Determination of potentially toxic elements in selected vegetables sampled from some markets in the Kumasi metropolis

Abstract

Lead, cadmium, arsenic, and copper are potentially toxic elements that are of concern to man because of their negative effects on human health. The Atomic Absorption Spectrometer was used to evaluate the quantities of these heavy metals, and a value of heavy metal consumption in human meals was derived to determine the danger to human health. The mean cadmium content in the three vegetables from the four market sites varied from 0.60 mg/kg to 1.33 mg/kg, above the Food and Agricultural Organization/World Health Organization permissible limit. Lead, arsenic, and copper had mean values ranging from 0.064 mg/kg to 0.106 mg/kg, 0.003 mg/kg to 0.004 mg/kg, and 12.35 mg/kg to 21.54 mg/kg, respectively, all of which were below the World Health Organization’s maximum limit for each heavy metal. The Target Hazard Quotient (THQ) was used to determine the health risk that these heavy metals posed to vegetable eaters. Cadmium and copper THQ levels were greater than the acceptable limit of 1 in both adults and children, whereas lead THQ values were higher in just children, indicating a considerable health risk to the population. Arsenic THQ levels in both populations and lead THQ values in the adult population were both below the acceptable limit of 1, indicating that there was no major health risk to the population.

Keywords:

• Heavy metals
• hazard quotient
• health risk
• Vegetables

Introduction

Contamination of vegetables with heavy metal(loid)s pose public health risks which has become a worldwide concern (Jehan et al., 2021). These heavy metal(loid)s are released from both natural and anthropogenic sources (Zheng et al., 2020) into the environment and they find their way into humans through oral ingestion of food (Muhammad et al., 2019). The intensive use of agrochemicals, thermal power plants, coal mining, gold mining, metal smelting and electronic waste processing are some of the anthropogenic sources of heavy metal(loid)s (Shahmoradi et al., 2020; Wang et al., 2021), while rock weathering and erosion are natural sources (Wang et al., 2021). Vegetables are also contaminated with heavy metal(loid)s by other non-point sources such as atmospheric deposition deposition, metal evaporation and metal corrosion by water sources (Kuerban et al., 2020). They are non-biodegradable, persistent and have extended biological half-lives (Heidarieh et al., 2013: Negahban et al., 2021). Metal pollution has become a major source of worry, particularly in agricultural production systems, due to its rising prevalence in humans. Arsenic (As), chromium (Cr), cobalt (Co), and lead are toxic heavy metal(loid)s that are commonly discovered as pollutants in vegetables (Pb; Muhammad et al., 2019). These heavy metal(loid)s can cause chronic and acute health problems in humans, including irritability, hypertension, kidney, liver and heart problems, nerve damage, anaemia, bone problems, intellectual and memory loss and carcinogenesis, especially at levels higher than the body’s minimal needs (Amin et al., 2021; Madkour, 2020). Under certain environmental conditions, heavy metals may accumulate to toxic concentrations and cause ecological damages (Freedman, 2013). The non- essential heavy metals are not necessary for plants but are taken up by plants in the toxic form very easily. Copper, iron, manganese, nickel, selenium and zinc are required by vegetables in trace quantities for the normal functioning of organs in humans (Ahmad et al., 2020; Muhammad et al., 2021).

Vegetables, which are an important element of the human diet, contain several minerals, essential metals, carbohydrates, proteins and vitamins that aid in illness prevention, cell repair, and immune system stimulation through the production of blood cells (Muhammad et al., 2019). They also act as buffering agents for acidic products formed during the digestion processes (Bahrami et al., 2021). They are more frequently ingested because of the presence of certain nutritionally significant substances that are required for human existence, and they are frequently referred to as protective foods since they may help to avoid illnesses in humans (Boeing et al., 2012). Vegetables, on the other hand, contain both necessary and harmful elements in varying amounts (Radwan & Salama, 2006). Metals can be hazardous to humans at high quantities exceeding the extremely low physiological requirement, but they can also be detrimental in low concentrations due to the lack of an effective excretion system (Ghosh et al., 2012). Vegetables grown in fields polluted with heavy metals or near sources of heavy metal pollution may accumulates higher levels of heavy metals than those grown in other fields. Heavy metals are naturally present in soils, but anthropogenic activities such as mining, industrialization, and urbanization have accelerated their accumulation (Facchinelli et al., 2001) The biotoxic effects of heavy metals are thought to be dependent on their quantities and oxidation states, as well as the kind of sources and mechanism of deposition (Duruibe et al., 2007).

