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 Table of Contents 
ORIGINAL ARTICLE
Year : 2014  |  Volume : 37  |  Issue : 3  |  Page : 120-131  

Assessment of airborne 238 U and 232 Th exposure and dust load impact on people living in the vicinity of a cement factory in Ghana


1 Nuclear Reactors Research Centre, National Nuclear Research Institute, Ghana Atomic Energy Commission, Legon, Accra, Ghana
2 Radiation Protection Institute, Ghana Atomic Energy Commission, Legon, Accra, Ghana
3 Institute of Environmental Science and Sanitaion Sudy, University of Ghana, Legon, Accra, Ghana
4 Radiation Technology Centre, Biotechnology and Nuclear Agriculture Research Institute, Ghana Atomic Energy Commission, Legon, Accra, Ghana
5 Environmental Protection and Waste Management Centre, Radiation Protection Institute, Ghana Atomic Energy Commission, Legon, Accra, Ghana

Date of Web Publication10-Apr-2015

Correspondence Address:
Peter Davor
Biotechnology and Nuclear Agriculture Research Institute, Ghana Atomic Energy Commission, P. O. Box LG 80, Legon, Accra
Ghana
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Source of Support: Carnegie Next Generation of Academic in Africa (CNGAA) for financing this work through the University of Ghana, Legon. The ethical support offered by the generality of the people of Akporkploe., Conflict of Interest: None


DOI: 10.4103/0972-0464.154865

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  Abstract 

Globally, the cement industry has been identified as one which causes significant particle pollution. In Ghana, environmental research in the neighborhood of the cement industry especially on human health is scanty. In the present work, attempts were made to evaluate the concentration of airborne dust at various distances and directions around the Diamond Cement Factory in the Volta Region of Ghana. The samples of dust were collected on filter papers and later analyzed for the concentration (mg/kg) of 232 Th and 238 U using neutron activation analysis. The principal objective of the study was to generate data intended at assessing the annual effective dose due to 232 Th and 238 U inhalation for both adult and children population living in the vicinity of cement factory. The data generated were supposed to assist in remediation decision making, if required. The study recored a few incidences of higher total dust load concentrations as compared to the permissible limit of 150 μg/m 3 specified by the Ghana Environmental Protection Agency. The calculated mean effective doses were 28.2 ± 1.06 μSv/year and 25.9 ± 0.91 μSv/year for both adult and child, respectively. From the radiological point of view, the study concluded that the people living in the vicinity of the cement factory are not at risk to significant radiological hazards. However, the study indicated the need to have a complete evaluation of the impact of the factory on the environment assessment programs which should include both chemical and radiological toxicity.

Keywords: Atmospheric pollutants, committed effective dose, hazard quotient, natural radionuclides, total suspended particle and cement dust


How to cite this article:
Addo MA, Darko E O, Gordon C, Davor P, Gbadago J K, Faanu A, Kpeglo D, Ameyaw F. Assessment of airborne 238 U and 232 Th exposure and dust load impact on people living in the vicinity of a cement factory in Ghana. Radiat Prot Environ 2014;37:120-31

How to cite this URL:
Addo MA, Darko E O, Gordon C, Davor P, Gbadago J K, Faanu A, Kpeglo D, Ameyaw F. Assessment of airborne 238 U and 232 Th exposure and dust load impact on people living in the vicinity of a cement factory in Ghana. Radiat Prot Environ [serial online] 2014 [cited 2020 May 30];37:120-31. Available from: http://www.rpe.org.in/text.asp?2014/37/3/120/154865


  Introduction Top


The cement manufacturing industry has been identified as one with great potential of causing atmospheric particle pollution. [1],[2],[3] The main airborne pollutants of cement production to the environment are the emission of dust and gases. [4] Cement manufacturing is through a series of process stages that include the mining, crushing and grinding of raw materials, blending and kiln burning to form clinker, cement milling, and packaging. [5] Dust is significantly emitted during all these processes exposing workers and the environment to particulate pollution. [6] It is reported that in a cement facility in Egypt, 1 kg of cement manufacture generates about 0.07 kg of dust which is lost to the atmosphere.

