Evolution of radionuclide concentration and the radiological hazards in building material: a review

Abstract

Residents are continuously exposed to ionizing radiation emitted from ²³⁸U, ²³²Th and ⁴⁰K maintained in building materials, which may cause health risks. Hence, these building materials need to routinely monitor. This study reports a building material that poses a radiation risk. A sample of 123 research papers evaluating concentrations of ²³⁸U, ²³²Th and ⁴⁰K in 7 types of building material from 38 countries was reviewed. This study revealed that granite had the highest concentrations of ²³⁸U, ²³²Th and ⁴⁰K, whereas the lowest concentration of ²³⁸U and ²³²Th were in marble, while ⁴⁰K was in gypsum. It was also observed that the granite had higher levels in seven hazard indices: radium equivalent index, gamma index, alpha index, external and internal hazard indexes, absorbed dose rate index and annual effective dose equivalent index than the recommended limits for each index, while the other materials were lower as hierarchal cluster analysis revealed as well.

GRAPHICAL ABSTRACT

KEYWORDS:

• Building materials
• radionuclide
• radiological hazard indices
• hierarchal cluster analysis

1. Introduction

Natural radioactivity in the environmental material stands behind the continuous exposure to the ionizing radiation emitted from naturally occurring radioactive materials (NORM) present on Earth that can have a potential for malignant neoplastic disease (cancer). Fundamentally, NORM contains unstable radionuclides that decay and emit ionizing radiation in alpha and beta particles, and gamma radiation. NORM consists mainly of the primordial natural radionuclides uranium (²³⁸U), thorium (²³²Th) and their decayed progeny (²²⁶Ra, ²²²Rn, ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, ²¹⁰Pb, etc.), and potassium (⁴⁰K), which exist in the earth's crust and the building materials even in our bodies [1–3] as well as anthropogenic radionuclides such as ⁹⁰Sr, ¹³⁷Cs, ²³⁹Pu, etc. produced by human activities. Their presence in the building material is due to the fact that some of these radioactive materials are incorporated into it during its formation or during the construction process.

Building materials are produced from a mixture of different geological raw materials, mainly soil and rock, and they are either naturally formed such as granite, marble, gypsum, etc., or manufactured such as bricks, cement, ceramic tiles, etc. Granite contains quartz, feldspar, mica and other minerals [4], marble as metamorphic rock consists of calcite or dolomite crystals, organic matters (such as coal), and other minerals [5], gypsum originates naturally from calcium sulphate and water (CaSO₄. 2H₂O) [6], bricks are mainly fabricated from clay, cement, or sand in different countries [7], cement consists of clay, iron ore, mudstone and silica [8], and ceramics is composed of clays, quartz materials and feldspar, in addition to Zircon in the glazed tiles [5]. Worth noting, these minerals are generally made up of different elements and present naturally as crystalline inorganic substances in sediments which are produced through weathering and erosion of rocks [9]. As building materials contain a variety of these minerals, which, in turn, have an effect on the material's properties such as improved durability, increased strength, flexibility, resistance to weathering and others. In addition, for industrial purposes, some additive radioactive materials are used in certain amounts in building materials as technically enhanced naturally occurring radioactive material (TENORM) [10]. These additives can be also industrial by-products enriched in some building materials (e.g. Coal fly ash, phosphogypsum, certain slags, zirconium and monazite material (glazed), marbles, granite and ceramics). Such additives of minerals when used in constructing the dwelling of inhabitants could maintain high concentrations of natural radionuclides and elevate the internal exposure risk due to alpha particles emitted from radon and its decay products as well as an external exposure risk due to the γ-rays emitted from NORM [1,10], which, in turn, results in serious hazards to the dwellers especially they spend major time (80%) in their dwellings [1]. Additionally, UNSCEAR (2000) reported that the ratio of indoor to outdoor radiation dose varied from 0.8 to 2.0 depending on the building material type.

Overall, the activity of primordial and anthropogenic radionuclides in building material types with their imparted or added mineral composition can affect the received dose to the public who stay in their houses and the material's properties. Thus, the selection of building material type is important in terms of the health and safety of occupants. That is, while radiological evaluation of radionuclides activity in building materials is useful to ensure that they do not pose a risk to human health, the mineral composition analysis assessment is useful to determine the quality of building material needed for the desired application that ensures human safety.

The evaluation of the γ-rays radiation dosage from natural sources is significant, as it is the greatest contributor to the world population's external exposure [11]. Consequently, in the past few decades, many researchers have highlighted the attention of concern of nations about the importance of doing studies in assessing the radiation hazards associated with the use of construction materials [12,13]. In fact, the study of natural radioactivity in materials makes it possible to improve and develop an understanding of the radiological implications of these content distributions [14]. These studies can be a reference and a guide for regulators to form regulating actions on radiation safety. For example, in manufacturing construction materials the uranium, thorium and potassium contents should be regulated [15].

The geographical and geological conditions of the area can be the main factor that affects the level of natural radioactivity concentrations so the soil and building materials have different levels of natural radioactivity according to their locations and the surrounding environmental conditions [1], the additive raw material(s) composition as well. In the same context, the majority of studies of radioactivity in building materials show that the values of radiological hazard indices, such as the radium equivalent content, external and internal hazard indices, γ-rays absorbed dose rate, the annual effective dose, etc., are varied according to building material types and origins [15–24], which affects the variation in the distribution of radionuclide contents in raw materials and manufactured products [25]. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) studied the estimation of the vulnerability of the chronic doses of radiation (NORM) to the public. UNSCEAR estimated the allowable security values using NORM content values. The permitted safety values of natural radionuclides ²³⁸U (²²⁶Ra), ²³²Th, ⁴⁰K and Ra_eq concentrations in building materials were established to be 50, 50, 500 and 370 Bq/kg, respectively [1,26]. Therefore, the building materials that contain the values of ²³⁸U (²²⁶Ra), ²³²Th, ⁴⁰K and Ra_eq less than permitted safety limits will have an external radiation dose less than 1.5 mSv/y. Therefore, they are suitable for safe uses without posing a radiological risk [27]. Nevertheless, due to the chronic radiation exposure from building materials, the worldwide average limit of indoor effective dose is 1 mSv/y [28,29]. UNSCEAR (1996) reported that the whole background radiation dose, that the public may be exposed from all radiation sources, was stated to be 2.4 mSv/y (i.e. a radiation dose of 1.1 mSv/y due to NORM and a 1.3 mSv/y from radon radioactive gas) [15,30]. The International Commission on Radiological Protection (ICRP) reported that a radiation dose higher than 1 mSv/y should not be administered by the public because of exposure to non-nuclear industrial occupations [31]. These efforts, including radiological studies and recommendations of international organizations regarding natural radiation, emphasize that it is critical from the point of view of selecting suitable building materials for safe use in construction, particularly those with a wide variation of their radiological hazards. For instance, the wide variation of radium equivalent suggests that for safe uses of building material, it is advisable to track the radioactivity levels of materials from a new source [32,33].

