Introduction
Natural radioactivity is mostly caused by the primordial radionuclides, namely 40K, and 238U-232Th decay series, which are present in earth formations. It is important to understand the concentrations and distributions of naturally occurring radioactive materials (NORM) since it gives helpful information to monitor environmental radioactivity. Artificial radionuclides such as 137Cs which are occasioned by anthropogenic activities also contribute to environmental monitoring [1].
Several human activities enhance the level of NORM in the environment. For example, there are the burning of fossil fuels and the unrestrained mining processes, etc. These radionuclides accumulated in the soil can reach foods or plants through metabolic processes and ultimately can be transferred to animals and subsequently to humans when they consume contaminated foods. The level of risk to human health is dependent on the type, amount of radionuclide, and the exposure time. Radiation exposure to humans differs from one place to another depending on the geographical and geological conditions [2, 3].
The key objective of this study is to calculate the activity concentration of radionuclides and evaluate the radiological hazards in some food products consumed by the Malian population using gamma spectrometry.
This study will offer a basis for more assessment of exposures to radionuclides in food products and useful data for the Malian regulatory body Agence Malienne de Radioprotection (AMARAP) and other relevant authorities.
Materials and Methods
1. Sample Collection, Preparations, and Measurement
Mali is the second largest importing country in West Africa. Around 13.91% of agricultural products are imported and 13.45% of this value is foods only [4]. Based on the collected data from Direction Généraledu Commerce, de la Consommation et de la Concurrence (DGCCC) and Office des Produits Agricoles du Mali (OPAM), four different kinds of the most consumed foods by the Malian population (powdered milk, rice, sugar, and wheat flour) were selected for sampling. Thirteen samples were collected: three from powdered milk, three from sugar, three from rice, and four from wheat flour from different markets, malls, and shops around Bamako city (Mali), between them, five samples were made in Mali (one rice, one sugar, and three wheat flour samples).
Sixteen samples were prepared and stored for almost 1 month for secular equilibrium between the U-Th series and their daughters to be reached. The activity concentration of samples was obtained using a gamma spectrometry system with a high-purity germanium (HPGe) detector at the Ghana Research Reactor-1 (GHARR-1) of the Ghana Atomic Energy Commission (GAEC). The gamma spectrometry system was made up of an HPGe detector connected to a computer-based multi-channel analyzer. The detector has a relative efficiency of 40% with an energy resolution of 2.0 keV at gamma-ray energy of 1,332 keV of 60Co. The individual radionuclides were identified by their characteristic gamma-ray energies and the quantitative analysis of radionuclides was done with the Genie 2000 gamma acquisition and analysis software (Mirion).
2. Calculations of Activity Concentration and Annual Committed Effective Dose
1) Activity concentration
After the secular equilibrium time, all the samples were read for 10 hours, the activity concentrations of 238U and 232Th were assessed by averaging the peaks of different daughters for the U-Th series, 40K and 137Cs were calculated using the photopeak in the spectrum. The activity concentrations of 238U were obtained by using the gamma-ray energies and the respective gamma-intensity of 214Bi at 609.3 keV (44.8%) and 214Pb at 351.9 keV (35.8%). That of 232Th was obtained using gamma energies of 228Ac at 911 keV (26.6%), 212Pb at 238.6 keV (43.3%), and 208Tl at 583 keV (30.1%), and 2,614.7 keV (35.3%) considering a branching ratio of 33.7% from 212Bi towards 208Tl. For 40K, it was estimated from its energy line at 1,460.8 keV (10.7%) while the 137Cs were calculated from the gamma energy line at 661.67 keV (85.1%). All the energies and yields of the various radionuclides were from a standard library.
Therefore, the activity concentration (AE,i) in Bq/kg, for a radionuclide i with a photopeak at energy E, was calculated as:
where NE,i is the net peak-area of i radionuclide at energy E, εE is efficiency at energy E, t is counting time (s), γd is the gamma emission probability, and M is the mass (kg).
2) Annual committed effective dose
The annual committed effective dose (AED) (Sv/yr) was calculated using the activity concentration of all radionuclides detected and analyzed in the samples as:
where e(g)j is the effective dose conversion factor for ingestion of nuclide j (Sv/Bq), Aj is the activity concentration of nuclide j (Bq/kg) and Uj is the consumption rate of food (kg/yr). In this study, the international values of annual consumption rate were used [5].
3) Internal hazard index
The internal exposure to radon and its short-lived daughters is also dangerous to the respiratory organs. To account for this threat the internal hazard index (Hin) was quantified using the activity concentration of 40K, 238U, and 232Th. It should be below one to provide safe levels for the respiratory organs of individuals. The internal exposure from the daughters of 238U, 232Th, and 40K, was calculated using Equation (3) [6]:
where AU is activity concentration of 238U (Bq/kg), ATh is activity concentration of 232Th (Bq/kg), and AK is activity concentration of 40K (Bq/kg).
