Natural radioactivity measurements and dose rate assessment of selected ceramic and cement types used in Riyadh, Saudi Arabia

1 Introduction

Human beings are constantly subjected to ionizing radiation from radionuclides in minerals and raw materials of natural origin. Naturally occurring radioactive materials (NORM) are the radionuclides found in the uranium and thorium decay series, and radioactive potassium presents a radiological risk to humans. NORM is the result of specific human activities that enhance human exposure to the radionuclides that emerge from the Earth’s crust; therefore, they can be found in water, air, food, building materials, and the human body (IAEA, 2015; NORM V, 2007; Raghu et al., 2017).

Individuals spend an average of 80% of their time indoors (UNSCEAR 1993; Stoulos et al., 2003). Therefore, it is essential to estimate how much radiation exposure from building materials they may experience. Building materials are normally extracted from rocks, sand, and soil and contain varying levels of radionuclides depending on the raw materials from which they are derived (Rahman et al., 2012; Lu et al., 2014). The radiation hazard is due to these radioactive isotopes via external and internal exposure. External exposure is related to direct gamma radiations emitted from isotopes in the above-mentioned series, as well as from the main ⁴⁰K gamma line. Internal exposure is caused by the inhalation of the radioactive inert gases radon ²²²Rn, thoron ²²⁰Rn, and their short-lived progeny radioisotopes (Ngachin et al., 2007; Al-Sulaiti et al., 2011; Khandaker et al., 2012).

Health hazards related to exposure to radiation from building materials used indoors have been studied by numerous researchers worldwide (Amana, 2017; Amin and Naji, 2013; Majid et al., 2013; Baz, 2015; Damla et al., 2010; El-Taher A., 2012; Hassan et al., 2015; Gbenu et al., 2016; Lu et al., 2014; Ravisankar et al., 2012; Righi and Bruzzi, 2006). In addition, international organizations such as the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), European Commission, and International Commission on Radiological Protection have produced several publications regarding limits on the constituent concentrations in some building materials and have issued restrictions on materials with excessive levels of radioactivity (EC, 1999; ICRP, 1999; ICRP, 2015; UNSCEAR, 2000). Over the past decades, the majority of published data on radiation health risks due to building materials did not present significant findings. However, the accumulation of radioactivity from building materials over time combined with poor ventilation may increase the probability of occurrence of cancer in the building occupants.

In recent decades, cities in Saudi Arabia have grown significantly, with the population of the capital city of Riyadh exceeding five million people. This growth was accompanied by a rising demand in the residential sector, and it is estimated that 150,000 to 200,000 housing units are being built annually. Presently, the most common material tiles used in Saudi Arabian dwellings are ceramic, with various new brand names entering the market, both local and imported. In addition, cement is still an essential material in the construction of houses and buildings all over the country. The cement sector in Saudi Arabia is the third largest in the Middle East and North Africa, with a cement capacity of over 70 million tons and a local demand of approximately 47 million tons in 2017 (Balakrishnan and Al-Moammar, 2018). In the case of ceramics, Saudi Arabia’s 2016 vol of imported ceramics was estimated to be approximately 167 million m³, which is approximately two thirds of the national consumption (ACIMAC, 2017).

The current study aimed to investigate ceramic tiles and cement for levels of natural radioactivity. The scope of this study is to determine the specific activity concentration of ²²⁶Ra, ²³²Th, and ⁴⁰K in different ceramic and cement samples commonly used in Riyadh. The measured values will be used to calculate the average radium equivalent activity (Ra_eq), external hazard index (H_ex), absorbed dose rate (D), and annual effective dose (AED) hazard indices, which will be compared with those published by other researchers and the UNSCEAR safety limits. The objectives of this work were to compare current data to previously published data on cement samples in Saudi Arabia and other parts of the world and to set a reference background for future extensive measurements of cement and ceramic bricks used as building materials in Saudi Arabia.

2 Materials and methods

3 Results and discussion

The natural radioactivity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K found in the 20 ceramic samples are listed in Table 1. The specific activity concentrations for ²²⁶Ra, ²³²Th, and ⁴⁰K were found to vary from 45.0 ± 4.2 (S1) to 177.8 ± 7.5 Bq/kg (E2) with a mean ± standard deviation of 89.0 ± 40.6 Bq/kg for ²²⁶Ra, 49.1 ± 2.6 (I1) to 228.4 ± 6.8 Bq/kg (I2) with a mean ± standard deviation of 105.5 ± 42.2 Bq/kg for ²³²Th, and 370.0 ± 5.3 (E1) to 1269.0 ± 12.2 Bq/kg (P1) with a mean ± standard deviation of 772.9 ± 304.3 Bq/kg for ⁴⁰K. The above specific activity concentrations were higher than the global average values of ²²⁶Ra, ²³²Th, and ⁴⁰K in the Earth’s crust, which are estimated to be 50, 50, and 500 Bq/kg, respectively (UNSCEAR, 1993). The results for the above radionuclides can be more clearly observed when plotted as a frequency distribution, as shown in Fig. 1. The range of 45–100 Bq/kg includes 70% and 50% of the samples for ²²⁶Ra and ²³²Th, respectively, while the range of 370–635 Bq/kg includes 80% of the samples for ⁴⁰K.

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Fig. 1

Frequency distribution of the activity concentrations of ²²⁶Ra,²³²Th, and ⁴⁰K (Bq/kg) for the ceramic samples.

