>eq</mtext></mrow></msub><mtext> </mtext><mo>−</mo><mtext> </mtext><msub><mi>N</mi><mn>0</mn></msub></mrow><mrow><msub><mi>N</mi><mrow><mtext>eq</mtext></mrow></msub></mrow></mfrac><mtext> </mtext><mo>×</mo><mtext> </mtext><mn>100.</mn></mrow></math> The radon emanation fraction from six different building materials was measured and presented in Fig. (2) Fig. (2) with values ranging from 10±0.4 % to 30±0.4 %. The reported high values for cement brick and sand suggest that the radium contents in these materials are concentrated near the grain surface. Another possible factor is cracks on the surface of the grain, which may be from the previous radioactive decays and weathering effects: it can affect the surface area of the grain and subsequently increase the radon emanation fraction [ 24 24 ] . The International Atomic Energy Agency (IAEA) reported the radon emanation fraction of natural materials such as uranium mine and mill tailings, rocks, soils, and minerals range from 10% to 30% with high values reported for rocks, soils, and mill tailings whereas low values less than 10% were reported for fly ash and minerals [ 24 24 ] . Sahoo et al. [ 26 26 ] reported an emanation fraction from Indian building materials using alpha-guard, a continuous radon monitor and their results range from 0.1 to 20.8 % with maximum and minimum reported values in lime and vetrified tile, respectively [ 26 26 ] . UNSCEAR [ 27 27 ] provided a representative value of 20 % for soil with a range of 1 to 80 %. Martino [ 28 28 ] reported radon emanation fraction of soils using an electrostatic collector coupled to a silicon detector and his results ranged from 6 to 60 % [ 28 28 ] . Bossew [ 29 29 ] reported the radon emanation fraction of building materials, soil, and rock using activated charcoal and his results ranged from 0.6 to 29.5 %, respectively. These reported findings are in accordance with the results of the present study. 3.2 Natural Radioactivity and Indoor Radon Concentration 2 The individual activity concentration (C i ) is calculated using Eq. (5) [ 5 5 , 21 21 ] : <math display="block" id="M5"><mrow><msub><mi>C</mi><mtext>i</mtext></msub><mtext> </mtext><mo>=</mo><mtext> </mtext><mfrac><mrow><msub><mi>M</mi><mrow><mtext>ref </mtext></mrow></msub><mo>×</mo><mtext> </mtext><msub><mi>A</mi><mtext>s</mtext></msub></mrow><mrow><msub><mi>M</mi><mtext>s</mtext></msub><mtext> </mtext><mo>×</mo><mtext> </mtext><msub><mi>A</mi><mrow><mtext>ref</mtext></mrow></msub></mrow></mfrac><mtext> </mtext><mo>×</mo><mtext> </mtext><msub><mi>C</mi><mrow><mtext>ref</mtext></mrow></msub><mo>,</mo></mrow></math> where C ref is the activity concentration of reference materials, M s and M ref are the Mass of the sample (kg) and reference materials, respectively, and A s and A ref are the activity (cps) of sample and reference materials, respectively. Radon is the most important natural radionuclide in the indoor environment. Generally, only a few people are exposed to high 222 Rn and the majority of the lung cancers are caused by low and moderate 222 Rn exposure [ 11 11 ] . All building materials contain some natural radionuclides such as 238 U and 232 Th and their decay series and singly occurring 40 K [ 1 1 , 11 11 ] . The concentrations of natural radioactivity in building materials vary significantly. Hence, this causes an excess indoor 222 Rn and it is calculated using Eq. (7) [ 2 2 , 30 30 , 31 31 ] : <math display="block" id="M6"><mrow><msub><mi>C</mi><mrow><mtext>Rn</mtext></mrow></msub><mo> </mo><mo>=</mo><mo> </mo><mfrac><mi>U</mi><mrow><msub><mi>λ</mi><mi>v</mi></msub><mi>V</mi></mrow></mfrac><mo>,</mo></mrow></math> where U is the rate of radon entry into the room from building materials (Bq h -1 ) and its given as [ 2 2 , 30 30 ] <math display="block" id="M7"><mrow><mi>U</mi><mo>=</mo><mi>λ</mi><mi>P</mi><msub><mi>C</mi><mrow><mtext>Ra</mtext></mrow></msub><mi>ρ</mi><mi>d</mi><mi>A</mi></mrow></math> where λ v and λ are the decay constant of radon (h -1 ) and air exchanged rate (h -1 ), respectively. V is the room volume (m 3 ), P is the emanation fraction which was calculated previously, ρ is the density of the materials (kg m -3 ), and d and A are the wall half-thickness (m) and exhaling area (m 2 ), respectively. A standard room with dimensions 4 m × 5 m × 2.8 m and an air exchange rate of 0.5 h -1 was assumed. The thickness of the wall and density of the building were 20 m and 2400 kg m -3 , respectively. On the other hand, the volume of the room and exhaling area are 56 m 3 and 20 m 2 , respectively. The natural activity concentrations of various building materials were measured and analyzed using gamma-ray spectroscopy. The geometric mean activity concentration of each of the building material under study is presented in Table 1 Table 1 with values ranging from 10 ± 1 to 223 ± 5, 9 ± 1 to 274 ± 8 and 46 ± 7 to 1589 ± 21 Bq kg -1 for 226 Ra, 232 Th, and 40 K, respectively. On the other hand, to evaluate the contribution of building materials to the indoor environment, a standard room with an air exchange rate of 0.5 h -1 was assumed as mentioned above. The variation of the indoor radon concentration ( 222 Rn) from various building materials is presented in Fig. (3) Fig. (3) with values ranging from 2.2 ± 0.1 to 54.9 ± 1.3 Bq m -3 and a geometric mean of 10 ± 9 Bq m -3 . However, the geometric mean of 222 Rn of each of the building materials is also presented in Table 1 Table 1 . It can be observed that the activity concentrations and 222 Rn vary significantly among the building materials. 