Potatoes from Nyandarua County—one of Kenya’s top producers—are increasingly coming under scrutiny for containing dangerous levels of banned pesticide residues, according to new research from Egerton University.
In a detailed study involving 275 potato farmers, researchers Millicent Kanario, Prof Joseph Wafula, and Dr John Masani discovered that nearly all farmers (98.8%) regularly used synthetic pesticides, often without following proper safety instructions. Many relied on advice from agrochemical dealers or fellow farmers instead of manufacturer guidelines, with only 12% adhering to label directions.
The team tested raw and cooked Shangi potato samples and found residues of chlorpyrifos and fenitrothion—insecticides banned in Europe and flagged for serious health risks such as brain damage and hormone disruption—at levels exceeding global safety limits set by the EU and Codex Alimentarius.
Though Kenya has begun phasing out chlorpyrifos (due by end of 2024), and fenitrothion has long been proposed for withdrawal, these harmful chemicals remain common in use. The risk to consumers is real: toxic residues may be reaching dinner plates despite cooking.
To assess whether cooking helps, the researchers tested five methods: frying, boiling, steaming, baking, and roasting. Frying was most effective, reducing some residues by over 90%, while boiling was somewhat helpful, especially for water-soluble pesticides. Roasting, however, was the least effective, leaving up to 80% of residue intact.
Alarmingly, even after frying or baking, residues of chlorpyrifos and fenitrothion often remained above safe levels, posing continued health risks.
Why are farmers overusing these chemicals? Mostly out of fear of crop loss. The Shangi variety is profitable but highly vulnerable to pests, leading farmers to spray weekly—even without visible pest presence—and often mix chemicals for faster results, worsening residue levels.
Harvesting also happens too soon after spraying, often within three weeks, not giving pesticides enough time to degrade.
Despite the risks, the study offers practical advice: consumers concerned about pesticide exposure should favor fried or well-boiled potatoes, and more importantly, invest in farmer education on safe pesticide practices