Latterly, the Kingdom of Bahrain concerned about the dangers of ionizing radiation emitted by environmental materials around the public, particularly construction materials. Thus, this study was conducted to show the level of radioactivity in building materials used in different countries around the world. This study will identify the highest-priority types of building materials with a high level of radioactivity. It will likewise demonstrate the worldwide safety limits according to UNSCEAR and ICRP. Moreover, it opens the door to deal with establishing nationwide or international baseline data on the levels of natural radioactivity materials that depend upon the conducted studies, especially needed for those countries that import or export building materials. Remarkably, it encourages researchers to conduct studies and easily compare their measurements of radioactivity from various countries.

1.1. Sampling procedures

In order to achieve the objectives of this study, 123 studies estimating the natural radioactivity in building materials in 38 countries using a gamma spectroscopy system equipped with high purity germanium (HPGe) or sodium Iodide (NaI) detectors, which was considered as the sample of this study as shown in Appendix Table A1. It was selected randomly from the electronic journals available online of keywords such as assessment, evaluation or measurements of the natural radioactivity, or natural radionuclides in commonly used building materials of bricks, cements, soils, sands, granites, gypsums, marbles and ceramics. The data includes the country, the average radionuclides concentration of ²³⁸U, ²³²Th and ⁴⁰K, and their associated radiological hazards. The data were summarized and listed into separate tables for each type of building material. It was noted that some subjects do not observe the error or standard deviation of the experimental average value of the radionuclides, so this study will concern only the mean value to preserve the reliability values in literatures. In this study, radium equivalent activity (Ra_eq) was also examined, summarized and compared against the established global limit [22]. In the current study, the appropriate statistical tools used for the purpose of the study are the average or the mean, which encloses the highest and lowest values and the range. For the purpose of analysing the data, the average of these parameters (C_Ra, C_Th, C_K and Ra_eq) were compared with their worldwide safety limit values of building materials of 50, 50, 500 and 370 Bq/kg, reported by UNSCEAR 1993 and 2000. In addition, the hazard indices of gamma index (I_γ), alpha index (I_α), external hazard index (H_ex), internal hazard index (H_in), absorbed dose rate (D(nGy/h)) index and annual effective dose rate equivalent index (AEDE)) were mathematically estimated and compared with acceptable limits reported by UNSCEAR 2000 & 2006, EC 1999 and ICRP-60 1991.

Table A1. Building materials of different countries.

Building materials	References	No. of countries	No. of studies
Bricks	Table A2	24	39
Cement	Table A3	20	40
Soil/sand	Table A4	18	34
Granite	Table A5	16	30
Gypsum	Table A6	12	18
Marble	Table A7	20	31
Ceramic	Table A8	19	32
Total without duplicate	Table A9	38 countries	123 studies

2. Results and discussion

The results of this study will focus on the discussion of the literature values of the ²²⁶Ra, ²³²Th, and ⁴⁰K activity concentrations and radiological hazard indices, Ra_eq, Iγ, Iα, H_ex, H_in, D(nGy/h) and AEDE, in the building materials that are commonly used in different countries around the world.

2.1. Activity concentrations of radium (²²⁶Ra)

Several studies focus on the estimation of the ²²⁶Ra concentration value instead of ²³⁸U because the ²²⁶Ra and its daughters present 98.5% of the radiological hazards associated with the ²³⁸U series [10,30,31]. The average values of the activity concentration of ²²⁶Ra of the building materials that are used in some international countries are summarized as shown in Appendix Tables A2–A9. It can be observed from Appendix Table A9 that the descending order of building materials according to their average activity concentration of ²²⁶Ra begins with granite (1073 Bq/kg), gypsum (140 Bq/kg), soil or sand (128 Bq/kg), ceramic (73 Bq/kg), bricks (45 Bq/kg), cement (41 Bq/kg) and finally marble (22 Bq/kg). In the same context, granite maintains the highest value of ²²⁶Ra content in a range of (27–15,772 Bq/kg) while cement maintains the lowest values that range from 22 to 77 Bq/kg. Then, the other materials were ranged (6–560 Bq/kg) for gypsum, (4–1042 Bq/kg) for soil or sand, (12–207 Bq/kg) for ceramic, (7–143 Bq/kg) for bricks and (18–65 Bq/kg) for cement.

Table A2. Range and mean activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, in (Bq/Kg), and associated radiological hazard indices for bricks in some world countries.

Country	^238U (^226Ra)	^232Th	^40K	Ra(eq)	Iγ	Iα	Hex	Hin	D (nGy/h)	AEDE (mSv/y)	Ref.
Albania	33.40	42.00	644.00	143.05	0.54	0.17	0.39	0.48	67.65	0.33	[34]
Algeria a (2)*	(47–65), 56**	(35–51), 43	(425–675), 550	159.84	0.59	0.28	0.43	0.58	74.78	0.37	[35]
Australia	41.00	89.00	681.00	220.71	0.81	0.21	0.60	0.71	101.10	0.50	[36]
Bangladesh a (4)	(29–45.9), 43.95	(52–105.6), 82.63	(292–1564.2),1121.83	248.48	0.93	0.22	0.67	0.79	116.99	0.57	[37–39]
Brazil	52.00	65.00	747.00	202.47	0.75	0.26	0.55	0.69	94.43	0.46	[40]
Cameroon	49.60	91.00	172.00	192.97	0.68	0.25	0.52	0.66	85.05	0.42	[41]
China a (4)	(41–49), 47.22	(28.4–52), 45.46	(137.4–717), 629.14	160.67	0.59	0.24	0.43	0.56	75.51	0.37	[42–45]
Cuba	57.00	12.00	857.00	140.15	0.54	0.29	0.38	0.53	69.32	0.34	[46]
Egypt a (8)	(8.36–288.50), 74.12	(14–77.77), 34.41	(155.67–909.50), 384.00	152.89	0.55	0.37	0.41	0.61	71.04	0.35	[7,17,21,47,48]
Greece	35.00	45.00	710.00	154.02	0.58	0.18	0.42	0.51	72.96	0.36	[49]
Hong Kong	143.00	158.00	850.00	434.39	1.55	0.72	1.17	1.56	196.94	0.97	[50]
India a (4)	(18.3–63.74), 34.35	(19.40–38.6), 26.33	(238.40–313.71), 280.70	93.62	0.34	0.17	0.25	0.35	43.48	0.21	[51–53]
Iraq a (3)	(13.49–27.09), 20.83	(17.85–27.89), 24.43	(123.86–223.27),170.10	68.86	0.25	0.10	0.19	0.24	31.47	0.15	[54]
Italy	57.57	51.43	472.86	167.53	0.61	0.29	0.45	0.61	77.38	0.38	[55]
Jordan	7.48	1.23	6.00	9.70	0.03	0.04	0.03	0.05	4.45	0.02	[56]
Kuwait	6.60	6.60	332.00	41.60	0.17	0.03	0.11	0.13	20.88	0.10	[57]
Malaysia	33	55	505	150.54	0.55	0.17	0.41	0.50	69.52	0.34	[58]
Netherlands	39.00	41.00	560.00	140.75	0.52	0.20	0.38	0.49	66.13	0.32	[59]
Pakistan a (3)	(23–58), 36.47	(35–84), 58.33	(431–567.70), 513.57	159.43	0.58	0.18	0.43	0.53	73.50	0.36	[60–62]
Palestine	32.80	7.20	1139.00	130.80	0.53	0.16	0.35	0.44	67.00	0.33	[15]
South Korea	33.30	79.80	698.00	201.16	0.74	0.17	0.54	0.63	92.69	0.45	[63]
Sri Lanka	35.00	72.00	585.00	183.01	0.67	0.18	0.49	0.59	84.05	0.41	[25]
Turkey a (5)	(21.92–69.90), 39.58	(19.14–69.60), 37.94	(368.8–923.40), 618.20	141.43	0.53	0.20	0.38	0.49	66.98	0.33	[64–66]
Vietnam	64.35	77.57	589.00	220.63	0.80	0.32	0.60	0.77	101.14	0.50	[67]
Mean	44.69	51.93	575.64	163.28	0.60	0.22	0.44	0.56	76.02	0.37
Max.	143.00	158.00	1139.00	434.39	1.55	0.72	1.17	1.56	196.94	0.97
Min.	6.60	1.23	6.00	9.70	0.03	0.03	0.03	0.05	4.45	0.02