1 1 Building Materials 1 1 No. of samples 1 1 226 Ra (Bq kg -1 ) 1 1 232 Th (Bq kg -1 ) 1 1 40 K (Bq kg -1 ) 1 1 222 Rn (Bq m -3 ) 1 1 Tile 1 1 40 1 1 85 ± 40 1 1 65 ± 25 1 1 649 ± 220 1 1 11.0 ± 5.1 1 1 Red Brick 1 1 5 1 1 33 ± 3 1 1 55 ± 8 1 1 505 ± 135 1 1 6.8 ± 0.6 1 1 Cement Brick 1 1 5 1 1 46 ± 9 1 1 59 ± 15 1 1 576 ± 158 1 1 17.8 ± 3.5 1 1 Sand 1 1 10 1 1 38 ± 20 1 1 40 ± 20 1 1 358 ± 355 1 1 12.3 ± 6.6 1 1 Cement 1 1 10 1 1 28 ± 7 1 1 30 ± 9 1 1 194 ± 71 1 1 6.2 ± 1.6 1 1 Gravel 1 1 10 1 1 49 ± 64 1 1 45 ± 47 1 1 612 ± 475 1 1 9.5 ± 12.4 1 1 White Cement 1 1 5 1 1 33 ± 15 1 1 35 ± 15 1 1 119 ± 12 1 1 8.2 ± 3.6 1 1 Fly Ash 1 1 5 1 1 35 ± 2 1 1 34 ± 2 1 1 407 ± 21 1 1 8.7 ± 0.5 1 1 Feldspar 1 1 3 1 1 147 ± 61 1 1 58 ± 20 1 1 698 ± 329 1 1 36.2 ± 15.0 1 1 Limestone 1 1 3 1 1 10 ± 1 1 1 11 ± 3 1 1 60 ± 19 1 1 2.5 ± 0.2 1 1 Kaolin 1 1 1 1 1 140 ± 1 1 1 274 ± 8 1 1 703 ± 65 1 1 34.5 ± 0.2 1 1 Pottery Stone 1 1 1 1 1 96 ± 2 1 1 54 ± 1 1 1 1114 ± 20 1 1 23.7 ± 0.4 1 1 Clay 1 1 2 1 1 100 ± 92 1 1 125 ± 72 1 1 533 ± 88 1 1 24.6 ± 22.7 1 1 Glaze 1 1 1 1 1 42 ± 2 1 1 51 ± 5 1 1 891 ± 1 1 1 10.4 ± 0.5 1 1 Talc 1 1 1 1 1 21 ± 3 1 1 36 ± 3 1 1 123 ± 13 1 1 5.2 ± 0.6 1 1 Geometric Mean 1 1 102 1 1 53 ± 48 1 1 49 ± 37 1 1 441 ± 325 1 1 10 ± 9 The highest activity concentrations of 226 Ra and 232 Th were obtained in the tile compared with other building materials under study and can be associated to 226 Ra and 232 Th contents of some of the tile raw materials such as feldspar, kaolin, and clay as shown in Table 1 Table 1 . However, 40 K is the most abundant radioactive element in nature: it can be found in water, brines, and some other minerals such as carnallite, feldspar, saltpetre, sylvite, and greensand. It is also an essential constituent of fertile soil and nutrients for plant growth and the human diet [ 32 32 ] . Therefore, it may not be surprising to find an elevated activity concentration of 40 K in some building materials such as pottery stone, glaze, and gravel. According to United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) [ 33 33 ] , the 226 Ra content of the building materials establishes the source of the radon emanation. High indoor 222 Rn may be found in buildings constructed using building materials with high 226 Ra content, although it also depends on other factors such as ventilation, porosity, radon emanation fraction, texture, density, permeability, and other environmental conditions that may influence the indoor 222 Rn levels in a room [ 33 33 ] . An example of the role of radon emanation fraction on 222 Rn level is shown in Table 1 Table 1 . It can be seen that tile has the highest 226 Ra content compared with sand and gravel, but the emanation fraction of tile is low. Hence, the 222 Rn level of sand and gravel is greater than that of tile. UNSCEAR [ 34 34 ] reported the activity concentrations of the Malaysia’s soils with mean values of 67, 82 and 310 Bq kg -1 for 226 Ra, 232 Th, and 40 K, respectively [ 34 34 ] . Jang et al. [ 35 35 ] reported the indoor exposure caused by building materials using an HPGe detector and inductively coupled plasma mass spectrometry (ICP-MS) with mean values 57, 39, and 323 Bq kg -1 for 226 Ra, 232 Th, and 40 K, respectively [ 35 35 ] . Ge and Zhang [ 36 36 ] reported the natural radioactivity concentration and associated radiological risks from building materials using gamma-ray spectroscopy. The average results are 39, 47, and 563 Bq kg -1 for 226 Ra, 232 Th, and 40 K, respectively. These findings are in agreement with the results reported in our study. Herein we reported the study and the comparison of the indoor 222 Rn concentration from some selected countries across Africa, Asia, Europe, and the Middle-East. And the results, as shown in Table 2 Table 2 , showed that, in Africa, the reported indoor 222 Rn from Sudan, which is about nineteen times higher than the value reported in our study, has exceeded the recommended level of 100 to 300, 200, and 200 to 600 set by WHO, EC, and ICRP Level [ 1 1 , 3 3 , 11 11 ] . However, the average value is less than the action level of WHO and ICRP [ 3 3 , 11 