Heavy metals health risk assessment for population via consumption of food crops and fruits in Owerri, South Eastern, Nigeria

Background This study assessed lead, cadmium, and nickel level in food crops, fruits and soil samples from Ohaji and Umuagwo and Owerri in South Eastern Nigeria and estimated the potential health risks of metals. Samples were washed, oven-dried at 70–80°C for 24 h and powdered. Samples were digested with perchloric acid and nitric acid. Metals were analysed with Unicam Atomic Absorption Spectrophotometer. Result The concentration of Pb, Cd, and Ni in Ohaji exceeded the maximum allowable concentrations for agricultural soil as recommended by EU. Lead, Cd, and Ni in the food crops were highest in Oryza sativa, Glycine max, and Pentabacta microfila respectively. Highest levels of Pb, Cd, and Ni, in fruits were detected in Canarium schweinfurthii, Citrus reticulata, Ananas comosus respectively. The true lead and cadmium intake for the rice based meal were 3.53 and 0.034 g/kg respectively. Whereas the true intake of lead and cadmium for the cassava based meal were 19.42 and 0.049 g/kg respectively. Conclusion Local food stuff commonly available in South Eastern Nigeria villages may contribute to the body burden of heavy metal. This is of public health importance.


Background
Contamination of foods by heavy metals has become an inevitable challenge these days. Air, soil, and water pollution are contributing to the presence of harmful elements, such as cadmium, lead, and mercury in foodstuff. The occurrences of heavy metals-enriched ecosystem components, firstly, arise from rapid industrial growth, advances in agricultural chemicalization, or the urban activities of human beings. These agents have led to metal dispersion in the environment and, consequently, impaired health of the population by the ingestion of victuals contaminated by harmful elements [1].
Flooding from heavy downpour may lead to horizontal leaching from dump sites causing metal uptake by root of crops; the rest may find their way into open water bodies and the entire aquatic ecosystem. The entry into food chain of these metals leads to increased susceptibility and exposure to metal poisoning of the local population.
A number of serious health problems can develop as a result of excessive uptake of dietary heavy metals. Furthermore, the consumption of heavy metal-contaminated food can seriously deplete some essential nutrients in the body causing a decrease in immunological defenses, intrauterine growth retardation, impaired psycho-social behaviors, disabilities associated with malnutrition and a high prevalence of upper gastrointestinal cancer [2]. In Nigeria there is lack of food intake diaries to monitor the intake of heavy metals and therefore their levels in blood and urine. It is advocated that any legislation to check lead exposure to humans should be based on genuine scientific evaluation of the available data [3].
We have estimated the heavy metal (lead, cadmium, nickel and mercury) concentrations in soil and food crops commonly grown or sold in South Eastern Nigeria with the aim of evaluating the potential dietary toxicity by determination of daily intake of these metals. The effect of transfer factors of heavy metals from different soil sites are also studied in the food crops and fruits to quantify the concentration of accumulated metals to which the local population are exposed.

Study area and sampling
Samples of some commonly grown fruits and food crops were collected from three different sites namely Ohaji (1, 2 & 3), Umuagwo and Owerri all in Imo State in southern Nigeria. For metal analysis, only the edible parts of food crops, and fruit samples were used.

Sample preparation
All the collected samples of various food crops samples were washed with deionised to remove airborne pollutants. The edible parts of the samples were weighed and air-dried for a day, to reduce water content. All the samples were then oven-dried at 70-80°C for 24 h, to remove all moisture. Dried samples were powdered using a pestle and mortar and sieved through Muslin cloth.

