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Health Risk Assessment of Food Crops Fumigated with Metal Based Pesticides Grown in North-Eastern Nigeria



U. Bawa, A. AbdulHameed, A.J. Nayaya and A.G. Ezra
 
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ABSTRACT

Background and Objective: Heavy metals contents in food crops and their associated human health through food consumption has been a major health concern. This study was designed to determine the concentrations of heavy metals (Cd, Pb, Cr, Cu, Zn) in food crops fumigated with pesticides as the only source of contamination and their associated human health risk. Materials and Methods: Samples of seven selected and commonly consumed food crops and their corresponding soil were randomly collected from farmlands fumigated with pesticides. Heavy metals were measured by atomic absorption spectrometry after acid digestion. The mean difference between the heavy metal in each plant part (root, stem, leave and fruit) was determined using Analysis of variance (ANOVA) with SPSS software. Health risk assessment and pollution indices from the consumption of each plant were computed using the standard formula as described USEPA. Results: The concentrations of heavy metals in most of the studied crops varied significantly in different parts of the plants and for different metals. Most of the studied crops showed bioaccumulation factor >1 indicating greater absorption of metals and bioaccumulation factor values were higher for Cd compared to other metals. The Estimated Daily Intake (EDI) of metals were above the oral reference dose (RfD) in all the studied crops for Cd and in Allium cepa, Zea mays and Oryza sativa for Pb, while hazard index showed slightly health risk from the consumption of all metals in Oryza sativa. Conclusion: The consumption of the studied food crops fumigated with pesticides in the study area may pose a great health risk and hence regular monitoring/screening of heavy metals in pesticides should be enforced.

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  How to cite this article:

U. Bawa, A. AbdulHameed, A.J. Nayaya and A.G. Ezra, 2021. Health Risk Assessment of Food Crops Fumigated with Metal Based Pesticides Grown in North-Eastern Nigeria. Research Journal of Environmental Toxicology, 15: 1-10.

DOI: 10.3923/rjet.2021.1.10

URL: https://scialert.net/abstract/?doi=rjet.2021.1.10
 

INTRODUCTION

Heavy metal contamination from pesticides of agricultural soil and crops is one of the serious ecological problems on a global scale, especially in developing countries like Nigeria1. However, there are limited studies on the health risk of heavy metals from the consumption of pesticides fumigated food crops. Most studies have attributed the sources of heavy metals in food crops from industrial irrigated wastewater and industrial contaminated sites, while sources of heavy metals from pesticides are mostly overlooked.

Agriculture in Northern Nigeria is the most fundamental form of economic activity and food crops produced in this region are usually distributed to not only the southern parts of the country but including neighboring countries like Niger, Chad, Cameroon etc. The production of chemical pesticides has been on the increase and new pesticides have been infiltrating the Nigerian markets whose chemicals contents for heavy metals and other chemicals unfriendly and toxic to the environment are not known. A survey on pesticide usage in Nigeria indicated that about 15,000 metric tons per annum of pesticides comprising about 135 pesticides chemicals are marketed locally under 200 different product brands and formulation, thus making Nigeria one of the largest pesticide users in sub-Sahara Africa2. These records are believed to have substantially increased as more people have gone into farming now. However, despite the beneficial contribution of pesticides in the control of pests and diseases of agricultural importance, studies have shown that they could be detrimental to the soil ecosystem and contamination of food crops through the food chain3. More so, studies have revealed the presence of heavy metals in pesticides even at the recommended dilution rate for farmers4,5. Heavy metal contents have been banned in pesticides even in developing countries like Nigeria but however, despite the ban, recent studies have revealed heavy metals in pesticides4,5. This is thus a case of indiscriminate and illegal infiltration and adulteration of agrochemicals and its massive usage in Nigeria.

