Assessments of Natural Radioactivity and Heavy Metals in Commonly Consumed Milk in Oke-Ogun Area, Nigeria and Estimation of Health Risk Hazard to the Population

Copyright: © 2014 Kolapo AA, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Assessments of Natural Radioactivity and Heavy Metals in Commonly Consumed Milk in Oke-Ogun Area, Nigeria and Estimation of Health Risk Hazard to the Population


Introduction
Many dangerous radionuclides, elements or compounds such as metals and metalloids, accumulate along the food chain. Their concentrations in the environment grow with the increase of urban, agricultural, and industrial emissions. The presence of metal pollutants, like Cd and Pb, aids their entry into the food chain and thereby increases the toxicity effects of the food in humans and animals diet. Milk is one of the most important foods for human nutrition; it is beneficial in human diet and mostly needed by infant and children during their growing age. Milk contains all the macronutrients namely protein, carbohydrates, fat, vitamins (A, D and B groups) and trace elements particularly calcium, phosphate, magnesium, zinc and selenium [1,2]. Milk is known as an excellent source of Ca, and it can supply moderate amounts of Mg, smaller amounts of Zn and very small amounts of Fe and Cu [3].
Heavy metals in human diets come mainly from milk, meat and other products frequently consumed. Investigation of heavy metals level in food products have been monitored in numerous regions of the world and there has been an increasing world-wide concern about quality of powdered milk, studies in recent years have investigated the quality of powdered milk in different parts of the world [4][5][6][7][8][9][10][11].
In Nigeria no record of radioactive contamination of the environment has been reported [12]. Some research works in radioactivity measurements of food samples in Nigeria, have been reported [11,[13][14][15]. Among different kinds of foodstuffs, milk is a reliable indicator of the general population intake of certain radionuclides, metals and metalloids, since it is consumed by a large segment of the population. So, the determination heavy metals and natural radionuclides levels in milk and milk products is important for controlling the toxicity levels of our diets and necessary in establishing rules and regulations relating to processing and manufacturing of products. Milk is an important vector of radionuclides and heavy metals to man and they may get into the environment from the mining activities, several nuclear weapon tests and numerous nuclear reactor accidents. Contamination of the food chain may occur as a result of direct deposition of radionuclides and heavy metals on plants leave and fruits tubers, root uptake from contaminated soil or water, and animals ingesting contaminated plants, soil or water. Another important factor of contamination is the importation of contaminated food from foreign countries where nuclear accident or fall-out had occurred which can indirectly affect people health around the world. Therefore, determination of radionuclides and heavy metals in milk is of inter est to know the adequate daily intake of these metals and also to determine and monitor the levels of toxicity in our diets because they can significantly influence human and animal health [16].
The aims of this study therefore are: to determine the concentration of natural radionuclides and heavy metals in 8 different brands of milk products consumed in Oke-Ogun area of Oyo State; the border between Nigeria and Benin Republic where different kinds of milk products were illegally imported into the country and to estimate the annual internal dose from the intake of the heavy metals in the milk products.
Powdered milk products were sampled because it is an important ingredient used in the making of cookies, ice cream, yogurt, chocolate, powdered chocolate, candy and many others, it is a cheap and easy to store milk product which inhabitants can afford. These samples were analysed using Flame Atomic Absorption Spectrophotometer model S4 series, Model (GBC 906) (USA) for the heavy metals and gamma ray spectrometer with NaI(Tl) detector for natural radionuclides.

Radionuclide analysis of samples
One hundred gram of each sample was weighed and parked in a plastic container. The plastic containers were hermetically sealed for 30 days for secular equilibrium to be reached between parents and their daughter radionuclides [17]. Detection and measurements of the radionuclides in the powdered milk samples were carried out by gamma ray spectrometer with a NaI(TI) detector. The counting assembly was a scintillation detector and a Canberra multichannel analyzer. The detector was a 7.6 × 7.6 cm 2 NaI(Tl) manufactured by Bicron. A cylindrical lead shield of approximately 5 cm thickness with a fixed bottom and a movable cover shielded the detector from background radiation. The detector assembly has a resolution of ~8% at 662 KeV of 137 Cs.
Samples were counted for 10 hours. The activity concentration of 214 Pb (352 KeV) and 214 Bi (609 KeV, 1120 KeV) were chosen to provide an estimate of 226 Ra, while that of the daughter radionuclides 208 Ti (2651KeV), 212 Pb (239KeV), 228 Ac (911KeV) were chosen as indicator of 232 Th, 40 K was directly measured using its single photo peak at 1460 KeV emitter. The background count was determined by counting an empty container of the same dimensions as the one containing the samples and subtracting from the gross count. The counting time was set at 36000 s (10 h) to obtain the gamma spectrum with good statistics. The activity concentrations in the samples were obtained using equation given elsewhere [18]. The detection limit (DL) given in Bq kg -1 , which is required to estimate the minimum detectable activity in a sample, was obtained using equation given in [18]. The detection limits obtained were 17.3 Bq kg -1 , 4.2 Bq kg -1 and 5.1 Bq kg -1 for 40 K, 226 Ra and 232 Th respectively.

