The findings, published in Discover Chemistry, challenge a widespread household assumption in resource-constrained settings: that washing grains in hot water before cooking reduces contaminant levels. In southeastern Nigeria, where access to clean water and reliable energy can be limited, many families routinely rinse staples with warm or hot water to shorten cooking times and save fuel. The practice is often believed to strip away impurities, including pesticide residues. But dieldrin, formed in the environment through the epoxidation of the related pesticide aldrin, is a notoriously stubborn molecule. Its high chemical stability, low volatility, and strong lipophilicity allow it to bind tightly to soil organic matter, persist in sediments for decades, and accumulate in the fatty tissues of living organisms, climbing the food chain through biomagnification.

To test whether domestic processing could dislodge the compound, the research team, led by Ogechi Irene Eboh-Ajoku of Nnamdi Azikiwe University, collected composite samples of local and imported rice, yellow and white corn, red and white guinea corn, pearl and finger millet, three wheat varieties, and several bean types from five Awka markets between May and August 2024, the peak rainy season when pesticide application is at its highest. The 162 samples were divided into three treatment groups: washing with tap water at ambient temperature around 25 degrees Celsius, mild heating at 50 degrees Celsius for fifteen minutes, and boiling at 100 degrees Celsius for ten minutes. These conditions were chosen to mirror real household practices, from warm-water pre-washing to standard cooking durations.

Residues were extracted using a modified QuEChERS method, a technique prized for being quick, easy, cheap, effective, rugged, and safe, and then quantified with gas chromatography coupled to mass spectrometry. The analytical method showed strong performance: calibration curves spanning 0.005 to 1.000 milligrams per kilogram displayed correlation coefficients between 0.992 and 0.999, detection limits ranged from 0.001 to 0.005 milligrams per kilogram, and mean recovery of spiked samples reached 95 percent. Procedural blanks and calibration verification standards were run throughout to guard against background contamination. Dieldrin was identified by matching retention times and mass spectra against certified reference standards and the National Institute of Standards and Technology spectral library.

The results were sobering. Concentrations ranged from undetectable to 0.376 milligrams per kilogram, with white soft wheat showing the highest contamination, followed by iron white beans at 0.258 milligrams per kilogram and brown beans at 0.244. Local Anam rice carried 0.030 milligrams per kilogram, more than double the 0.013 found in imported foreign rice, a disparity the authors attribute to differences in agricultural practices, pesticide application intensity, storage conditions, and the stricter adherence to international residue standards that imported rice may enjoy. Yellow corn registered 0.071 milligrams per kilogram. Across all treatment conditions, approximately 66.6 percent of food crop samples met or exceeded the FAO/WHO maximum residue limit of 0.01 milligrams per kilogram.

Thermal processing barely moved the needle. Local rice held steady at 0.030 milligrams per kilogram through ambient washing and 50-degree heating, dipping only slightly to 0.027 after boiling. Iron white beans fell from 0.258 to 0.213 milligrams per kilogram at 100 degrees, a marginal reduction that leaves the residue far above the safety threshold. An independent samples t-test comparing concentrations at 50 and 100 degrees found no statistically significant difference, with p-values of 0.128 to 0.135 across standard, unequal-variance, Monte Carlo permutation, and exact permutation tests. The explanation lies in dieldrin’s molecular architecture: as a highly chlorinated cyclodiene compound with a rigid structure and strong carbon-chlorine bonds, it resists breakdown at kitchen-scale temperatures. Meaningful degradation of such compounds typically requires advanced oxidation conditions far beyond any cooking process.

Perhaps the most intriguing finding concerns the cooking wastewater itself. Dieldrin is poorly soluble in water, yet the researchers detected it in the liquid left behind after washing, at concentrations ranging from 0.001 to 0.017 milligrams per liter depending on the crop and temperature. In some samples, such as white hard wheat, wastewater concentrations rose from 0.004 to 0.015 milligrams per liter as heating progressed. The authors caution that these variations were not statistically significant and likely represent minor numerical fluctuations rather than thermally enhanced desorption. Still, the detection of dieldrin in wastewater suggests that thermal washing primarily induces partitioning, redistributing a fraction of the compound from the food matrix into the aqueous phase, rather than eliminating it. That raises the possibility of secondary exposure pathways, since contaminated cooking water is often discarded into household surroundings or reused.

