INTERNATIONAL JOURNAL OF RESEARCH AND INNOVATION IN APPLIED SCIENCE (IJRIAS)  
ISSN No. 2454-6194 | DOI: 10.51584/IJRIAS |Volume X Issue XI November 2025  
Concentration and Health Risk Assessment of Organophosphorus  
Pesticide Residues in Selected Grains: Maize (Zea Mays L.) And  
Beans (Vigna Unguiculata (L.) Walp.) Sold in Karu Market,  
Nasarawa State, Nigeria  
*Adeshola Rebecca Akinwola., Abdullahi Danjuma Kassim., Ishegbe Eko Joyce  
Department of Chemistry, Nasarawa State University, Keffi, Nigeria  
*Corresponding Author  
DOI: https://dx.doi.org/10.51584/IJRIAS.2025.101100047  
Received: 25 November 2025; Accepted: 01 December 2025; Published: 10 December 2025  
ABSTRACT  
This study provides a comprehensive assessment of the concentration and potential health risks of  
organophosphorus pesticide (OPPs) residues in two widely consumed grains purchased from major trading  
centers (market) in Nasarawa State. For this study a total of twelve grain samples were collected using stratified  
random sampling to ensure market-level representation where homogenization and extraction using the  
QuEChERS method, followed by a dispersive solid-phase extraction SPE clean-up. Quantification and  
identification of nine target OPPs were performed using Gas Chromatography–Mass Spectrometry (GC–MS)  
calibrated with multilevel standard. The analytical results showed that OPPs residues were widespread across all  
samples, though concentrations varied markedly by grain type and market location. Notably, methyl parathion  
and azinphos-methyl pesticides banned or severely restricted due to their high toxicity were detected in several  
samples, indicating possible illegal application or improper storage practices. White and yellow maize recorded  
higher contamination than beans, with yellow maize from Orange Market containing the highest oxydisulfoton  
concentration (11.96 mg/kg) and white maize from Karu Market showing extremely elevated ronnel levels  
(50.63 mg/kg). Health risk assessment was conducted using Estimated Daily Intake (EDI) and Target Hazard  
Quotient (THQ) for both adults and children. Results revealed that multiple pesticides exhibited THQ values  
significantly greater than 1, signaling substantial non-carcinogenic health risks from chronic exposure. Maize  
particularly from Orange and Karu markets posed the highest risk burden, while beans showed moderate to high  
risk depending on pesticide type and concentration. In conclusion, the study findings highlight the urgent need  
for stricter enforcement of pesticide regulations, routine monitoring of pesticide residues in grain markets, and  
comprehensive farmer and trader sensitization on the dangers of banned and excessive pesticide use.  
Implementing these measures is essential to safeguard food safety and reduce long-term health risks associated  
with dietary pesticide exposure.  
Key word: Health, Risks, Organophosphorus, Pesticide, Exposure, Monitoring.  
INTRODUCTION  
Agricultural production has been considerably improved with the help of pesticides but their unrestricted and  
extreme use is polluting the atmosphere, foodstuffs, and aquatic and agricultural products (Aljerf, 2018). The  
usage of pesticides has increased recently as a result of the growing global population, the need to produce more  
food, and the need to prevent crop losses. Pesticides known as organophosphates (OPPs) are a class of  
insecticides that are most commonly employed. They are generated from phosphorous compounds, specifically  
phosphoric and phosphorothioic acids. In 2020, the yearly rise of these compounds was anticipated to be 3.5  
million tons, of which approximately 40% represent OPPs. Currently, approximately 2 million tons of pesticides  
are used worldwide each year (Sharma et al., 2019; Derbalah et al., 2019). It is estimated that over 3 million  
individuals are exposed to OPs annually, resulting in 300,000 fatalities globally (Robb, 2021). Pesticides are  
accepted as a lucrative way of controlling pests and improving yield and food quality.  
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The most widely used pesticides in developing countries are primarily organochlorines, organophosphorus,  
synthetic pyrethroids, and carbamates (Mohammed et al., 2019). Insecticides consist of a varied class of  
compounds but the most protuberant for plant treatment are chlorine and phosphorus having compounds.  
Organophosphorus pesticides (OPPs) are slowly replacing organochlorines due to their high bio accumulation  
effect (Suryono et al., 2019). OPPs are derivatives of phosphoric or phosphonic acid in form of amides, esters,  
or thiols. They do not persist in the environment for longer periods as they are easily hydrolyzed (Sidhu et al.,  
2019; Bala et al., 2019). Nevertheless, their numerous poisonousness (they act as inhibitors of  
acetylcholinesterase enzyme), as well as the likelihood of build-up in the food chain, can cause a menace to  
human health (Iskra et al., 2020; Mukherjee, and Rinkoo, 2020; Biziuk and Stocka, 2015). Organophosphate  
pesticides interfere with growth-stimulating mechanisms by hindering various enzymes, transcuticular diffusion,  
and penetrability which is vital for the growth of plants. They can also deter the activities of the enzyme in  
aquatic and terrestrial organisms leading to, hepatic reproductive, respiratory, nervous, and renal anomalies.  
