2.1 Sample Preparation

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 ^oC 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].

2.2 Instrumentation

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]. 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

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 ²²Na, ⁵⁷Co, ⁶⁰Co, and ¹³⁷Cs. 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 ²²Na, ⁵⁷Co, ⁶⁰Co, and ¹³⁷Cs, 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 ²²⁶Ra, ²³²Th, and ⁴⁰K 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 ²²⁶Ra and ²³²Th are 1764.5 keV (²¹⁴Bi) and 2614.5 keV (²⁰⁸Tl), respectively, while singly occurring ⁴⁰K was identified directly from its gamma-ray energy peak of 1461 keV. The activity concentration of individual radionuclides was calculated using Eq. (5)^[5,21]. The minimum detectable activity (MDA) was calculated based on Curie’s derivation^[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.

3.1 Determination of Radon Emanation Fraction (P)

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 ²¹⁴Pb and/or ²¹⁴Bi were counted in an interval of two days before and after achieving radioactive equilibrium between the ²²⁶Ra 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. When the ²²²Rn and its progeny are in secular equilibrium, then the activities of ²²²Rn and its progeny are equal^[23]. Moreover, the activity concentration of ²²⁶Ra 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 ²²⁶Ra nuclide. Since only relative counts are needed, no calibration is required for this method^[24].

Fig. 1.Full-screen View

An example of the growth of the radon daughters in a closed container for a grinded red brick sample. The solid lines indicate the fitting curves.

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]:

It follows that,

Eq. (4) was further deduced as:

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 N₀ 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 N₀ to Eq. (3). Eq. (3) was used to determine the grow of ²²²Rn decay daughters in a closed counting container. An example of measured data and fitting curves are shown in Fig. (1). It could be observed that the fitting was successful. Hence, the radon emanation fraction P (%) was calculated using Eq. (4)^[24,25]:

The radon emanation fraction from six different building materials was measured and presented in 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].

Fig. 2.Full-screen View

Radon emanation fraction of various building materials

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]. Sahoo et al.^[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]. UNSCEAR^[27] provided a representative value of 20 % for soil with a range of 1 to 80 %. Martino^[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]. Bossew^[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

The individual activity concentration (C_i) is calculated using Eq. (5) ^[5,21]:

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 ²²²Rn and the majority of the lung cancers are caused by low and moderate ²²²Rn exposure^[11]. All building materials contain some natural radionuclides such as ²³⁸U and ²³²Th and their decay series and singly occurring ⁴⁰K^[1,11]. The concentrations of natural radioactivity in building materials vary significantly. Hence, this causes an excess indoor ²²²Rn and it is calculated using Eq. (7)^[2,30,31]:

where U is the rate of radon entry into the room from building materials (Bq h^-1) and its given as^[2,30]

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 (m³), 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 (m²), 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 m³ and 20 m², 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 with values ranging from 10 ± 1 to 223 ± 5, 9 ± 1 to 274 ± 8 and 46 ± 7 to 1589 ± 21 Bq kg^-1 for ²²⁶Ra, ²³²Th, and ⁴⁰K, 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 (²²²Rn) from various building materials is presented in 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 ²²²Rn of each of the building materials is also presented in Table 1. It can be observed that the activity concentrations and ²²²Rn vary significantly among the building materials.

Fig. 3.Full-screen View

Indoor Radon Concentration and World Average Value

The highest activity concentrations of ²²⁶Ra and ²³²Th were obtained in the tile compared with other building materials under study and can be associated to ²²⁶Ra and ²³²Th contents of some of the tile raw materials such as feldspar, kaolin, and clay as shown in Table 1. However, ⁴⁰K 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]. Therefore, it may not be surprising to find an elevated activity concentration of ⁴⁰K 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], the ²²⁶Ra content of the building materials establishes the source of the radon emanation. High indoor ²²²Rn may be found in buildings constructed using building materials with high ²²⁶Ra 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 ²²²Rn levels in a room^[33]. An example of the role of radon emanation fraction on ²²²Rn level is shown in Table 1. It can be seen that tile has the highest ²²⁶Ra content compared with sand and gravel, but the emanation fraction of tile is low. Hence, the ²²²Rn level of sand and gravel is greater than that of tile.

UNSCEAR^[34] reported the activity concentrations of the Malaysia’s soils with mean values of 67, 82 and 310 Bq kg^-1 for ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively^[34]. Jang et al.^[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 ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively^[35]. Ge and Zhang^[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 ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively. These findings are in agreement with the results reported in our study.

Herein we reported the study and the comparison of the indoor ²²²Rn concentration from some selected countries across Africa, Asia, Europe, and the Middle-East. And the results, as shown in Table 2, showed that, in Africa, the reported indoor ²²²Rn 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^[1,3,11]. However, the average value is less than the action level of WHO and ICRP^[3,11]. 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[11,34]. 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^[37-39]. In the European site, though the reported indoor ²²²Rn 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^[40-43]. The indoor ²²²Rn reported in our study is about 3 and 2 times less than the values reported from Saudi Arabia and Kuwait, respectively^[44,45]. While the values from Turkey^[46] are about eight times greater than our values, our values are greater than those reported from Yemen by 4 times^[47].

Generally, if the world average value (40 Bq m^-3) is taken as a reference value in the comparison, the indoor ²²²Rn 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]. Other factors that may lead to the variation include: ventilation and radon emanation fraction. Therefore, the indoor ²²²Rn concentration reported in our study is in accordance with published results from various parts of the world.