The health risk calculations are where the study becomes genuinely alarming. Using United States Environmental Protection Agency guidelines, the team estimated daily intakes based on a consumption rate of 0.33 kilograms per person per day and a standard adult body weight of 65 kilograms, then divided by the World Health Organization’s acceptable daily intake of 0.0001 milligrams per kilogram of body weight to yield a hazard index. Any value above unity signals potential non-carcinogenic risk. At ambient temperature, white soft wheat scored a hazard index of 19.09 and white hard wheat 10.92, while iron white beans reached 13.10 and brown beans 12.39. Local rice and yellow corn also exceeded the threshold, at 1.52 and 3.60 respectively. Even after boiling, local rice (1.37), yellow corn (3.35), white soft wheat (18.02), white hard wheat (10.71), iron white beans (10.81), and brown beans (10.71) remained above the risk line. The authors acknowledge that their uniform consumption and body weight assumptions are conservative and do not capture variability among sensitive populations such as children, meaning actual risks for some groups could differ.

The toxicological stakes are considerable. Dieldrin is associated with carcinogenic, teratogenic, endocrine-disrupting, and reproductive effects. Experimental studies cited in the paper show it interferes with steroidogenesis, estrogen signaling pathways, and thyroid hormone homeostasis, contributing to reduced fertility, altered fetal development, spontaneous abortion, and gestational abnormalities. Its teratogenic effects include skeletal malformations, cleft palate, and increased fetal resorption. Acute neurotoxicity arises from the compound’s ability to interfere with neuronal action potentials, producing symptoms ranging from headache, dizziness, and nausea to muscle twitching, tremors, convulsions, and, in extreme cases, death. Because dieldrin resists both abiotic and microbial degradation, it poses long-term ecological hazards to fish, birds, mammals, and humans alike.

A multivariate layer of the analysis added further texture. Bray-Curtis hierarchical clustering grouped the food crops into four clusters with remarkably high similarity coefficients of 98 to 99 percent, pairing local rice with iron white beans and white soft wheat with yellow corn, among others. The wastewater samples formed three distinct clusters. The authors interpret these groupings as reflecting similar contamination levels rather than confirmed shared sources, noting that dieldrin’s hydrophobicity and persistence promote consistent adsorption across different crop matrices, and that shared post-harvest handling and market aggregation may contribute. The study’s limitations are acknowledged: sampling was confined to Awka markets during a single four-month window, so regional and seasonal variation remains unexplored. Even so, the baseline it establishes is unsettling. If routine cooking cannot mitigate exposure to a pesticide banned across much of the world, the burden of protection falls squarely on monitoring, agricultural reform, and regulatory enforcement, and the researchers argue that without targeted interventions, chronic dietary exposure will continue to accumulate among populations who depend on these staples most.

Subject of Research: Dieldrin pesticide residues and dietary health risks in Nigerian staple crops and cooking wastewater

Article Title: Multivariate analysis and dietary risk assessment of dieldrin in Nigerian staple crops and cooking wastewater

Article References: Eboh-Ajoku, O. I., Nduka, J. K., Anagboso, M. O., & Offor, C. C. (2026). Multivariate analysis and dietary risk assessment of dieldrin in Nigerian staple crops and cooking wastewater. Discover Chemistry, 3(1), Article 556. https://doi.org/10.1007/s44371-026-01012-w

Image Credits: AI Generated

DOI: 10.1007/s44371-026-01012-w

Keywords: dieldrin, organochlorine pesticides, food safety, Nigeria, staple crops, thermal processing, GC-MS, QuEChERS, hazard index, dietary exposure, bioaccumulation, maximum residue limits