Metals such as mercury, lead, cadmium, tin, and arsenic are toxic in two ways: they have no metabolic function, but when present in the human body, they disrupt normal cellular processes, causing toxicity in a variety of organs; and they have the potential, to accumulate in the body through a process called bioaccumulation. This occurs because the metal, once absorbed into the body, accumulates in certain organs such as the liver or kidney, and is eliminated at a slower pace than it was absorbed (Jehan et al., 2021). People in cities such as Kumasi normally patronize the markets for their vegetable supplies that come from different farms across the country. Vegetables are bought and consumed without recourse to their source of production. Some of these lands could be contaminated with heavy metal(loid)s which could find their way into vegetables and pose adverse health risks to consumers. It is therefore imperative that the Heavy metal(loid)s content of vegetables from markets within the Kumasi metropolis are assessed and a human health risk assessment conducted to verify the adverse health risk posed by heavy metal(loid)s through consumption of vegetables to people. This study therefore assessed the heavy metal(loid)s load of vegetables from selected markets in the Kumasi metropolis and their human health risk.

1. Materials and method

1.1. Study area

The research was carried out in Kumasi, the Ashanti region’s capital. It is strategically positioned in south-central Ghana, covering an area of around 230 to 240 km² (Obuobie et al., 2006). Kumasi has a population of 2.5 million people and is growing at a rate of about 3.4 percent per year (Ghana Statistical Service, 2012). It is located between Latitude 6.35° N and 6.40° S and Longitude 1.30° W and 1.35° E and is approximately 270 km north of the national capital, Accra.

1.2. Sampling

Three markets in the Kumasi city, namely Ayiga, Asafo, and Adwaase, were chosen for sampling of three vegetables: cabbage, carrots, and green beans. In each of the three marketplaces, the three vegetables were purchased at random from three vendors. A total of twenty-seven samples were obtained, with three of each vegetable.

1.3. Digestion procedure

All glass wares were immersed in a weak HNO₃ solution for 24 h. The sliced samples were dried in the oven for three days at a temperature of 60 °C. The dried samples were crushed into powder form and placed in plastic containers for examination after drying. A mixture of concentrated triacid (Aquaregia) containing HNO₃, H₂SO₄, and HCLO₄ in the ratio of 9:4:1 was added to 1.00 g of powdered material in a digestion tube. The mixture was boiled until it became a clear solution. The sample was allowed to cool to ambient temperature before being filtered with Whatman filter paper no. 42. The mixture was then transferred to a 50 ml volumetric flask and distilled water was used to get it to the desired consistency.

1.4. Determination of heavy metals

The metals in the digested sample were determined using a UNICAM 979 Atomic Absorption Spectrometer. The heavy metals were analyzed using an air-acetylene flame and a Hitachi single element hollow cathode lamp as the source of radiation. A calibration curve was created for each of the heavy metals to be investigated using the stock standards, and the amounts of these metals were calculated using the known absorbance of each heavy metal. This was done three times for each sample, and the mean concentration was recorded as the concentration.

1.5. Quality control

The correctness of the observed values and analytical procedures were validated by analyzing a known reference material BRM# 12 (BAM, Germany).