11 ] . On the other hand, the difference on the average values reported from Nigeria, Egypt, and Ghana are insignificant when compared with the values obtained from our study (about three times greater). They are less than the representative value of 40 Bq m -3 [ 11 11 , 34 34 ] . In Asia, the values reported from India and Korea are approximately the same with the value reported in our study, however, the values reported from Japan and Thailand are 35 % greater than the value reported in our study [ 37 37 - 39 39 37-39 ] . In the European site, though the reported indoor 222 Rn from Cyprus and Lithuania are comparable to the values obtained from our study, our value is however 17 and 6 times smaller than the reported values from Bulgaria and Romania, respectively [ 40 40 - 43 43 40-43 ] . The indoor 222 Rn reported in our study is about 3 and 2 times less than the values reported from Saudi Arabia and Kuwait, respectively [ 44 44 , 45 45 ] . While the values from Turkey [ 46 46 ] are about eight times greater than our values, our values are greater than those reported from Yemen by 4 times [ 47 47 ] . CR-39 and LR-115 Type II are alpha track detectors, CRM is the continuous radon monitor, IC is the ionization chamber, and PT-DM is the passive-type radon/thoron descriptive monitor (Rn Monitor). 2 1 Country 1 1 222 Rn (Bq m -3 ) 1 1 Techniques 1 1 Source 2 1 Africa 1 1 1 1 Egypt 1 1 44 (15 to 132) 1 1 LR-115 Type II 1 1 [ 49 49 ] 1 1 1 1 Ghana 1 1 35 (11 to 111) 1 1 LR-115 Type II 1 1 [ 50 50 ] 1 1 1 1 Nigeria 1 1 39 (5 to 255) 1 1 CR-39 1 1 [ 51 51 ] 1 1 1 1 Sudan 1 1 245 (128 to 506) 1 1 CR-39 1 1 [ 52 52 ] 2 1 Asia 1 1 1 1 India 1 1 16 (9 to 23) 1 1 LR-115 Type II 1 1 [ 53 53 ] 1 1 1 1 Japan 1 1 20 (3 to 125) 1 1 Alphagurad 1 1 [ 39 39 ] 1 1 1 1 Korea 1 1 13 (0 to 88) and 71 (7 to 572) 1 1 HPGe and ATD 1 1 [ 35 35 , 54 54 ] 1 1 1 1 Thailand 1 1 20 (10 to 48) 1 1 IC 1 1 [ 37 37 , 38 38 ] 2 1 Europe 1 1 1 1 Bulgaria 1 1 227 (20 to 1117) 1 1 LR-115 Type II 1 1 [ 40 40 ] 1 1 1 1 Cyprus 1 1 19 (4 to 100) 1 1 CRM 1 1 [ 41 41 ] 1 1 1 1 Lithuania 1 1 14 (7 to 19) 1 1 Alphaguard 1 1 [ 42 42 ] 1 1 1 1 Romania 1 1 83 1 1 CR-39 1 1 [ 43 43 ] 2 1 Middle East/Western Asia 1 1 1 1 Kuwait 1 1 25 (6 to 78) 1 1 Alphaguard 1 1 [ 44 44 ] 1 1 1 1 Saudi Arabia 1 1 36 (21 to 52) 1 1 CR-39 1 1 [ 45 45 ] 1 1 1 1 Turkey 1 1 98 (39 to 405) 1 1 LR-115 Type II 1 1 [ 46 46 ] 1 1 1 1 Yemen 1 1 3 (1 to 5) 1 1 CR-39 1 1 [ 47 47 ] 2 1 Representative Values and Present Study 1 1 1 1 World Average Value 1 1 40 1 1   1 1 [ 34 34 ] 1 1 1 1 Malaysia Average Value 1 1 14 1 1   1 1 [ 34 34 ] 1 1 1 1 Malaysia 1 1 13 (2 to 55) 1 1   1 1 Present study Generally, if the world average value (40 Bq m -3 ) is taken as a reference value in the comparison, the indoor 222 Rn reported is comparable with the values reported from Nigeria, Ghana, India, Korea, Japan, Thailand, Cyprus, Lithuania, Kuwait, Saudi Arabia, and Yemen, based on the world average value, the value reported is similar with eleven out of sixteen studies under comparison corresponding to 70% of the studies under comparison. The variation observed with other studies may be due to the geological variables of the materials beneath the dwellings, the type of building materials used in the construction, geographical condition, and atmospheric variables such as temperature and pressure differences [ 48 48 ] . Other factors that may lead to the variation include: ventilation and radon emanation fraction. Therefore, the indoor 222 Rn concentration reported in our study is in accordance with published results from various parts of the world. 3.3 Radiological Risks Analysis 2 The main contributions of the radiation dose received by the general public is originated from 226 Ra, 232 Th and 40 K release mainly from building materials. Many countries have established guidelines to restrict the amount of natural radionuclides in building materials [ 13 13 ] . European Commission (EC) and IAEA showed that guidelines for building materials are primarily based on the activity concentration index ( ACI ) to the natural radionuclides of 226 Ra, 232 Th, and 40 K in the building materials [ 1 1 ] , [ 13 13 ] . 