The use of large quantities of agrochemicals such as metal based pesticides plays an important role in the contaminating different food crops6. Heavy metals have the ability to accumulate in food crops in a contaminated environment and may be toxic to humans, plants even at low concentrations2. Heavy metals contamination through the ecological food chain is the main contributing pathway accounting for more than 90% to human exposure6,7. Heavy metals are hazardous contaminants in food and the environment and they are non-biodegradable with long biological half-lives8. Previous reports showed the persistence of heavy metals in the environment in both non-biodegradable and non-thermo-degradable category which lead to accumulation readily reaching to toxic levels9. Excessive accumulations of heavy metals in agricultural soils through the use of agrochemicals particularly pesticides may not only result in soil contamination but also lead to elevated heavy metals uptake by crops and thus affect food quality and safety10. Consequently, food crops safety and its associated risk have become an important food quality attribute and a global public health issue within the last decade11,12. Crop contamination by heavy metals from pesticides cannot be ignored since crops are crucial to human diet as they comprise important constituents desired by human body, like carbohydrates, protein, vitamins, minerals and trace elements13. The aim of this research was to determine the concentrations of heavy metals (Cd, Pb, Cr, Cu, Zn) from pesticides in rain fed agricultural crops and their associated human health risk in some farms in the north eastern part of Nigeria.

MATERIALS AND METHODS

Study area: The study was carried out from February, 2018 to March, 2019. Random samples of leaves, stems, roots and fruits were collected from matured seven crop plants from farms in Gombe. The sampling areas are located at 32P N0749836, 32P E11366869 and N0749941, E1134986. All sampling was replicated five times. At each sampling sites, 20 g each of the seven crops namely (Spinacia oleracea, Capsium cerasiforme, Solanum lycopersium, Lactuca sativa, Allium cepa, Zea mays and Oryza sativa) were collected from three different locations in each farm to provide representative samples of each crop. Samples were collected from the roots, stems, leaves and fruits of each of these crops and their corresponding soil. The crops and soil samples were collected in a clean brown envelope, labeled and transported to the laboratory of the Department of Biological Sciences, Abubakar Tafawa Balewa University Bauchi, Nigeria.

Samples preparation: Each plant parts were chopped into small pieces and oven-dried at 80°C, pulverized and passed through a 2 mm sieve at Biology laboratory ATBU. The resulting fine powder was kept at room temperature before analysis. Soil samples were air-dried in the laboratory, crushed and passed through a 2 mm mesh size sieve and stored at room temperature before analysis.

Heavy metal analysis: Samples (1 g) of both plant and soil were digested using 15 mL each of tri- acid mixture (70% high purity (Nitric acid) HNO3, 65% (Perchloric acid) HClO4 and 70% (Sulfuric) H2SO4 in 5:1:1 ratio) The mixture were digested at 80°C till the solution became transparent. The resulting solution was filtered and diluted to 50 mL using deionized water and was analyzed for Cr, Cu, Cd, Zn and Pb using an atomic absorption spectrophotometer14.

Health risk assessment
Daily intake of metal: The Daily Intake of Metals (DIM) was calculated to determine the health risk from consuming vegetables contaminated with pesticides with heavy metal contents using the formula given below14:

Image for - Health Risk Assessment of Food Crops Fumigated with Metal Based Pesticides Grown in North-Eastern Nigeria

where, M is the concentration of heavy metals in plants (mg kg–1), K is the conversion factor, I is the daily intake of vegetables and W is the average body weight

Fresh weight of vegetables was converted to dry weight using the conversion factor 0.085 as described in another study3. Where, the daily vegetable Intake (I) for adults in Nigeria was taken as 0.086 kg day–1 in accordance with previous study15, Adult body weight was taken as 60 kg.

Hazard quotient: The health risks to the local inhabitants from the consumption of vegetables were evaluated based on the Hazard Quotient, which is the ratio between exposure and oral reference dose (RfD).

Given as:

Image for - Health Risk Assessment of Food Crops Fumigated with Metal Based Pesticides Grown in North-Eastern Nigeria

The (RfD) is an estimation of human daily exposure that is likely to pose an appreciable risk of adverse health effects during a lifetime. Oral Reference Dose (mg kg–1 b.wt./day) for heavy metals was taken as Cr = 1.5, Cd = 0.001, Pb = 0.004, Cu = 0.04, Zn = 0.316.

