Public Health Risk Assessment of Heavy Metal Uptake by Vegetables Grown at a Waste-water-Irrigated Site in Dhaka, Bangladesh

Industrial or municipal wastewater is mostly used for the irrigation of crops due to its easy accessibility, difficulty of disposal and scarcity of fresh water. Irrigation with wastewater is known to contribute significantly to the heavy metals content of soil.1 Wastewater may contain various heavy metals, including zinc (Zn), copper (Cu), lead (Pb), arsenic (As), nickel (Ni), chromium (Cr), and cadmium (Cd), depending Background. Wastewater is often used in developing countries for irrigation of crops. This wastewater often contains a number of heavy metals which are harmful to human health due to their non-biodegradable nature, long biological half-lives and their potential to accumulate in different body parts. There are many potential risks to human health from heavy metal contamination of vegetables grown at waste-water-irrigated sites. Objectives. This study was carried out to assess the concentration of heavy metals and the risk to human health by heavy metals (lead (Pb), nickel (Ni) and arsenic (As)) through the intake of locally grown vegetables collected from wastewater irrigated agricultural fields. The objectives of the present study were to determine concentrations of heavy metals in vegetables collected from the wastewater-irrigated fields, to calculate daily intake of heavy metals from the consumption of vegetables for both adults and children, and to evaluate their potential health risk. Methods. Twenty-seven samples of nine different types of vegetables were analyzed by an Atomic Absorption Spectrometer (AAS) (Varian AAS 240 F S). Results. The range of various metals in waste-water irrigated vegetables were not detected (ND)-0.188, 0.072-1.069 and ND-0.076 mg/kg for Pb, Ni and As, respectively. The highest mean concentration of all metals was detected in jute leaf, except for Pb which was found in the stem amaranth leaf. The mean concentration of all metals in all vegetables was within the safe limits of the World Health Organization/Food and Agricultural Organization (WHO/ FAO) and China’s national standards. The health risk index was more than 1 for As in jute leaf for both adults and children. The metal pollution index was highest (0.16 mg/kg) in jute leaf, whereas green papaya showed the lowest metal pollution index (MPI) value (0.005 mg/kg). Among all vegetables tested, the highest intake values of Ni and As were from consumption of jute leaf and the highest intake values of Pb from consumption of stem amaranth leaf for both adults and children. Conclusions. Higher values in the metal pollution index and health risk index indicate heavy metal contamination in wastewater-irrigated soils that present the potential for a significant negative impact on human health. Competing Interests. The authors declare no competing financial interests.


shakhaoat@juniv.edu
Hossain et al upon the type of activities associated with it. Continuous irrigation of agricultural land with sewage and industrial wastewater may cause heavy metal accumulation in the soil and vegetables. [2][3][4] Heavy metals are very harmful due their non-biodegradable nature, long biological half-lives and their potential to accumulate in different body parts. Most heavy metals are extremely toxic because of their solubility in water. Even low concentrations of heavy metals have damaging effects to humans and animals because there is no good Zinc mechanism for their elimination from the body. 1 Intake of heavy metals through the food chain by human populations has been widely reported throughout the world. 5 Individual metals exhibit specific signs of toxicity. The nature of their effects can be toxic (acute, chronic or sub-chronic), neurotoxic, carcinogenic, mutagenic or teratogenic. 6 Vegetables are very important because they contain essential components of protein, vitamins, iron, calcium and other nutrients. 7 However, their nutritional value and consumer acceptance as food must be taken into consideration, because vegetables can contain both essential and nonessential elements over a wide range of concentrations. 8, 9 It is well established that the daily intake of heavy metalcontaminated vegetables may pose a risk to human health. This is because heavy metals can accumulate in living organisms and at elevated levels, they can be toxic. It has been reported that prolonged consumption of unsafe concentrations of heavy metals through foodstuffs may lead to the chronic accumulation of metals in the kidney and liver of humans, causing disruption of numerous biochemical processes, leading to cardiovascular, nervous, kidney, and bone diseases. 10 Therefore, the aim of present study was: (1) to determine concentrations of heavy metals in vegetables collected from a waste water-irrigated agricultural field, (2) to calculate daily intake of heavy metals through the consumption of vegetables for both adults and children, and (3) to evaluate the potential health risks to local consumers.