*Country a (n): "a" means include more than one study at that country, and "n" means number of data taken from these studies.

**(Range), average concentration: (Range) means the range of values of nuclide concentrations among these studies, and "average concentration" is the calculated average of the values taken from these studies.

Table A3. Range, mean activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, in (Bq/Kg), and associated radiological hazard indices for cements in some world countries.

Country	^238U (^226Ra)	^232Th	^40K	Ra(eq)	Iγ	Iα	Hex	Hin	D (nGy/h)	AEDE (mSv/y)	Ref.
Albania	55.00	17.00	179.70	93.15	0.33	0.28	0.25	0.40	43.17	0.21	[34]
Algeria	41.00	27.00	422.00	112.10	0.41	0.21	0.30	0.41	52.85	0.26	[35]
Australia	51.80	48.10	115.00	129.44	0.45	0.26	0.35	0.49	57.78	0.28	[36]
Bangladesh a (2)	(60.5–61.1), 60.80	(64.7–79.9), 72.30	(952.2–1132.6), 1042.40	244.45	0.91	0.30	0.66	0.82	115.23	0.57	[38,39]
Brazil	61.70	58.50	564.00	188.78	0.69	0.31	0.51	0.68	87.36	0.43	[15]
Cameroon	27.00	15.00	277.00	69.78	0.26	0.14	0.19	0.26	33.08	0.16	[41]
China a (7)	(29.1–118.7), 64.69	(15.8–62), 39.89	(137.4–444), 227.39	139.23	0.49	0.32	0.38	0.55	63.46	0.31	[42–45,68]
Cuba a (2)	(23–44.5), 22.50	(11–21.8), 10.93	(99–467), 188.67	52.66	0.19	0.11	0.14	0.20	24.87	0.12	[46,69]
Egypt a (9)	(17.45–134), 50.37	(3.59–88), 23.94	(4.09–416), 109.75	93.05	0.32	0.25	0.25	0.39	42.31	0.21	[16,17,21,47]
India a (2)	(35.73–54), 44.87	(37.75–65), 51.38	(159.83–440), 299.92	141.44	0.51	0.22	0.38	0.50	64.27	0.32	[51,52]
Iraq a (3)	(20.87–29.75), 24.75	(21.72–32.18), 27.86	(153.58–201.58), 179.66	78.42	0.28	0.12	0.21	0.28	35.75	0.18	[54]
Malaysia	34.70	32.90	190.60	96.42	0.34	0.17	0.26	0.35	43.85	0.22	[70]
Nigeria a (8)	(26–66, 44.61	(21.5–101), 63.11	(71.7–850), 337.19	160.83	0.58	0.22	0.43	0.55	72.79	0.36	[24,71,72]
Pakistan a (3)	(25–37), 32.83	(28–37), 31.03	(200–245), 219.97	94.15	0.34	0.16	0.25	0.34	43.09	0.21	[61,62,73]
Palestine a (2)	(29.8–85.1), 57.45	(28.9–67.6), 48.25	(18.5–31.5), 25.00	128.37	0.44	0.29	0.35	0.50	56.73	0.28	[15]
Portugal and Spain	28.00	12.00	213.00	61.56	0.22	0.14	0.17	0.24	29.07	0.14	[33]
Saudia Arabia a (14)	(8.58–38.4), 18.28	(7.44–45.3), 17.38	(86–236), 142.73	54.78	0.20	0.09	0.15	0.20	25.17	0.12	[74–77]
South Korea	34.50	19.40	241.00	80.80	0.29	0.17	0.22	0.31	37.71	0.18	[63]
Turkey a (6)	(24.70–71.55), 45.13	(16.30–47.83), 28.09	(99.2–2493.10), 661.85	136.25	0.51	0.23	0.37	0.49	65.41	0.32	[64–66,78,79]
Vietnam	39.86	25.46	243.50	95.02	0.34	0.20	0.26	0.36	43.95	0.22	[67]
Mean	41.99	33.50	294.02	112.53	0.41	0.21	0.30	0.42	51.89	0.25
Max.	64.69	72.30	1042.40	244.45	0.91	0.32	0.66	0.82	115.23	0.57
Min.	18.28	10.93	25.00	52.66	0.19	0.09	0.14	0.20	24.87	0.12

Table A4. Range, mean activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, in (Bq/Kg), and associated radiological hazard indices for sand/ soil in some world countries.