Hazard index: Potential risk to human health due to more than one heavy metal known as the Hazard Index (HI) was calculated16, which is the total sum of all the Hazard Quotients as shown in the Eq:

Image for - Health Risk Assessment of Food Crops Fumigated with Metal Based Pesticides Grown in North-Eastern Nigeria

where, HI is the Hazard Index and HQ is the Hazard Quotients

The hazard index assumes that the magnitude of the adverse effect will be proportional to the sum of the multiple metal exposures.

Estimated daily intake (EDI): The degree of toxicity of heavy metals to human upon their daily intake (mg kg–1/day) known as the estimated daily intake was computed for each element16:

Image for - Health Risk Assessment of Food Crops Fumigated with Metal Based Pesticides Grown in North-Eastern Nigeria

Cmetal is concentration of heavy metals in plants (mg kg–1), Average daily intake of vegetable in Nigeria for adult was taken as 0.086 kg day–1 in accordance with Hart et al.15 Adult body weight was taken as 60 kg. To determine the degree of toxicity the computed EDI values were compared with an acceptable level of oral reference dose (Rfd)16.

Pollution index (PI): Pollution index (PI) is the ratio of metal concentration in a biotic or abiotic medium to that of the regulatory Standard of international bodies such as the World Health Organization (WHO), United States Environmental Protection Agency (USEPA) was observed. Values of PI<1 value indicates that the soil or plant material is not yet contaminated whereas PI>1 indicates pollution. On the other hand, PI = 1 reveals a critical state which makes the involved plant useful for environmental monitoring17.

Mathematically, PI was computed as:

Image for - Health Risk Assessment of Food Crops Fumigated with Metal Based Pesticides Grown in North-Eastern Nigeria

where, PI is the individual pollution index of study metal, Cplant is the concentration of the metal in plant and CUSEPA is the value of the regulatory limit of the heavy metal by USEPA16.

Whereas the USEPA, 2006 Oral Reference Dose (mg kg–1 b.wt./day) for heavy metals was taken as Cr = 1.5, Cd = 0.001, Pb = 0.004, Cu = 0.04, Zn = 0.316.

Statistical analysis: The data were analyzed using statistical software “R” 2014 version and SPPS version 8.1. Analysis of variance (ANOVA) was applied for evaluating the significance difference at 0.05 level between heavy metal concentrations in different parts of each vegetable. One way ANOVA was also applied to determine the significance difference between heavy metals concentration in plants tissues and the concentration in corresponding soil for each plant.

RESULTS AND DISCUSSION

Heavy metals concentration in crops: The heavy metal concentrations in different parts of the crops under investigation (roots, stems, leaves and fruits) ranged from 7.67-0.33, 8.17-0.50, 1.17-0.33, 32.83-6.33, 186.08-19.00 mg kg–1 for Cd, Pb, Cr, Cu and Zn, respectively (Table 1). There is significant (p<0.05) variation in the concentrations of heavy metals in different parts of most of the crops. Variability exist in the uptake and concentration of heavy metals among the different types of crops which could be attributed to individual differences in metal uptake and tolerance coupled with soil types18. The finding of this research showed a decreasing order of concentration via Zn>Pb>Cd>Cu>Cr (Table 1). Cadmium concentrations varied from (7.67 mg kg–1) in Spinacia oleracea root to (0.33 mg kg–1) in Solanum lycopersicum root. The concentrations of cadmium in all the crops under investigation were above the permissible limits of 0.2 mg kg–1 19, indicating high contamination of the crops. Previous research findings found cadmium as a component of pesticides used by farmers in Northern Nigeria20. The cadmium concentrations in this study were higher (0.47 mg kg–1) than what was reported21 in Tunisia and (1.57 mg kg–1) in Pakistan22. However, the cadmium concentrations reported in the present study were comparatively similar to 4.09 mg kg–1 reported in Bangladesh23 and 2.97 mg kg–1 in southern Nigeria by1. Conversely, significantly higher concentrations (900 mg kg–1) of cadmium was reported in China14, in different crops compared to the cadmium concentrations reported in this study. Cadmium is a non-essential element, which has no function in the human body but can exhibit toxic tendencies even in low concentrations22. Cadmium has been showing to accumulates in the human bones, lungs, liver, kidneys, nerve tissues, leading to their damage and malfunction24-26.

