Effect of human body position on gamma radiation dose rate from granite stones

Abstract

The use of granite stones as building materials in homes or offices can result in the residents’ long-term whole-body exposure to gamma radiation. Although the whole-body annual dose has been investigated in the literature, it is obvious that different human organs receive different equivalent dose due to different position respect to the walls and floor covered by granite stones. In this paper, the effect of distance from the walls and floor of a room on the equivalent dose is investigated by using MCNPX code. An “ORNL” phantom is simulated in three situations; standing (P1), sleeping one meter above the floor (P2) and sleeping on the floor (P3) and the equivalent dose in different organs is calculated. Excess lifetime cancer risk (ELCR) is calculated in the whole of the body for these three positions. By the results, the value of ELCR in the third position is more than the average world value (2.9 × 10⁻⁴). The results show the importance of considering body position in dose determination, especially in some organs such as the brain and eyes which are close to the granite stones in certain positions such as sleeping.

Introduction

Natural radioactive materials have existed since the Earth’s formation. Some radioelements having very long half-lives remain in the Earth’s crust and have been decaying for many years. These radioelements have been found in all-natural materials, for example, soil, drinking water, breathing, foods, the stones used in the buildings, plants, river, ocean, etc. Our bodies - muscles, bones, and tissue contain naturally occurring radioactive elements. In addition to receiving the radiation from these sources, we receive the radiation from outer space called cosmic radiation or cosmic rays.

A variety of materials may be used to make countertops, including artificial materials, quartz, marble, slate, and granite. Granite is one of the best stones that is used in the home, hospitals, schools, offices, etc. Some granites have been reported to have higher radioactivity, leading to increased safety concerns. Therefore, it is important to study the radiation of these types of stones around the human body. Potassium-40 (⁴⁰K), series of Uranium-238 (²³⁸U) and Thorium-232 (²³²Th) are the most important radionuclides available in the granite stone which produced gamma radiation with energy distribution from 30 keV to 2.6 MeV. Some works have studied the radiation of different granites and found that the average concentration of these radioelements in some granites is more than others [1–13].

Due to the high concentration of radionuclides in these granites, which leads to an increase in the effective annual dose and absorbed dose, investigation of the gamma absorbed dose by granite in the human body is necessary especially in some sensitive organs such as bone, lung, and eyes. Although there are some studies about the absorbed dose in some organs of the human body due to building materials and soil [6–10], in our view, there are no studies about the effect of different position of human organs on the absorbed dose, for example, due to the different distance from the radiation source or different states of the human body, such as standing, sitting and sleeping. More research is needed to address the comparative effectiveness of effective annual dose versus the human body position.

The distance of the organs of the body from the floor and the wall of a room is different and depends on the position of the human body. Therefore, the dose received in different organs of the body varies due to being located in places where the walls or floor are covered with granite. Due to the high activity of some types of granite stones, the proximity of different human organs to granite in different positions of the body may increase the dose to more than the recommended level. This study focuses on absorbed dose rate and effective annual dose rate from granite stone as building materials and their dependency on the distance of different organs of the body from the granite at different positions of the human. We will also address the question of how different granite, depending on the position of the human body, leads to increase in the Excess lifetime cancer risk (ELCR) to an excessive extent or not.

Materials and methods

The granite stone which has been collected for this experiment was pulverized and sieved through a 0.2 mm mesh. The sample was dried in an oven at 110 °C for 24 h to remove moisture content. It was packed into cylinder plastic bottle of volume 1000 ml, kept sealed for four weeks to reach secular equilibrium between ²²⁶Ra and ²³²Th with their decay products. Activity measurements were performed using a high-resolution HPGe gamma-spectroscopy system. Having relative efficiency of 2.7% at 121.78 keV and resolution (FWHM) of 980.67 at 121.78 keV with ¹⁵²Eu isotope. Equation 1 has been used to calculate specific activities of natural radioactivity levels in this granite:

$$A\left(\mathit{Bq}{\mathit{kg}}^{-1}\right)=\frac{\left(n-n_0\right)}{\left(\epsilon \times \gamma \times m\right)}$$ 1

where n and n₀ are count rate and background count rate (counts per second); ε is the efficiency of detector for energy line E, γ is gamma abundance for energy line E and m is mass of the sample (kg).