3.3 Radiological Risks Analysis

The main contributions of the radiation dose received by the general public is originated from ²²⁶Ra, ²³²Th and ⁴⁰K release mainly from building materials. Many countries have established guidelines to restrict the amount of natural radionuclides in building materials ^[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 ²²⁶Ra, ²³²Th, and ⁴⁰K in the building materials ^[1],[13]. ²²⁶Ra content criterion should be added to the building materials with high emanation fraction ^[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]. 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],[13]. The ACI and AED_in expressions are given in Eq. (9) and (11), respectively ^{[1],[30],[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 ²²⁶Ra, ²³²Th, and 40K as initiated by EC and given in Eq. (10) ^[1].

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 and 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), 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) 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], Malaysia is among the countries with high D_in from the building materials^[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) 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] 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]. 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].

Fig. 4Full-screen View

Activity concentration index

Fig. 5Full-screen View

Activity concentration index, indoor air absorbed dose rate and annual effective dose

3.4 Statistical Analysis of Activity Concentrations and Radiological Risk Variables

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. The frequency distribution histograms of the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K are shown in 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, 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 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 ²²⁶Ra, ²³²Th, and ⁴⁰K 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 ⁴⁰K and indoor annual effective dose were a partially fitted lognormal curve. Some researchers such as Do Carmo Leal and Da Costa Lauria ^[56], Ravisankar et al. ^[57], and Ravisankar et al. ^[58] concluded that the activity concentration of ⁴⁰K is normally distributed in most of the earth materials including building materials.

Fig. 6Full-screen View

Frequency distribution histogram of activity concentrations and radiological risk parameters

3.5 Radiological Health Effect of Indoor Radon Concentration

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]:

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^[59,60]. 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]. In most cases, the indoor F is assumed to be 0.4 when estimating dose parameters due to radon exposure^[60]. The EEC is calculated by multiplying the calculated C_Rn by F^[27]. EEC can be written as:

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].

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]. 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:

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. (8), 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. 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^[34,62].

Fig. 7.Full-screen View

Variation of mass exhalation rate

Fig. 8.Full-screen View

Equilibrium equivalent concentration and world average value

Fig. 9.Full-screen View

Annual effective dose due to radon and its world average value

To assess the exposure caused by ²²²Rn 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 ²²²Rn and the results reported herein were all below the representative value^[27,34].

The compared annual effective dose due to inhalation of ²²²Rn is shown in 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.

In general, the recommended exemption value of the annual effective dose due to inhalation of ²²²Rn is 3 mSv y^-1 while the action level is 10 mSv y^-1[3]. However, the representative value is 1 mSv y^-1 which is equivalent to the indoor ²²²Rn concentration of 40 Bq m^-3[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)

Radon is an inert noble gas which is not chemically bound in body tissues. However, the activity concentration of ²²²Rn in the environmental air (EEC) is proportional to the activity concentration of ²²²Rn in a tissue, T, due to its restricted saturation solubility. Inhaled ²²²Rn 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 ²²²Rn gas. A quality factor of 20 for α radiation is applied for both soft tissues and lung, respectively^[27,59]. For equilibrium conditions, the dose equivalent rate for risk relevant soft tissues is given as^[59]:

$H_{\text{ST}}\left({\text{nSv h}}^{-1}\right)=0.09\times EEC\left({\text{Bq m}}^{-3}\right).$ (12)

In the case of the lungs, in addition to the dissolved ²²²Rn, the air containing ²²²Rn in the lung must be taken into account. The air volume in the lung is assumed to be 3.2 × 10^-2 m³ for the reference man and the short-lived decay products was assumed to stay in the lungs^[27,59]. The dose equivalent rate for the lung is^[59]:

$H_{\text{L}}\left({\text{nSv h}}^{-1}\right)=0.8\times EEC\left({\text{Bq m}}^{-3}\right).$ (13)

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^[27,59]:

$H_{\text{eff}}\left({\text{nSv h}}^{-1}\right)=0.18EEC\left({\text{Bq m}}^{-3}\right).$ (14)

The detailed derivation of Eq. (12) to (13) are presented in the ICRP publication 32^[59] and UNSCEAR^[27].

It is convenient to assess the doses from the inhalation of ²²²Rn 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]. 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. (11), 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, 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].

Table 6Full-screen View

Geometric mean of dose equivalent rates for soft tissues and lung and effective dose equivalent

Building materials	No. of samples	Hst (nSv h-1)	HL (nSv h-1)	Heff (nSv h-1)
Tile	40	0.4	3.5	0.8
Red brick	5	0.3	2.2	0.5
Cement brick	5	0.7	5.7	1.3
Sand	10	0.5	3.9	0.9
Cement	10	0.2	2.0	0.4
Gravel	10	0.4	3.0	0.7
White cement	5	0.3	2.6	0.6
Fly ash	5	0.3	2.8	0.6
Feldspar	3	1.4	11.6	2.6
Limestone	3	0.1	0.8	0.2
Kaolin	1	1.4	11.1	2.5
Pottery stone	1	0.9	7.6	1.7
clay	2	1.0	7.9	1.8
Glaze	1	0.4	3.3	0.7
Talc	1	0.2	1.7	0.4
Geometric mean	102	0.4±0.4	3.3±2.9	0.7±0.7

Show more

Fig. 10Full-screen View

Variation of dose equivalent rate for soft tissues

Fig. 11Full-screen View

Variation of dose equivalent rate for lung

Fig. 12Full-screen View

Variation of effective dose equivalent