The concentrations of lead ranged from 8.17 mg kg–1 in Oryza sativa root to 0.50 mg kg–1 in Capsicum cerasiforme root (Table 1). The concentration of lead in all the studied crops exceeded the permissible limits of 0.3 mg kg–1 19. This is an indication of the potential toxicity and health risk to humans. This contamination is due to the presence of lead contents in the pesticides20 use indiscriminately in the study area. The result revealed high contamination in Zea mays, Oryza sativa and Spinacia oleracea compared to other crops. Interestingly the Lead concentrations recorded in this study were higher compared to 0.034 mg kg–1 reported in China27, 2.2 mg kg–1 in Romania28 and 0.09 mg kg–1 reported2 in Nigeria. However, other findings reported higher values. For example 28.14 mg kg–1 was reported in Bangladesh23 and 374 mg kg–1 in Indonesia was reported previously29. Lead is non-essential metal that has no function in the human body and exposure to lead has been linked to incidence of neurological disorders, hypertension, cognitive impairment, renal dysfunction, arthritis, hallucination and vertigo30.

Chromium concentrations varied from 1.17 mg kg–1 in Allium cepa stem to 0.33 mg kg–1 in Capsicum cerasiforme stem (Table 1). The concentrations of chromium in all the crops under investigation were above the permissible limits of 2.3 mg kg–1 19. This chromium contamination could be a potential risk for human consumption. This contamination could be attributed to the use of pesticides with high chromium contents by farmers in the study area. Studies have shown the presence of heavy metals in pesticides at levels above the recommended farmer’s dilution rate4,5,20. The concentration of chromium in this study was higher compared to 0.077 mg kg–1 in China27, 0.84 mg kg–1 in Nigeria2 and 0.32 mg kg–1 reported in India31. Conversely, chromium concentrations in this study were significantly lower than concentrations of 17.8 mg kg–1 in Romania28, 10.5 mg kg–1 in Pakistan22 and 33.16 mg kg–1 in Bangladesh23. The concentration in this study was within the range reported of 2.5 mg kg–1 reported in Bangladesh32. The chromium concentrations are thus a likely potential health risk to its consumers. Chromium has been shown to cause ulcer, perforation of nasal septum, respiratory cancer and alterations in replication and transcription of DNA and chromosomal aberrations in humans33,34.

The concentration of copper in the studied crops ranged from (32 mg kg–1) in Spinacia oleracea root to (6.33 mg kg–1) in Solanum lycopersicum root (Table 1). The concentrations of copper in different parts of the studied crops are shown in (Table 1). Copper concentrations in this study were below the permissible limits of 40 mg kg–1 19, indicating non Cu contamination. This might infer low copper contents in the pesticides used in the study area. The concentration of copper in this study is higher than (1.68 mg kg–1) in Kenya35, (8.56 mg kg–1) in Pakistan22 and (25.45 mg kg–1) in Bangladesh by23. Conversely, copper concentrations in this study were substantially lower than (62.58 mg kg–1) in Tunisia21 and (46.53 mg kg–1) in Nigeria by1. Copper is an essential metal and a component of several enzymes necessary for normal metabolic functions in humans36,37. But excessive uptake of copper in humans can cause temporary gastrointestinal distress with symptoms such as nausea, vomiting, abdominal pain, liver toxicity37.