To investigation the absorbed dose in different organs of the human body based on using granite stones in buildings, the effects of distance between the organ and the floor and walls must be considered. To study the variation of dose versus the position of the organs of the human body, we simulate a room in MCNPX 2.6 code with 2.8 m height where the wall and floor of the room have been covered by granite stone with 3 cm thickness. For definition of the granite stone, a typical granite stone was used (see Table 1). Table 2 shows the activity concentration of 232Th, 226Ra and 40 K (Bq/kg) in Nehbandan granite stone in Iran and three different types of rocks. The Nehbandan granite stone was selected due to its highest activity concentration from the other granite samples in Iran.

Table 1.

Compositions (in %) of typical granite stone [1]

Fe2O3	FeO	CaO	Na2O	K2O	Al2O3	SiO2
1.22	1.68	1.82	3.69	4.12	14.42	72.04

Table 2.

Activity concentration of ²³²Th, ²²⁶Ra and ⁴⁰K (Bq/kg) in the simulated granite stone based on Nehbandan stone in Iran and three different types of rocks [1]

Sample code Granite stone name		Activity concentration (Bq/kg)
		232Th	226Ra	40K
G1	Nehbandan	172.2	94.2	1532.2
G2	Germez yazd	59.2	29.9	1047.2
G3	Morvarid sabz	6.5	3.8	556.9
G4	Alborz	166.6	79.2	1234.6

We are also interested in studying the dependency of dose changes versus height, room dimension, granite activity and the composition of human organs. To study dose variations in terms of distance from the granite, the deposited photon energy in a cube placed at the center of the room from a height of 170 cm to a height of 10 cm is calculated by using F6 tally in MCNP code.

For room dimension, five different floor area 5 × 5, 4 × 4, 3 × 3, 2 × 2 and 1 × 1 m² have been considered. Also, we investigate two different cases for walls, with and without granite. For activity concentration, three different activity is considered to find how dose rate changes relative to height depends on the granite stone activity. All of these studies are evaluated for different organs mentioned in Table 3.

Table 3.

Elemental compositions of some organs of the human body [14]

		Elemental compositions (% by mass)
Compound	Density (g/cm3)	H	C	N	O	Na	P	S	Cl	K	Ca	Fe	I
Adipose	0.95	11.4	59.8	0.7	27.8	0.1	–	0.1	0.1	–	–	–	–
Heart	1.06	10.3	12.1	3.2	73.4	0.1	0.1	0.2	0.3	0.2	–	0.1	.
Brain	1.04	10.7	14.5	2.2	71.2	0.2	0.4	0.2	0.3	0.3	–	–	–
Liver	1.04	10.2	13.9	3.0	71.6	0.2	0.3	0.3	0.2	0.3	–	–	–
Cartilage	1.10	9.6	9.9	2.2	74.4	0.5	2.2	0.9	0.3	–	–	–	–
Skin	1.09	10.0	20.4	4.2	64.5	0.2	0.1	0.2	0.3	0.1	–	–	–
Soft tissue	1.05	10.5	12.50	2.60	73.50	0.2	0.20	0.18	0.22	0.21	0.01	0.01	0.01
Lung	0.29	10.13	10.23	2.86	75.75	0.1	0.08	0.22	0.26	0.19	0.14	0.03	0.01