Study Area and Sampling
Twenty-seven samples ( Quality control measures were taken to assess contamination and reliability of data. Accuracy of the analytical method was performed as percent recoveries for each of the elements.

Metal Pollution Index (MPI)
To examine the overall heavy metal concentrations of vegetables, the metal pollution index (MPI) was computed. This index was obtained by calculating the geometrical mean of concentrations of all the metals in the vegetables, cereals and milk. 11

Daily Intake of Metals (DIM)
The daily intake of metals (DIM) was determined by the following equation:

Daily intake of metal (DIM) = (Concentration of metal (mg/kg) × Daily food intake)/(Average body weight)
The average adult and child body weights were considered to be 55.9 and 32.7 kg respectively, while average daily vegetable intakes for adults and children were considered to be 0.345 and 0.232 kg/person/day, respectively. 12-13

Health Risk Index (HRI)
The health risk index (HRI) was calculated from the ratio of estimated exposure of test vegetable and oral reference doses. 14 Oral reference doses were 3 × 10 -4 mg kg -1 day -1 for As, and 0.004 and 0.02 mg kg -1 day -1 for Pb and Ni, respectively. 15, 16 The oral reference dose (RfD) for As has been determined based on critical health effects like hyperpigmentation, keratosis and possible vascular complications. 15 The RfD for Pb has been determined based on its particular health effects such as neurotoxicity, developmental delay, hypertension, impaired hearing acuity, impaired hemoglobin synthesis, and male reproductive impairment, and the RfD for Ni has been determined based on critical health effects such as decreased body and organ weights. 16 Estimated exposure is obtained by dividing the daily intake of heavy metals by their safe limits. An index of more than 1 is considered to be unsafe powdered using a pestle and mortar and sieved through muslin cloth.
For each vegetable sample, approximately 0.5 g dried samples were acid digested in a microwave digestion system (Berghof, Speed Wave) using nitric acid: hydrogen peroxide

Quality Control Analysis
Analytical reagent blanks were prepared with each batch of digestion set and then analyzed for the same element in each of the samples. The analytical procedures

Health risk index (HRI) = DIM/RfD
Where DIM is daily intake of metal and RfD is the reference oral dose for each metal.

Statistical Analysis
All analyses were performed in triplicate. Results were expressed as means with ± (standard deviation) SD. The statistical software package Statistical Package for Social Sciences Version 16.0 (SPSS 16.0) was used for the data analysis.

Heavy Metal Concentrations of Vegetables
The mean concentrations of heavy metals in edible parts of various vegetables collected from wastewaterirrigated agricultural fields in Savar Upazila of Dhaka District, Bangladesh are shown in Table 2. The results reveal significant levels (P< 0.05) of identified metals in different vegetable samples. The application of wastewater leads to changes in the physicochemical characteristics of soil and consequently leads to heavy metal uptake by vegetables. 1 In general, leafy vegetables (red amaranth, radish, Indian spinach (green), stem amaranth leaf, jute leaf and coriander) accumulated much higher concentrations of Pb, Ni and As compared to non-leaf vegetables (bottle gourd, green papaya and pointed gourd) grown on wastewaterirrigated soils at the same site. Leafy vegetables generally accumulate metals at a higher rate than roots/ tuberous vegetables, which is due to the fact that leafy vegetables have high translocation, high transpiration and fast growth rates. 5,17,18 The mean concentration of metals in radish, jute leaf and stem amaranth leaf are shown in Table 2. Metals were detected in radish, jute leaf and stem amaranth leaf in the following decreasing order: Ni > Pb > As, Ni > As > Pb and Ni > Pb > As, respectively. For the rest of the vegetables, the order of metal ion concentration was Ni > Pb > As. Islam