Table 1: Mean concentration of heavy metals (mg kg1) in crops
Heavy metals (mg kg1)
Sampling site Name of sample Botanical name Hausa name Cd Pb Cr Cu Zn
Gombe Spinach Spinacia oleracea Alayyaho
Root 7.67a 5.33a 0.17a 32.83b 19.00a
Stem ND ND ND ND ND
Leaf 1.33b 2.33b 0.58a 10.92a 42.67b
Sweet pepper C. cerasiforme Tattase
Root ND ND ND ND ND
Stem 1.83a 0.50a 0.33a 9.50a 68.33a
Leaf 1.33a 3.33a 0.67a 17.58b 66.50a
Fruit ND ND ND ND ND
Tomato S. lycopersicum Tomatur
Root 0.33a ND 0.50a 6.33a 26.83a
Stem 1.42b 4.92a 0.83a 16.83a 61.83a
Leaf 1.42b 6.33a 1.00a 14.25a 112.92a
Fruit ND ND ND ND ND
Lettuce Lactuca sativa Salad
Root ND ND ND ND ND
Stem ND ND ND ND ND
Leaf 1.17 2.5 1 23.67 56.17
Onion Allium cepa Albasa
Root 0.75a 4.42a 0.83a 26.50a 73.92a
Stem ND ND ND ND ND
Leaf 1.00a 3.50a 1.17b 22.00a 99.33a
Maize Zea mays Masara
Root ND ND ND ND ND
Stem 0.50a 0.50a 0.5 12.83a 37.50a
Leaf ND ND ND ND ND
Fruit 1.42a 7.17b ND 10.92a 186.08b
Rice Oryza sativa Shinkafa
Root 2.00a 8.17a ND 22.33a 84.50a
Stem ND ND ND ND ND
Leaf ND ND ND ND ND
Fruit 7.33b 3.50b 1.17 24.83a 54.00b
Safe limita 0.2 0.3 2.3 40 60
Mean followed with the same letter across the column are not significantly different p>0.05

The concentration of zinc in the studied crops is shown in (Table 1). Zinc concentrations varied from (186.08 mg kg–1) in Zea mays fruit to (19 mg kg–1) in Spinacia oleracea root. The zinc concentrations were above the 60 mg kg–1 permissible limits19, in Zea mays fruit (186.08 mg kg–1), Solanum lycopersicum leave (112.92 mg kg–1), Allium cepa leave (99.33 mg kg–1), Oryza sativa root (84.50 mg kg–1), Allium cepa root (73.92 mg kg–1), Capsicum cerasiforme root (68.33 mg kg–1) and Capsicum cerasiforme leave (66.50 mg kg–1). Thus the consumption of these crops for zinc could pose potential health risk in humans. This could be attributed to the presence of zinc contents in the pesticides which are used indiscriminately by farmers in the study area (personal communication). The concentration of zinc in the present study was comparatively higher than 32.44 mg kg–1 reported in Tunisia21, 88.4 mg kg–1 in Pakistan22 and 1.68 mg kg–1 in Kenya35. However, zinc concentration in this study was similar to 171.03 mg kg–1 in vegetables from Titagarh, India38. Zinc is an essential trace element and has a rather low toxicity39. Studies have shown that high dietary doses of zinc can cause clinical symptoms of gastrointestinal distress38.

The mean concentrations of Cd, Pb, Cr and Co reported in this study were lower compared with some reports from China, Pakistan, Bangladesh, Indonesia and Romania could be due to industrialization, waste water irrigation, mine land sites. However, the higher Zn concentrations observed in this with reports from some countries could be due to indiscriminate use of metal based pesticides20.

Heavy metals concentrations in soil: The mean concentration of heavy metals in the corresponding soil of the crops under investigation is shown in (Table 2).

Table 2: Mean concentrations of heavy metals (mg kg1) in soils of corresponding crops
Heavy metals (mg kg1)
Sampling site Soil from the land of Botanical name
Hausa name
Cd
Pb
Cr
Cu
Zn
Gombe Spinach Spinacia oleracea
Alayyaho
1.92abc
2.83a
0.58a
5.58a
190.83b
Sweet pepper C. cerasiforme
Tattase
0.17ab
4.50a
1.00a
7.50b
48.42b
Tomato S. lycopersicum
Tomatur
3.67c
2.33a
0.58a
5.17a
26.67a
Lettuce Lactuca sativa
Salad
1.50ab
4.00a
1.00a
4.83a
30.83a
Onion Allium cepa
Albasa
1.67ab
2.92a
0.92a
10.83b
37.25a
Maize Zea mays
Masara
2.50ab
2.67a
0.75a
3.58a
39.17a
Rice Oryza sativa
Shinkafa
1.50b
4.00a
1.00a
4.83a
30.83a
Safe limitsa
3
100
100
100
300
aSource: UNEP et al. 40. Mean followed with the same letter across the column are not significantly different p>0.05