Digestion of the samples
For each food crop, three powdered samples from each soil site (0.5 g each) were accurately weighed and placed in crucibles, three replicates for each sample. The ash was digested with perchloric acid and nitric acid (1:4) solution. The samples were left to cool and made up to a final volume of 25 ml with deionised water. The hydrolysed samples were well shaken and transferred to a centrifuge tube for centrifugation at the rate of 3000 rpm to remove solid particles. The resulting homogenised samples were thoroughly mixed before sub-samples were taken for analysis to ensure homogeneity of the mixture. The presence of lead, cadmium and nickel were analysed in samples using the Unicam Atomic Absorption Spectrophotometer (AAS) Model 929) at 217.0, 228.8 and 232.0 nm wavelength respectively . The limit of detection for lead, cadmium, and nickel were all 0.001 ppm with blank values reading as 0.00 ppm for all the metals in deionized water with electrical conductivity value of lower than 5 μS/cm. Samples were analysed in triplicates.

Quality control
Appropriate quality procedures and precautions were carried out to assure the reliability of the results. Reagents used to calibrate the instrumentation were of analytical grades. A spike-and-recovery analysis was performed to assess the accuracy of the analytical techniques used. Post-analysed samples were spiked and homogenized with varying amounts of the standard solutions of the different metals. The spiked samples were then processed for the analysis by the dry ashing method. Quality control measures were taken to assess contamination and reliability of data. The coefficients of variation of replicate analysis were determined for precision of analysis; the variations were found to be less than 10%.

Data analyses
The daily intake rate of metals (DIR) was calculated by the following equation: Where C metal is Heavy metal concentration in plants (μg/g) D food intake is daily intake of vegetable (kg/person) B average weight is average body weight. The average adult body weights were considered to be 55.9kg, while average daily vegetable intakes for adults is considered to be 0.345 kg/person/day, respectively [4,5].
The true intake using the arithmetic mean according to Parkhurst (1998) [6] was calculated by multiplying contaminant level i.e. (yearly intake of rice, cassava and fruits multiplied by heavy metal contaminant for each of the food items). In all the estimated or calculated levels of lead and cadmium in the food samples yearly averages as quoted by various workers were used [7,8]. Table 1 shows the lead, cadmium nickel and mercury levels (mg/kg) in commonly consumed food crops in Nigeria. Lead ranged from 0.00 -61.17 mg/kg found in Oryza sativa. The range of various metals in food crops were 0.00-61.17, 0.00-0.24, and 0.00-3.13 mg/kg and for lead (Pb), cadmium (Cd), and nickel (Ni) respectively. The highest levels of Pb, Cd, and Ni in the food crops were detected in rice (Oryza sativa), soybean (Glycine max), and Pentabacta microfila respectively. Table 2 shows the lead, cadmium and nickel levels (mg/kg) in fruits. The range of various metals in fruits was 0.00-4.23, 0.00-0.24,and 0.00-1.76 mg/kg and for Pb, Cd, and Ni respectively. The highest mean levels of Pb, Cd, and Ni, in the fruits were detected in Canarium schweinfurthii, tangerine Citrus reticulata, and pineapple Ananas comosus respectively. Taken together, about 36% of the sampled food crops, and fruits had non-detectable levels of lead, 49% had non-detectable levels of cadmium, and 15.7% had non-detectable levels of nickel. Forty-five percent of the sampled food crops violated the permissible limits of 0.01 mg/kg and 0mg/kg for lead as prescribed by WHO, EU and EPA respectively, 49% violated the limits for cadmium and 82% violated the limits for nickel.