In general, common materials used to manufacture cement include limestone, shells, and chalk or marble. These ingredients are derived from rock and soil and contain mainly natural radionuclides of the uranium ( 238 U) and thorium ( 232 Th) series and the radioactive isotope of potassium ( 40 K). [7] The isotopes of 238 U, 232 Th, and 40 K can be a source of external radiation exposure through gamma-ray emission whereas internal exposure occurs through the inhalation of radon gas. [8]

These facts about radioactivity indicate that significant radioactive contamination may result from industrial processes in which natural radionuclides are concentrated to levels that cannot be disregarded on health and environmental grounds. [9] Such processes are not normally considered within the ambit of nuclear regulation (the exception being uranium mining), and the associated hazard may well remain undetected for many years. For instance, having identified that cement contain naturally occurring radionuclide, there is no difficulty in the understanding that the dust may also have these same radionuclide constituents. The radiation level in the dust is what should now constitute scientific interest to study the radiological impact on health.

Considering the fact that humans breathe in and out approximately every 4 s which equates to over 8 million in a year, airborne pollutants can therefore be regarded as an important environmental issue since it has a direct effect on human health. [10] For instance, airborne uranium can be inhaled and may enter the human system. Uranium, in its soluble form, is chemo-toxic and can primarily affect the kidney, causing damage to the proximal tubule while this metal has been also identified as a potential reproductive toxicant. [11],[12] Most of the uranium intake is removed by the kidneys and excreted in the urine in a few days. However, small amounts of this element may accumulate in some tissues of mammals, especially in bone. [13] It means that in some cases, chemical toxicity may be of concern than the radiological effects.

Atmospheric pollutants like natural radionuclides in a dusty environment may affect public health, particularly of young children, because of the greater probability of contact with the suspended dust. [14]

A little over a decade ago, a cement plant has been operating near Aflao, the Ketu South district, in the Volta Region of Ghana. The Diamond Cement (DIACEM) Factory has been the only industrial facility in the whole district. In recent years, public concern over adverse health effects for the population surrounding the facility has increased. For instance, data from the only health facility (Akporkploe Health Centre) in the area indicated that reported cases of respiratory symptoms increased by 100% between year 2011 and 2012. What exacerbates the problem is that the scarce investigation of airborne particulates and trace metal analysis leading to the evaluation of the human health risk of the surrounding population in the area. In response to these concerns, it was felt that data related to total suspended particulates (TSP) would be very useful and informative geared toward having some insight into the situation. The initial response was the conduction of air quality survey in order to determine levels of TSP and levels of 238 U and 232 Th contamination in the ambient air in various directions and radii distances from the DIACEM factory. The main objectives were to determine the inhalation risk due to uranium and thorium and also estimate the annual effective dose of the exposed population.


  Materials and methods Top


Study area

The study area is located in the South Eastern part of Ghana in the Ketu South district of the Volta region. The area is geographically enclosed between Latitudes 06.13400 N and 06.16650 N and Longitudes 01.16100 E and 01.19911 E, which is part of Ketu South Municipal Assembly. The area is bounded on the: North by the eastern boarder of the Republic of Togo; east by the Aflao township; west by Akplorkploe; and south by a lagoon which floods a wide area. The DIACEM factory is located 3 km north of the Aflao township [Figure 1]. The cement factory plays a significant role in the local building industry and the economy of Ghana. The factory was established in 2002 and was a major employer in the area. The area lies within the dry equatorial climate of the region. It has two rainy seasons with the major rains in April to June, and the minor rains between September and November. Minimum temperatures in the investigated area are 13.5°C and occur between the months of August and September, and the average maximum of 40°C is experienced between February and March.
Figure 1: Map of the study area showing the location of the cement factory