Besides of studying the effects of height on the absorbed dose, we simulate an ORNL phantom in the middle of a room with standard dimensions in three cases; standing, sleeping at height 1 m and sleeping on the floor. Calculating the dose absorbed in phantom of the body in these conditions enables us to examine the rate of dose increase in various organs of the body, especially important organs such as the eyes and brain due to their reduced distance to granite in situations such as sleeping. Following equation is used for annual dose rate calculation:

$$\mathrm{E}\left(\mathrm{mSv}{\mathrm{y}}^{-1}\right)=\mathrm{D}\left(\mathrm{nGy}{\mathrm{h}}^{-1}\right)24\left(\mathrm{h}{\mathrm{Day}}^{-1}\right)\times 365\left(\mathrm{Day}{\text{year}}^{-1}\right)\times 0.7\left(\mathrm{Sv}{\mathrm{Gy}}^{-1}\right)\times 0.8$$ 2

where E is the effective annual dose rate, 0.8 is used as indoor occupancy factor, D is absorbed dose rate calculated by MCNPX code and the final part is the absorbed dose conversion to the effective dose for an adult, according to UNSCEAR 2000 [15].

Effective annual dose for ORNL phantom is calculated according to Eq. 3:

$$E_{\mathit{\text{total}}}=\sum _{\mathrm{T}}{W}_T{H}_T$$ 3

where H_T is equivalent dose in tissue and W_T is tissue weight factor. The values of W_T are given in ICRP publication 60 [16].

Excess lifetime cancer risk for the whole of the phantom is also calculated by using the following equation [17]:

$$\text{ELCR}={\mathrm{E}}_{\text{total}}\left(\mathrm{mSv}{\mathrm{y}}^{-1}\right)\times 70\left(\mathrm{y}\right)\times 0.05\left({\mathrm{Sv}}^{-1}\right)$$ 4

where 70 y is the duration of life and 0.05 Sv⁻¹ is the risk factor fatal cancer risk per Sievert. This value is used for stochastic effects and is given in ICRP 103 [18].

Results and discussion

Figure 1 shows the semi-logarithmic variation of effective annual dose rate versus the voxel height for different area of the floor. Here, only the floor has been covered by granite, not the wall. Results show the linear variation of the Energy logarithm versus the height as follows:

$$\mathrm{Log}\left(\mathrm{E}\left(\mathrm{mSv}{\mathrm{y}}^{-1}\right)\right)=\mathrm{a}\times \mathrm{H}\left(\mathrm{cm}\right)+\mathrm{b}$$ 5

Fig. 1.

Variation of effective annual dose rate (in mSv y⁻¹) versus height (in cm) for different areas of the floor covered by granite (not the wall). The sloop of the fitted lines are −0.0058, −0.0043, −0.0034 and − 0.0027 for areas 2 × 2, 3 × 3, 4 × 4 and 5 × 5 m², respectively

where b = 0.3 is approximately independent of the area, and a is an area-dependent variable which is −0.0058, −0.0043, −0.0034 and − 0.0027 for areas 2 × 2, 3 × 3, 4 × 4 and 5 × 5 m², respectively. Using Eq. 5, by knowing the dose in any height, the dose can be found at other heights.

In Fig. 2, similar calculations have been conducted where the wall, likes the floor, has been covered by granite stone. It is assumed that the lower part of the walls (with 1 m height) has granite coating. In this case, the results show that the effective annual dose linearly depends on height as follows:

$$\mathrm{E}\left(\mathrm{mSv}{\mathrm{y}}^{-1}\right)=\mathrm{\alpha }\times \mathrm{H}\left(\mathrm{cm}\right)+\mathrm{\beta }$$ 6

Fig. 2.

Effective annual dose rate (in mSv y⁻¹) versus height (in cm) where both floor and wall have been covered by granite. The sloop and intercept of the fitted lines are (−0.0022,0.54), (−0.0016,0.50), (−0.0013,0.48) and (−0.0011,0.46) for floor areas 2 × 2, 3 × 3, 4 × 4 and 5 × 5 m², respectively

where (α,β) are (−0.0017,0.43), (−0.0013,0.40), (−0.0010,0.38) and (−0.0008,0.36) for 2 × 2, 3 × 3, 4 × 4 and 5 × 5 m² dimension of the floor.