Table 3: Bioaccumulation factor (BAF) of heavy metals in the edible parts of crops
BAF
Sampling site Name of sample Botanical name
Hausa name
Cd
Pb
Cr
Cu
Zn
Gombe Spinach Spinacia oleracea
Alayyaho
2.35
1.35
0.64
3.92
0.16
Lettuce Lactuca sativa
Salad
0.78
0.63
1
4.9
1.82
Onion Allium cepa
Albasa
0.48
3.96
1.2
3.16
1.93
Maize Zea mays
Masara
0.64
0.96
0.5
2.46
3.63
Rice Oryza sativa
Shinkafa
14
1.46
1.17
4.88
2.25
BAF value greater than >1 is indicating high uptake of metal

The result showed that the mean heavy metal concentrations in the corresponding soils of the crops under investigation ranged from 3.67 mg kg–1 in Solanum lycopersicum soil to 0.17 mg kg–1 in Capsicum cerasiforme soil for Cd, 4.50 mg kg–1 in Capsicum cerasiforme soil to 2.33 mg kg–1 in Solanum lycopersium soil for Pb, 1.00 mg kg–1 in Lactuca sativa soil to 0.58 mg kg–1 in Spinacia oleracea soil for Cr, 10.8 mg kg–1 in Allium cepa soil to 3.58 mg kg–1 in Zea mays soil for Cu and 190.83 mg kg–1 in Spinacia oleracea soil to 26.67 mg kg–1 in Solanum lycopersicum soil for Zn. The mean concentration of all heavy metals in the corresponding soils of all the studied crops was below the permissible limits set40 for agricultural soils except for Cd in Solanum lycopersicum. The lower mean concentration of metals in soil of the studied crops could be due to the higher uptake of heavy metals by the crops and lower retention in soil. Previous41 have shown that higher concentration of heavy metals in crops tissues than their corresponding soil reflects relatively poor retention in soil and greater efficiency of crops to absorb metals. The mean soil concentration of cadmium in this study was higher than what was reported in some parts of the world. For example, 0.46 mg kg–1 was reported in Tunisia21, 0.001 mg kg–1 in Nigeria by2. Lead soil concentrations in the present study were similar to 6.53 mg kg–1 reported2 and 2.31 mg kg–1 in India22 but lower compared to 70.36 mg kg–1 in China11 and 559.5 mg kg–1 from near chemical facility in Romania28. Chromium concentrations in soils of this study were comparatively lower than 8.9 mg kg–1 in Nigeria2 and 525.8 mg kg–1 from chemical facility in Romania28. While the soil concentrations of Cu were lower compared to 62.87 mg kg–1 reported in Tunisia by21, however Zn concentrations in soil of the present were higher than 21.42 mg kg–1 reported in Tunisia by21.

Heavy metals soil plant transfer bioaccumulation factor (BAF): The values of the estimated bioaccumulation of heavy metals concentration in the corresponding soil of each crop are presented in (Table 3). The bioaccumulation factor of heavy metals is measured by ratio of metal concentrations in plants to metal concentrations in soil18. Higher soil to plant transfer of heavy metals means strong accumulation of these metals in plants tissues14. The result showed variability in BAF values among the different crops and heavy metals. The result revealed that among the five metals the highest BAF was observed for Cd (14.00) followed by Cu (4.88) in Oryza sativa, for Pb (3.96) in Allium cepa, for Zn (3.63) in Zea mays and (1.17) in Oryza sativa for Cr. The trend of metal transfer from the soil to crops was in the order of Cd>Cu>Pb>Zn>Cr and most of the crops showed BAF>1 indicating higher absorption of heavy metals from soil. This is worrisome as bioaccumulation (BAF) has been showing to be a key path way of human exposure to heavy metals through food chain18. The variation observed in BAF values among the crops and metals could be due to several factors such as metal concentration in the soil, their chemical forms, the difference in uptake capability and growth rate of different plant species42.