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The factory's surrounding area is essentially rural with minor agricultural activities. Settlements are scattered houses at varying distances with the nearest settlement at about 300 m from the factory. The surrounding vegetation is made up of several shrubs and grasses and lies within the coastal savanna agro-ecological zone. The geological formations of the investigated area are rocks of the Dahomeyan series of the Precambrian age. These rocks consist of dense aggregate of essential stable minerals which are bounded and have medium to coarse-grained granite texture. The Dahomeyan series are seismically stable and, therefore, there is no history of earthquake in the area. The soil types are mainly lateritic sandy soils, tropical black clays, tropical grey earths, and sodium vleisols. These soil types are suitable for the cultivation of different types of crops. [15]

Sampling and analytical method

Sample collection and preparation


Air samples were collected around the DIACEM factory. The sampling sites were selected in such a manner to cover the entire geographical vicinity of the cement factory. To provide a satisfactory environmental representation of the study area, concentric circles of radii 150 m, 300 m, 500 m, and 700 m around the main stack of the cement facility were taken into consideration. On each circle, eight sampling sites representing the north, north-east, east, south-east, south, south-west, and west and north-west directions were created on a map [Figure 2]. On the field, these predetermined points were located by means of a geographical positioning system. Sampling points which were located within the facility and a water-logged area were ignored. At least two sampling points representing each geographical axes of the factory were sampled, and the results computed into averages.
Figure 2: Study area showing points around the current facility

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To establish the average concentration of cement dust at the sampling sites, air particulate matter (dust) was collected on a 0.45 μm pore size Hollingsworth and Vose type HD-2064 with approximately 105 mm diameter filter paper with the aid of SAIC Air Sampler 12/24 VDC model H-809C to determine TSP, the air sampler was equipped with flow rate meter (rotometer). The instrument with a flow rate of 350 L/min (0.35 m 3 /min) has been calibrated by the manufacturer with SAIC-RADệCO CP-200 iodine cartridge and a 47 mm 0.3 μm filter paper. The sampler was generally positioned at a height of 1.6 m from the ground on an adjustable tripod stand and generally isolated from buildings and obstacles which may hinder maximum dust from reaching the sampler. The sampling time was approximately 4 h during the day (between 8.00 a.m. and 5.30 p.m.). Two different measurements were taken per each location, and the concentration results averaged out for the site. Before sampling, the filter was conditioned in a desiccator at 50% relative humidity for 24 h and weighed accurately on an analytical balance. The flow rate which was power supply dependent was measured at the start, during and at the end of the sampling period. At the end of each sampling period, the filter paper was removed from the sampler and placed in a 20 cm × 30 cm transparent polyethylene envelope. For easy identification, the sampling point codes were indicated on the envelopes. For each site, the sampling was repeated in order to determine the average of dust or TSP sampled.

The mass of particulate matter on the filters was weighed using a microbalance with an accuracy of ±0.0001 g. All samples were weighed 3 times in a row for repeatability, and the average of the three weights was used as the pre- or post-sampling weight for gravimetric calculations. Filter paper tongs were used to handle all filter papers to prevent dirt from the hand contaminating the sample.

The amount of total dust collected during sampling was calculated as the average of the three filters weighing of mass postsampling minus the average mass presampling. The average dust concentration for each personal sampler was then calculated using Equation 1:

(1)



Where:

Mdust= Mass of dust collected on each filter, g

C = Average dust concentration, μg/m 3

Q = Average sampler pump flow rate during the sample period, L/min

t = Length of the sampling period, min.

Instrumental neutron activation analysis of filters

The elemental analysis of the filter papers was carried out using instrumental neutron activation analysis (INAA). INAA has previously been shown to be feasible for multi-elemental determination in geological samples. [16] High accuracy, minimum sample handling, no added reagent, multi-elemental capability, and low detection limits are among the advantages of this analytical technique. As a result of such numerous advantages, it has been widely used in various elemental investigations. [17],[18],[19],[20]

Having determined the amount of dust in the filter papers, they were folded into multiple equal sectors, and the whole piece pushed into a 9.5 cm 3 rabbit capsule and sealed thermally before irradiation. The samples were then coded accordingly as the sampling points from where they were collected.