1.6. Health risk assessment

The Target Hazard Quotient and the hazard index were developed to determine the hazards associated with the consumption of heavy metal contaminated vegetables. The Target Hazard Quotient is the proportion of heavy metal exposure to the reference dosage (Guerra et al., 2012). If the Target Hazard Quotient (THQ) is less than one (1), the entire population is unlikely to incur negative consequences within their lifetime (Mahmood & Malik, 2014; Zhuang et al., 2009). When the THQ is discovered to be more than one, the sensitive sub-population are likely to develop adverse health effects. The equation used to quantify the possible risk to human health from eating heavy metal-contaminated plants is described in equation 1(1) $\mathrm{T}\mathrm{H}\mathrm{Q}=\mathrm{A}\mathrm{D}\mathrm{D}/\mathrm{R}\mathrm{f}\mathrm{D}$(1) :

(1) $\mathrm{T}\mathrm{H}\mathrm{Q}=\mathrm{A}\mathrm{D}\mathrm{D}/\mathrm{R}\mathrm{f}\mathrm{D}$(1)

Where ADD is the Average Daily Dose and the RfD is the Reference Dose. The average daily dose is calculated using the equation 2(2) $\mathrm{A}\mathrm{D}\mathrm{D}=\left({\mathrm{C}}_{\mathrm{i}}\times \mathrm{I}\mathrm{R}\times \mathrm{E}\mathrm{F}\times \mathrm{E}\mathrm{D}\right)/\mathrm{B}\mathrm{W}\times \mathrm{A}\mathrm{T}$(2) below;

(2) $\mathrm{A}\mathrm{D}\mathrm{D}=\left({\mathrm{C}}_{\mathrm{i}}\times \mathrm{I}\mathrm{R}\times \mathrm{E}\mathrm{F}\times \mathrm{E}\mathrm{D}\right)/\mathrm{B}\mathrm{W}\times \mathrm{A}\mathrm{T}$(2)

Where C_i is the concentration of metal in vegetable, IR is the ingestion rate, EF is the exposure frequency, ED is the exposure duration, BW is the body weight of consumer and AT is the average time. The health risk was assessed in relation to it’s non-carcinogenic effects based on the calculation of the ADD estimates and defined toxicity according to (USEPA, 2011; Wongsasuluk et al., 2014). The values of the parameters used are given in Table 1.

Table 1. Input parameters to characterize the ADD value (Wongsasuluk et al., 2014)

Exposure parameters	Symbols	Units	Value
Concentraton	C	mg/kg	Table 2
Ingestion rate	IR	g/day	2.2
Exposure frequency	EF	days/year	365
Exposure duration	ED	Years	70
Adult BW	BW	kg	70
Child BW	BW	kg	16
Average time	AT	Years	25,550 days

Hazard index (HI) was used to estimate the potential health risk when more than one heavy metal is involved and it is calculated as the sum of THQ_S in equation 3:

(3) $\mathrm{H}\mathrm{I}=\left(\mathrm{T}\mathrm{H}{\mathrm{Q}}_{\mathrm{i}}+\mathrm{T}\mathrm{H}{\mathrm{Q}}_{\mathrm{i}\mathrm{i}}+\mathrm{T}\mathrm{H}{\mathrm{Q}}_{\mathrm{i}\mathrm{i}\mathrm{i}}\dots ..\mathrm{T}\mathrm{H}{\mathrm{Q}}_{\mathrm{n}}\right)=\sum \mathrm{T}\mathrm{H}\mathrm{Q}$(3)

Table 2. Mean concentrations (mg/kg) of Cd, Pb, As, Cu in vegetable samples

LOCATION (MARKET)	SAMPLE	Pb	Cd	As	Cu
	Cabbage	0.08	0.66	0.003	18.70
Ayigya	Carrot	0.07	1.33	0.003	18.25
	Green beans	0.06	0.65	0.002	13.25
	Cabbage	0.09	0.92	0.004	20.25
Asafo	Carrot	0.08	0.60	0.004	17.05
	Green beans	0.11	0.96	0.003	21.54
	Cabbage	0.08	0.84	0.003	19.25
Adwaase	Carrot	0.07	0.98	0.003	12.35
	Green beans	0.10	1.13	0.003	22.75

The Permissible limits of Cadmium, Lead, Arsenic and Copper are 0.02, 0.30, 0.007 and 40 mg/kg respectively (Kowalska, 2021).

According to (Lim et al., 2008) a ratio of HI/HQ < 1 means an acceptable risk of non-carcinogenic effect on human health whilst HI/HQ> 1 means an unacceptable risk of non-carcinogenic effect.