Digestion of milk samples and elemental analysis
One gram each of the powdered milk samples was weighed into a 100 mL digestion tube. Each of the tubes was labelled to avoid mix-up. A total of 5 mL each of nitric acid and hydrogen peroxide were added, while SO 4 was added in small amount and the mixture was thoroughly stirred until white fumes evolved. The process continued until the solution is clear. The solution was decanted and diluted with deionized water up to 100 mL before being filtered. The procedure was repeated for all the samples. For each sample three determinations were performed and average results were reported. Before metal concentrations were determined, standard solutions were prepared in each case and were used to eliminate sample standard matrix indifferences.
The concentration of the metals in the samples were obtained from the readings of Atomic Absorption Spectrophotometer (AAS) S4 Model (GBC 906) as shown in equation (1) Concentration of sample = AAS reading × 100 [1] Weight of samples.

Average Daily dose and Health risk index (HRI)
The health risk from heavy metal intake through ingestion may be characterized using hazard index, which is the ratio of the Average Daily Dose (ADD in mg per kilogram of the body weight per day) of a chemical to an oral reference dose (Rf Do; in mg per kg per day) [19]. Oral reference dose is defined as the maximum tolerable daily intake of specific metal that does not result in any deleterious health effects. According to USEPA 2002 [20], oral reference doses are 0.4 × 10 -3 , 0.7 and 1× 10 -3 mg kg -1 day -1 for Cu, Fe and Cd respectively and 3.5 × 10 -3 mg kg -1 day -1 for Pb [21].
ADD is expressed as: where C metal is the geometric mean concentration of metal in the sample; D intake is the average daily intake of the radionuclides from the consumption of the milk sample and B average is the is the average weight of the adults taken as 70 kg.
Exposure to heavy metal is therefore estimated as: An index more than 1 implies that the ADD of a particular metal exceeds the RfDo which mean that there is a potential risk associated with that metal and is considered as not safe for human health [20]. Table 1 show the results of the gamma-ray analysis of the eight powdered milk samples analyzed. The radionuclides observed with reliable regularity in the samples belonged to the decay series chain of 226 Ra, 232 Th and the non-series 40 K. The activity concentrations of 226 Ra and 232 Th in all the samples have value below detection limit (BDL). The activity concentration of 40 K in the samples ranged from BDL to 195.0 ± 9.5 Bqkg -1 with means as shown in column 3 in Table 1. The highest activity concentration of 40 K was detected in sample M5 (195.0 ± 9.5 Bqkg -1 ) and the lowest activity concentration was obtained to be BDL in sample M8. The overall mean of 40 K is 39.6 ± 12.6 Bqkg -1 .

Internal dose of the radionuclide from the ingestion of the milk
The radiation dose to the population from the intake of radionuclide in foods was calculated from the equation reported in [22] as: where E is the annual effective dose by ingestion of the radionuclide (Svy -1 ), A is the activity concentration of the radionuclides in the sample (Bqkg -1 ), C is the internal conversion factor by ingestion of the radionuclide (Sv Bq -1 ) [23] and I is the annual intake of milk (Kgy -1 ) which depends on a given age [23,24]. Annual effective ingestion dose due to milk consumption is strongly dependent of the milk consumption [25]. In this study, the average mass of the milk consumed by the adults is taken as 13 Kg y -1 according to [26,27]. In the calculation of the total dose, the sum of the contributions from each radioisotope in the samples Table 1 with the recommended conversion factors for adults given as 6.2 x 10 -9 , 2.3 x 10 -7 and 2.8 x 10 -7 for 40 K, 232 Th and 226 Ra, respectively [23] was used.
The contributions of each radionuclide in the estimated annual effective ingestion dose are presented in Table 2 Table 2 while the mean contribution of 40 K is 3.2 µSvy -1 in column 3, Table 2. The total average ingestion dose due to all the radionuclide in the powdered milk sample is < 33.7 µSvy -1 . These results are within the typical worldwide range of annual dose (200-800 mSv) due to the ingestion of all natural radiation sources reported by UNSCEAR 2000 [22].
The comparison of the results of the average activity concentrations of 226 Ra, 232 Th and 40 K obtained in this work with similar published data in literature are presented in Table 3. The results obtained for the activity concentration of 226 Ra and 232 Th in all the samples are below detection limit (BDL) and so their values are taken as BDL. The average activity concentrations for 40 K in the milk samples are found to be lower than the values published in Indian [28], Egypt [29], Saudi Arabia [23] and in Iran [30] as shown in Table 3.