Country	^238U (^226Ra)	^232Th	^40K	Ra(eq)	Iγ	Iα	Hex	Hin	D (nGy/h)	AEDE (mSv/y)	Ref.
Algeria	12.00	7.00	74.00	27.71	0.10	0.06	0.07	0.11	12.86	0.06	[35]
Australia	3.70	40.00	44.40	64.32	0.23	0.02	0.17	0.18	27.72	0.14	[36]
Bangladesh a (3)	(49.4–59.2), 53.30	(71.6–135), 96.20	(927.2–1592),1185.00	282.11	1.05	0.27	0.76	0.91	132.14	0.65	[38,39]
Cameroon	14.00	31.00	586.00	103.45	0.40	0.07	0.28	0.32	49.63	0.24	[41]
China a (6)	(22.4–63), 37.50	(21.5–70), 45.02	(302.6–789.3), 633.83	150.68	0.56	0.19	0.41	0.51	70.95	0.35	[42,43,45,50,80,81]
Cuba	17.00	16.00	208.00	55.90	0.21	0.09	0.15	0.20	26.19	0.13	[46]
Egypt a (5)	(9.19–16.63), 12.57	(1.87–15.43), 7.99	(15.43–433), 153.66	35.83	0.13	0.06	0.10	0.13	17.04	0.08	[16,21,47,82]
Greece	18.00	17.00	367.00	70.57	0.27	0.09	0.19	0.24	33.89	0.17	[49]
Hong Kong	24.30	27.10	841.00	127.81	0.50	0.12	0.35	0.41	62.66	0.31	[50]
India a (2)	(90.27–116.1), 103.19	(43.51–101.67), 72.59	(280.71–300.07), 290.39	229.35	0.80	0.52	0.62	0.90	103.63	0.51	[52]
Indonesia	1042.29	1756.12	231.80	3571.39	12.33	5.21	9.65	12.46	1551.90	7.61	[83]
Iraq a (4)	(11.24–29.35), 22.07	(9.12–33.53), 24.22	(80.15–221.51), 167.40	69.59	0.25	0.11	0.19	0.25	31.80	0.16	[54,84]
Pakistan a (3)	(20–27), 32.67	(29–40.63), 46.21	(383–566.94), 470.65	111.69	0.42	0.12	0.30	0.37	52.43	0.26	[13,61,73]
Palestine	20.60	18.80	26.30	49.51	0.17	0.10	0.13	0.19	21.97	0.11	[15]
Saudi Arabia a (4)	(12.30–41.1), 32.58	(19.7–41.9), 28.60	(182.50–260), 227.98	91.03	0.33	0.16	0.25	0.33	41.83	0.21	[77,85]
South Korea	28.98	56.37	1008.00	187.21	0.71	0.14	0.51	0.58	89.47	0.44	[63]
Turkey a (5)	(22.9–1559.1), 415.70	(26.4–142), 60.10	(471.4–1711), 878.52	569.29	1.98	2.08	1.54	2.66	264.99	1.30	[64–66,86]
Vietnam	26.74	39.77	506.90	122.64	0.46	0.13	0.33	0.40	57.51	0.28	[67]
Mean	106.01	132.23	438.93	328.89	1.16	0.53	0.89	1.17	147.15	0.72
Max.	1042.29	1756.12	1185.00	3571.39	12.33	5.21	9.65	12.46	1551.90	7.61
Min.	3.70	7.00	26.30	27.71	0.10	0.02	0.07	0.11	12.86	0.06

Table A5. Range, mean activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, in (Bq/Kg), and associated radiological hazard indices for granite in some world countries.

Country	^238U (^226Ra)	^232Th	^40K	Ra(eq)	Iγ	Iα	Hex	Hin	D (nGy/h)	AEDE (mSv/y)	Ref.
Brazil a (3)	(10–91), 49.87	(13–288.2), 151.07	(51–1819), 1068.33	348.16	1.28	0.25	0.94	1.07	158.84	0.78	[40,87]
Cyprus	57.00	37.00	1228.00	204.47	0.78	0.29	0.55	0.71	99.89	0.49	[88]
Egypt a (5)	(13–78571), 15772.036	(14–3820), 810.844	(405–4819), 1951.51	17081.81	57.28	78.86	46.16	88.79	7857.81	38.55	[17,21,47,89,90]
Greece (2)	(35–64), 49.5	(17–81), 49	(383–1104), 743.5	176.82	0.66	0.25	0.48	0.61	83.47	0.41	[49,91]
Hong Kong	202.00	140.00	1030.00	481.51	1.72	1.01	1.30	1.85	220.84	1.08	[50]
India a (3)	(6.17–64), 28.13	(4.62–76), 33.05	(139.17–1096), 503.58	114.18	0.43	0.14	0.31	0.38	53.96	0.26	[5,92]
Iran (3)	(25–69), 69	(23–62), 62	(556–1422), 1078.67	240.72	0.90	0.35	0.65	0.84	114.31	0.56	[93,94]
Jordan	41.52	58.42	897.00	194.13	0.73	0.21	0.52	0.64	91.87	0.45	[56]
Nigeria	56.26	29.4	301.42	121.51	0.44	0.28	0.33	0.48	56.32	0.28	[95]
Pakistan a (2)	(27.32–659), 343.16	(50.07–598), 324.04	(953.1–1218), 1085.55	890.12	3.13	1.72	2.40	3.33	399.52	1.96	[96,97]
Palestine a (15)	(35.1–151.2), 68.67	(20.5–140.8), 77.87	(400–1256.8), 804.09	241.95	0.89	0.34	0.65	0.84	112.29	0.55	[15,98]
Portugal and Spain	116.00	53.00	1290.00	291.12	1.08	0.58	0.79	1.10	139.40	0.68	[33]
Saudi Arabia a (6)	(10–56.4), 27.26	(22.9–80), 39.58	(340–1906), 949.75	156.99	0.61	0.14	0.42	0.50	76.11	0.37	[77,99–101]
Spain	80.00	123.00	1289.00	355.14	1.31	0.40	0.96	1.18	165.00	0.81	[87]
Turkey a (3)	(67–306), 146.83	(77.4–248), 159.47	(915.3–1266), 1079	458.01	1.65	0.73	1.24	1.63	209.18	1.03	[64,102]
Vietnam	59.48	85.17	992	257.66	0.95	0.30	0.70	0.86	120.29	0.59	[67]
Mean	1072.92	139.56	1018.26	1350.89	4.61	5.36	3.65	6.55	622.44	3.05
Max.	15772.04	810.84	1951.51	17081.81	57.28	78.86	46.16	88.79	7857.81	38.55
Min.	27.26	29.40	301.42	114.18	0.43	0.14	0.31	0.38	53.96	0.26

Table A6. Range, mean activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, in (Bq/Kg), and associated radiological hazard indices for gypsum in some world countries.

Country	^238U (^226Ra)	^232Th	^40K	Ra(eq)	Iγ	Iα	Hex	Hin	D (nGy/h)	AEDE (mSv/y)	Ref.
Australia	451.40	11.10	-	467.27	1.56	2.26	1.26	2.48	215.25	1.06	[36]
Bangladesh a (2)	(58.4–254.53), 156.47	(21.35–91.2), 56.28	(120.75–1101), 110.93	245.48	0.84	0.78	0.66	1.09	110.90	0.54	[37,39]
Brazil	6.00	-	154.00	17.86	0.07	0.03	0.05	0.06	9.19	0.05	[40]
Egypt a (5)	(0.24–105), 42.48	(4.58–45), 21.38	(33.47–500), 233.68	91.04	0.33	0.21	0.25	0.36	42.28	0.21	[17,21,47,103]
Germany	305	20	110	342.07	1.15	1.53	0.92	1.75	157.58	0.77	[20]
India	8.30		26.70	10.36	0.04	0.04	0.03	0.05	4.95	0.02	[104]
Iraq a (4)	(9.55–29.12), 21.65	(16.18–28.71), 23.51	(115.83–179.84), 152.10	66.98	0.24	0.11	0.18	0.24	30.54	0.15	[54,84]
Italy	6.00	2.00	32.00	11.32	0.04	0.03	0.03	0.05	5.31	0.03	[105]
Palestine	78.20	9.80	2.70	92.42	0.31	0.39	0.25	0.46	42.16	0.21	[15]
Saudi Arabia	33.30	47.20	88.00	107.57	0.38	0.17	0.29	0.38	47.56	0.23	[77]
Spain	560.00	8.00	30.00	573.75	1.92	2.80	1.55	3.06	264.80	1.30	[106]
Turkey a (2)	(5.2–17.1),11.15	(5.2–14.2), 9.70	(59.0–98.67), 78.84	31.09	0.11	0.06	0.08	0.11	14.30	0.07	[64,65]
Mean	140.00	20.90	92.63	171.43	0.58	0.70	0.46	0.84	78.74	0.39
Max.	560.00	56.28	233.68	573.75	1.92	2.80	1.55	3.06	264.80	1.30
Min.	6.00	2.00	2.70	10.36	0.04	0.03	0.03	0.05	4.95	0.02

Table A7. Range, mean activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, in (Bq/Kg), and associated radiological hazard indices for marble in some world countries.

