The research team, led by Omolara Jemimah Ojezele of the Federal College of Animal Health and Production Technology in Ibadan and Matthew Obaineh Ojezele of Delta State University, focused on Amaranthus cruentus, a leafy green vegetable that is a dietary staple across Nigeria and much of sub-Saharan Africa. Sampling was carried out at 30 sites spread across three major vegetable-producing settlements in Oyo State, yielding 240 matrix-season samples of water, sediment, soil, and vegetable tissue. After analytical duplicates were averaged, the dataset comprised 480 independent pesticide-specific sample-level observations. All samples were analysed using validated chromatographic methods, providing the analytical foundation for the layered statistical and risk assessments that followed.

The study’s first major finding concerns environmental partitioning, the tendency of a chemical to accumulate in particular compartments of an ecosystem. Glyphosate, a systemic herbicide that binds strongly to soil particles, predominated in soil and sediment samples, consistent with its known physicochemical behaviour. Cypermethrin, a synthetic pyrethroid insecticide, showed a different pattern, occurring at comparatively higher concentrations in the edible Amaranthus cruentus tissue itself. This divergence matters because it shapes the primary route of human exposure: while glyphosate residues largely remain locked in the ground, cypermethrin residues travel directly onto the plates of consumers through the vegetable crop, bypassing many of the natural attenuation processes that limit exposure to soil-bound chemicals.

To understand whether contamination was random or clustered, the researchers applied Moran’s I spatial autocorrelation analysis, a statistical technique that tests whether values measured at nearby locations are more similar than expected by chance. The results were striking for one compound: glyphosate in sediment displayed significant spatial autocorrelation, with a Moran’s I statistic of 0.783 and a p-value of 0.0003. In practical terms, this means that sites with high glyphosate contamination in sediment tended to sit near other highly contaminated sites, forming identifiable hotspots rather than a scattered pattern. Such clustering suggests that localised factors, such as application intensity, topography, and runoff pathways, drive glyphosate accumulation in sediments, information that could help regulators target interventions at specific zones rather than imposing blanket restrictions.

The team complemented the spatial analysis with hierarchical mixed-effects modelling and principal component analysis, statistical tools that allowed them to separate the influence of different environmental and seasonal variables on residue concentrations while accounting for the nested structure of the sampling design. Together with a structured survey of vegetable farmers, these methods revealed a troubling gap between residue levels in crops and the practices that produced them. The survey documented limited pesticide stewardship among growers, including short pre-harvest intervals, meaning farmers were applying pesticides and harvesting their crops with little time for residues to degrade. Substantial exceedances of selected European Union maximum residue limit benchmarks for spinach were recorded in the vegetable samples, underscoring the consequences of these practices.

The heart of the study lies in its dietary risk assessment, which the researchers conducted in two complementary ways. The first was a deterministic approach, which uses single point estimates of residue concentration and consumption to calculate a hazard quotient, the ratio of estimated exposure to a reference dose below which no adverse effects are expected. A hazard quotient above one signals a potential health concern. The deterministic results indicated acceptable dietary risk for adults for both pesticides, and acceptable risk for children for glyphosate. However, cypermethrin exceeded the hazard threshold in children, with a hazard quotient of 1.818, nearly double the level of concern for the youngest and most vulnerable consumers.

Because deterministic assessments rely on single values, they can obscure the true range of possible exposures. To address this, the researchers turned to Monte Carlo simulation, a probabilistic technique that runs thousands of virtual exposure scenarios by randomly drawing residue concentrations and consumption rates from their measured distributions. Each of the 10,000 iterations represents one plausible combination of real-world conditions, and the resulting spread of hazard quotients reveals how often risky exposures might actually occur. The simulation results sharpened the study’s central warning: hazard quotients above one appeared in 15.09 percent of adult cypermethrin exposure scenarios and a remarkable 70.47 percent of child exposure scenarios. For glyphosate, by contrast, no simulated exposure scenario for either group exceeded the threshold, suggesting the herbicide poses comparatively limited dietary risk under the conditions measured.

The contrast between the two pesticides carries important implications. Cypermethrin is prized by farmers for its effectiveness against the insect pests that devastate vegetable crops, but its persistence on leafy surfaces means that harvest timing is critical to food safety. The finding that roughly seven in ten simulated child exposure scenarios exceeded the hazard threshold points to a systemic problem rather than isolated lapses, one rooted in the short intervals between spraying and harvest documented in the farmer survey. Children are particularly sensitive in such assessments because their lower body weight means the same residue concentration in food delivers a higher dose per kilogram of body mass, and because developing systems may be more vulnerable to neurotoxic compounds such as pyrethroids.

The study’s authors argue that their findings support a coordinated response spanning pesticide stewardship, residue surveillance, farmer education, and regulatory oversight. In many smallholder farming systems across sub-Saharan Africa, extension services that once trained farmers in safe application practices have contracted, leaving growers to rely on agrochemical vendors and informal knowledge networks. The short pre-harvest intervals recorded in this study are a classic symptom of that gap, reflecting neither malice nor ignorance of risk so much as the absence of accessible, practical guidance. Simple interventions, such as clear labelling in local languages, community-level training on waiting periods, and routine market testing of leafy vegetables, could substantially reduce exposure without requiring farmers to abandon chemical pest control altogether.

The research also demonstrates the value of methodological rigour in regions where pesticide monitoring data have historically been sparse. By pairing validated chromatographic analysis with spatial statistics, mixed-effects modelling, and Monte Carlo simulation, the study transforms a set of field measurements into a nuanced risk picture that distinguishes between compounds, environmental compartments, and consumer groups. The strong spatial clustering of glyphosate in sediment, for instance, would have been invisible to a conventional point-sampling survey, and the probabilistic approach revealed a child-specific risk that a deterministic calculation alone might have understated or overstated. As pesticide use intensifies across African agricultural landscapes to meet growing food demand, studies of this kind provide the evidence base that national regulators and international bodies such as the Joint FAO/WHO Meeting on Pesticide Residues need to set meaningful, locally calibrated safety standards. For now, the message from Oyo State is clear: the greens on the market stall are nutritious and affordable, but for the children eating them, the invisible residues of an everyday insecticide may be crossing a line that better farming practice could hold firmly in place.

Subject of Research: Pesticide residue occurrence and dietary health risk assessment in agricultural ecosystems of southwestern Nigeria

Article Title: Environmental occurrence, spatial distribution, and probabilistic dietary risk assessment of cypermethrin and glyphosate residues in agricultural ecosystems of southwestern Nigeria

Article References: Ojezele, O. J., & Ojezele, M. O. (2026). Environmental occurrence, spatial distribution, and probabilistic dietary risk assessment of cypermethrin and glyphosate residues in agricultural ecosystems of southwestern Nigeria. Environmental Monitoring and Assessment, 198(11), Article 1171. https://doi.org/10.1007/s10661-026-15973-z

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15973-z

Keywords: cypermethrin, glyphosate, pesticide residues, Amaranthus cruentus, food safety, dietary risk assessment, Monte Carlo simulation, spatial autocorrelation, Nigeria, vegetable farming, pesticide stewardship, environmental monitoring