Seasonal variation in pesticide residue concentrations and associated human health risks in tomatoes from the southern highlands of Tanzania using the QuEChERS analytical method with LC–MS/MS detection

Introduction Pesticides are commonly used in vegetable production to control pests and enhance crop yields. However, excessive or inappropriate application can lead to the accumulation of pesticide residues in food, thereby posing potential risks to human health. This study investigated the occurrence, seasonal variation, spatial distribution, and health risks associated with pesticide residues in tomatoes ( Solanum lycopersicum ). Methods A total of 60 composite tomato samples were collected during the dry season (July–October) and the wet season (December–March) from six sampling locations using stratified sampling. Samples were analyzed for pesticide residues using liquid chromatography–tandem mass spectrometry (LC–MS/MS), and the results were evaluated against European Union Maximum Residue Limits (EU MRLs). Health risk assessment was conducted using the Estimated Daily Intake (EDI), Hazard Quotient (HQ), and Hazard Index (HI) to determine potential dietary exposure risks for adults and children. Results Seventeen unique pesticide active ingredients were detected in the 60 tomato samples, comprising 11 targeted and 6 non-targeted compounds. Mean total pesticide residue concentrations were substantially higher in the dry season than in the wet season (0.392 ± 0.302 vs. 0.021 ± 0.018 mg/kg; p < 0.001). Profenofos had the highest EU MRL exceedance frequency (46.7%), followed by chlorpyrifos (35.0%), lambda-cyhalothrin (28.3%), metalaxyl (20.0%), and cypermethrin (15.0%). Chlorpyrifos was the major individual contributor to non-carcinogenic risk, with HQs of 2.386 in children and 1.790 in adults, while the cumulative Hazard Index (HI) was 3.321 for children and 2.491 for adults, both exceeding 1. During the dry season, chlorpyrifos and cypermethrin contributed 45 and 30% of total HQ, respectively, whereas imidacloprid and bendiocarb contributed 40 and 35%, respectively, during the wet season. Residue complexity varied substantially across locations, with Image Tomato Farm containing 10 residues and Lyamkena Farm containing 17 residues, indicating high cumulative exposure potential, particularly at Lyamkena. Conclusion Tomatoes from Tanzania’s Southern Highlands showed substantial multi-residue pesticide contamination, with markedly higher residue levels and more EU MRL exceedances during the dry season. Chlorpyrifos was the major contributor to individual dietary risk, while cumulative Hazard Index values exceeded 1 for both children and adults, indicating potential concern from combined pesticide exposure. The high residue complexity at Image and Lyamkena Tomato Farms highlights important contamination hotspots. Strengthened seasonal residue monitoring, pesticide-use regulation, farmer training, and Integrated Pest Management are needed to reduce dietary pesticide exposure and improve tomato food safety.