226 Ra content criterion should be added to the building materials with high emanation fraction [ 13 13 ] . To investigate the compliance of guidelines for the EC member states, the exemption level and upper level of ACI were given as 0.5 (exemption level) and 1 (upper level) for the building materials used in bulk quantities such as brick, cement, sand, gravel etc. While the ACI of 2 (exemption level) and 6 (upper level) for the limited used building materials such as tiles or ceramic materials and board [ 1 1 ] . The exemption and upper level of ACI is equivalent to 0.3 (exemption level) and 1 mSv y -1 (upper level) from external exposure to γ-radiation, which is called an indoor annual effective dose ( AED in ) in this study [ 1 1 ] , [ 13 13 ] . The ACI and AED in expressions are given in Eq. (9) and (11), respectively [ 1 1 ] , [ 30 30 ] , [ 34 34 ] . To determine AED in , an indoor air absorbed dose rate (D in ) must be estimated at 1 m above the ground surface by applying the conversion factors 0.92, 1.1, and 0.08 for 226 Ra, 232 Th, and 40K as initiated by EC and given in Eq. (10) [ 1 1 ] . <math display="block" id="M8"><mrow><mi>A</mi><mi>C</mi><mi>I</mi><mo> </mo><mo>=</mo><mo> </mo><mfrac><mrow><msub><mi>C</mi><mrow><mtext>Ra</mtext></mrow></msub></mrow><mrow><mn>300</mn></mrow></mfrac><mo> </mo><mo>+</mo><mo> </mo><mfrac><mrow><msub><mi>C</mi><mrow><mtext>Th</mtext></mrow></msub></mrow><mrow><mn>200</mn></mrow></mfrac><mo> </mo><mo>+</mo><mo> </mo><mfrac><mrow><msub><mi>C</mi><mtext>K</mtext></msub></mrow><mrow><mn>3000</mn></mrow></mfrac></mrow></math> <math display="block" id="M9"><mrow><msub><mi>D</mi><mrow><mtext>in</mtext></mrow></msub><mrow><mo>(</mo><mrow><msup><mrow><mtext>nGy h</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow><mo>)</mo></mrow><mo> </mo><mo>=</mo><mo> </mo><mn>0.92</mn><mo> </mo><msub><mi>C</mi><mrow><mtext>Ra</mtext></mrow></msub><mo> </mo><mo>+</mo><mo> </mo><mn>1.1</mn><mo> </mo><msub><mi>C</mi><mrow><mtext>Th</mtext></mrow></msub><mo> </mo><mo>+</mo><mo> </mo><mn>0.081</mn><mo> </mo><msub><mi>C</mi><mtext>K</mtext></msub></mrow></math> <math display="block" id="M10"><mrow><mi>A</mi><mi>E</mi><msub><mi>D</mi><mrow><mtext>in</mtext></mrow></msub><mrow><mo>(</mo><mrow><msup><mrow><mtext>mSv y</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow><mo>)</mo></mrow><mo> </mo><mo>=</mo><mo> </mo><mn>4.91</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup><mo> </mo><mrow><mo>(</mo><mrow><msup><mrow><mtext>Sv h Gy</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow><mo>)</mo></mrow><mo> </mo><mo>×</mo><msub><mi>D</mi><mrow><mtext>in</mtext></mrow></msub><mrow><mo>(</mo><mrow><msup><mrow><mtext>nGy h</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow><mo>)</mo></mrow></mrow></math> where C Ra , C Th , and C K were previously defined. The calculated D in was converted into AED in using a conversion factor of 0.7 Sv Gy -1 and other relevant parameters such as indoor time fraction (0.8 × 8765 hours). Multiplying 0.7 Sv Gy -1 by 7012 gives the multiplying factor 4908 Sv Gy -1 h in Eq. (8). The multiplying factor was derived based on the assumption that an individual spent 80 % of their indoors. The variations of the ACI , D in and AED in are presented in Fig. 4 Fig. 4 and Fig. 5 Fig. 5 . The calculated values range from 0.1 ± 0.01 to 2.1 ± 0.1, 22 ± 2 to 494 ± 12 nGy h −1 , and 0.1 ± 0.01 to 2.4 ± 0.6 mSv y -1 with geometric mean values of 0.6 ± 0.4, 144 ± 97 nGy h −1 , and 0.7 ± 0.5 mSv y -1 for ACI, D in and AED in , respectively. It would be observed that from Fig. (4) Fig. (4) , almost all calculated ACI are below the recommended maximum value for building materials used in bulk quantity except for some ceramic raw materials such as feldspar, kaolin, and clay which slightly exceeded the recommended maximum value of 1. However, these ceramic raw materials are in the region of limited used building materials and therefore based on their criteria, they are all below the exemption level of 2. Fig. (5a) Fig. (5a) shows the variation of D in across various building materials and most of the values are above the world average value of 84 nGy h -1 . Though, according to an UNSCEAR report [ 34 34 ] , Malaysia is among the countries with high D in from the building materials [ 34 34 ] . Therefore, it may not be surprising to obtain high values of D in in the current study. Additionally, the calculated values of AED in shown in Fig. (5b) Fig. (5b) indicated that almost all the main building materials are below the recommended maximum value of 1 mSv y -1  except for tile, feldspar, kaolin, pottery stone and clay which are slightly higher than 1 mSv y -1 . Therefore, the ceramic raw materials are the major contributors of the high level radiation dose observed in the tile. One of the latest studies entitled "Use of NORM-Containing Products in Construction" reported by Schroeyers et al. [ 55 55 ] focused on the building materials originated from industrial by-products from various countries around the world. Their results of ACI for coal fly ash is 0.8 while for ceramic range from 0.3 (Italy) to 1.9 (China) [ 55 55 ] . The average ACI for fly ash reported herein is 0.4 and ceramic range from 0.1 (Limestone) to 2.1 (Kaolin). Despite this, the highest value reported in the present study is higher than the value reported from China, but still about 90 % of our reported values are within the range stated from various countries in the study [ 55 55 ] . 