Public exposure to natural radioactivity and radon exhalation rate in construction materials used within Greater Accra Region of Ghana

The natural radioactivity of 238 U, 232 Th, 40 K, radiological hazards and 222 Rn exhalation rate in building materials used within Greater Accra Region of Ghana, have been measured using Gamma spectrometry and CR-39. The results obtained are as follows 238 U (2.6–47.1 Bq/kg), 232 Th (3.6–43.0 Bq/kg), 40 K (62.8–1222.2 Bq/kg), radium equivalent (32.7– 174.3 Bq/kg), internal hazard index (0.04–0.51), external hazard index (0.04–0.23), activity concentration index (0.10–0.63 mSv/y) absorbed dose rate (9.5–76.3 nGy/h), outdoor effective doses (11.6–93.6 μSv/y), indoor effective doses (46.5–374.1 μSv/y,) and excess lifetime cancer risk (0.04 × 10 −3 –0.33 × 10 −3 ). 222 Rn exhalation rate (3.1 × 10 −5 –11.4 × 10 −5 Bq/m 2 h), 222 Rn activity (17.4–42.6 Bq/m 3 ), effective radium (0.19–0.64 Bq/kg). Positive correlation was found between 238 U and 222 Rn. The results were discussed in terms of limits to the accepted natural radioactivity levels and compared with similar studies reported in other countries. The gneiss rocks recorded excess lifetime cancer risk values of (0.32 × 10 −3 and 0.33 × 10 −3 ) greater than the world value of 0.29 × 10 −3 proposed by UNSCEAR, 20 0 0. With exception gneiss rocks from Shai hills which recorded high value of cancer risk as compare to the world average value, all the studied building materials do not pose any radiological effects to the people of Greater Accra Region when used for construction. © 2018 The Authors. Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative. This is an open access article under the CC BY license. ( http://creativecommons.org/licenses/by/4.0/ )


Introduction
Studies on naturally occurring radioactive materials (NORMS) and radon exhalation rate in geological and processed building materials are of particular interest because possible human exposure to 238 U natural background radiation and constitutes the largest source of radiation exposure to the public [1] . Human exposure is principally to the whole body from external gamma rays of 238 U, 232 Th, 40 K and to the lungs from internal alpha particles such as 222 Rn. Radon is a gas that emanates from soil and rock in the ground as well as building materials as a result of decay series of 238 U [2,3] . Radon decays to form radioactive particles that can enter the body by inhalation. Inhalation of these short-lived decay products of radon has been associated to an increase in the risk of developing lung cancer [1] . The radiation exposure of humans may increase for those living in houses built from materials with radiation dose rate values above normal background radiation levels [1,4,5] . The construction materials commonly known to the general public in Ghana are soil, sand, clay bricks, cement, tiles, rocks, sandcrete and concrete blocks. The usage of these building materials depends on the prevailing environmental conditions and availability. Natural radioactivity and radon linked with Ghanaian building materials have not yet been evaluated. At the present time, Ghana does not have national guidelines specifying the acceptable radioactivity and radon levels in local geological and processed building materials but have been using international limits. Most of the studies done in Ghana have concentration of 238 U, 232 Th and 40 K on soil, rocks and other environmental samples [6][7][8][9][10] . The present work on natural radioactivity in geological and processed building materials in Ghana comes as a part of a broader project that has just been initiated on public and occupational radiation exposure control programme in soils, building materials, indoor, food and water. This study will aid in decision-making processes in setting up national guidelines for the control of radiation exposure in building materials in Ghana. Gamma spectrometry and solid nuclear track techniques are the most widely used methods [3,[11][12][13][14][15] . In this study, gamma spectrometric and alpha techniques have been applied in the analysis of the concentration of the natural radionuclides of U, Th series and isotope of 40 K, and 222 Rn exhalation rate from building materials.