Results
Heavy metal concentrations (mg/kg) in soil samples are shown on Table 3. The range of various metals in the soil samples was 0.00-3.53, 0.00-0.18, and 0.26-1.56 for lead (Pb), cadmium (Cd), and nickel (Ni) respectively. The concentration of Pb, Cd, and Ni in Ohaji 1, 2 and 3 soil samples exceeded the maximum allowable concentrations for agricultural soil as recommended by EU but lower than Canadian human quality health soil quality guideline when compared with Table 4. Tables 5 and 6 show the daily intake rate (g person -1 day -1 ) of lead, cadmium, nickel and mercury DIR through consumption of food crops, and fruits respectively. Rice (Oryza sativa) and Wall nut (Tetracarpidium conophorum) had the highest DIR of 0.3775 and 0.0526 for lead respectively. The highest DIR (0.0015) for cadmium were seen in Soybean (Glycine max) and Breadfruit (Artocarpus altilis), while the highest DIR for nickel were Vigna subterranean and Beans (Phaseolus vulgaris) with DIR of 0.0163 and Wall nut (Tetracarpidium conophorum) (0.0118).
The estimated or the calculated intake of lead and cadmium according to Parkhurst equation is contained in Example of calculating true metal intake. The calculated amount of lead and cadmium for a person on rice, and tangerine (Citrus reticulate) and another person on Cassava (Manihot spp) and Bannana (Musa spp). The true lead and cadmium intake for the rice based meal were 3.531g/kg and 0.034 g/kg respectively. Whereas the true intake of lead and cadmium for the cassava based meal were 19.42 g/kg and 0.049 g/kg respectively.

Discussion
The thrust of this study was to determine the levels of lead and cadmium, and nickel in commonly ingested farm produce that form the staple foods of Nigerians  with a view to lending credence to the assertion by Gidlow (2004) [3] that irrespective of the pressure to reduce lead exposure in the general population and working environment, legislation must be based on genuine scientific evaluation of the available evidence. We have presented data on lead, cadmium, and nickel levels in food crops and fruits of southeastern Nigeria and the soil levels in corresponding farmlands. These values are higher in the sampled food crops and fruits compared to the soil samples. Some plants are capable of taking up lead from soil through their root systems, although this uptake does not appear to be appreciable [9] Soil samples from different farm lands where crops and fruits were harvested showed the presence of Pb, Cd and Ni. The concentration of soil lead was highest followed by Ni, and Cd in a descending order. The concentration of Pb, Cd, and Ni in Ohaji 1, 2 and 3 soil samples exceeded the maximum allowable concentrations for agricultural soil as recommended by EU but lower than Canadian human quality health soil quality guideline when compared with Table 4. Cereals are a major component of the human diet and a source of essential nutrients, antioxidants and metabolites [10]. However, intake of toxic metal-contaminated cereals may pose a risk to human health. Agricultural activities have been identified as contributors to increasing toxic metal contamination through the application of various types of pesticides and fertilizers [11]. Results from present and previous studies [12,13] demonstrate that the foods grown on contaminated soils are more contaminated with heavy metals, which pose a major health concern.
In the present study the high percentage of food crops and fruits that violated the permissible limits of lead and cadmium as set by WHO, EU and EPA respectively is of public health concern. Among possible target organs of heavy metals, are soft tissues such as the kidney and liver and the central nervous system appear to be especially sensitive [14]. Some patients develop vesicular type of hand eczema following the ingestion of nickel in diet [15]. Although rare, chronic urticaria, a type 1 hypersensitivity response, has been attributed to dietary nickel [16]. The mean total dietary intake of nickel has been reported to be between 0.12-0.21 mg in UK [17], 0.13 mg in Finland [18], 0.17 mg in US [19] and between 0.207-0.406 mg in Canada [20]. The true intake of nickel in two Nigerian staple foods namely rice and cassava based meals calculated in this study appear to be within this range. This could be higher depending on food-fruits combinations. The high lead level seen in fruits especially local pear Canarium schweinfurthii is in an agreement with our previous finding of high lead level in same local pear in another city of South Eastern Nigeria [21].