2. Results and discussion

The mean concentrations of the heavy metals in vegetables sampled from the three markets are represented in Table 2 below.

The vegetable samples from all the markets were found to be rich in copper, with mean values ranging from 17.05 mg/kg in carrot from Asafo market to 22.75 mg/kg in green beans, from Adwaase market. However, copper levels in this study were below the acceptable limit (40 mg/kg) in all of the vegetable samples. Copper is a common and important metal that exists in nature. Copper is ingested by organisms through food, and it is used to start a variety of biological processes. It is a structural component of many macromolecules, ensuring that a significant number of enzymes and other proteins function normally (Addae, 2015).

Elevated levels of cadmium were found in the vegetable samples, with mean values ranging from 0.65 mg/kg in green beans from Ayigya market to 1.33 mg/kg for cabbage and green beans, respectively. The cadmium content in the vegetables analyzed were all higher than FAO/WHO acceptable limit of 0.02 mg/kg. The high cadmium content in the vegetables might have come from human activities as well as the usage of wastewater in irrigating the vegetables at locations where the vegetables were grown. Cadmium is a relatively mobile metal that is quickly absorbed by the aerial sections of vegetables, which might explain why it was found in greater concentrations in the vegetable samples (Asdeo & Loonker, 2011). When the concentration of cadmium in vegetables collected from all three markets was analyzed, it was discovered that the vegetables from Ayeduase had a greater cadmium content than the other markets. Cadmium is a recognized carcinogen, and when ingested can harm important organs such as the kidney and liver (Sharma et al., 2016). Ametepey et al. (2018) assessed heavy metals in vegetables from markets in Tamale metropolis in Ghana and found far lower levels of cadmium in cabbage (0.05 mg/kg), carrot (0.04 mg/kg) and green pepper (0.05 mg/kg) than recorded in this study (0.65–1.33 mg Cd/kg). However, (Perveen et al., 2012) reported a higher level of Cd (2.84 mg/kg) than this study in a research done on vegetables in Khyber Pakhtunkhwa, Pakistan.

Lead followed closely with mean concentration ranging from 0.07 mg/kg to 0.11 mg/kg in green beans sampled from Ayigya and Asafo market respectively. Green beans and cabbage from Asafo market and green beans from Adwaase market were the most lead-contaminated vegetables, with mean amounts of 0.11 mg/kg, 0.09 mg/kg, and 0.11 mg/kg, respectively. The lead levels in the aforementioned veggies were much below the legal limit. Green beans from Ayigya market and carrot from Ayeduase had low lead amounts of 0.06 mg/kg and 0.07 mg/kg, respectively, which were similarly below the permissible range in the products. Lead poisoning is caused by ingesting too much lead, which causes harm to the neurological, skeletal, circulatory, and immunological systems.

Humans have been shown to have altered neurobehavioral functioning as a result of lead poisoning. Due to the bioaccumulative potential in children, children are more susceptible to the negative effects of heavy metal than adults (Elledge et al., 2014).

In all of the veggies tested, the concentration of arsenic was the lowest (Table 2). The ability of plants to absorb only soluble arsenic from the environment, and hence the limited availability of arsenic for absorption by vegetables at diverse places, might explain the low arsenic concentrations reported in the vegetable samples. In its inorganic form, arsenic is a known carcinogen with a lengthy history of neurotoxicity. Ingesting large amounts of inorganic arsenic can cause major health problems, such as gastrointestinal symptoms, cardiovascular and neurological system disturbances, and peripheral vascular dysfunction, which can lead to infection in extreme cases (black foot disease, reported in Taiwan; Järup, 2003).

2.1. Health risk assessment

When analyzing the health risk associated with the intake of heavy metal contaminated vegetables, the level of exposure to these environmental pollutants, as well as the amount of heavy metal absorbed by the body, are taken into account. The Target Hazard Quotient and Average Daily Dose were calculated for both adults and children. The Average Daily Dose of the vegetable samples from the various market sites is shown in Table 3. For adults (70 kg) and children (16 kg) in the community, it was calculated as a function of mean metal concentrations, daily vegetable consumption, and expected body weight.

