Nyandarua’s Dirty Secret: Poison in the Potatoes
Experts warn of health risks from banned pesticides still found in Kenya’s favourite tuber
Your next plate of chips might come with an unwanted side of poison.
A new study by researchers from Egerton University has uncovered worrying levels of pesticide residues—some of them banned internationally—in potatoes grown in Nyandarua County, Kenya’s top potato-producing region.
The research, conducted by Millicent Kanario, Prof Joseph Wafula, and Dr John Masani, involved 275 potato farms and revealed that nearly all the farmers (98.8%) used synthetic pesticides. Alarmingly, most of them neither followed dosage instructions nor observed the proper intervals between spraying and harvest.
“Only 12 per cent of the farmers followed label instructions. The rest relied on advice from agrochemical retailers or other farmers,” said Kanario.
The study, titled “Influence of On-Farm Pesticide Practices and Processing Methods on Pesticide Residue Levels in Potato Tubers (Solanum tuberosum L.) in Nyandarua County, Kenya,” has been accepted for publication by The Journal of Food Protection.
Dangerous Chemicals in Your Dinner
Researchers tested raw and cooked samples of the popular Shangi variety and found residues of chlorpyrifos and fenitrothion—two pesticides that are banned or restricted in many parts of the world, including the European Union.
These chemicals are associated with serious health conditions, including brain damage, hormone disruption, and organ toxicity. Worryingly, the residue levels exceeded both European Union and Codex Alimentarius maximum residue limits (MRLs)—the global food safety benchmark.
“Even after frying, baking, or roasting, these insecticides remained above safe levels, indicating potential health risks,” the study notes.
Can Cooking Help?
Yes—but not always. The researchers tested five common cooking methods—frying, boiling, steaming, baking, and roasting.
Frying came out best, cutting pesticide levels by up to 92% for some chemicals like azoxystrobin (a commonly used fungicide). Boiling also helped, especially with water-soluble pesticides. However, roasting was the least effective, leaving more than 80% of the residues intact in some samples.
“Roasting doesn’t involve oil or water, which are key in breaking down or drawing out pesticide residues,” the report explained.
Still, even the best cooking method can’t fix poor practices on the farm.
Spraying Out of Fear
The overuse of pesticides is driven partly by fear—fear of pests, disease, and crop failure. Many Nyandarua farmers spray their potatoes weekly, often mixing several chemicals together without understanding the risks.
One farmer confessed to spraying “every seven days like clockwork.” The study found that most farmers harvested their potatoes just three weeks after spraying, giving the toxic substances little time to break down.
This excessive spraying and poor timing are creating a silent food safety crisis—one that is largely invisible to the average Kenyan consumer.
A Call for Urgent Reform
While cooking methods can reduce some of the danger, the researchers say the real solution lies at the source: education.
“Expanded educational initiatives to promote good agricultural practices among potato farming communities are urgently needed,” the report concludes.
In 2021, Kenya’s Pest Control Products Board began a phased ban on chlorpyrifos, with a full withdrawal planned by December 2024. Fenitrothion has been proposed for removal since 2000, but it still appears on shelves and in farms.
For now, the potatoes may still be on your plate—but so is the risk.
In other news: Fire Guts Section of Gikomba Market in Nairobi Yet Again
Nyandarua’s Dirty Secret: Poison in the Potatoes