Country	^238U (^226Ra)	^232Th	^40K	Ra(eq)	Iγ	Iα	Hex	Hin	D (nGy/h)	AEDE (mSv/y)	Ref.
Algeria	23.00	18.00	310.00	72.61	0.27	0.12	0.20	0.26	34.43	0.17	[35]
Cameroon	8.21	0.35	18.75	10.15	0.04	0.04	0.03	0.05	4.79	0.02	[41]
Cyprus	14.20	0.64	15.60	16.32	0.06	0.07	0.04	0.08	7.60	0.04	[107]
Egypt a (3)	(4.04–205), 76.70	(0.67–115), 49.56	(666.5–865), 765.77	186.88	0.76	0.38	0.56	0.77	97.30	0.48	[17,21,108]
Greece	1.80	2.20		4.95	0.02	0.01	0.01	0.02	2.16	0.01	[109]
India	9.25	61.63	1366.85	202.63	0.79	0.05	0.55	0.57	98.50	0.48	[5]
Iraq a (4)	(5–70.5), 35.46	(15.30–160), 62.33	(933–1953), 1331.00	227.07	0.87	0.18	0.61	0.71	109.53	0.54	[110,111]
Italy a (2)	52.5	24.5	345	114.10	0.41	0.26	0.31	0.45	53.44	0.26	[112]
Japan	33.7	46.3	943.6	4.20	0.66	0.17	0.47	0.56	82.88	0.41	[113]
Jordon a (2)	(2.84–19.3), 11.07	(1.24–11.9), 6.57	(18–93.3), 55.65	24.75	0.09	0.06	0.07	0.10	11.40	0.06	[56,114]
Kuwait	3.60	0.22	3.70	4.20	0.01	0.02	0.01	0.02	1.95	0.01	[57]
Malaysia	19.00	16.50	243.30	61.33	0.23	0.10	0.17	0.22	28.89	0.14	[115]
Nigeria a	(2.4–61), 39.80	(0.7–96), 52.23	(6.6–330), 158.87	126.73	0.45	0.20	0.34	0.45	56.56	0.28	[24,116]
Pakistan a (4)	(7.90–33), 21.68	(2.7–32), 19.28	(25–299), 110.28	57.73	0.21	0.11	0.16	0.21	26.25	0.13	[117–120]
Portugal and Spain	6.10	1.90	34.00	11.44	0.04	0.03	0.03	0.05	5.38	0.03	[33]
Saudi Arabia a (4)	(2–12.7), 6.93	(0.1–13.2), 3.63	(0.8–64), 16.67	13.40	0.05	0.03	0.04	0.05	6.09	0.03	[77,99]
Serbia	37.00	19.20	31.00	66.84	0.23	0.19	0.18	0.28	29.98	0.15	[121]
Syria	18.00	1.00	4.00	19.74	0.07	0.09	0.05	0.10	9.09	0.04	[122]
Tunis	33.24	8.01	116.98	53.70	0.19	0.17	0.15	0.23	25.07	0.12	[123]
Turkey a (3)	(5.4–13), 8.87	(4.9–24), 11.47	(28–58.1), 45.27	28.75	0.10	0.04	0.08	0.10	12.91	0.06	[22,65,124]
Mean	23.00	20.28	311.38	65.37	0.28	0.12	0.20	0.26	35.21	0.17
Max.	76.70	62.33	1366.85	227.07	0.87	0.38	0.61	0.77	109.53	0.54
Min.	1.80	0.22	3.70	4.20	0.01	0.01	0.01	0.02	1.95	0.01

Table A8. Range, mean activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, in (Bq/Kg), and associated radiological hazard indices for ceramic in some world countries.

Country	^238U (^226Ra)	^232Th	^40K	Ra(eq)	Iγ	Iα	Hex	Hin	D (nGy/h)	AEDE (mSv/y)	Ref.
Algeria	55.00	41.00	410.00	145.20	0.53	0.28	0.39	0.54	67.27	0.33	[35]
Bangladesh	60.90	70.80	1000.20	239.16	0.89	0.30	0.65	0.81	112.61	0.55	[125]
Cameroon	12.22	20.40	318.76	65.94	0.25	0.06	0.18	0.21	31.26	0.15	[41]
China	58.20	161.50	455.70	324.23	1.15	0.29	0.88	1.03	143.44	0.70	[126]
Cyprus	75.60	84.00	620.00	243.46	0.88	0.38	0.66	0.86	111.52	0.55	[107]
Egypt a (4)	(51.12–126), 74.46	(29.52–72), 47.28	(300–703.23), 591.67	187.62	0.68	0.37	0.51	0.71	87.63	0.43	[17,21,47,108]
India a (2)	(28.2–42), 35.10	(57–63.7), 60.35	(24.3–528), 276.15	142.66	0.51	0.18	0.39	0.48	64.18	0.31	[52,104]
Iraq a (4)	(21.21–102.12), 72.61	(23.84–87.53), 47.98	(162.01–401), 299.04	239.16	0.56	0.28	0.42	0.57	70.97	0.35	[10,127,128]
Italy a (4)	(48–114), 74.75	(42–66), 53.75	(460–890), 672.25	203.38	0.74	0.37	0.55	0.75	95.03	0.47	[55,129,130]
Serbia	67.00	50.00	500.00	177.00	0.64	0.34	0.48	0.66	82.00	0.40	[131]
Jordon a (2)	(33.86–63.75), 48.81	(28.82–93.65), 61.24	(180.9–411), 295.95	159.16	0.57	0.24	0.43	0.56	71.87	0.35	[56,132]
Nigeria a (5)	(24–74), 59.24	(76–173.9), 108.78	(490–850), 643.4	270.71	0.98	0.28	0.73	0.88	122.51	0.60	[24,126,133,134]
Pakistan	84	86	403	237.01	0.84	0.42	0.64	0.86	107.10	0.53	[73]
Portugal and Spain	139.00	46.00	653.00	255.06	0.91	0.70	0.69	1.06	119.23	0.58	[33]
Saudi Arabia	89.00	105.50	772.90	299.38	1.08	0.45	0.81	1.05	137.07	0.67	[75]
Syria	59.00	40.00	292.00	138.68	0.49	0.30	0.37	0.53	63.59	0.31	[122]
Turkey a (2)	(31–81.2), 56.10	(28–65.4), 46.70	(358–450), 404.05	239.16	0.56	0.28	0.42	0.57	70.97	0.35	[22,124]
Vietnam	95.14	74.64	658.70	252.60	0.91	0.48	0.68	0.94	116.51	0.57	[67]
Yemen	207.2	75.2	400.70	345.59	1.20	1.04	0.93	1.49	157.86	0.77	[2]
Mean	68.23	65.79	504.76	205.23	0.72	0.34	0.54	0.73	92.31	0.45
Max.	207.20	173.90	1000.20	354.61	1.26	1.04	0.96	1.49	157.86	0.77
Min.	12.22	20.40	24.30	65.94	0.24	0.06	0.18	0.21	30.95	0.15

Table A9. Summary of range, mean activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, in (Bq/Kg), and associated radiological hazard indices for building materials used in some international countries.