3.4 Statistical Analysis of Activity Concentrations and Radiological Risk Variables 2 The frequency distribution histogram, stem-and-leaf plot, P-P plot, and Q-Q plot are employed for visually normality examination. In this study, we employed frequency distribution histograms in addition to numerical tests such as the Shapiro-Wilk test ( S-W test), Anderson-Darling test ( A-D test), and Lilliefors test or modified Kolmogorov-Smirnov test ( K-S test). The numerical tests of the activity concentrations and radiological risk parameters showed that the data were not significantly drawn from the normally distributed population as shown in Table 3 Table 3 . The frequency distribution histograms of the activity concentrations of 226 Ra, 232 Th, and 40 K are shown in Fig. 6a, 6b, and 6c Fig. 6a, 6b, and 6c , respectively. While, the frequency distribution histogram of ACI , D in , and AED in are shown in Fig. 6d, 6e, and 6f Fig. 6d, 6e, and 6f , respectively. The frequency distributions of the aforementioned parameters indicated that none of the parameters are normally distributed. Furthermore, for data to be normally distributed or drawn from the normal distribution skewness and kurtosis of the data have to be zero or very close to zero. When the skewness is positive this indicates the data is right-handed skewed, or if it is negative it indicates the data is left-handed skewed. Similarly, positive kurtosis indicates a heavy tail or outliers and negative kurtosis indicates light tail or low outliers. The activity concentrations and radiological risk parameters showed positive skewness and kurtosis as shown in Table 3 Table 3 Furthermore, the numerical tests including the S-W test, A-D test, and K-S test in addition to skewness and kurtosis indicate that the activity concentrations of 226 Ra, 232 Th, and 40 K and radiological risk parameters are asymmetrically distributed. The frequency distribution histograms were then compared with a lognormal curve and found that most of the parameters fit the lognormal distribution curve while some few parameters such as 40 K and indoor annual effective dose were a partially fitted lognormal curve. Some researchers such as Do Carmo Leal and Da Costa Lauria [ 56 56 ] , Ravisankar et al. [ 57 57 ] , and Ravisankar et al. [ 58 58 ] concluded that the activity concentration of 40 K is normally distributed in most of the earth materials including building materials. 2 1 Statistical Variables 1 1 226 Ra (Bq kg -1 ) 1 1 232 Th (Bq kg -1 ) 1 1 40 K (Bq kg -1 ) 1 1 ACI 1 1 D in (nGy h -1 ) 1 1 AED in (mSv y -1 ) 2 1 Geometric Mean 1 1 53 1 1 49 1 1 441 1 1 0.6 1 1 144 1 1 0.7 2 1 STD 1 1 48 1 1 37 1 1 325 1 1 0.4 1 1 97 1 1 0.5 2 1 Min 1 1 10 1 1 9 1 1 46 1 1 0.1 1 1 22 1 1 0.1 2 1 25th 1 1 32 1 1 37 1 1 304 1 1 0.4 1 1 99 1 1 0.5 2 1 Median 1 1 54 1 1 54 1 1 600 1 1 0.7 1 1 166 1 1 0.8 2 1 75th 1 1 90 1 1 68 1 1 690 1 1 0.8 1 1 207 1 1 1.0 2 1 Max 1 1 223 1 1 274 1 1 1589 1 1 2.1 1 1 494 1 1 2.4 2 1 Interquartile Range 1 1 58 1 1 31 1 1 387 1 1 0.4 1 1 108 1 1 0.5 2 1 Skewness 1 1 1.2 1 1 2.8 1 1 0.8 1 1 1.3 1 1 1.2 1 1 1.2 2 1 Kurtosis 1 1 1.0 1 1 12.3 1 1 0.9 1 1 2.4 1 1 2.1 1 1 2.1 3 1 Goodness-of-Fit Tests for Normality 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Shapiro-Wilk Test 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 1 1 Anderson-Darling Test 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 1 1 Lilliefors Test 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 1 1 < 0.01 3.5 Radiological Health Effect of Indoor Radon Concentration 2 The radon mass exhalation rate ( E m ) is defined as the release of radon activity per unit time from unit mass of the matrix if the sample thickness is ≤ 10 times the radon diffusion length in the medium. Therefore, E m is referred to as the radon production rate in the pore volume. In practical cases, measured E m from the samples is referred to as radon production rate in the pore volume. E m can be written as [ 27 27 ] : <math display="block" id="M11"><mrow><msub><mi>E</mi><mtext>m</mtext></msub><mo>=</mo><mi>P</mi><msub><mi>C</mi><mrow><mtext>Ra</mtext></mrow></msub><mi>λ</mi><mo>.</mo></mrow></math> The equilibrium equivalent concentration ( EEC ) of radon corresponding to a non-equilibrium mixture of short-lived radon daughters in air is defined as the radon activity concentration in radioactive equilibrium with its short-lived progeny. The short-lived progeny has the same potential alpha (α) energy concentration as the actual non-equilibrium mixture to which EEC refers [ 59 59 , 60 60 ] . The equilibrium factor ( F ) is the ratio of the entire potential α energy for the actual radon daughter concentrations to the entire potential α energy of the radon daughters which would be in equilibrium with radon concentration [ 60 60 ] . In most cases, the indoor F is assumed to be 0.4 when estimating dose parameters due to radon exposure [ 60 60 ] . The EEC is calculated by multiplying the calculated C Rn by F [ 27 27 ] . EEC can be written as: <math display="block" id="M12"><mrow><mi>E</mi><mi>E</mi><mi>C</mi><mo> </mo><mo>=</mo><mo> </mo><mi>F</mi><mo>×</mo><msub><mi>C</mi><mrow><mtext>Rn</mtext></mrow></msub><mo>.