Table 4: Pollution indices of heavy metals in the edible parts of crops
Pollution index
Sampling site Name of sample Botanical name
Hausa name
Cd
Pb
Cr
Cu
Zn
Gombe Spinach Spinacia oleracea
Alayyaho
6.67
7.78
0.25
0.27
0.71
Lettuce Lactuca sativa
Salad
5.83
8.33
0.43
0.59
0.94
Onion Allium cepa
Albasa
5.00
11.67
0.51
0.55
1.66
Maize Zea mays
Masara
7.08
23.89
0.00
0.27
3.10
Rice Oryza sativa
Shinkafa
36.67
11.67
0.51
0.62
0.9
N.B. PI value greater than >1 is indicating high pollution load


Table 5: Daily intake of metals (EDI) (mg kg1 b.wt./day) through consumption of crops
Estimated daily intake
Botanical name
Hausa name
Cd
Pb
Cr
Cu
Zn
Spinacia oleracea
Alayyaho
0.00191
0.00334
0.00084
0.01565
0.06116
Lactuca sativa
Salad
0.00167
0.00358
0.00143
0.03392
0.08051
Allium cepa
Albasa
0.00143
0.00502
0.00167
0.03798
0.14238
Zea mays
Masara
0.00203
0.01027
0.00000
0.01565
0.26672
Oryza sativa
Shinkafa
0.01051
0.00502
0.00167
0.03559
0.0774
RfDa
0.001
0.004
1.5
0.04
0.3
aSource: USEPA16

The BAF values in this study were higher compared to 0.16, 1.18, 1.19, 1.72 and 0.36 for Cr, Zn, Cd, Cu and Pb reported in Pakistan22. Similarly, the Cd BAF values in this study were similar to the 16.79 BAF Cd value reported in Pakistan21.

Pollution index determination: The Pollution Index (PI) is measured by the ratio of metal concentration in plant to that of the regulatory standard (WHO). The PI<1 indicates that the plant is not yet contaminated whereas PI>1 indicates pollution. The result of pollution index in the crops is presented in the (Table 4). The result showed that PI values ranged from 36.67 in Oryza sativa to 5.00 in Allium cepa for Cd, 23.89 in Zea mays to 7.78 in Spinacia oleracea for Pb, (0.51) in Oryza sativa to, 0.25 in Spinacia oleracea for Cr, 0.62 in Oryza sativa to, 0.27 in Zea mays for Cu and 3.10 in Zea mays to 0.71 in Spinacia oleracea for Zn. The result suggest that PI values for all the studied crops for Cd and Pb were >1 indicating high level contamination for these metals and hence unsuitable for human consumption. This is due to the presence of high contents of Cd and Pb in the pesticide use in the study area. However, PI values were <1 in all the studied crops for Cr, Cu, Zn except in Allium cepa and Zea mays. The trend of PI value was in decreasing risk Cd>Pb>Zn>Cu>Cr.

Health risk assessment
Estimated daily intake (EDI) of heavy metals: The EDI of the heavy metals was computed based on the heavy metals concentration in the edible part of each food crop and individual consumption rate for adults (Table 5). The result showed that the EDI ranged from 0.01051-0.0014, 0.01027-0.00334, 0.00167-0.00084, 0.03798-0.01565 and 0.26672-0.06116, for Cd, Pb, Cr, Cu and Zn, respectively. The highest daily intake of Cd, Pb and Zn were observed in Zea mays and the least was observed in Spinacia oleracea for Cr. This could be attributed to higher metals uptake efficiency of Zea mays in comparison other studied crops32. The result showed all the studied crops exceeded the oral reference dose RfD (the daily exposure of individuals to toxins or pollutants that can pose no appreciable hazard over life time) and are likely to cause human health risk. In the entire crop under investigation Cd exceeded the RfD while Allium cepa, Zea mays and Oryza sativa only Pb exceeded the RfD. The computed daily intake of all the crops for Cr, Cu and Zn were below the RfD dose and are unlikely to cause health risk to humans. The result showed a decreasing order based on daily intake through food consumption as Cd>Pb>Zn>Cu>Cr. This result suggest that Cd, Pb were the major metals contributing significantly to the potential health risk through consumption of the studied food crops in the study area.