Geology of the study area
The Greater Accra Region is bordered on the north by the Eastern Region, on the east by the Volta region, on the south by the Gulf of Guinea, and on the west by the Central Region. The geology of the study area is made up of different types of rocks and soils. The main rock types of the study area are Precumbrian Dahomeyan schists, granodiorites, granitic gneiss, and amphibolites to late Precambrian Togo series comprising mainly quartzite, phillites, phylitones, and quartz breccias [16,17] . The soil of the study area is classified into four main groups: drift materials resulting from deposits by windblown erosion; alluvial and marine motted clays of comparatively recent origin derived from underlying shales; residual clays and gravels derived from weathered quartzites, gneiss and schist rocks, and lateritic sandy clay soils derived from weathered Accraian sandstone bedrock formations. In many low lying poorly drained areas, pockets of alluvial 'black cotton' soils are found. These soils have a heavy organic content, expand, and contract readily causing major problems with foundations and footings. In some areas, lateritic soils are strongly acidic and when saturated are prone to attack concrete foundations causing honeycombing. Near the foothills are the large areas of alluvial laterite gravels and sands [16,17] . Many of these deposits are being exploited in an uncontrolled manner for constructional purposes.

Sample collection and preparation
Geological and processed materials mostly used for construction in Ghana are soil, clay bricks, sand, cement (Portland, Dangote and Diamond Cement), rock, ceramics, gravel aggregate, concrete and sandcrete blocks. Geological and processed building materials data were obtained from Building and Road Research Institute (BRRI) of the Council for Scientific and Industrial Research, Kumasi, Ghana.
The data was used to determine the types of samples to be collected and the sampling points in selected areas in the Greater Accra, and Central regions of Ghana. Seventeen different types of construction materials weighing 1-2 kg were collected from different locations as shown in the Fig. 1 .
Rocks and soils were collected from quarry, open-pits and construction sites. Granite and gneiss were from greater Accra and central part of Ghana. Cements and block samples were collected from Antis Cement Depot and Ghana Atomic Energy Commission (GAEC) block factory located in Haatso and Atomic respectively. Cement commonly used in Ghana for building are Portland cement from Ghana cement company, Diamond manufactured in Aflao in the Volta region of Ghana while Dangote cement from Lagos, Nigeria. Beach sand were collected along the coastal parts of the region. Though the tiles used for this study were collected within the region, their countries of origin are China, Spain and Brazil. The samples were taken from at least three (3) different locations of the same area into labeled black polythene bags and kept separated. Fifty one (51) samples were then sent to Centro Regionale di Radioprotezione, Agenzia Regionale la Protezione dell' Ambientale del Friuli Venzia Giulia, via 42 Colugna, 33,100 Udine, Italy for analysis.
The rocks, tiles and blocks samples were pulverized, homogenized, air-dried and sieved to a uniform mixture with a particle size of about 5 μm and 50 ml geometry respectively. The sealed samples were weighed stored at room temperature for a period of 3-4 weeks to allow 238 U and 232 Th decay series to reach radioactive equilibrium with the short-lived progenies and also to prevent escape of radon gas [6,7,10,13] .

Measurements of activity concentration
The measurement of the activity concentration of the 238 U, 232 Th was by way of the daughter products. For a nuclide having more than one peak in the spectrum, the activity concentration was obtained as the weighted average activity at each peak.  63, 300.09 keV) were used to evaluate the activity of 232 Th. 40 K was determined using its only γray line of peak energy 1460.82 keV. Prior to sample measurement, the background was determined with an empty Marinelli beaker under identical measurement conditions as the samples. Counting time was 72,0 0 0 s. The data acquisition, display and on-line spectrum analysis were carried out using the Genie 20 0 0 V3.3 (1) spectroscopy software from Canberra. The activity concentration ( A C ) (Bq/kg) of each radionuclide in any given sample was calculated from the spectrum using the following analytical expression Ac( 238 U , 232 Th , 40 where, M sam (kg) is the mass of sample, N sam (cps) is the net peak area for the sample in the peak range, P(E) is the gamma emission probability, T c (s) is the counting time in seconds, and Ƞ (E) is the photo peak efficiency which had been obtained from the standard solution.