</mo></mrow></math> Generally, F is defined within the range from 0 to 1. Therefore, C Rn would be always higher than the concentration of its decay daughters [ 61 61 ] . Indoor annual effective dose ( E Rn ) is the total dose due to radon received by the dweller in a year and it was estimated following UNSCEAR [ 34 34 ] . Applying the following parameters: - indoor equilibrium factor as 0.4, the indoor occupancy time as 7008 hours, and dose conversion factor as 9 nSv (Bq h m -3 ) -1 , and can be written as: <math display="block" id="M13"><mrow><msub><mi>E</mi><mrow><mtext>Rn</mtext></mrow></msub><mo> </mo><mo>=</mo><mo> </mo><msub><mi>C</mi><mrow><mtext>Rn</mtext></mrow></msub><mo> </mo><mo>×</mo><mo> </mo><mn>2.523</mn><mo> </mo><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup><mo> </mo><msup><mrow><mtext>mSv Bq</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup><msup><mrow><mtext> m</mtext></mrow><mn>3</mn></msup><mo>.</mo></mrow></math> The radiological health effect of indoor radon concentration was assessed through E m , EEC , and E Rn , while the variation of E m , EEC , and E Rn are presented in Fig. (7) Fig. (7) , Fig. (8) Fig. (8) , Fig. (9) Fig. (9) , respectively. The aforementioned parameters show a non-uniform variation across various building materials and even within materials of the same kind. The results range from 33 to 674 mBq kg -1 h -1 , 0.9 to 22 Bq m -3 , and 0.1 to 1.4 mSv y -1 for E m , EEC , and E Rn , respectively. The fractional contribution of each of the building materials under study is shown in Table 4 Table 4 . Over 98 % of EEC and E Rn reported herein are below the world average value of 16 Bq m -3 and 1 mSv y -1 , respectively [ 34 34 , 62 62 ] . 1 1 Building Materials 1 1 No. of samples 1 1 E m (mBq h -1 kg -1 ) 1 1 EEC (Bq m -3 ) 1 1 E Rn (mSv y -1 ) 1 1 Tile 1 1 40 1 1 258 ± 120 1 1 4.4 ± 2.1 1 1 0.3 ± 0.1 1 1 Red Brick 1 1 5 1 1 98 ± 8 1 1 2.7 ± 0.2 1 1 0.2 ± 0.01 1 1 Cement Brick 1 1 5 1 1 138 ± 27 1 1 7.1 ± 1.4 1 1 0.4 ± 0.1 1 1 Sand 1 1 10 1 1 115 ± 61 1 1 4.9 ± 2.6 1 1 0.3 ± 0.2 1 1 Cement 1 1 10 1 1 85 ± 22 1 1 2.5 ± 0.6 1 1 0.2 ± 0.04 1 1 Gravel 1 1 10 1 1 147 ± 193 1 1 3.8 ± 5.0 1 1 0.2 ± 0.3 1 1 White Cement 1 1 5 1 1 100 ± 44 1 1 3.3 ± 1.4 1 1 0.2 ± 0.1 1 1 Fly Ash 1 1 5 1 1 107 ± 6 1 1 3.5 ± 0.2 1 1 0.2 ± 0.01 1 1 Feldspar 1 1 3 1 1 445 ± 185 1 1 14.5 ± 6.0 1 1 0.9 ± 0.4 1 1 Limestone 1 1 3 1 1 31 ± 2 1 1 1.0 ± 0.1 1 1 0.1 ± 0.004 1 1 Kaolin 1 1 1 1 1 424 ± 2 1 1 13.8 ± 0.1 1 1 0.9 ± 0.004 1 1 Pottery Stone 1 1 1 1 1 291 ± 5 1 1 9.5 ± 0.2 1 1 0.6 ± 0.01 1 1 Clay 1 1 2 1 1 302 ± 278 1 1 9.8 ± 9.1 1 1 0.6 ± 0.5 1 1 Glaze 1 1 1 1 1 127 ± 6 1 1 4.1 ± 0.2 1 1 0.3 ± 0.01 1 1 Talc 1 1 1 1 1 64 ± 8 1 1 2.1 ± 0.3 1 1 0.1 ± 0.02 1 1 Geometric Mean 1 1  102 1 1 161 ± 144 1 1 4.1 ± 3.6 1 1 0.3 ± 0.2 To assess the exposure caused by 222 Rn progeny, the EEC parameter plays a significant role in estimating the dose equivalent rate for organs and/or tissues such as lung and soft tissues. It can also be used to determine the effective dose equivalent when appropriate quality factors and dose coefficients are applied. However, E Rn is one of the reference indicators for the assessment of exposure caused by 222 Rn and the results reported herein were all below the representative value [ 27 27 , 34 34 ] . The compared annual effective dose due to inhalation of 222 Rn is shown in Table 5 Table 5 . The comparison is based on the recently published studies from some selected countries in Africa, Asia, Europe, and the Middle-East. The results showed that, most of the results can be compared with the results obtained from our study with the exception of results from Sudan, Turkey, and Romania which are relatively large compared with all other studies under comparison. Generally, the values in our results are only slightly higher than the values reported from Yemen and Serbia, otherwise our result is compared with 77% of the reviewed literatures from the selected countries. CR-39 and LR-115 Type II are alpha track detectors, CRM is the continuous radon monitor, IC is the ionization chamber, and PT-DM is the passive-type radon/thoron descriptive monitor (Rn Monitor). 