To validate the accuracy of the instrumentation, a series of geological standard reference materials (SRMs) were prepared, irradiated, and analyzed in the same manner as the analytical samples. The geological standards used were: Rock reference materials GBW07106 and GBW07107, all purchased from the Institute of Marine Geology in China; and IAEA CRM SL-1 Lake Sediment. In the preparation of the standards, blank filter paper was treated by spreading the standard material of approximately 0.010 g on it and folded into a rabbit capsule as was done to the analytical samples. All the analytical and standard samples were herein prepared in five replicates and analyzed accordingly. The standards were used to validate the accuracy of the analytical procedure employed for this work.

Irradiation and counting

Both the analytical samples and the SRMs were analyzed by INAA. The neutron flux used for the irradiation was approximately 5.0 × 10 11 n/cm 2 s. The samples were sent into the miniature neutron source reactor by means of a pneumatic transfer system operating at a pressure of 25 atmospheres. The scheme of the irradiation was chosen so as to take into account the half-lives of the radionuclides of interest. In that regard, the following irradiation times were selected: 10 s for the short-lived radionuclide ( 238 U); and 14,400 s for the long-lived radionuclide ( 232 Th).

After a short decay period, the activity of the gamma-ray emitting radionuclides with short half-lives was measured. Similarly, the gamma spectral intensities for the long half-life radionuclides were also measured between 2 and 4 weeks decay period, respectively. The experimental parameters for the NAA are captured in [Table 1]. [Figure 3]a and b shows the different spectra lines for the determination of 232 Th and 238 U.
Figure 3: (a) Spectrum showing the peak energy of U-238. (b) Spectrum showing the peak energy of Th-232

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Table 1: Experimental parameters for INAA


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The measurements of the gamma-ray spectral intensities were made using a spectroscopy system of the high purity germanium N-type coaxial detector model GR2581; high voltage power supply model 3105; and a spectroscopy amplifier model 2020 (all manufactured by Canberra Industries, Inc.,). The detector system at fixed geometry was coupled to an 8 k Ortec multi-channel analyzer (MCA) emulation card and a 486 microcomputer. The resolution of the detector system which operates at a bias voltage of − 3000 V full width at half maximum was 1.8 keV for 60 Co 1332 keV gamma-ray with 25% relative efficiency.

The output spectral intensities of both the analytical samples and standards were processed and stored in the microcomputer software by means of the MCA card. Qualitative analysis of the radioisotopes was achieved by means of identifying their spectral intensities. The evaluation of the areas through integration under the photopeaks of the identified elements was converted into their concentrations using the comparator method [21] which is given by the comparator Equation 2 below:

(2)



Where, Cstd = Known standard concentration in mg/kg

Csam = Known analyte concentration in mg/kg

Astd = Specific net area of standard element

Asam = Specific net area of analyte element.

In deriving Equation 2, it is assumed that the neutron flux (ncm−2 /s), cross sections, irradiation times (s), and all other variable parameters associated with counting are constant for both the standard and sample.

Health risk assessment model

In order to assess the health risks due to 238 U and 232 Th inhalation for both children and adults, the USEPA risk assessment guidance for superfund methodology, which has been widely applied in human risk assessment investigations was used. [22],[23],[24],[25],[26],[27] The expression to calculate the daily exposure (Dinh ) to U and Th through air inhalation is shown in Equation 3:

(3)



IR: Inhalation rate, in this study, 7.6 m 3 /day for children and 20 m 3 /day for adults. [28] EF: Exposure frequency, in this study, 180 day/year for both children and adults. [23] ED: Exposure duration, in this study, 6 years for children and 24 years for adults (USEPA, 2001). PEF: Particle emission factor, in this study, 1.36 × 10 9 m 3 /kg (USEPA, 2001). BW: Average body weight; in this study, 15 kg for children and 70 kg for adults (USEPA, 1989). AT: Averaging time; for noncarcinogens, ED × 365 days; for carcinogens, 70 × 365 = 25,550 days. C is exposure point concentration measured in mg/kg.