Regular uses of organophosphate pesticides decrease the microbial community that affects soil fertility (Sidhu  
et al., 2019; Ali et al., 2019). Many countries have set rules and regulations in monitoring pesticide residues in  
food commodities to confirm whether the levels adhere to the national or international guidelines (John et al.,  
2017).  
Long-term exposure to pesticides is increasingly suspected of being linked to a broad spectrum of medical  
problems such as cancer, neurotoxic effects, reproductive health concerns and endocrine disruption, particularly  
for specific populations (Mostafalou, and Abdollahi, 2013; George and Shukla, 2011). Apart from farmers, those  
working on the farm and those living within the vicinity of the farm (they are at risk of greatest exposure) that  
these pesticides are used, consumers are also at risk of these pesticidal residues in fruits and vegetables because  
their major mode of exposure is through consumption (Wolejko et.al., 2014) of the agriculture products that this  
class of pesticides are used on.  
However, misuse or excessive application of these compounds results in accumulation of toxic residues in food  
crops, posing serious public health concerns (Akoto et al., 2013). Cowpea (Vigna unguiculata) - white and brown  
beans are common leguminous crops and the most important staple plant protein food crops in sub-Sahara Africa  
(Abebe & Alemayehu, 2022). In Nigeria, grains such as maize (Zea mays L.) and beans (Vigna unguiculata (L.)  
Walp.) constitute staple foods, increasing the potential risk of dietary exposure to pesticide residues. Despite  
regulatory efforts by national agencies, enforcement remains limited. Recent studies document notable pesticide  
contamination in grains across Nigerian markets (Abdullahi et al, 2019; Maina et al, 2024), emphasizing the  
need for continuous residue monitoring. This study evaluates Organophosphorus pesticide (OPPs) residue levels  
in selected grains from Karu and surrounding markets, determines associated health risks, and contributes to  
evidence-based recommendations for safer pesticide use.  
MATERIALS AND METHODS  
Study Area  
The research was conducted in the Karu Urban Area, located within Nasarawa State, adjacent to Nigeria’s  
Federal Capital Territory. The area experiences a tropical climate with an average annual rainfall of 1,100–2,000  
mm and supports intensive agricultural activity. Major markets sampled were Orange Market, Karu Market, and  
Mararaba Market, all known for grain trading and storage.  
Figure 1: Map of sampling locations in Nasarawa state.  
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Sample Collection and Preparation  
Samples of white and yellow maize (Zea mays L.), and white and brown beans (Vigna unguiculata (L.) Walp,  
were collected randomly from the three markets. Each sample (2 kg) was stored in sealed bags, labelled, and  
transported to the Analytical Laboratory, Ahmadu Bello University, Zaria for pesticide residue analysis. Samples  
were cleaned, milled, and homogenized. Precautionary measures were taken to prevent cross-contamination  
during handling (González-Curbelo, 2022).  
Extraction and Clean-Up  
The QuEChERS technique (Quick, Easy, Cheap, Effective, Rugged, and Safe) method described by  
(Anastassiades, 2003), is a very feasible method and can extract varied classes of pesticide remains in numerous  
agricultural and horticultural matrices (Rada and Tijana, 2014; Arsand et al., (2023). It requires minimum reagent  
consumables and a short pretreatment period, so it is accepted by many experimenters and international  
associations like AOAC and European standardization committee (Gonz´alez et al., 2015). The major steps in  
both methods includes acetonitrile extraction, dispersive solid phase clean up using primary secondary amine  
and magnesium sulfate. The purposes of further method modification are decreasing time of sample preparation  
and analysis, minimize extraction solvents and glassware used and upsurge the sensitivity of the method. The  
procedure involved: 10 g sample plus 10 mL acetonitrile, addition of 4 g anhydrous MgSO₄ and 1 g NaCl,  
vortexing and centrifugation at 4,000 rpm for 5 minutes, clean-up using dispersive SPE containing 150 mg  
MgSO₄, 50 mg PSA, and 50 mg C18, Filtration using 0.22 µm PTFE filter prior to GC-MS analysis.  
Instrumental Analysis  
Analyses were performed using an Agilent 7890B-5977A GC-MS system equipped with a DB-5 capillary  
column. Calibration curves were prepared using standard solutions (50–250 µg/mL). The GC oven was  
programmed from 80°C to 290°C, with helium as the carrier gas. Residues were identified by retention time and  
mass spectra comparison with standard references.  