Table 3. Estimated average daily dose (ADD) of vegetables in adult and children population

Heavy Metal	Market	Vegetable	Average Daily Dose in adult population (mg/70/day (adult))	Average Daily Dose in children population (mg/16/day (children))
Lead (Pb)	Ayigya	Cabbage	2.62 × 10^−3	1.14 × 10^−2
		Carrot	2.27 × 10^−3	9.91 × 10^−3
		Green beans	2.01 × 10^−3	8.77 × 10^−3
	Asafo	Cabbage	2.93 × 10^−3	1.28 × 10^−2
		Carrot	2.37 × 10^−3	1.04 × 10^−2
		Green beans	3.32 × 10^−3	1.45 × 10^−2
	Ayeduase	Cabbage	2.67 × 10^−3	1.16 × 10^−2
		Carrot	2.11 × 10^−3	9.24 × 10^−3
		Green beans	3.19 × 10^−3	1.14 × 10^−2
Cadmium	Ayigya	Cabbage	2.02 × 10^−2	9.10 × 10^−2
		Carrot	4.17 × 10^−2	1.83 × 10^−1
		Green beans	2.05 × 10^−2	8.97 × 10^−2
	Asafo	Cabbage	2.89 × 10^−2	1.27 × 10^−1
		Carrot	1.88 × 10^−2	8.21 × 10^−2
		Green beans	3.01 × 10^−2	1.32 × 10^−1
	Ayeduase	Cabbage	2.62 × 10^−2	1.15 × 10^−1
		Carrot	3.07 × 10^−2	1.34 × 10^−1
		Green beans	3.54 × 10^−2	1.55 × 10^−1
Arsenic	Ayigya	Cabbage	9.43 × 10^−5	4. 13 × 10^−4
		Carrot	1.04 × 10^−4	4.54 × 10^−4
		Green beans	6.29 × 10^−5	2.75 × 10^−4
	Asafo	Cabbage	1.10 × 10^−4	4.81 × 10^−4
		Carrot	1.10 × 10^−4	4.81 × 10^−4
		Green beans	1.10 × 10^−4	4.81 × 10^−4
	Ayeduase	Cabbage	8.46 × 10^−5	3.71 × 10^−4
		Carrot	8.17 × 10^−5	3.58 × 10^−4
		Green beans	1.04 × 10^−4	4.54 × 10^−4
Copper	Ayigya	Cabbage	5.88 × 10^−1	2.57
		Carrot	5.74 × 10^−1	2.51
		Green beans	4.16 × 10^−1	1.82
	Asafo	Cabbage	6.36 × 10^−1	2.78
		Carrot	5.36 × 10^−1	2.34
		Green beans	6.77 × 10^−1	2.96
	Ayeduase	Cabbage	6.05 × 10^−1	2.65
		Carrot	3.88 × 10^−1	1.70
		Green beans	7.15 × 10^−1	3.13

The ADD of cadmium contaminated vegetables from the market sites were higher than the reference dose of cadmium (3.0 × 10⁻³ mg/kg/day) in both the adult and children population as shown in Table 3. This puts the population at risk of adverse health effects of cadmium. The ADD of arsenic contaminated vegetable samples was found to be lower than the reference dose (0.05 mg/kg/day) in both the adult and children population.

With copper, it was observed that the ADD was higher than the reference dose (4.2 × 10⁻²) in all vegetables in both the children and adult population. The Target Hazard Quotient was used in estimating the potential health risk that the population may be exposed to. The Target Hazard Quotient (THQ) is a ratio of the determined dose of the heavy metal pollutant to the oral reference dose. Factors such as exposure duration, exposure frequency, intake of the heavy metal, body weight, average time and the oral reference significantly affect the THQ. The THQ values for the vegetable in all three market sites is given in Table 4 below.

