Naturally occurring radioisotopes (²³⁸Th, ²³²U series and ⁴⁰K) are the main source of both external and internal radiation exposure in humans, whence the radiation finds its way into building materials, air, water, food, and eventually the human body. Human beings are exposed to external radiation from cosmic rays and terrestrial radiation. These isotopes enter the human body by intake from food and inhalation and result in exposure to internal radiation [1] [2] . According to the report by UNSCEAR, the total exposure per person resulting from ingestion of terrestrial radioisotopes was 0.3 mSv, of which 0.17 mSv comes from ⁴⁰K and 0.12 mSv comes from thorium and uranium series. Exposure from inhalation of terrestrial radioisotopes contributes another 0.01 mSv. Thus, the internal dose of terrestrial radioisotopes can be estimated from the concentrations of natural radioisotopes in foods [3] . Internal radiation exposure, mainly affecting various levels of the respiratory tract, is due to the deposition of short lived progeny products of radon, which are exhaled from building material into indoor air [4] [5] . Radon (²²²Rn), a gaseous product of decay of ²²⁶Ra, is extremely important in an indoor exposure. Radon easily diffuses from the ground and building materials to indoor air and is the source of exposing bronchus and lungs [6] - [8] . The isotope ¹³⁷Cs is produced anthropogenically by several types of nuclear activity, including past testing of nuclear weapons, accidents in nuclear facilities, reprocessing of spent nuclear fuel and nuclear power reactors. All these materials contain some amount of natural radionuclides that cause exposure of people to ionizing radiation. The measurement of activity concentrations of radionuclides in polishing materials is important in the assessment of population exposures, as most individuals spend 80% of their time indoors [9] . Long exposures to low levels of ionizing radiation and poor ventilation of residential buildings can seriously increase health risks to polishing materials [10] . Polishing materials have a variety of chemical natures and some of them are toxic to humans and may cause sickness including cancers and death [11] . Even nontoxic polishing materials can pollute the land or water. Therefore, care needs to be exercised when using and disposing such materials [12] . The present study was undertaken with the purpose of determining natural radioactivity in some polishing materials. The data obtained are essential for the development of standards and guidelines concerning the use and management of polishing materials.

Twenty-two samples representing commercial species of the most commonly consumed polishing materials were collected from Saudi Arabia local market. They were being used for cleaning and polishing purposes of marble, granite, ceramic, tiles, stucco, brick, porcelain, chrome and metal, houses and vehicles. Weighed samples were stored in standard polyethylene Marinelli beakers of 650 cm³ volume each. The beakers were tightly sealed for 4 - 5 weeks to reach secular equilibrium where the rate of decay of the progeny becomes equal to that of the parent (radium and thorium) [13] . Table 1 shows samples description.

The activity concentrations of ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs were measured with gamma-spectrometric system based on HPGe. The samples were counted using a gamma spectrometry (Canberra coaxial-type Model GC2520) with relative efficiency of 25% and resolution FWHM of 0.2 keV for 1332.5 keV gamma-ray peak of ⁶⁰Co and a peak to

Compton ratio of 50:1. The gamma-spectrometry system consists of HPGe connected to a desk top computer provided with a Canberra multichannel analyzer (MCA) in conjunction with a configuration software for spectrum acquisition and evaluation [14] [15] .

The specific activities were averaged from gamma-ray photo peaks at several energies. The gamma-ray lines at 295.2 and 351.9 keV from ²¹⁴Pb and at 609.3 and 1764.5 keV from ²¹⁴Bi were used to determine the specific activity of ²²⁶Ra. The gamma ray lines of 338.4 keV; 911.2 keV and 968.97 keV from ²²⁸Ac, the 238.58 keV and 727.3 keV from ²¹²Bi and 583.2 keV from ²⁰⁸Tl were used to determine the specific activity of ²³²Th. The specific activities of ⁴⁰K and ¹³⁷Cs were measured directly by its own gamma-ray lines at 1460.8 and 661.66 keV, respectively [13] . The minimum detectable amount (BDLs) for ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs were 0.312 ± 0.16, 0.340 ± 0.16, 1.660 ± 1.10 and 0.45 ± 0.06 Bq/l respectively. Radioactivity concentrations of each sample were measured for about 23 h. The data acquisition, display and spectrum analysis were carried out using a dedicated software program [16] , which enabled also the concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K to be calculated. The degree of secular equilibrium reached between ²²⁶Ra and ²³²Th and their decay products was taken into account during the concentration calculations.