2 1 Country 1 1 E Rn (mSv y -1 ) 1 1 Techniques 1 1 Source 2 1 Africa 1 1 1 1 Egypt 1 1 0.8 (0.6 to 0.9) 1 1 LR-115 type II 1 1 [ 49 49 ] 1 1 1 1 Ghana 1 1 1.2 (0.4 to 3.4) 1 1 LR-115 type II 1 1 [ 50 50 ] 1 1 1 1 Nigeria 1 1 1.9 (1.0 to 3.1) and 1.0 1 1 CR-39 1 1 [ 50 50 , 59 59 ] 1 1 1 1 Sudan 1 1 6.9 (3.6 to 14.2) 1 1 CR-39 1 1 [ 52 52 ] 2 1 Asia 1 1 1 1 China 1 1 0.8 1 1 CR-39 1 1 [ 64 64 ] 1 1 1 1 India 1 1 0.5 (0.3 to 0.7) 1 1 LR-115 type II 1 1 [ 53 53 ] 2 1 Japan 1 1 0.4 (0.4 to 0.5) and 0.2 1 1 PT-DM 1 1 [ 39 39 , 65 65 ] 1 1 1 1 Thailand 1 1 0.5 (0.3 to 1.2) 1 1 IC 1 1 [ 37 37 , 38 38 ] 2 1 Europe 1 1 1 1 Romania 1 1 2.4 1 1 LR-115 type II 1 1 [ 43 43 ] 1 1 1 1 Serbia 1 1 0.1 1 1 LR-115 type II 1 1 [ 66 66 ] 2 1 Middle East 1 1 1 1 Saudi Arabia 1 1 0.6 (0.4 to 0.9) 1 1 CR-39 1 1 [ 45 45 ] 1 1 1 1 Turkey 1 1 5.1 (0.4 to 8.4) 1 1 CR-39 1 1 [ 67 67 ] 1 1 1 1 Yemen 1 1 0.06 (0.03 to 0.13) 1 1 CR-39 1 1 [ 47 47 ] 2 1 Representative Value and Present Study 1 1 1 1 World Average Value 1 1 1.0 1 1   1 1 [ 34 34 ] 1 1 1 1 Malaysia 1 1 0.3 (0.1 to 1.4) 1 1   1 1 Present study In general, the recommended exemption value of the annual effective dose due to inhalation of 222 Rn is 3 mSv y -1 while the action level is 10 mSv y -1 [ 3 3 ] . However, the representative value is 1 mSv y -1 which is equivalent to the indoor 222 Rn concentration of 40 Bq m -3 [ 34 34 ] . About 60% of the studies under comparison are below the representative value, however only studies reported from Sudan and Turkey exceeded the exemption value which amount for 15% of the studies under comparison. Therefore, all studies including the value reported in our study are below the action level. 3.5.1 Dose Equivalent Rate for Soft Tissues (H ST ) and Lung (H L ) and Effective Dose Equivalent (H eff ) 3 Radon is an inert noble gas which is not chemically bound in body tissues. However, the activity concentration of 222 Rn in the environmental air ( EEC ) is proportional to the activity concentration of 222 Rn in a tissue, T , due to its restricted saturation solubility. Inhaled 222 Rn is continuously present in the air volume of the lungs at the concentration in the environmental air which is partly dissolved in soft tissues. Moreover, the solubility factor is taken to be 0.4 for soft tissues and the short-lived daughters were assumed to decay in the same tissue as the parent 222 Rn gas. A quality factor of 20 for α radiation is applied for both soft tissues and lung, respectively [ 27 27 , 59 59 ] . For equilibrium conditions, the dose equivalent rate for risk relevant soft tissues is given as [ 59 59 ] : <math display="block" id="M14"><mrow><msub><mi>H</mi><mrow><mtext>ST</mtext></mrow></msub><mtext> </mtext><mrow><mo>(</mo><mrow><msup><mrow><mtext>nSv h</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mtext> </mtext><mn>0.09</mn><mtext> </mtext><mo>×</mo><mtext> </mtext><mi>E</mi><mi>E</mi><mi>C</mi><mtext> </mtext><mrow><mo>(</mo><mrow><msup><mrow><mtext>Bq m</mtext></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></mrow><mo>)</mo></mrow><mo>.</mo></mrow></math> In the case of the lungs, in addition to the dissolved 222 Rn, the air containing 222 Rn in the lung must be taken into account. The air volume in the lung is assumed to be 3.2 × 10 -2  m 3 for the reference man and the short-lived decay products was assumed to stay in the lungs [ 27 27 , 59 59 ] . The dose equivalent rate for the lung is [ 59 59 ] : <math display="block" id="M15"><mrow><msub><mi>H</mi><mtext>L</mtext></msub><mtext> </mtext><mrow><mo>(</mo><mrow><msup><mrow><mtext>nSv h</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mtext> </mtext><mn>0.8</mn><mtext> </mtext><mo>×</mo><mtext> </mtext><mi>E</mi><mi>E</mi><mi>C</mi><mtext> </mtext><mrow><mo>(</mo><mrow><msup><mrow><mtext>Bq m</mtext></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></mrow><mo>)</mo></mrow><mo>.</mo></mrow></math> Furthermore, applying the tissue weighing factor of 0.12 for lung and 0.88 for other risk-relevant tissues, an effective dose equivalent rate is given as [ 27 27 , 59 59 ] : <math display="block" id="M16"><mrow><msub><mi>H</mi><mrow><mtext>eff</mtext></mrow></msub><mtext> </mtext><mrow><mo>(</mo><mrow><msup><mrow><mtext>nSv h</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow><mo>)</mo></mrow><mtext> </mtext><mo>=</mo><mtext> </mtext><mn>0.18</mn><mtext> </mtext><mi>E</mi><mi>E</mi><mi>C</mi><mtext> </mtext><mrow><mo>(</mo><mrow><msup><mrow><mtext>Bq m</mtext></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></mrow><mo>)</mo></mrow><mo>.