Material	Stat.	^238U (^226Ra)	^232Th	^40K	Ra(eq)	Iγ	Iα	Hex	Hin	D (nGy/h)	AEDE (mSv/y)
Bricks	Mean	44.69	51.93	575.64	163.28	0.60	0.22	0.44	0.56	76.02	0.37
	Max.	143.00	158.00	1139.00	434.39	1.55	0.72	1.17	1.56	196.94	0.97
	Min.	6.60	1.23	6.00	9.70	0.03	0.03	0.03	0.05	4.45	0.02
Cement	Mean	41.99	33.50	294.02	112.53	0.41	0.21	0.30	0.42	51.89	0.25
	Max.	64.69	72.30	1042.40	244.45	0.91	0.32	0.66	0.82	115.23	0.57
	Min.	18.28	10.93	25.00	52.66	0.19	0.09	0.14	0.20	24.87	0.12
Soil/sand	Mean	106.01	132.23	438.93	328.89	1.16	0.53	0.89	1.17	147.15	0.72
	Max.	1042.29	1756.12	1185.00	3571.39	12.33	5.21	9.65	12.46	1551.90	7.61
	Min.	3.70	7.00	26.30	27.71	0.10	0.02	0.07	0.11	12.86	0.06
Granite	Mean	1072.92	139.56	1018.26	1350.89	4.61	5.36	3.65	6.55	622.44	3.05
	Max	15772.04	810.84	1951.51	17081.81	57.28	78.86	46.16	88.79	7857.81	38.55
	Min	27.26	29.40	301.42	114.18	0.43	0.14	0.31	0.38	53.96	0.26
Gypsum	Mean	140.00	20.90	92.63	171.43	0.58	0.70	0.46	0.84	78.74	0.39
	Max	560.00	56.28	233.68	573.75	1.92	2.80	1.55	3.06	264.80	1.30
	Min	6.00	2.00	2.70	10.36	0.04	0.03	0.03	0.05	4.95	0.02
Marble	Mean	23.00	20.28	311.38	65.37	0.28	0.12	0.20	0.26	35.21	0.17
	Max.	76.70	62.33	1366.85	227.07	0.87	0.38	0.61	0.77	109.53	0.54
	Min.	1.80	0.22	3.70	4.20	0.01	0.01	0.01	0.02	1.95	0.01
Ceramic	Mean	68.23	65.79	504.76	205.23	0.72	0.34	0.54	0.73	92.31	0.45
	Max.	207.20	173.90	1000.20	354.61	1.26	1.04	0.96	1.49	157.86	0.77
	Min.	12.22	20.40	24.30	65.94	0.24	0.06	0.18	0.21	30.95	0.15

Figure 1 shows the average value of the activity concentrations of ²²⁶Ra for the studied building materials. Furthermore, it shows that most of the building materials studied are above the recommended limit of 50 Bq/kg, with the exception of bricks, cement and marble [1,26]. The obtained results suggest that since granite materials maintained the highest radioactivity; then, it is recommended to be applied as decorative building materials (superficial uses) whereas the lower radioactivity of other construction materials can be used without posing a radiation hazard.

Figure 1. ²²⁶Ra concentrations for building materials.

2.2. Activity concentrations of thorium (²³²Th)

Appendix Tables A2–A9 summarize the average concentrations of ²³²Th in construction materials, which are used in various countries around the world. The activity concentrations of ²³²Th in building materials vary from the highest value to the lowest value as 140 Bq/kg for granites, 134 Bq/kg for soils or sands, 62 Bq/kg for ceramic, 52 Bq/kg for bricks, 34 Bq/kg for cement, 21 Bq/kg for gypsum and 18 Bq/kg for marble, respectively, as seen from Appendix Table A9. In addition, ²³²Th concentration ranged from 29 to 811 Bq/kg for granite (the highest value), 7 to 1756 Bq/kg for soil/ sand, 20 to 162 Bq/kg for ceramic, 1 to 158 Bq/kg for bricks, 11 to 72 Bq/kg for cement, 2 to 56 Bq/kg for gypsum and 0.22 to 62 Bq/kg for marble (the lowest value), as shown in Appendix Tables A2–A9.

Figure 2 shows the average activity concentrations of ²³²Th for the building materials utilized in the analysed samples. Figure 2 suggests that there are three types of the studied building materials; granites, sands/soils and ceramics, which contain a radioactivity higher than the recommended safety limits of 50 Bq/kg while the remaining types have less values [1,26]. As seen in Appendix Tables A2–A9 and Figure 2, granites and ceramics are far from the recommended safety limit, hence they pose radiological hazards. As a result, granite and ceramics should be used as decorative building materials, whereas other building materials that maintain low radioactivity can be used without radiation hazards.

Figure 2. ²³²Th concentrations for building materials.

2.3. Activity concentrations of potassium (⁴⁰K)

The average activity concentration of ⁴⁰K in the studied samples of building materials; granites, bricks, ceramics, sands/soils, marbles, cements and gypsums are (in ascending order), 1018 Bq/kg, 576 Bq/kg, 490 Bq/kg, 462 Bq/kg, 305 Bq/kg, 294 Bq/kg and 93 Bq/kg, respectively, as shown in Appendix Tables A2–A9. It was noticed that granites have the maximum highest range of ⁴⁰K activity concentration of 301–1952 Bq/kg, while the other materials were in the ranges, in Bq/kg, 6–1139 for bricks, 292–1000 for ceramics, 26–1185 for soils/ sands, 4–1367 for marbles, 25–1042 for cement and the lowest value of gypsum 3–234. Figure 3 shows the average activity concentrations of ⁴⁰K of the studied building material samples. Three types of building materials, namely granite and bricks, have higher activity concentrations of ⁴⁰K than the recommended safety limit of 500 Bq/kg, while the other types have low concentrations of ⁴⁰K [1,26]. The average concentration of granite is clearly far from the recommended limit, but the average of the bricks is slightly higher than the recommended limit. Granites are posing a radiation risk whilst the others do not.

Figure 3. ⁴⁰K concentrations for building materials.