Determination of radon exhalation rate
Radon exhalation rate in geological and processed materials have been determined using tightly closed vessel technique with cylindrical jar. A known weight of a sample was placed at the bottom of the cylindrical jar and completely sealed for 1 month in order to establish equilibrium between radium and radon. Detection procedures were done by installing CR-39 detector at the top of chamber covering at a distance of 22 cm from the surface of the sample in order to count for only radon ( 222 Rn) and prevent thoron from evading the surface of the detectors [3,12] . The radon exhalation measurements were performed by placing the construction material at the bottom of glass containers with 10 cm diameter ( D ) and 25 cm height by the cylindrical vessel for 90 days. After the exposure, the detectors were removed and etched in 6.25 N solution of 90 °C for 4 h and 15 min at constant temperature followed by 15 min neutralization with of 36 ml of 96% diluted acetic acid. Finally detectors were washed in distilled water for (15) minutes to wash any excess chemicals and dried for four (4) days. The latent tracks formed on the detectors were scanned and counted in 144 fields using an optical microscope of 40 × magnification objective lens. The tracks density left on track films were then used to evaluate the radon concentration. The following mathematical models were used to estimate radon parameters: Activity of radon concentration C Rn Bq / m 3 = εt (2) where, ɛ is the calibration factor of detector (track/cm 2 d/(Bq/m 3 ), ϱ is the measured surface density of tracks (tracks/cm 2 ) and t is the exposure time where M = mass of building materials sample (kg) where, V c volume of diffusion chamber (m 3 ), S a is the surface area of the sample (m 2 ), λ Rn is the decay constant of radon (1/ s ) and T c is the effective exposure (s) in the diffusion chamber.

Activity concentration
The activity concentration of 238 U and 232 Th have been determined using their daughter products as well as the isotope of 40 K, with only one peak. The results of the activity concentrations and radium equivalent activity in Bq/kg measured are presented in the form of range and mean values as shown in Table 1 . The 40 K was found to be the most abundant and contributed significantly to the activity concentration in the building materials as compared to 238 U and 232 Th. The average activity concentrations of 40 K vary from 62.8 ± 12.5 Bq/kg in geological sample of sandstone from Tesano to 1222.2 ± 96.30 Bq/kg in gneiss rocks from Shai Hills. The highest average activity concentration of 238 U, was recorded as 47.10 ± 2.8 Bq/kg in processed sample of Dangote cement with the lowest value of 2.6 ± 0.5 Bq/kg coming from beach sand in Nugua. For 232 Th, the average activity concentration ranged from 3.6 ± 0 . 8 Bq/kg in beach sand from Labadi with a value of 45.6 ± 18.6 Bq/kg in granite from Dominase in the central part of Ghana. Highest activity concentration of radionuclides of 40 K, 238 U and 232 Th were obtained in Gneiss rocks, Diamond cement, granite rocks while the least radionuclides were found in sandstones, beach sand respectively. Beach sand appear to be the building material that contain the lowest radionuclides of 238 U and 232 Th. The reason for the difference in variation of the minimum and maximum activity concentrations of the 238 U, 232 Th and 40 K of the building materials were due to the topographical and geological differences within the sampling locations of the origin. Except Diamond, Portland, Dangote, clay brick and the soil from Oyibi, the average activity concentration of 238 U in all the construction materials in this studied were lower than world average value of 25 Bq/kg for soil [18] . Dangote cement recorded activity concentration of 1.9 times greater than the word average value for soil. The 232 Th activity concentration in building materials were also found to be less than 25 Bq/kg except granite from Kasoa, Dominase and gneiss rocks from Shai Hills in the central and Greater Accra Region of Ghana. The gneiss, granite, quartzite from Atomic and McCarthy, soil from Oyibi, tiles from China recorded activity concentration of 40 K far greater than world average of 370 Bq/kg [18] . Shai hills recorded highest 40 K concentration while sandstone from Tesano recorded 3.3 times greater and less than 5.9 times the world average value of soil proposed by UNSCEAR, 1988 respectively. The Table 2 showed comparison studies of average activity concentration with other studies in different countries.

Radium equivalent activity
Activity concentration of 226 Ra, 232 Th and 40 K are not uniformly distributed in building materials. In order to compare the specific activities of construction materials containing different concentrations of 226 Ra (radium is equivalent to uranium under the secular equilibrium condition), 232 Th and 40 K, the radium equivalent activity Ra eq is used. It was defined on the assumption that 370 Bq/kg of 226 Ra, 259 Bq /kg of 232 Th and 4810 Bq/kg of 40 K produce the same gamma-ray dose equivalent. It was calculated using the following equation [5,[19][20][21] .
R a eq = C Ra + 10 7 where, C Ra , C Th , and C K are the mean activities of 226 Ra, 232 Th, and 40 K (Bq/kg), in the building materials respectively. The maximum value of Ra eq must be 370 Bq/kg to keep the external dose to 1.5 mSv/y [15] . The building material whose radium equivalent index exceeds 370 Bq/kg must be discarded to reduce radiation hazards associated with materials. It has been Table 1 Activity concentration, radium equivalent activity, radon activity, radium concentration, radon exhalation rate in investigate materials.    ( continued on next page )  [1,5] . It can be concluded that, all the investigated building materials would not pose any significant radiological hazard when used for construction. However, from the results, it is noticed that the Ra eq varies considerably in the different materials and even in the same type of geological and processed materials from different areas as shown in Table 1 . The studied building materials were also compared with different countries and revealed that radium equivalent activity differ from one country to another even for the same type of construction materials as shown in Table 2 .