After calculating the three pathways of exposure, a hazard quotient (HQ) based on noncancer toxic risk was then calculated by dividing the average daily value by a specific reference dose (RfD): [29]







The RfD (mg/kg/day) is an estimation of the maximum permissible risk on human population through daily exposure, taking into consideration a sensitive group during a lifetime. [30] The threshold value of RfD can be used to indicate whether there was an adverse health effect during a lifetime. The RfD used for 238 U calculation was 300 ng/m 3 /day. The RfD for 232 Th is yet to be established and, therefore, the HQ estimations were ignored.

Annual effective dose model

The annual effective dose (mSv/year) owing to inhalation of U and Th in the ambient air surrounding the cement facility was calculated as: [11]

(5)



Where J = U, Th and DCF j.inh (U, Th) is the dose conversion factor for U, Th in mSv/Bq, Cj is the activity concentration of radionuclide in Bq/m 3 , Tex is the exposure time (h), Br is the breathing rate and Fr is the respirable fraction. The exposure time (Tex ) was fixed at 8760 h. For the adults and children, the breathing rate for light activity from International Commission on Radiological Protection (ICRP) 66 [31] of 0.36 and 0.27 m 3 /h was used, respectively. The dose conversion factor for both radionuclides and respirable fractions were obtained from the IAEA basic safety standard no. 115. The corresponding dose conversion factor used for calculating the annual effective dose for a child was ( 232 Th = 2.6 × 10−5 ; 238 U = 1.0 × 10−5 ) and ( 232 Th = 2.5 × 10−5 ; 238 U = 8.0 × 10−6 ) for an adult.

In the estimation of the HEinh, the relationship between the concentration of uranium and thorium in soil and corresponding activities of 238 U and 232 Th were considered as follows: [32 ]











Where (eU) and (eTh) represent equivalent uranium and thorium, respectively.


  Results and discussion Top


Total suspended particulate matter concentration

Results of the ambient air quality measurement around the vicinity of the DIACEM factory are shown in [Table 2]. In all 20 sampling points representing various directions and distances were chosen for the study. The levels of dust concentration ranged from 218 to 1771 μg/m 3 which were higher compared to the permissible limits of 150 μg/m 3 specified for residential and rural areas by the Ghana Environmental Protection Agency.
Table 2: Concentration of TSPM among sampling points in the study area


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The air quality monitoring data indicated that the average value obtained for the study area was 2.4 times higher than those recorded in the same periods in the reference area. This was expected and clearly indicated that the cement factory contribution to dust must be responsible for the high atmospheric pollution load in the study area.

Cement dust comprises both suspended particulate matter (SPM) (particle diameter >10 μm) and respirable particulate matter (RPM) (particle diameter <10 μm). The ambient air concentration of dust around the cement factory can be said to be very disturbing, during the sampling period. Under long-term exposure, there is a correlation between particle concentration and mortality from lung diseases. [33] Health effects of SPM in humans depend on particle size, concentration and exposure time. [34] Exposure of 200 μg/m 3 of particulate matter for a long-time can cause upper respiratory disease and between 294 and 470 μg/m 3 depress immune function in children. [33 ] The contribution of SPM and RPM from the dust concentration in the ambient air of the study area exceeds standard values for residential and rural areas throughout the study period.

The total dust concentration analysis with respect to distance from the factory indicated that, in general, the dust level reduces as one moves away from the facility. The average concentration range of the dust in respect of radii distance between 300 m and 700 m is 721 and 341 μg/m 3 , respectively. The average peak concentration (1032 μg/m 3 ) of the dust is toward the north although the predominant wind direction in the area is toward the north-east which registered average concentration of 883 μg/m 3 .

It is worth comparing the finding of the total dust levels of the current study with those of others around cement facilities. For instance, the average mean concentration in this study was higher than those conducted in Algeria [35] and south-west Nigeria. [36] Similarly, the total dust level by this study was found less than those performed in Iran and Ethiopia. [37],[38] The difference in levels of total dust may probably due to newer technology and effective control measures adopted by individual countries [5] and more importantly the environmental conditions.