2.4. Radiological hazard indices

2.4.1. Radium equivalent activity (Ra_eq)

It is difficult to compare the specific activities of building material types containing a non-uniform distribution of various levels of ²²⁶Ra, ²³²Th and ⁴⁰K concentrations so a radium equivalent index was introduced by UNSCEAR 1993 and 2000. Radium equivalent concentration (Ra_eq) was calculated from the following equation [12,24]. The radium equivalent activity (Ra_eq) index was established to describe the radioactivity level in building materials by a single quantity [33]. (1) $\begin{array}{r}\text{R}{\text{a}}_{\text{eq}}=C_{\text{Ra}}+1.43C_{\text{Th}}+0.07C_{\text{k}}\end{array}$(1) where C_Ra, C_Th and C_K are the activity concentrations for the natural radionuclides ²²⁶Ra, ²³²Th and ⁴⁰K, respectively [26]. The calculation of radium equivalent concentration was established on the assumption that 370 Bq/kg of ²²⁶Ra, 259 Bq/kg of ²³²Th and 4810 Bq/kg of ⁴⁰K produce the same γ-rays radiation dose of 1.5 mSv/y. The average concentration in radium equivalent (Ra_eq) of the 7 types of building materials is given in Appendix Table A9. The radium equivalent concentration (Ra_eq) of the 7 types of building materials that contain various levels of ²²⁶Ra, ²³²Th and ⁴⁰K concentrations has an average of 1351 Bq/kg for granite, 324 Bq/kg for soils/sands, 231 Bq/kg for ceramics, 171 Bq/kg for gypsums, 163 Bq/kg for bricks, 113 Bq/kg for cement and 61 Bq/kg for marble.

It can be remarkably seen that granite has the maximum range of Ra_eq measurement, which ranged from 114 to 17,082 Bq/kg, followed by the other lower materials which ranged, in Bq/kg, from 28 to 3571 for soils/sands, 66 to 346 for ceramics, 10 to 574 for gypsums, 10 to 434 for bricks and 53 to 245 for cements, while the minimum type is marbles that ranged from 4 Bq/kg to 227 Bq/kg. Figure 4 presents the average radium equivalent concentration of construction material samples studied in some countries around the world.

Figure 4. Ra(_eq) index for building materials.

Figure 4 shows that there are six types of building materials that have relatively low average concentrations of natural radionuclides in comparison to the recommended level (370 Bq/kg), such as marbles (61 Bq/kg), cements (113 Bq/kg), bricks (163 Bq/kg), gypsum (171 Bq/kg), ceramics (231 Bq/kg) and soils /sands (324 Bq/kg). As a result, six types do not constitute radiation or a danger to the public and hence can be used safely in the housing. Despite this result, granite materials have radium concentrations higher than the recommended safe limit of 370 Bq/kg, indicating a radiation hazard (Tables A2–A9). Thus, granite (1351 Bq/kg) comes in the first rank that has the highest in its NORM of natural radioactivity and radium equivalent concentration in comparison with other materials so it calls for serious monitoring of this type of construction fabric.

This result is consistent with what has been indicated by some studies that granite, as an igneous rock, has a high concentration than sedimentary rocks, except for some shale and phosphate rocks [135,136]. The higher concentration in granite rocks is due to enriched concentrations of uranium and thorium, in spite of low abundances in the mantle and crust [88]. Geologists demonstrate that the partial melting and fractional crystallization of magma allows ²³⁸U and ²³²Th to concentrate in the liquid phase and be incorporated into silica-rich products [89]. The variation of radionuclide concentrations as well as radium equivalent concentration may be due the variation of the geographical and geological conditions of the raw material used to fabricate these building materials as well as the additive materials (with high minerals content) such as Coal fly ash, phosphogypsum, certain slags, zirconium and monazite material (glazed) that used to enhancement the shaping and appearance properties of building materials so that the increase of mineral content reflects an increasing of the activity concentrations.

2.4.2. Gamma index (I_γ)

The gamma index (I_γ) is used to estimate the level of gamma radiation from building materials. It was calculated based on the concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K from the following equation (2)(2) $\begin{array}{r}{I}_{\gamma }=\frac{C_{\text{Ra}}}{300}+\frac{C_{\text{Th}}}{200}+\frac{C_{\text{k}}}{3000}\end{array}$(2) derived by the European Commission [23,107]: (2) $\begin{array}{r}{I}_{\gamma }=\frac{C_{\text{Ra}}}{300}+\frac{C_{\text{Th}}}{200}+\frac{C_{\text{k}}}{3000}\end{array}$(2) The mean values of I_γ calculated for building materials from their measurements of activity concentrations are presented in Appendix Tables A2–A8 and summarized in Appendix Table A9. In Table A9, it is obvious that there are variations in the mean values of I_γ, in the range 0.28–4.61, for the building materials types, which can be sorted ascendingly and compared with the world limit due to their mean values of gamma index as granite, and soil/sand are greater than the unity, while the ceramic, bricks, gypsum, cement and marble are lower the unity.

2.4.3. Alpha index (I_α)

Alpha index (I_α) is used to assess the excess alpha radiation due to the radon inhalation originating from building materials, which was estimated by the following equation [7,133]: (3) $\begin{array}{r}{I}_{\alpha }=\frac{C_{\text{Ra}}}{200}\end{array}$(3)

Appendix Tables A2–A8 shows the mean values of I_α calculated for building materials based on the measurements of radium concentration and summarized in Appendix Table A9. In Appendix Table A9, the mean values of Iα of studying building materials types varied in the range 0.12–5.36, which can be sorted descending as granite, gypsum, soil/sand, ceramic, bricks, cement and marble. All material types are lower than the world safety limit, the unity, except the granite material. Thus, it is possible that the radon exhalation level from granite could pose indoor radon concentration to exceed 200 Bq/m³.

2.4.4. External hazard index (H_ex)

The external hazard index (H_ex) is used to assess the suitability of materials for construction and can be calculated with the following equation [110,133]: (4) $\begin{array}{r}{H}_{\text{ex}}=\frac{C_{\text{Ra}}}{370}+\frac{C_{\text{Th}}}{259}+\frac{C_{\text{k}}}{4810}\end{array}$(4) The index value should be less than unity for the radiation hazard to be neglected. Appendix Tables A2–A8 list the mean values of H_ex computed for building materials using the measurements of radionuclides activity concentrations, ²²⁶Ra, ²³²Th and ⁴⁰K, and summarized in Appendix Table A9. As it can be seen in Appendix Table A9, the mean values of H_ex for the descending order of building materials types, such as soil/sand, ceramic, gypsum, bricks, cement, and marble, were found to be less than the unity, except granite material (3.65).

2.4.5. Internal hazard index (H_in)

The internal radiation hazard index (H_in) gives an estimate of the exposure to radon and its daughters and was computed by the following [12,110,133]. Appendix Tables A2–A8 show the computed values of H_in for the studied building materials types. (5) $\begin{array}{r}{H}_{\text{in}}=\frac{C_{\text{Ra}}}{185}+\frac{C_{\text{Th}}}{259}+\frac{C_{\text{k}}}{4810}\end{array}$(5) H_in ≤1 is the safe limit adopted on the building materials for safe use in construction dwellings. Appendix Table A9 shows the mean values of H_in for granite, and soil sand are 6.55 and 1.17, respectively, which are higher than safe limit, while the others are lower than the unity. Thus, granite with higher values probably causes problems for the respiratory organs.