Provisional tolerable weekly intake (PTWI) depends on the amount, consumption period and contamination level of consumed food. The FAO/WHO in 1993 established a provisional tolerable weekly intake (PTWI) of 25 μg lead/kg body weight for humans, equaling 1500 μg lead/week for a 60 kg person [22]. In 1995, the WHO  estimated that total lead intake in adults worldwide range from 105 to 2212 μg/week [22]. In Canada the dietary intake of lead may be calculated to 168 μg/week for a 60 kg person [23]. The PTWI of cadmium has been set at 7 μg/kg body weight [24], equaling 420 μg cadmium/ week for a 60-kg person. The dietary intake of lead, cadmium and nickel seem to be higher than the FAO/ WHO PTWI. Nigeria's food regime is based essentially on two foods: grains, which provide 46% of calories and 52% of proteins consumed, and root crops/tubers, which provide 20% of calories and 8% of proteins consumed. Consumption of grains and root crops/tubers amounts to 150 kg and 214 kg respectively per person and per year [8]. The average Nigerian now consumes 24.8 kg of rice per year, representing 9% of total caloric intake [7]. The WHO/ FAO report, "Diet, Nutrition and the Prevention of Chronic Diseases" recommends a population dietary intake goal of more than 400 g per day for fruits and vegetables. Many advanced counties have already launched campaigns for promoting the consumption of fruits and vegetables, especially in the framework of the International Fruits and Vegetables Alliance (IFAVA) [25,26]. In addition to the FAO-WHO initiative, such an approach is supported by the Global Horticultural Initiative (GlobalHort) and is now considered globally as a good way for reaching the United Nations Millenium Development Goals (MDGs) [26]. Should Nigeria adhere to this recommendation, it will be worthwhile to ascertain the safety of fruits consumption with respect to these heavy metals especially lead and cadmium. Although the present study has not employed the use of food intake diaries, the calculated/estimated intake of lead and cadmium using arithmetic mean, for a Nigerians on the popular staple foods namely rice or cassava were 3.53 -19.42g/kg and 0.034 -0.049 g/kg for lead and cadmium respectively. It could be feared therefore that if from staple foods alone excluding, the body burden of lead in an average Nigerian exceeds that of values obtained in Europe and America, a cumulative amount from other sources may make it even higher. Only recently Orisakwe 2009 [27] noted that while blood lead levels (BLLs) in many western countries have progressively declined over the years, in Nigeria high BLL continue to be documented not only in exposed workers but also in "unexposed" control subjects. There exist many sources of environmental lead exposure in Nigeria. At the top of the list is leaded gasoline. Although there was a plan to reduce the lead content of Nigerian gasoline from 0.74 g/L to 0.15 g/L by 2002 there is a doubt that it was implemented [28].
The degree of toxicity of heavy metals to human being depends upon their daily intake. Heavy metals intake through consumption of various types of food stuffs grown and sold in Southeastern Nigeria showed large variations. The standard of FAO/WHO (1999) [29] has  established a reference value for tolerable daily intake. Our estimated daily intake rate for lead and cadmium DIR were above the tolerable daily intake rates for some of the food stuffs. In addition, the body weight of the human can influence the tolerance of pollutants. The DIM values for heavy metals were high when based on the consumption of food crops and fruits grown in the soils sampled in this study. The highest intakes of Pb, Cd, Ni and Hg were from the consumption of Rice (Oryza sativa), Soybean Soybean (Glycine max), oil bean (Pentabacta microfila) and (Vigna subterranean) respectively.

Conclusion
Among other routes, food is one of the main sources of consumer exposure to heavy metals. Since increased dietary metals intake may contribute to the development of various disorders, there is a necessity for monitoring of these substances in the human diet. Heavy metals show a significant build-up with contamination and long-term accumulation of heavy metals in soils has led to contamination of food crops in the study area. All the food crops and fruits containing heavy metals were higher than the recommended tolerable levels proposed EU, USEPA and WHO. Local food stuff commonly available in South Eastern Nigeria villages may contribute to the body burden of heavy metal. This is of public health importance. It is recommended that people living in this area should not eat large quantities of these foods, so as to avoid excessive accumulation of heavy metals in the body. Dietary intake of food results in long-term low level body accumulation of heavy metals and the detrimental impact becomes apparent only after several years of exposure. Thus regular monitoring of these toxic heavy metals from effluents and sewage, in foods is essential, to prevent their excessive build-up in the food chain.