The specific activity of ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs, in the samples are shown in Table 2. The highest average concentrations of ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs (13.60 ± 0.0015, 18.43 ± 0.0031, 342.59 ± 0.0091and 1.47 ± 0.0005 Bq/l respectively) were found in samples P5,

ND: Not detected.

P6, P5 and P8 respectively, whereas the lowest average concentrations ²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs (0.60 ± 0.0015, 0.78 ± 0.0011, 2.47 ± 0.0005, 0.55 ± 0.0008 Bq/l respectively) were found in samples P3, P17, P15 and P5. The ⁴⁰K activity concentration dominated over that of the ²²⁶Ra and ²³²Th, as normally happens in samples, while the highest activity level of ⁴⁰K (342 Bq/l) which was recorded in sample P5 was lower than that of the global average of 400 Bq∙kg⁻¹ [17] .

The variation among the activity levels in different samples may be attributed to the pH, and chemical composition of the raw materials from which they derived [18] . Some studies present the concentration values of ²²⁶Ra only and do not consider the ²³⁸U presence taking into account the fact that 98.5% of the radiological effects of the uranium series are produced by radium and its daughters [19] .

The activity concentrations of Ra, presented here, fall within the range reported by other authors. Both Th and K in this work revealed higher concentration levels compared to those level for painting oxides and paints. However, their levels were lower than in the case of titanium enamel frits (Table 3). According to the recommended reference level of 30, 25 and 370 Bq/kg for ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, for the world average concentrations published by UNSCEAR [1] , it was noted that the obtained results in all samples are lower than the recommended reference level.

Uniformity of the distribution of ²²⁶Ra, ²³²Th and ⁴⁰K in materials with respect to radiation exposure has been described by a common index. This index is called the radium equivalent (Ra_eq) activity and is defined as a weighted sum of the activity concentrations of the above three radionuclides. Ra_eq is given in Bq∙l⁻¹ to compare the specific activity of materials containing different amounts of ²²⁶Ra, ²³²Th and ⁴⁰K. It was assumed that 370 Bq∙kg⁻¹ of ²²⁶Ra, 259 Bq∙kg⁻¹ of ²³²Th or 4810 Bq∙kg⁻¹ of ⁴⁰K produce the same γ-ray dose rates [20] ; Ra_eq is calculated through the following relationship:

where C_Ra, C_Th, and C_K are the specific activities of ²²⁶Ra, ²³²Th, and ⁴⁰K, respectively, expressed in Bq∙l⁻¹. The maximum value of Ra_eq in building materials must be less than 370 Bq∙kg⁻¹ for safe use of materials in the construction of buildings [1] , to keep the external dose below 1.5 mSv∙y⁻¹ [21] . The results obtained by us showed that the lowest

Ra_eq was equal to 1.24 Bq∙l⁻¹ in sample P17, while the highest value reached 51.37 Bq∙l⁻¹ in in sample P5 (Table 2). All the studied samples showed lower values than the recommended worldwide mean value 370 Bq kg⁻¹. Thus, every material used to clean and polish of marble, granite, ceramic and tile can be considered to does not reveal a significant radiological hazard.

The activity levels of the natural terrestrial radionuclides as ²²⁶ Ra, ²³² Th, ⁴⁰K and artificial ¹³⁷Cs were determined using a gamma-ray spectrometry in 22 samples of polishing materials collected from local stores. The obtained values were, in general, comparable to the corresponding ones obtained from other studies, and they all fell within the average worldwide ranges. Basing on the measurement results, the values of the radium equivalent activities Ra_eq were calculated to assess the radiological hazard for inhabitants of Jeddah. The obtained values displayed to be below the permissible limiting values.

Alharbi, W.R. (2016) Investigation on ²²⁶Ra, ²³⁸Th, ⁴⁰K and ¹³⁷Cs Concentrations in Common Polishing Materials Consumed by Inhabitants in Saudi Arabia. Journal of Environmental Protection, 7, 1466-1472. http://dx.doi.org/10.4236/jep.2016.711123