4) Risk assessment calculation
Risk is estimated by the assumption that a linear dose-effect relationship has no threshold according to the International Commission on Radiological Protection (ICRP) [7]. Lower doses have a fatal cancer risk factor (RF) of 0.05 Sv−1 reported by International Atomic Energy Agency (IAEA) [7]. This RF determines the likelihood of an individual dying of cancer by a 5% increase of 1 Sv dose received throughout his entire life.
The AED (Sv/yr) in food is estimated to determine the cancer risk of an adult using Equation (4).
Results and Discussion
1. Results
1) Activity concentration
In the present study, 16 food samples were analyzed. Table 1 shows the activity concentrations of 238U, 232Th,40K, and 137Cs. The minimum detection activity obtained for 238U, 232Th,40K, and 137Cs were 0.038, 0.044, 0.31, and 10−3 Bq/kg, respectively.
2. Radiological Hazard Indices
2) Internal hazard index
The Hin was determined in the various samples of powdered milk, rice, sugar, and wheat flour as presented in Table 3.
3) Risk assessment
The RA was calculated using Equation (4) above and the results are shown in Table 4.
3. Discussion
The range of activity concentrations for 238U varies from 1.1±0.1 to 32.8±2.3 Bq/kg, for 232Th from 0.01±0.001 to 33.9±2.2 Bq/kg, for 40K from 3.2±0.2 to 382.2±19.5 Bq/kg, and for 137Cs from 0.0025±0.0004 to 0.0044±0.0006 Bq/kg. The peaks of 40K and 137Cs for most of the rice, wheat flour, and sugar samples were below the detection limit as indicated in Table 1.
The result of samples showed that S-01, S-03, R-01, WF-04 represented the lowest activities concentrations level of 238U, 232Th, 40K, and 137Cs, respectively; M-01, WF-02, M-03, and R-03 represented the highest values of 238U, 232Th, 40K, and 137Cs, respectively. The results proved that the samples of powdered milk contained the maximum values of activity concentrations of NORM; the R-03 contains the maximum value of 137Cs. All of which are imported from Europe and Asia. Special or more investigation must be carried out on the milk because they are more consumed by infants and children.
The activity concentrations of measured radionuclides in the food samples are much lower than the Malian standard 1,000 Bq/kg for 238U and 232Th, 10,000 Bq/kg for 137Cs, and 100,000 Bq/kg for 40K and in comparison, to other works as shown in Table 5 [1, 9–13].
The AED from ingestion is calculated for various age groups (from infant to adult). The range of values for AED (μSv/yr) of samples was; for powdered milk 248.8 (M-03) to 2,201.3 (M-03), for rice 2.2 (R-01) to 432.6 (RF-03), for sugar 0.2 (S-03) to 25.1 (S-02), and for wheat flour 9.2 (WF-04) to 1,155.9 (WF-02), as shown in Table 2. The highest values of AED were reported again in powdered milk. The AED’s mean value for the consumption of these foods was around 0.33 mSv/yr. This compared well with the UNSCEAR worldwide average annual dose due to the ingestion of foodstuffs of 0.29 mSv/yr.
Based on the calculated values (Hin<1), the obtained values of this index due to the ingestion of these food products were below one. For consumption of these foods products, the calculated Hin means good safe levels for the respiratory organs due to internal exposure to radon and its short-lived progenies as in Table 3.
The RA was calculated for different ages from 1 year to more than 17 years, the range of RA varied from 7.07×10−4 (for 1 to 2 years) to 1.5×10−3 (for 2 to 7 years). It is 4 to 8.5 times less than the risk (6.0×10−3) from all natural radiation sources based on the global average annual radiation dose which is 2.4 mSv/yr to man [5]. The mean value of RA is 9.28×10−4, which is 6.5 times less than the risk 6.0×10−3 as mentioned above as shown in Table 4.
The cancer risk is based on an annual dose limit of 1 mSv for the public, which gives an annual death probability of 10−5, i.e., 1 in 100,000 reported by ICRP in 1990 [14]. That means the death probability by cancer due to the ingestion of these food products is much lower than the estimated value reported by ICRP in 1990 [14].
Conclusion
Natural and artificial radionuclides (137Cs) in selected foods were studied for their radiological hazards. The radionuclides of 238U, 232Th (through their daughters) and 40K, and 137Cs were the main radionuclides detected in the food samples in this research. The analysis revealed that the results of activity concentrations of the 238U, 232Th, 40K, and 137Cs in samples were generally lower than the national standard and global reference. The calculated AED values were less than the authorized limit for the public which is 1 mSv/yr. The mean radiological parameters such as AED, Hin, and RA were below the dose criteria limits given by international organizations (ICRP and UNSCEAR) and national standards.
The results of this study proved that these foods are safe for human consumption even if the risk (stochastic effect) associated with internal exposure due to low dose intakes exists. Based on obtained values, the probability of someone dying of cancer due to the ingestion of these foods is less than 10−5 in the Malian population.
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