Radon exhalation rate, radon activity concentration and effective radium contents
The radon exhalation rate ( E Rn ), radon activity concentration ( C Rn ) and effective radium content ( C Ra ) varies from 3.1 × 10 −5 to 11.4 × 10 −5 Bq/m 2 h, 17.4 to 42.6 Bq/m 3 and 0.19 to 0.64 Bq/kg, in building materials used for this study in Table 1 . The maximum value of radon exhalation, radon concentration and radium contents were found in dangote cement while beach sand recorded the lowest of the three parameters. These results indicated that radon exhalation, activity concentration and radium contents are directly related to each in the investigated construction materials. The difference in the radon exhalation, concentration and radium contents could be due to minerals composition and geological formation of the origin of the materials [59][60][61] . It was observed that the radon exhalation rates from this study were less than the world average value of 1.25 × 10 −4 Bq/m −2 h [61] . This indicated that building materials will not pose any radiological effect when used as construction materials. The radon risk associated with the building materials was examined by establishing the relationship between the uranium activity concentration and radon exhalation rates in construction materials. A positive correlation of R 2 = 0.77 was found as shown the Fig. 2 , indicating that measured 222 Rn isotope was dependent on the decay of the 238 U radionuclide in the building materials.

Gamma activity concentration index
The gamma hazards of the building materials were calculated by adopting the following equation below, which is widely used at the investigation level for practical monitoring purposes [31] .  where, C Ra , C Th , C K are the radium, thorium and potassium activity concentrations (Bq/kg) in the construction material respectively. The European Commission [62] , proposed that activity concentration hazard could be classified into four classes, leading to two categories of materials used in bulk amounts and materials with superficial or restricted uses ( Table 3 ). Activity Concentration index ( I x ) from this study was found to be in range of 0.10-0.63. Concrete block and gneiss obtained highest and lowest values respectively. This result therefore, indicating that building materials from this region are suitable for use in bulk amounts without restrictions. It is also met the dose criterion proposed by European Commission and annual effective dose of 1mSv [62] .

External and internal hazards indices
To evaluate the external γ -radiation dose from building materials, according to ICRP (1991) the upper limit of radiation dose arising from building materials can be reduced to 1.5 mSv/y by using the conservative model proposed by [62,63] . This is by considering a finite thickness of walls and the existence of windows and doors. Taking these conditions into account, the equation used for the calculation of external hazard index ( H ex ) is given as where, AC Ra , AC Th , and AC K are the activity concentrations of 226 Ra, 232 Th, and 40 K, respectively, expressed in Bq/kg. This index must be less than unity so that the annual effective dose due to radioactivity in the construction material will be less or equal to 1.5 mSv/y [5] . The internal hazard index ( H in ) was also calculated using the expression [63] .
where, AC Ra , AC Th and AC K are the activity concentrations of 226 Ra, 232 Th, and 40 K, expressed in Bq/kg. The values of H in must also be less than unity to have negligible hazardous effects of radon and its short-lived progeny to the respiratory organs. The summary of the result from Table 3 , indicated that external and internal hazards ranged between 0.04-0.23 and 0.04-0.51. Concrete block from GAEC block factory and gneiss rocks from Shail Hills recorded lowest and highest hazards indices. Generally, it was found out that, values in this study do not exceed the recommended limits, indicating that the hazardous effects of these construction materials are negligible.
Where C Ra , C Th and C K , are the activity concentrations of 226 Ra, 232 Th and 40 K expressed in (Bq/kg) in the construction materials. The absorbed dose ( D R ) associate with this materials ranges from 9.5 to 76.29 nGy/h. It is observed that gneiss samples recorded the highest value (77.8 nGy/h), whereas the lowest value was found in concrete block (9.5 nGy/h). The studied values ( Table 4 ) were lower or comparable to that of the published world average dose rate of 84 nGy/ h [61] .