2.4.6. Absorbed gamma dose rate (D(nGy/h))

The absorbed gamma dose rate(D (nGy/h)) above 1m from the ground was computed by using the following equation [7,12]: (6) $\begin{array}{r}\text{D}(\text{nGy/h})=0.46C_{\text{Ra}}+0.62C_{\text{Th}}+0.042C_{\text{K}}\end{array}$(6) The calculated values of D(nGy/h) are listed in Appendix Tables A2–A8 and summarized in Appendix Table A9. The recommended safety limit for this index as reported by UNSCEAR 2000 is 84 nGy/h. Appendix Table A9 shows the main value of D(nGy/h), which is varied in the lowest ranges, below the safety limit, which lies between 35.21 and 76 nGy/h for the sorted order marble, cement and bricks. In contrast, the higher ranges lie above the safety limit, which lie between 76.02 and 92.31 nGy/h for the ascending order gypsum, ceramic, soil/sand and granite.

2.4.7. Annual effective dose rate equivalent (AEDE)

For indoor use of the building material, the UNSCEAR (2000) reports that the annual effective dose AEDE (mSv/y) has a value of 0.7 Sv/Gy for the conversation coefficient from the absorbed dose in the air to the effective dose received by adults, and the indoor occupancy factor equal 80%. The AEDE was calculated from the following equation [12,83,110]: (7) $\begin{array}{rl}\text{AEDE}(\text{mSv/y})& =\text{D}(\text{nGy/h})\times 8760\text{h/y}\times 80\mathrm{\%}\\ & \times 0.7\text{Sv/Gy}\times {10}^{-6}\end{array}$(7) The recommended upper safety limit for this index is 1 mSv/y for individuals indoors as reported by UNSCEAR 2000. The mean of annual effective dose rate equivalent (AEDE) values of studied building materials is tabulated in Appendix Tables A2–A8 and summarized in Appendix Table A9. Appendix Table A9 shows that granite is the highest value (3.05 mSv/y) among the studied building material types and above the recommended safety limit, while the rest materials are below the recommended safety limit fall in the range of descending order that between 0.72 and 0.17 mSv/y as soil/sand, ceramic, gypsum, bricks, cement and marble.

2.5. Hierarchical cluster analysis

The consequence can be also summarized using a dendrogram diagram of hierarchical cluster analysis. Cluster analysis is a significant semi-quantitative statistical technique used to classify objects or cases into optimal clustering according to their similarities in linkage. Figures 5 and 6 represent hierarchical clusters using Word's method linkage to explore the connections between the variables, the primordial radionuclides, Ra_eq and the studied building materials due to their Ra_eq along with square Euclidean distance, respectively. It should be noted that Figures 5 and 6 are produced through the Statistical Package for the Social Sciences (SPSS) version 23.

Figure 5. Ward's method dendrogram for primary radionuclides and Ra_eq.

Figure 6. Dendrogram of ward's method for building materials and their Ra_eq values.

In Figure 5, Group 1 comprises and is relative to the concentrations of ²³⁸U, ⁴⁰K and Ra_eq. Obviously, ⁴⁰K concentrations are more related than ²³⁸U (zero distance, 100% similarity). In another hand, cluster 2 is uniquely formed by ²³²Th concentrations that are identified far distance from cluster 1, but it is more closely related to ⁴⁰K and Ra_eq than ²³⁸U concentration levels. This means that ²³⁸U and ⁴⁰K have the most significant impact on the Ra_eq concentration above ²³²Th in the study sample. Therefore, ²³²Th has the weakest association with Ra_eq due to its distance from each other.

Figure 6 generally shows the dendrogram of the radiological risk parameter Ra_eq for the types of materials used to construct study samples using Ward's method. Two statistically significant clusters are observed. Cluster 1 combines six building materials: bricks, gypsum, ceramic, cement, marble and soil/sand. They are closer to one another in their Ra_eq value and underpin a group (100% similarity and no distance). In contrast, granite formed cluster 2, which is a greater distance than cluster 1, and the farther away from the bricks and closer to the soil/sand. Thus, according to the distance of each building material type, it can be deduced the sequential of them due to their association with each other and their Ra_eq values, from the highest to the lowest as granite, soil/sand, marble, cement, ceramic, gypsum and bricks. This result is perfectly similar to that shown in Figure 4 for granite and soil/sand in their position and order, while ceramic, gypsum and bricks come successively also but not in accordance with the position. This difference could be attributed to changes in the association of primary concentrations in each construction material.

3. Conclusion and recommendations

There were vast fluctuations in the radioactivity of compiled data found in the conducted studies on the building material types, due to the geological and geographical origin of the materials, as well as anthropogenic factors that affect their distribution of radionuclides and mineral composition. This study concerns the radioactivity and radiological measurements of natural radionuclides content in seven different types of building materials, namely; bricks, cement, soils/sands, gypsums, marbles, ceramics and granites. The radioactivity content of these materials was examined in this article as part of a comparison with the permissible limits (global limits) for the use of these construction materials. High purity germanium detectors (HPGe) (major studies) and NaI detectors (minor studies) were used to measure radioactivity during the investigated studies. Some conducted studies reported that granite, as an igneous rock, has higher radioactivity with consideration of some exceptions of shale and phosphate rock. However, the current review study confirms that granite has the highest activity concentrations among all the studied building materials, whereas the concentrations of ²²⁶Ra and ²³²Th in marble and ⁴⁰K concentration in gypsum were the lowest. The highest concentration of granite is attributed to the highest concentration of ²³⁸U and ²³²Th in the liquid phase as explained by geologists. Soils, sands, ceramics, gypsums, bricks, cement and marbles were in the descending order of relatively low radium equivalent concentration, while granite has a higher level than the worldwide limit of 370 Bq/kg. Furthermore, granite material was the highest among the other investigated six hazard parameters indices. Thus, the appropriate amount of radioactivity and quantity of granite should be taken into account when used internally for decorative purposes. Generally, for the sake of safe uses of these building materials in the construction of dwellings, these materials have to be monitored and addressed to avoid threatening hazards of radiation to the public. Furthermore, it is recommended to classify the available building materials in the markets, according to their maintained radioactivity (i.e. radium equivalent concentration and radiological indices), which, in turn, improves the sustainable consumption and production patterns of building materials by public and factories. Moreover, this study can provide baseline data for the level of radioactivity in building materials, which is useful for the national level and orientation of radioactivity. The study results recommend a routine evaluation of the radionuclide concentration of the commonly used building materials for the sake of radiation protection precautionary to the public as well as to the workers.

Authors’ contributions

Jameel E. Salman: conceptualization, formal analysis, investigation, resources, data curation, calculations, writing the original draft, review & editing, visualization and publication. Nabil M. Hassan: proposed the idea of a review, writing the final version – review & editing, visualization, supervision and project administration.

Acknowledgement

The authors thank the University of Bahrain for their effective support, encouragement of research and electronic library services.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors note that statistical data supporting the findings of the article are available in the Tables section.

Appendix