According to the results, the dose varies exponentially with height when only the floor of the room is covered with granite, while when both the floor and the wall are covered with granite, the changes are linear. Figure 3 shows the reason where the effective annual dose has been plotted separately.

Fig. 3.

Effective annual dose rate for three different types of covers in the 4 × 4 m² room. Floor: Just the floor has been covered by granite. Wall: Just the wall (with 1 m height) has been covered. Floor and wall: Both floor and wall have been covered by granite

To investigate the variation of effective annual dose versus height for different granite stones, we have conducted the same calculation as Fig. 1 for the granite stones mentioned in Table 2. By the result, showed in Fig. 4, the sloop of semi-logarithmic variations does not depend on the activity concentration of the stones. Also, difference between the dose rates is due to the difference between the stones’ total activities, not the radioelements activities.

Fig. 4.

Effective annual dose rate at a different height for different granite stones of Table 2. Here, the floor size is 4 × 5 m² and just the floor is covered by granite stones, not the wall

The results show that by reducing the distance from granite, from 170 cm to 10 cm, the dose increases up to about 5 times. It shows that different positions of the human body may be lead to significant changes in the absorbed dose in different organs. However, as the distance of organs from the floor or walls of a room depends on their location in the body, the absorbed dose changes significantly depend on the location of the tissue. To investigate this dependency, an ORNL phantom is simulated in three different situations; standing, sleeping one meter above the floor and sleeping on the floor, named P1, P2, and P3, respectively. Table 4 shows the effective annual dose rate for each organs in these three cases where only the floor with 4 × 5 m² dimensions, has been covered by Nehbandan granite stone.

Table 4.

Effective annual dose rate for different organs of ORNL phantom in three different positions (P1: Standing, P2: Sleeping 1 m above the floor and P3: Sleeping on the floor)

Organs	Effective annual dose rate in three positions (×10−3 mSv y−1)			The relative difference with P1 (%)
	P1	P2	P3	between P1 and P2	between P1 and P3
Legs	2.38	1.60	3.07	−33	29
Head	0.908	1.78	3.81	96	322
Skin	2.35	2.03	4.24	−14	81
Right lung	14.4	23.4	43.8	62	203
Left lung	14.2	23.9	44.9	68	215
Liver	5.10	6.53	12.0	28	136
Stomach	16.6	17.5	32.3	5	95
Urinary bladder	6.76	5.08	9.04	−25	34
Brain	0.812	1.66	3.90	105	382
Esophagus	3.89	7.15	13.7	84	252
Colon	17.6	18.5	32.7	5	86
Leg bones	3.12	2.11	3.88	−33	25
Arm bones	1.64	2.15	4.21	31	157
Clavicle	2.23	2.90	6.39	30	186
Scapulae	0.134	2.84	0.560	111	317
Pelvis	3.36	4.55	8.22	35	145
Rib cage	2.31	3.38	6.78	47	194
Spine	1.17	2.48	4.65	112	298
Skull-cranium	0.503	9.49	2.14	89	326
Facial skeleton	1.52	2.74	6.59	81	334
Thyroid	1.29	2.13	4.78	64	269
Kidneys	2.43	4.53	8.23	86	238
Pancreas	2.01	3.06	5.45	52	171
Spleen	2.29	3.88	7.08	70	209
Thymus	2.03	2.03	4.11	0	103
Adrenals	2.07	4.13	7.41	99	258
Gall blander	4.93	5.54	10.0	12	103
Heart	2.10	2.86	5.47	36	160
Small intestine	2.47	2.87	5.01	16	103
Trunk	1.86	2.36	4.51	27	143
Total	124.51	164.03	309.03

According to Table 4, the effect of radioactive stones in the third case is more than two other cases. The dose rate in P2 situation is more than P1 except for some organs like legs, since the distance between legs and floor increases by changing the body situation form standing to sleep one meter above the floor.