238 U and 232 Th contents of the total suspended particulates

Validation of the analytical method


The method employed in the elemental analysis of the dust samples was validated by the determination of 232 Th and 238 U in SRMs GBW07106 (Institute of Marine Geology in the Republic of China). The result of the validation studies is presented in [Table 3]. In all cases, the results overlapped with reference values indicating favorable relationship between observed and reported values.
Table 3: Comparison of elemental concentration in various GBW7106 reference materials with measured laboratory values


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Concentrations in ambient air

Concentrations of 238 U and 232 Th measured in the samples of filter papers collected during the sampling period at the designated locations are reported in [Table 4]. The natural radionuclide concentrations increases or decreases with time and vary from the sampling points. The relationship between the radionuclide contents and time for the locations was different for each analyzed air sample. Descriptive statistics of the particulate matter associated with the concentrations of the two radionuclides is also presented in [Table 4].
Table 4: Concentration (mBq/m3) of 232Th and 238U in the airborne samples for the study area


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For the two radionuclides, uranium was the major contributor to natural radiation in the area for the sampling period. The concentration range of 238 U is from 7.79 to 62.6 mBq/m 3 with a mean value of 34.1 mBq/m 3 . 232 Th registered a mean of 4.39 mBq/m 3 with a concentration range of 0.6 to 1.2 mBq/m 3 in the filter samples. The peak values of both elements occurred in the north-western direction of the factory. The discrepancies in the elemental concentration distribution phenomena among the sampling sites might be occasioned by the cement dust free flow resistant plants which form a complete lining of the inner walls of the facility. The heights of these plants are enough to alter the wind speed and direction within the immediate vicinity of the factory and subsequently particles of the loose dust emanating from the factory.

Enrichment factor for airborne trace metals

In order to get a more comprehensive understanding of the origin of the trace metals in the atmospheric particulate matter, the concept of the  enrichment factor (EF) was introduced. The idea was to determine whether a particular metal is found in greater abundance in air or expected to be crustal in origin. [39],[40],[41] EFs are usually taken as double ratios of the target element and a reference element in aerosols and earth crust. [42] The reference element must be one that is, overwhelmingly derived from a natural source. The EF is calculated by means of Equation 8:

(8)



Where [X/Sc] TSP and [X/Sc] crust refers, respectively, to the ratios of the mean concentration of the target element X and Sc in atmospheric particulate matter and continental crust. The EFs were calculated on the basis of continental crust abundance for the metals given by Taylor. [43] EF values between 0.5 and 1.5 indicate the element is entirely from crustal materials or natural processes, whereas EF values >1.5 suggests that the sources are more likely to be anthropogenic. [44] It has been emphasized that these sources include among others human activities related to combustion, automobile and industrial emissions, and agricultural activities. [41]

The EF for various locations of the study area was calculated and shown in [Figure 2]. The analysis of the EFs revealed that though the concentration of an element could be high, it may not necessarily, be enriched. Large variations of the EF values were encountered in the natural radionuclides at various distances and directions from the cement factory. Meanwhile, the EF values have confirmed 238 U as the most contributor to ambient air natural radiations as it ranged between 2.03 and 11.88 with a mean value of 3.87 ± 2.23, while 232 Th ranged between 0.01 and 2.13 with a mean value of 0.57 ± 0.37.

Locations 16 (150 m, north-west) followed by location 35 (700 m, north) reported elevated EFs of 11.88 and 7.19, respectively, for 238 U. This situation apart from possible emission from cement dust contents may be due to the re-suspension of cement infested road dust from untarred routes by vehicular movement activities. 232 Th is typically dominated by crustal contributions, where it exists in the +4 valence state in most of its compounds and forms a number of minerals.