Estimated hazard quotient (HQ) and hazard index (HI): The potential health to humans through the intake of the contaminated crops was evaluated using the Hazard Quotient (HQ) are presented in (Table 6). The computed hazard quotient from the consumption of the studied crops ranged from 0.8934 in Oryza sativa to 0.12183 in Allium cepa for Cd, 0.21822 in Zea mays to 0.07107 in Spinacia oleracea for Pb, (9.47E-05) in Oryza sativa to (4.73E-05) in Spinacia oleracea for Cr, (0.0807) in Allium cepa to (0.03325) in Spinacia oleracea for Cu and (0.07557) in Zea mays to (0.01733) in Spinacia oleracea for Zn. The result showed a decreasing trend of health risk from the consumption of the crops viz Cd>Pb>Cu>Zn>Cr.

Table 6: Hazard quotient and hazard index for adult population through the consumption of crops
Hazard quotient (HQ)
Name of sample Botanical name
Hausa name
Cd
Pb
Cr
Cu
Zn
Hazard index (HI)
Spinach Spinacia oleracea
Alayyaho
0.16244
0.07107
4.73E-05
0.03325
0.01733
0.28414
Lettuce Lactuca sativa
Salad
0.14214
0.07615
8.12E-05
0.07208
0.02281
0.31326
Onion Allium cepa
Albasa
0.12183
0.1066
9.47E-05
0.08071
0.04034
0.34959
Maize Zea mays
Masara
0.1726
0.21828
ND
0.03325
0.07557
0.4997
Rice Oryza sativa
Shinkafa
0.89344
0.1066
9.47E-05
0.07564
0.02193
1.09771

This result revealed <1 HQ values through the consumption of all the investigated crops, indicating that the consumption of these crops will not pose any health risk to human.

The Hazard Index (HI) value expresses the combined non carcinogenic effects of multiple heavy metals (Table 6). The estimated hazard index varied from the highest (1.0977) in Oryza sativa to the least (0.28414) in Spinacia oleracea. It was observed that current estimates of (HI) from the consumption of all the studied heavy metals in Oryza sativa were slightly greater than one (1.0977) and will pose a potential health risk through continues intake. High HI values were reported in Bangladesh 5.2332 and (1.48) in Nigerian by2 compared to estimate in this study.

CONCLUSION

The present study revealed that the concentrations of heavy metals Cd, Pb in all the studied crops fumigated with pesticides as the only source of contamination. The study showed variability in the uptake of heavy metals in different parts of crops and in different plant species. Heavy metals soil plant transfer showed BAF at its highest for Cd in Oryza sativa compared to other metals. The pollution indices revealed that all the crops were contaminated with Cd and Pb and Oryza sativa and Allium cepa were contaminated with Zn hence considered unsafe for human consumption. The estimated daily intake in all the crops for Cd was higher than the RfD while in Allium cepa, Zea mays and Oryza sativa Pb was higher than the RfD values and therefore had the major contribution to potential health risk through food consumption in the study area. Hazard quotient for adult indicated these crops as safe for human consumption. However, the potential risk from the consumption of all the heavy metals combined which is the realistic scenario showed slightly health risk from the consumption of Oryza sativa. Thus regular screening of heavy metals in crops and pesticides is suggested.

SIGNIFICANCE STATEMENTS

This study has uncovered that pesticides application are major contributors of heavy metals accumulation in crops and their link with health risk to humans that consumed them. The study also explores that contaminated irrigated water is not the only source of heavy metal burden in crops. The outcome of this study will help researchers and particularly medical personnel in a proper diagnosis of symptoms associated with heavy metal toxicity in humans. The outcome of this finding will help spark a new frontier of research on the development of new alternative and environmentally safe or less harmful pesticides for now and in the future safe biopesticides. It will also provide clue to government and other environmental and health agencies on the regulation of import and judicious application of these chemical pesticides.

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