Heavy metal composition and health risk index of the milk samples
The heavy metal contents vary widely due to many factors such as differences between species, characteristics of the manufacturing practises and possible contamination coming from the equipments during the process [31]. The concentrations of heavy metals (Pb, Cd, Fe and Cu) analysed in the samples are presented in Table 4 , respectively for Pb, Cd, Fe and Cu. The mean concentrations of each heavy metal were compared with the permissible limit recorded in literature [32] and were found to be within the safe limit. Figure 1 compared the mean concentrations of the heavy metal in the samples. It was found that Fe was highest in all the samples but still within the permissible limit. Iron is an essential element that contributes to many important physiologic functions in the body and in addition 4.8mg has been recommended as the acceptable daily intake of Fe [33]. Traces of Pb found in the samples may be due to contamination of original cow's milk used in the manufacture of these products.
The ADDs and hazard indices in the milk samples were calculated using equation and 4 respectively and are presented in Table 5. The intake (mg/day) was taken from the acceptable daily intake per 70 kg person (FAO/WHO, 1989 and 1999) [33,34] and is presented in column 3 in Table 5. The mean ADD calculated for the four heavy metals analyzed are 0.1490, 0.0010, 0.0002 and 0.0003 mgkg -1 for Fe, Cu, Pb and Cd, respectively and are presented in column 4 of Table 5. The hazard index of the heavy metals in sampled powdered milk are 0.212, 0.025, 0.005 and 0.300, respectively, for Fe, Cu, Pb and Cd, and are lower than unity as can be seen in column 5 in Table 5. The hazard less than 1 implies that they do not pose any significant threat.

Conclusion
In this study, the radionuclide contents present in eight powdered milk commonly consumed by adults in Oke-Ogun region, Nigeria were determined using gamma ray spectroscopy. Mainly, the activity concentration of 40      detection limit (BDL). Therefore, the radionuclides level in the studied powdered milk samples for adults is obviously below the maximum level permitted by International Atomic Energy Agency (1989) [32]; so these kinds of milk can therefore be normally consumed. The total average annual effective dose due to the ingestion of all three natural radionuclides are found to be < 33.7µSvy -1 which is far below the 1mSvy -1 recommended limit by WHO and ICRP for radiological safety. Comparison of the results obtained with those reported in literature revealed that natural radionuclides in the samples are well within the worldwide ranges.
The heavy metal analysis revealed that there were traces of heavy metal in the samples although their mean concentrations were lower than the permissible limit. Fe has the highest concentration in all the samples because it is an essential element that contributes to many important physiologic functions in the body. Average Daily Dose (ADD) was estimated taking into account the geometric mean concentration of the heavy metal. The mean ADDs in the milk samples were 0.1490, 0.0010, 0.0002 and 0.0003 for Fe, Cu, Pb, and Cd, respectively which are lower than the safe limit. The hazard index which is the exposure due to the heavy metals in sampled powdered milk is 0.212, 0.025, 0.005 and 0.300, respectively, for Fe, Cu, Pb and Cd. These values were lower than unity as required which implies heavy metals in the powdered milk sample do not pose any significant threat and can therefore be normally consumed.
Regular monitoring of radionuclide and heavy metals in milk sample should be emphasized to prevent excessive build-up of the toxic metals in the food chain. Also movement of food stuff illegally into the country should be monitored. The data generated in this study will provide assistance in the development of future guidelines in Nigeria for radiological protection of the population.  Table 5: Average Daily Dose (ADD) and hazard index of heavy metals due to ingestion of the milk samples.