On the whole, judging from [Figure 4], it can be seen that the anthropogenic contribution to atmospheric radiation contamination is more pronounced in the area than those from natural processes. The main source of air pollution in the study area comes from cement production and airborne soil dust due to the movement of vehicles especially over unpaved road. Most of the sampling locations are found in settled areas of the study area. Therefore wind action and re-suspension of soil dusts contribute to significant inhalation of these contaminants, all of which have the potential of causing respiration problems to the rural dwellers.
Figure 4: Enrichment factor analysis for 238U and 232Th in the study area (Query)

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Effective dose assessment from ambient dust

The radiological dose exposure to the population around the DIACEM facility due to the inhalation of ambient dust was assessed. The people involved may be exposed to several broad categories of radioactive sources including the naturally occurring radioactive materials in cement dust. The assessment was made by the evaluation of the effective dose at points represented by various distances and directions from the cement factory, and this is represented in [Table 5] below.
Table 5: Effective dose (μSv/year) results at sites represented by various distances and directions from the DIACEM factory


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In determining the effective dose, Equation 7 was used for two age groups, adults (>18 years) and young persons (children) between 7 and 12 years. The radionuclides involved in the calculation were 232 Th and 238 U whose concentrations in μg/m 3 were converted to Bq/m 3 using Equations 6 and 7.

The results of the effective dose ranged between 11.9 and 50.7 μSv/year for adults and between 12.6 and 42.5 μSv/year for children indicating that it fell far below the international regulatory limit of 1.0 mSv/year for the public. The mean values were 28.2 ± 1.06 μSv/year and 25.9 ± 0.91 μSv/year for both adult and child, respectively. The trend of the dose does not follow any regular pattern in terms of radii distance and directions from the cement factory. This indicated that the radionuclides involved may not only be originating from the cement factory or through other natural processes, but also other anthropogenic activities might have contributed to their distribution. It is also possible that the highly nonuniform distribution of the natural radionuclides in the soil as the activity concentrations in the soil varied significantly within a small area can also affect the dose distribution pattern.

From the results, it can be inferred that an adult living for 1-year at a distance of 500 m in the eastern direction of the factory, receive a dose of 50.7 μSv. The result for a child may be similar to the adult according to information provided by [Table 5]. This reference site is a settled area with a cluster of rural dwellings. A serious radiological consequence is therefore not envisaged for the area in the medium term as the current results indicated for the people continuous exposure to respirable cement dust.

Noncarcinogenic risk assessment

The diameter of cement particles makes it a potential health hazard as these are respirable in size and reaches in internal organs particularly lungs leading to occupational lung diseases. Cement production of which, limestone is the basic raw material, has been identified as an important releaser of dust, the geochemical content of the parent material is a key issue to understand the release of elements originally contained in rocks. Dust concentrations of many elements usually reflect those of the local geology, [45] as it is known that granite rocks have higher amounts of uranium among other elements.

The natural radionuclides present in cement dust consist mostly of members of the 238 U and 232 Th decay chains, which are considered to be in equilibrium with their parent radionuclides. The radionuclides of the 235 U chain are negligible because of the small contribution of this isotope to natural uranium. Furthermore, 40 K has been disregarded in this study because its intake in the human body is being under homeostatic controls (Zeevaert et al.). [46]

The objective of the human health risk assessment for the population living around the DIACEM facility was to develop quantitative and qualitative estimate of potential noncarcinogenic risk for human receptors potentially exposed to the natural radionuclides of 232 Th and 238 U in airborne dust. These estimates were developed to support remediation decision-making. Noncarcinogenic risk is expressed in terms of HQs and hazard indices.

It is important to note that the specific toxicity data for inhalation are only available for 238 U. It is also important to note that the mean airborne value of 238 U exceeds that of the exposure concentration due to inhalation with the reference concentration (RfC) is set at 300 ng/m 3 by the USEPA. [47] The RfC of 232 Th is yet to be established; therefore the HQ could not be evaluated.