Health Risk Assessment  
The US EPA (2000) highlighted a range of standards and instructions on calculating the health risks associated  
with the consumption of grains contaminated with environmental pollutants. An assessment of health risks can  
be performed by comparing mean levels of contaminants with international guidelines. However, these  
assessments do not consider variables such as consumption rates and eating habits (Fagbohun et al., 2024). The  
estimated daily intake (EDI) of the 9 OPPs pesticides were determined based on their mean concentration in  
each grains and beans varieties and the daily intake in grams. The food supply value of cowpea was 18  
kg/capita/year according to the Food and Agriculture Organization (FAO), 2000, the food supply value was  
divided by the number of days in the year (365 days). The result obtained was an intake of 0.04932,  
approximately 0.049 kg/capita/day, which is the food ingestion rate (FIR) of cowpeas in Nigeria (Antoine et al.,  
2017).  
The Estimated Daily Intake (EDI) and Target Hazard Quotient (THQ) were calculated using USEPA and WHO  
methodologies (FAO/WHO, 2022).  
퐶 × 퐹  
퐸퐷퐼 =  
… … … … … … … … … … . 퐸푞푛 (1)  
… … … … … … … … … … . 퐸푞푛 (2)  
퐵푊  
퐸퐷퐼  
퐴퐷퐼  
푇퐻푄 =  
where C = concentration of pesticide (mg/kg), F = daily food intake (kg/day), BW = body weight (kg), and ADI  
= acceptable daily intake. A THQ > 1 indicates potential health risk.  
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Statistical analysis  
The results obtained from the matrices were statistically analyzed through MS Excel and SPSS version 21.  
Elements of descriptive statistics of samples generated included mean, range, minimum, maximum and standard  
deviations. The concentration of OC pesticide residues in cowpea samples was compared with the MRLs  
recommended by the European Union (2011). MRL of a pesticide is the maximum concentration of its residue  
that is legally permitted to remain in food after it has been treated with the pesticide (FAO, 2022).  
RESULTS AND DISCUSSION  
Pesticide Residue Concentrations  
The mean concentrations of nine (9) OPPs residues in the samples (Table 1 - 3) were found to varying  
concentrations across markets and sample types. The major residues identified were Oxydisulfoton is highest in  
yellow maize (11.96 µg/mL) from Orange Market, Ronnel is highest in white maize (50.63 µg/mL) from Karu  
Market, Methyl Parathion was detected in all maize and bean samples (0.001–9.5 µg/mL), Azinphos-methyl  
(C₁₀H₁₂N₃O₃PS₂), Dichlorvos and Azinphos-methyl: Detected at trace levels (<0.1 µg/mL). Similarly, bean  
samples in a study by Umar et al., (2024) recorded the highest level of dizinon and dichlorvos above the  
acceptable limits of 5.00E-02 and 1.00E-02 mg/kg in Kaduna South and Zangon Kataf, respectively.  
Table 1. Concentration (ug/mL) of Organophosphorus Pesticide Residue (OPPs) in Maize and Beans from  
Orange market.  
Note: ND = Not Detected  
Note: ND= Not Detected  
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Note: ND= Not Detected  
Tables 1–3 present concentrations across markets with Orange Market recorded the highest levels of residues,  
especially oxydisulfoton and methyl parathion. The persistence of methyl parathion and azinphos-methyl both  
restricted internationally suggests ongoing misuse and poor regulatory compliance (Maina et al., 2024).  
Comparative evaluation with existing literature shows similar patterns. Akoto et al., 2013 and Abdullahi et al.,  
2019 reported high organophosphorus pesticide (OPPs) presence in grains sold in West African markets. Maina  
et al., (2024) also found elevated oxydisulfoton levels in cereals across northern Nigeria.  
From the above result, the analysis of maize (yellow and white varieties) and beans (brown and white) from  
Orange, Karu, and Mararaba markets revealed the presence of multiple organophosphorus pesticide residues,  
with varying concentrations across commodities and locations Akan et al.,, 2024. All samples from Orange  
Market showed significantly elevated Organophosphorus pesticide (OPPs) concentrations, suggesting possible  
overuse or contamination during storage. According to Oshatunberu et al., (2023), OPPs used in this study have  
been documented in Nigerian rice and beans. These results are consistent with those reported by Tutuwa et al.  
(2024), who observed that the high use of several pesticides during plantation, culture, and storage may have  
contributed to the bioaccumulation of these substances in individual grains.  
A heat map (Figure 2) summarizing OPP residue distribution indicates consistently higher contamination in  
Orange Market samples. Similar contamination clustering was reported in Ghanaian cereal markets (Akoto et  
al.,, 2013) and urban vegetable markets in Lagos (Oluwoyo, 2024).  
