Occurrence and levels of pesticide residues in tomato sold in supermarkets and open-air markets in Nairobi, Kenya

Background Non-compliant residues are a concern anywhere, regardless of the presence of an effective regulatory system. In Kenya, tomatoes are widely consumed, but data on pesticide contamination across different retail outlets remain limited. Objective This study aimed to assess the levels and types of pesticide residues in tomatoes sold in Nairobi, Kenya. Additionally, it sought to compare contamination levels between informal traditional open-air markets and formal modern supermarket outlets. Methods A cross-sectional study was conducted from January 2017 to June 2017. Tomato samples were collected twice a month from three open-air markets and two supermarkets in Nairobi. A total of 240 whole tomato samples were pooled into 66 composite samples and analyzed for 98 pesticide residues using gas chromatography–mass spectrometry (GC–MS) and liquid chromatography–tandem mass spectrometry (LC–MS/MS). The data were compared against the maximum residue limits (MRLs) set by the European Union (EU) and Codex Alimentarius. Results Pesticide residues were detected in 49% of the pooled tomato samples, with 27% containing a single residue and 21% containing multiple residues. Seven samples exceeded the EU’s MRLs, while one exceeded Codex MRLs. Carbendazim was the most frequently detected residue, and difenoconazole, a pesticide not approved for use in Kenya, was also found. Notably, tomatoes from supermarkets had a higher frequency of pesticide residues (75%) compared to those from open-air markets (64%), challenging the assumption that formal retail outlets offer safer food. Conclusion The study findings indicate that pesticide residues are prevalent in tomatoes sold in Nairobi, with both formal and informal retail outlets presenting contamination risks. Contrary to popular belief, supermarkets were not safer than traditional open-air markets. These results underscore the need for improved pesticide regulation, farmer training, and enhanced monitoring of both formal and informal food supply chains to ensure consumer safety.