Table 4. Target Hazard Quotient values for vegetables in all three market sites

Heavy metal	Market site	Vegetable	THQ values for adult population	THQ values for children population
Pb	Ayigya	Cabbage	0.75	3.26
		Carrot	0.65	2.83
		Green beans	0.57	2.51
	Asafo	Cabbage	0.84	3.66
		Carrot	0.68	2.97
		Green beans	0.95	4.14
	Ayeduase	Cabbage	0.76	3.31
		Carrot	0.60	2.64
		Green beans	0.91	4.00
Cd	Ayigya	Cabbage	6.93	30.00
		Carrot	13.90	60.83
		Green beans	6.83	29.90
	Asafo	Cabbage	9.63	42.17
		Carrot	6.27	27.37
		Green beans	8.73	38.27
	Ayeduase	Cabbage	10.23	44.77
		Carrot	11.80	51.57
		Green beans	6.93	30.00
As	Ayigya	Cabbage	0.009	0.008
		Carrot	0.002	0.009
		Green beans	0.001	0.005
	Asafo	Cabbage	0.002	0.010
		Carrot	0.002	0.010
		Green beans	0.002	0.010
	Ayeduase	Cabbage	0.002	0.007
		Carrot	0.002	0.007
		Green beans	0.002	0.009
Cu	Ayigya	Cabbage	13.99	61.22
		Carrot	13.66	59.75
		Green beans	9.91	43.38
	Asafo	Cabbage	15.15	66.30
		Carrot	12.76	55.82
		Green beans	16.12	70.52
	Ayeduase	Cabbage	14.41	63.02
		Carrot	9.24	40.43
		Green beans	17.02	74.48

In general, it was discovered that the THQ values for adults and children from different research sites differed significantly. THQ values, which indicate the danger presented to consumers over the course of their lives, were found to be worrisome in vegetables containing copper and cadmium. The calculated ADD of the copper and cadmium contaminated vegetables from the market sites were higher hence, the higher THQ and the estimation of the health risk. With the exception of arsenic in both groups and lead in the adult population, all heavy metals had a THQ value more than the acceptable limit of THQ < 1 in both adult and child populations. Lead, a non-essential heavy metal, was found to have a high THQ in vegetables taken from all three market sites, but exclusively in the children’s population. Lead THQ values were two to three times greater in the children than in adults. This means that the potential health risk of lead intoxication for children is two to three times higher than for adults. Cadmium THQ readings were also found to be greater above the safe limit of one in vegetables from all three market locations. The figures for the children’s population were significantly higher than those for the adults, however both populations’ levels above the maximum limit. As shown in Table 4, the THQ values for arsenic in vegetable samples were below the safe limit of one, indicating an acceptable level of non-carcinogenic adverse health risk. For both children and adults, all of the vegetables selected from the research locations were below the oral reference dosage of 0.05 mg/kg/day. Copper, a necessary heavy metal, was found to have THQ values greater than the allowed limit of one in both children and adults, implying that, although being an essential metal, copper provides a possible health risk to both adults and children over the course of their lives.

3. Conclusion

This study has determined the levels of heavy metal in regularly consumed vegetables, as well as the health concerns connected with their use. With the exception of cadmium, which surpassed the allowed level, heavy metal concentrations in the diverse vegetables were all within the joint FAO/WHO standard.

With the exception of arsenic, the THQ values of the heavy metals tested varied from 2.83 to 74.48, suggesting unacceptable level of non-carcinogenic adverse health effect are probable. These high THQ values for heavy metals found in all three vegetables (cabbage, carrot, and green beans) indicate a significant likelihood of putting the consumer’s health at risk. Differences in body weight, ingestion rate, and exposure period might explain the discrepancy in THQ levels between adults and children. Heavy metals are absorbed through a variety of mechanisms, including food consumption, water consumption, skin contact, occupational exposure, and inhalation. Heavy metal poisoning can be acute or chronic following long-term exposure, causing significant damage to important organs such as the liver, kidneys, and lungs, as well as disorders in the body.

Acknowledgements

The research team is thankful to the chemistry department at KNUST in Kumasi for providing lab space and consumables.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The author(s) reported there is no funding associated with the work featured in this article.