Vegetable Table 2 -Mean Concentration of Heavy Metals (mean ± standard deviation, mg/kg -1 ) in Vegetable Samples (fresh weight basis) *ND-Not Detected
Research other hand, Ni was detected in the range of 0.072 to 1.069 mg/kg, with the highest concentration (1.069 mg/kg) in jute leaf and lowest concentration (0.072 mg/kg) in bottle gourd. The mean concentration range of As in all vegetables analyzed was ND to 0.076 mg/kg, with the highest concentration (0.076 mg/kg) in jute leaf and lowest concentration (ND less than 0.001 mg/kg) in Indian spinach (green), green papaya and bottle gourd. In the present study, the mean concentrations of all studied heavy metals in all vegetables were much lower than the concentrations reported in a previous study due to less wastewater contamination, less frequency of irrigation with wastewater, and thus lower duration of exposure to wastewater.

Discussion
The mean concentrations of metals were compared with the safe limits given by the World Health Organization/Food and Agricultural Organization (WHO/FAO) ( The results of the present investigation agrees with the findings of previous studies with regard to heavy metal contamination in the edible parts of vegetables produced in wastewaterirrigated sites. 25,26 Table 3  demonstrated that plants grown on wastewater-irrigated soils are generally contaminated with heavy metals, which pose a major health concern. 5,21,26

Metal Pollution Index and Health Risk Assessment
The MPI is a reliable and precise method for monitoring metal pollution in vegetables. 11 The MPI of vegetables is presented in Figure  1. Among the different vegetables in the present study, jute leaf showed the highest MPI (0.16), and green papaya showed the lowest MPI (0.005). Shigh et al. reported that the MPI of 9.74 mg/kg for radish and 10.22 mg/ kg for bottle gourd was higher than the findings in the present study of 0.11 mg/kg for radish and 0.011 mg/ kg for bottle gourd. 20 The higher MPI for jute leaf and radish suggests that these vegetables may present more of a risk to humans due to a higher accumulation of heavy metals in the edible portion. 20 To appraise the health risk associated with heavy metal contamination of vegetables, the daily intake of metals and risk index were calculated. There are several possible pathways of exposure to humans, but the food chain is the most important. The daily intake of metals was calculated according to the average vegetable consumption for both adults and children ( Table 4). Among all vegetables tested, the highest intake values of Ni and As were from consumption of jute leaf and the highest intake of Pb came from consumption of stem amaranth leaf for both adults and children. The daily intake of metals in present study is similar to the provisional tolerable daily intake for adults (PTDI) set by the WHO/FAO. 27 The health risk index of metals with regard to noncarcinogenic effects from

Table 3 -Comparison of Studies of Metal Concentrations in Vegetables *ND-Not detected. All values are presented as (mg/kg fw)
Research consumption of vegetables for both adults and children is presented in Table 5. The results revealed that As contamination in jute leaf posed the greatest health risk to both adults and children. There is growing evidence that chronic exposure to inorganic arsenic (iAs) may increase the risk of keratosis, hyperpigmentation, and cardiometabolic (CM) disorders, including diabetes mellitus (DM) and cardiovascular diseases (CVD). 15, 31-33 The health risk index was less than 1 for Pb, As and Ni in all vegetables, except for As in jute leaf for both adults and children. Singh et al. found that the health risk index was less than 1 for Pb and Ni in bottle gourd, radish and pointed gourd, which is similar to the results in the present study. 20 In the present study, Pb and Ni were not found to present any risk to the local population. In contrast, Cui et al. reported that local residents of an area near a smelter in Nanning, China were exposed to Pb through consumption of vegetables and there was a potential health risk to the local population. 14

Conclusion
The build-up of heavy metals due to continuous irrigation with wastewater has led to the contamination of vegetables in the study area. The highest mean concentration of all metals was detected in jute leaf, except for Pb, which was found in the highest concentrations in stem amaranth leaf.  Table 4

-Daily Intake of Metals for Individual Heavy Metals in Different Vegetables for Adults and Children *ND-Not detected. Units of daily intake are expressed as mg/kg/day
Hossain et al Research for a significant negative impact on human health. Therefore, regular monitoring of these toxic heavy metals from industrial effluents in vegetables and other food materials is essential to prevent their excessive build-up in the food chain.