Figure 2: Heat map showing the level of organophosphorus pesticide in each grain sample.  
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Health Risk Assessment  
Target hazard quotient (THQ) computations indicate that oxydisulfoton and methyl parathion pose the greatest  
risks, particularly for children. Similar findings of elevated THQs among children have been documented in  
multiple African dietary risk assessments (Yusuf et al., 2023; Aina et al, 2025). From this study, Estimated daily  
intake (EDIs) and Target harzard quotient (THQs) calculated for both adults and children revealed the following:  
the high risk (THQ > 100) for oxydisulfoton and methyl parathion in maize, moderate to high risk for ronnel in  
white maize and beans. Children consistently recorded higher target hazard quotients (THQs) than adults due to  
lower body mass, Dichlorvos posed negligible risk (THQ < 1). Overall, the highest cumulative risk was observed  
in maize from Orange Market and white maize from Karu Market.  
Table 4: Summary of Health Risk Assessment of Organophosphorus Pesticide Residues in Maize and Beans  
samples from all three market  
Pesticide  
Max Detected Conc. EDI (mg/kg ADI  
(mg/kg HQ  
Risk Level  
(mg/kg)  
bw/day)*  
bw/day)  
(EDI/ADI)  
0.0756 (Beans – Karu)  
0.00025  
0.004  
0.063  
Low risk (HQ < 1)  
High risk (HQ >> 1)  
High risk (HQ >> 1)  
Dichlorvos  
11.96 (Maize – Orange) 0.0598  
0.0005  
0.0003  
119.6  
Oxydisulfoton  
9.50 (Maize – Orange)  
50.63 (Maize – Karu)  
0.16 (Maize – Orange)  
0.0475  
158.3  
Methyl  
Parathion  
0.253  
0.005  
50.6  
1.6  
Very High risk (HQ  
>> 1)  
Ronnel  
0.0008  
0.0005  
Moderate risk (HQ >  
1)  
Azinphos-  
methyl  
ND  
ND  
ND  
–
–
–
0.01  
0.01  
0.01  
–
–
–
Not detected (Safe)  
Not detected (Safe)  
Not detected (Safe)  
Chlorpyrifos  
Ethoprophos  
Dichlofenthion  
DISCUSSION  
The detection of multiple organophosphorus residues in grains sold in Karu and neighboring markets aligns with  
previous studies across Nigeria, which documented similar contamination patterns in cereals and vegetables.  
Despite international bans on methyl parathion and azinphos-methyl, their persistence in local markets indicates  
ongoing illegal usage and inadequate enforcement of pesticide regulations.  
High target hazard quotient (THQ) values, particularly for oxydisulfoton and ronnel, raise significant public  
health concerns. These compounds are known neurotoxins capable of causing both acute and chronic effects,  
including endocrine disruption and neurodegeneration. The elevated risk in children underscores the  
vulnerability of this demographic to dietary pesticide exposure. The lack of chlorpyrifos detection may suggest  
recent compliance with its restriction or rapid degradation during grain storage. However, the continued presence  
of other high-toxicity pesticides calls for urgent regulatory intervention.  
RECOMMENDATIONS  
This study confirms that grains (maize and beans) sold in Karu, Mararaba, and Orange markets are contaminated  
with organophosphorus pesticide residues, many exceeding international safety limits. The high Target Hazard  
Quotients indicate that chronic exposure through grain consumption poses significant health risks, especially to  
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children. Urgent measures are required to monitor, regulate, and educate stakeholders on safe pesticide use.  
Therefore the flowing recommendations: Strengthen regulatory enforcement: Agencies such as NAFDAC, SON,  
and NESREA should increase field inspections and residue monitoring. Promote farmer education: Conduct  
awareness campaigns and training on Integrated Pest Management (IPM) and safe pesticide application.  
Encourage alternative pest control: Introduce bio-pesticides and organic farming techniques to reduce reliance  
on synthetic chemicals. Establish regional laboratories: Set up pesticide testing centers in agricultural regions to  
support regular surveillance. Consumer sensitization: Educate the public on washing, sorting, and food  
preparation practices that minimize exposure.  
CONCLUSION  
In conclusion, the study demonstrates that consumers in the Karu axis are exposed to hazardous levels of  
Organophosphorus pesticide OPPs residues through staple grains, underscoring a critical public health concern.  
The findings highlight the urgent need for stricter enforcement of pesticide regulations, routine monitoring of  
pesticide residues in grain markets, and comprehensive farmer and trader sensitization on the dangers of banned  
and excessive pesticide use. Implementing these measures is essential to safeguard food safety and reduce long-  
term health risks associated with dietary pesticide exposure.  
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