</mo></mrow></math> The detailed derivation of Eq. (12) to (13) are presented in the ICRP publication 32 [ 59 59 ] and UNSCEAR [ 27 27 ] . It is convenient to assess the doses from the inhalation of 222 Rn gas, since the gas is soluble in the body fluids and tissues, and it can be easily transported all over the body. With this, doses would be delivered from decay of the gas itself and its short-lived decay daughters [ 62 62 ] . Therefore, the fractional contribution of dose equivalent rates for soft tissues ( H ST ) and lung ( H L ) and effective dose equivalent ( H eff ) were scrutinized. The variations of the calculated doses are presented in Fig. (10) Fig. (10) , Fig. (11) Fig. (11) , Fig. (12) Fig. (12) , with values ranging from 0.1 ± 0.01 to 2.2 ± 0.1, 0.7 ± 0.03 to 17.6 ± 0.4, and 0.2 ± 0.01 to 4.0 ± 0.1 nSv h -1 for H ST , H L , and H eff , respectively. On the other hand, the contributions of each of the building materials are presented in Table 6 Table 6 , and it can be seen that the variations of the doses are not uniform throughout the building materials. In other words, each building material has a unique fractional contribution to the doses received by an individual. The H L reported herein from all building materials were all greater than the representative value of 0.8 nSv h -1 while over 85% of the H ST reported herein were below the representative value of 1.2 nSv h -1 [ 62 62 ] . 1 1 Building materials 1 1 No. of samples 1 1 H st (nSv h -1 ) 1 1 H L (nSv h -1 ) 1 1 H eff (nSv h -1 ) 1 1 Tile 1 1 40 1 1 0.4 1 1 3.5 1 1 0.8 1 1 Red brick 1 1 5 1 1 0.3 1 1 2.2 1 1 0.5 1 1 Cement brick 1 1 5 1 1 0.7 1 1 5.7 1 1 1.3 1 1 Sand 1 1 10 1 1 0.5 1 1 3.9 1 1 0.9 1 1 Cement 1 1 10 1 1 0.2 1 1 2.0 1 1 0.4 1 1 Gravel 1 1 10 1 1 0.4 1 1 3.0 1 1 0.7 1 1 White cement 1 1 5 1 1 0.3 1 1 2.6 1 1 0.6 1 1 Fly ash 1 1 5 1 1 0.3 1 1 2.8 1 1 0.6 1 1 Feldspar 1 1 3 1 1 1.4 1 1 11.6 1 1 2.6 1 1 Limestone 1 1 3 1 1 0.1 1 1 0.8 1 1 0.2 1 1 Kaolin 1 1 1 1 1 1.4 1 1 11.1 1 1 2.5 1 1 Pottery stone 1 1 1 1 1 0.9 1 1 7.6 1 1 1.7 1 1 clay 1 1 2 1 1 1.0 1 1 7.9 1 1 1.8 1 1 Glaze 1 1 1 1 1 0.4 1 1 3.3 1 1 0.7 1 1 Talc 1 1 1 1 1 0.2 1 1 1.7 1 1 0.4 1 1 Geometric mean 1 1 102 1 1 0.4±0.4 1 1 3.3±2.9 1 1 0.7±0.7 4 Conclusion 1 In this study, the activity concentrations of 226 Ra, 232 Th, and 40 K in various building materials were measured using gamma-ray spectroscopy coupled with an HPGe detector. The activity concentrations were further evaluated for probable radiological health hazards by assessing ACI , D in and AED in with results ranging from 0.1 ± 0.01 to 2.1 ± 0.1, 22 ± 2 to 494 ± 12 nGy h −1 , and 0.1 ± 0.01 to 2.4 ± 0.6 mSv y -1 , respectively. The calculated P and 222 Rn values range from 10.0 ± 0.4 to 30.0 ± 0.4 % and 2.2 ± 0.1 to 54.9 ± 1.3 Bq m -3 , respectively. On the other hand, E m , EEC , and E Rn were computed and the results range from 33 to 674 mBq h -1 kg -1 , 0.9 to 22 Bq m -3 and 0.1 to 1.4 mSv y -1 , respectively. The internal doses caused by inhalation of radon and its progeny were also estimated and the results range from 0.1 ± 0.01 to 2.2 ± 0.1, 0.7 ± 0.03 to 17.6 ± 0.4, and 0.2 ± 0.01 to 4.0 ± 0.1 nSv h -1 for H ST , H L , and H eff , respectively. The reported results for external dose ( AED in ) and internal dose ( E Rn ) were below the recommended maximum limit of 1 and 10 mSv y -1 , respectively. The results reported herein are in agreement with other previous studies including representative values reported by international organizations dealing with radiation protection. Therefore, the radiological health risks attributed to the building materials under study are negligible. However, different individuals may have a different tolerance to the radiation exposure and there is no specific level below which the indoor radon level may be considered safe. References EC , Radiation Protection 112: Radiological Protection Principles concerning the Natural Radioactivity of Building Materials, Finland, 1999 . https://ec.europa.eu/energy/sites/ener/files/documents/112.pdf https://ec.europa.eu/energy/sites/ener/files/documents/112.pdf . 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Internal exposure from building materials exhaling 222Rn and 220Rn as compared to external exposure due to their natural radioactivity content M. Kıldır , İ. Gökmen , A. Gökmen , Indoor radon concentrations and radon doses at three districts of Ankara, Turkey and raising public awareness on the issue , J. Radioanal. Nucl. Chem . 307 , 777 - 786 ( 2016 ). doi: 10.1007/s10967-015-4489-3 http://doi.org/10.1007/s10967-015-4489-3 . Indoor radon concentrations and radon doses at three districts of Ankara, Turkey and raising public awareness on the issue A part of this research was supported by Universiti Kebangsaan Malaysia and Lynas Advanced Material Plant under grant number GGPM-2017-084 and ST-2017-012, respectively. The authors would like to acknowledge all lab technicians of the Nuclear Science Program, UKM for their technical support throughout the works. Shittu Abdullahi also wishes to appreciate and acknowledges Gombe State University, Gombe, Nigeria for providing the Ph.D fellowship. 10.1007/s41365-019-0569-3 2018-07-11 2018-08-20 2018-08-21 2019-02-13 2019-03-01 2021-04-10 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2019 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-019-0569-3 http://dx.doi.org/10.1007/s41365-019-0569-3 . 2019 Nuclear Science and Techniques 03 30