Annual effective dose equivalent (AEDE)
The annual effective dose equivalent (AEDE) in μSv/y to the public is calculated from the absorbed dose rate by using dose conversion factor of 0.7 Sv/Gy and the occupancy factor for outdoor and indoor 0.2 (5/24) and 0.8 (19/24) respectively [61] .
The annual effective dose is determined using the following equations The calculated indoor and outdoor AEDE values are presented in Table 4 . The minimum and maximum value for outdoor and indoor were found to be (11.6-93.6 μSv/y) and (46.5-374.1 μSv/y, respectively. All the studied samples recorded values far lower than the outdoor average effective dose of 460 μSv/y [61] . The values were also found to be less than the limit for public exposure control set by the International Commission on Radiological Protection and Organization for Economic Cooperation and Development-Nuclear Energy Agency [5,61] .

Excess lifetime cancer risk (ELCR)
The probability of developing excess cancer as a result of radiation exposure to radioactive materials over a lifetime at a given exposure level is called Excess Lifetime Cancer Risk (ELCR) [1] and the mathematical expression is given as: Where, AEDE is the annual effective dose equivalent, DL is the average duration of life time (estimated to be 70 years) and RF is the risk factor i.e., fatal cancer risk per sievert. For stochastic effects, ICRP uses RF as 0.05 for the public [64] .
The values obtained for ELCR ranged from 0.04 × 10 −3 to 0.33 × 10 −3 . The quartzite from Weija and Gneiss from Shai Hills recorded lowest and the highest values respectively. All the values obtained were found to be lower than the world average value with exception of the gneiss rocks from Shai hills which recorded values (0.32 × 10 −3 and 0.33 × 10 −3 ) greater than the world value of 0.29 × 10 −3 [61] . It can be attributed to the high activity concentration of 40 K content present in the rocks within the areas.

Conclusion
The activity concentration of 238 U, 232 Th, 40 K and 222 Rn exhalation rate and its related hazards were determined from the construction materials within Greater Accra Region of Ghana using gamma and alpha techniques. The average activity concentrations for 238 U, 232 Th and 40 K were found in the range of 2.6 ± 0.5-47.1 ± 2.8, 3.6 ± 0.8-45.6 ± 18.6, 62.8 ± 12.5-1222.2 ± 96.3 Bq/kg, respectively. The lowest 238 U and 232 Th activity concentrations were found in both beach sand from Labadi and Sakumono. The Dangote cement and granite from Dominase recorded highest activity concentration. The lowest and highest activity concentration of 40 K were determined in quartzite from Weija and the gneiss from Shail hills. The radium equivalent activity ranging from 32.7 to 174.3 Bq/kg, which are lower than the maximum acceptable value of 370 Bq/kg reported by UNSCEAR. The radon exhalation rate, radon activity concentration and effective radium content varies from 3.1 × 10 −5 to 11.4 × 10 −5 Bq/m 2 h, 17.4 to 42.6 Bq/m 3 and 0.19 to 0.64 Bq/kg. Radon exhalation rate measured for this study was less than the average value of 1.25 × 10 −4 Bq/m 2 h. The radiation hazards indices, were found to range from 0.04 to 0.23 for external and 0.04 to 0.51 for internal hazards. The activity concentration index was also found to vary from 0.10 to 0.63 lower than world acceptable value of 1.5 recommended by OECD. The absorbed dose rate vary from 9.5 to 76.3 nGy/h, and the corresponding effective dose for both indoor and outdoor dose rate were found to be 46.5-374.1 μSv/y and 11.6-93.6 μSv of which both exposures fell below the limit of 1.5 mSv/y recommended by the OECD. Its related Excess Lifetime Cancer Risk was also calculated to be 0.04 × 10 −3 to 0.33 × 10 −3 with gneiss rocks from Shai hills recording values greater than the world average value of 0.29 × 10 −3 recommended by UNSCEAR.
From the above mentioned results, with exception gneiss rocks from Shai hills which recorded high value of cancer risk as compare to the world average value, all the studied building materials do not pose any radiological effects to the people of Greater Accra Region when used for construction.

Conflict of interest
Financial support for this study was provided by the International Atomic Energy Agency (IAEA) and Radiation Protection Institute, Ghana Atomic Energy Commission. There are no perceived or conflicts of interest.