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Fig. 2 presents the concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in the four cement samples. The specific activity concentrations for ²²⁶Ra, ²³²Th, and ⁴⁰K were found to vary from 11.4 ± 2.0 (S-W) to 28.7 ± 5.3 Bq/kg (S-B) with a mean ± standard deviation of 21.7 ± 7.4.6 Bq/kg for ²²⁶Ra, 8.4 ± 1.3 (B-B) to 10.8 ± 1.1 Bq/kg (Q-B) with a mean ± standard deviation of 9.8 ± 1.0 Bq/kg for ²³²Th, and 50.7 ± 2.1 (B-B) to 209.7 ± 3.5 Bq/kg (Q-B) with a mean ± standard deviation of 101.6 ± 73.8 Bq/kg for ⁴⁰K. It is apparent that the specific activity concentrations of the three measured radionuclides in the cement samples are all below the global averages published by UNSCEAR (1993).

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Fig. 2

Concentrations of ²²⁶Ra,²³²Th, and ⁴⁰K (Bq/kg) for the cement samples.

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Radiation hazard indices were calculated for both the ceramic and cement samples. Table 2 presents the Ra_eq values for the studied ceramic samples. The Ra_eq results ranged from 157.2 (I1) to 530 Bq/kg (I2), with a mean value of 299.4 Bq/kg. This is lower than the recommended world value for their safe use in building materials: 370 Bq/kg (El-Taher et al., 2010). Five of the ceramic samples, however, exceeded this limit: C2: 392.4 Bq/kg, O1: 397.9 Bq/kg, E2: 413.2 Bq/kg, G1:430.5 Bq/kg, and I2: 530.2 Bq/kg. The Ra_eq values for the cement samples ranged from 25.8 (S-W) to 43.2 Bq/kg (S-B) with a mean value of 36.8. Bq/kg. The Ra_eq results of all the cement samples were well below the global safe value.

As shown in Table 2, the samples with higher Ra_eq values also had high H_ex values that exceeded the H_ex ≤ 1 limit. The H_ex minimum, maximum, and mean values were 0.4, 1.4, and 0.8, respectively. Table 3 shows the H_ex values for the cement samples with their minimum, maximum, and mean values of 0.09, 0.14, and 0.12, respectively.

Table 2 also shows the D values for the ceramic samples, which were found to vary from 73.3 to 239.4 nGy/h, with a mean value of 138.8 nGy/h. The obtained values of D were all greater than the recommended value of 55 nGy/h (UNSCEAR, 1993). However, Table 3 also shows that all the cement samples have safe D values, with the minimum, maximum, and mean values of 15.15, 25.47, and 20.35 nGy/h, respectively.

The AED was calculated using Eq. (4), and the results for the ceramic samples are presented in Table 2. The admissible AED value of 1 mSv/y (UNSCEAR, 2000) was satisfied by all the ceramic samples, except sample I2, which also exhibited the highest Ra_eq and H_ex index values. The AED range and mean for the ceramic samples were 0.36–1.17 and 0.68 mSv/y, respectively. Table 3 also shows the AED values for the cement samples; it is apparent that all the indicated values are significantly lower than the safe world limit value. The AED range and mean values for the cement samples were 0.07–1.12 and 0.10 mSv/y, respectively.

Comparisons of the data reported in this study with recently published studies in different countries are presented in Tables 4 and 5 for the ceramic and cement samples, respectively. Table 4 demonstrates that although the mean concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in this study were greater than those reported previously, they are still comparable with some recently reported values (Michael et al., 2010; Amin and Naji, 2013; Gbenu et al., 2016). Table 5 shows that the specific activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in the studied cement samples compare well with other recent studies conducted in Saudi Arabia and other countries.

Table 4 presents the Ra_eq values of the ceramic samples, wherein the results of approximately half of the published studies compare well with the present study. Table 5 presents the corresponding Ra_eq values of the cement samples wherein the values obtained in this study are the lowest, although they are close to the most recent study conducted by Al Mugren and El-Taher (2016) in Saudi Arabia.

4 Conclusions

A typical high-resolution gamma-ray spectrometry system was used for the activity measurements of ²²⁶Ra, ²³²Th, and ⁴⁰K in ceramic and cement samples collected from local markets in Riyadh, Saudi Arabia. The radiation hazard indices for the cement samples were all found to be well below the recommended world limits and confirm the works recently published by other researchers in this country. Therefore, they can be considered to be safe for residential use.

The average specific activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K of the twenty ceramic samples were found to be 89.0 ± 40.6, 105.5 ± 44.2, and 772.9 ± 304.3 Bq/kg. In the case of the four cement samples, these values were 21.7 ± 7.4, 9.8 ± 1.0, and 101.6 ± 73.8 Bq/kg, respectively. The Ra_eq, H_ex, D, and AED indices were calculated from the above measurements in order to assess the radiological hazard associated with the studied building materials. The average values of these radiological indices for the ceramic samples were 299.4 ± 94.8 Bq/kg, 0.8 ± 0.3, 138 ± 42.4, and 0.68 ± 0.21, respectively. The corresponding average values of the cement samples were 36.8 ± 7.74 Bq/kg, 0.12 ± 0.02, 20.35 ± 4.39, and 0.10 ± 0.02, respectively.

Five ceramic samples (C2, O1, E2, G1, and I2) slightly exceeded the Ra_eq and H_ex standard global limits by factors of (6%, 10%), (8%, 10%), (12%, 10%), (16%, 20%), and (43%, 40%), respectively.

The obtained values of D for the ceramic samples were all greater than the recommended value of 55 nGy/h, with their mean value exceeding the recommended value by a factor of 152%. The AED of 1 mSv/y was met for all the ceramic samples except for sample I2, which had an AED value of 1.17.

Based on the above results, the majority of the ceramic samples can be considered to be marginally safe for use as building materials for residential construction, while five of the samples require further assessment before usage to minimize long-term effects due to chronic radiation exposure.

This work on cement and ceramic tiles could be used as reference data for future studies. More extensive measurements are required to be conducted with the aim of minimizing public exposure as low as reasonably achievable.