The calculated inhalation pathway due to 232 Th and 238 U exposure for both adults and children is given in [Table 6], and the HQ for 238 U is represented in [Table 7]. In the study, no HQ exceeds unity for both age groups which indicate inhalation noncarcinogenic safety due to the radionuclides involved. However, generally, larger HQs indicate greater levels of concern.
Table 6: Inhalation exposure (mg/kg/day) of 238U and 232Th in airborne dust in various directions regarding the DIACEM facility


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Table 7: Uranium - 238 HQ values for adults and children regarding various directions from DIACEM


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It is important to state that despite the HQs around the cement facility was less than unity could mean overall safety for airborne 232 Th and 238 U since ingestion pathway and dermal contact on the human receptors were not considered in the analysis. However, it could therefore be predicted that directions in north, south-east, and north-west of the cement would have suffered higher risk due to their HQs which were 5 times of magnitude less than the maximum threshold established at unity [Table 7]. With the exception of the south-east, all directions specified in respect of the factory are areas where settlements are concentrated, indicating that a concern in relation to the noncarcinogenic risk estimation. A very important deduction was that in all cases, the noncancerous risk for the adult population was higher than that of the children. The result is contrary to what [27] and [23] observed in noncarcinogenic studies regarding street dust. The difference here may be attributed to the fact that in the two previous studies nonradioactive elements were used for the noncarcinogenic inhalation studies.

In general, 238 U exhibits HQ > 0.1 for the adult population which is higher than those of the children. All that notwithstanding, HQ values for 232 Th and 238 U on airborne dust exposure to adults around the Diacem Facility are <1, indicating that there is little adverse health risk due to the dusty environment. However, some natural radionuclides could be accumulated in the human body for a long time, and especially the noncancer adverse effects of uranium to the tissues of adults can be quite serious. [48] Since these are the two natural radionuclides and some of their progenies are toxic, it is advisable that their environmental levels around cement operational facilities should be continuously monitored.


  Summary and conclusions Top


The current study has corroborated global perceptive that the cement industry is one of the most atmospheric particle pollution causing industries in that the dust level around the DIACEM factory far exceeds the local regulatory limit. Constituent of the airborne dust includes natural occurring radionuclides of 232 Th and 238 U and possibly their decay progenies, and also radioactive isotope of 40 K. These radionuclides are noted to contribute significant natural radiation to mankind. However, 40 K, although a radioactive element, was excluded from our analysis because it is vital for good health as it supports growth and maintenance of the human body. The intakes of these radionuclides account for a substantial part average radiation doses to various organs of the body and also represent one important pathway for long-term health effects.

The study measured the concentration of airborne dust around the DIACEM facility during day times only and also determined the levels of 232 Th and 238 U in the dust using INAA techniques. Annual effective dose and noncarcinogenic risk due to inhalation were the tools employed to identify where public health regarding the population around the facility is most likely to be impacted upon. The results indicated that the mean effective dose for children (28.2 ± 1.06 μSv/year) and adults (25.9 ± 0.91 μSv/year) was <1 mSv/year dose limit for members of the public as suggested by the ICRP. Furthermore, the noncarcinogenic inhalation risk estimate for both age groups shows that there is no significant radiological impact from airborne dust to the population from radiation protection point of view.

The current results cannot be used to generalize the natural radiological levels around the local cements industry in Ghana, as the radiological contents of cement dust may depend on the geology from where the raw materials for the cement are derived from. This is so because natural radionuclide contents of rocks and soils depend on their geographical origin. To have a full picture about noncarcinogenic risk of radiological effects, ingestion and dermal contact pathways needed to be included in future assessment of the facility.

The study is of the view that to have a reasonable knowledge of the inhalable impacts of inhalables on human receptors regarding airborne dust around the current factory, it is advisable that future assessment programs should include both chemical and radiological toxicity considerations. Furthermore, the average 4 h sampling method used in this work was not enough to give a deeper insight of the problem at hand. Hence, long-term exhaustive studies are required to arrive at a meaningful conclusion.


  Acknowledgement Top


The authors would like to thank Carnegie Next Generation of Academic in Africa (CNGAA) for financing this work through the University of Ghana, Legon. The ethical support offered by the generality of the people of Akporkloe is also acknowledged.

 
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  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]


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