The results show that the maximum dose rates are for lung and colon (about 0.04 and 0.03 mSv y⁻¹) in P3 situation. The most increasing dose rate due to the changing of the organ position is for facial skeleton and brain, which is reasonable due to the maximum changing of the head position. Considering approximately five times increase of the dose rate in the brain between P1 and P3 situations and 8 h of sleep during the day, the annual dose rate in brain increases from 0.0008 to 0.0039 mSv y⁻¹ if the floor of a bedroom is covered by granite stones,. Those values of effective annual dose are less than the value of indoor exposure (0.41 mSv/y) recommended by UNSCEAR 2000 [15]. According to Fig. 5, the value of ELCR in P3 is more than the value recommended in ICRP 90 [17]. The recommended value is 2.9 × 10⁻⁴.

Fig. 5.

Comparison between value of excess lifetime cancer risk in this paper and ICRP publication 90 [16]

The results of this study are comparable to other researchers’ reports. In comparison, Ref. [19] reported the annual effective dose rate due to building materials between 0.28 mSv y⁻¹ for marble sample and 0.42 mSv y⁻¹ for ceramic samples. Ref. [20] measured 0.86 mSv y⁻¹ indoor gamma radiation dose rate in Iran. The annual effective dose rate of building materials calculated in Weinan, China and Adana, Turkey were reported from 0.295 mSv y⁻¹ in the sand to 1.875 mSv y⁻¹ in tiles and 0.025–0.124 mSv y⁻¹ in the granite to 0.063–0.082 mSv y⁻¹ in the ceramic floor tile, respectively [21, 22].

In other recently reported results, only the annual dose for different stones has been reported regardless of body location. For example, Ref. [23] reported 0.003–0.646 mSv y⁻¹ annual effective dose for twenty-four granite samples used in Saudi Arabia. Ref, [24] reported maximum 1.77 mSv y⁻¹ annual effective dose in 20 granite samples obtained from the Erongo Mountain Belt and Usakos Dome of the Erongo region, Namibia. In Ref. [25] the measurements annual dose in some granite samples from China, Finland, Spain, India, Sweden and Polish were reported (between 0.1–25 mSv y⁻¹). Even in the latest articles reported in this field, the effect of body position on the dose has not been reported. The results of our research show that changing the position of the body may increase the dose by up to 2.5 times. Therefore, the position of the body in calculating the dose in places where granite has been used, especially in hospitals or work rooms, should be considered.

Conclusion

So far, a lot of research has been done on the radioactivity of different types of granite rocks in different countries. In some of these samples, such as Nehbandan stone of Iran, the annual dose is close to the recommended limit, and the use of these stones may cause the gamma dose to be exceeded of the recommended values. Studies in various countries so far have not examined the effect of human body location on the delivered dose. Due to the fact that the distance between different organs of the body from the floor and walls of a room is different, in this paper, the effect of changing the distance between different parts of the body from granite on the delivered dose was investigated.

The dose received in a voxel in the middle of a room whose floor and walls are covered with granite showed that by changing the height of the voxel from 170 cm to 10 cm, the dose increases between 3 and 10 times depending on the area of the floor. The results showed that the dose changes exponentially with height where only the floor is covered with granite. While for granite walls and floors, the changes are linear. Also, ORNL phantom was simulated in three different positions; standing, sleeping one meter above the floor and sleeping on the floor, to examine dose changes in different organs of the body. Comparing the whole-body equivalent dose between standing and sleeping situation showed the increasing of the annual dose from 0.12 mSv y⁻¹ to 0.30 mSv y⁻¹. Therefore, the widespread use of granite in places such as work room, bedroom or hospital, where the body position is more in a sitting or sleeping position, leads to an increase in the annual dose even higher than the recommended limit and can be a threat to people’s health in the long run.

Declaration

Conflict of interest

The authors declare that they have no conflict of interest.